Post-Frame Building Design Manual - [PDF Document] (2023)

National Frame Builders Association

Post-Frame Building Design Manual

Chapter 1: INTRODUCTION TO POST-FRAME BUILDINGS1.1 General1.1.1Main Characteristics. Post-frame buildings are structurallyefficient buildings composed of main members such as posts andtrusses and secondary components such as purlins, girts, bracingand sheathing Snow and wind loads are transferred from thesheathing to the secondary members. Loads are transferred to theground through the posts that typically are embedded in the groundor surface-mounted to a concrete or masonry foundation. Figure 1.1illustrates the structural components of a post-frame building.1.1.2 Use. Post-frame construction is wellsuited for manycommercial, industrial, agricultural and residential applications.Post-frame offers unique advantages in terms of design andconstruction flexibility and structural efficiency. For thesereasons, post-frame construction has experienced rapid growth,particularly in nonagricultural applications.

Roof cladding

Ridge cap

Purlin

Truss

Wall cladding

Doorway Pressure preservative treated post Pressure preservativetreated splash board Concrete footing Wall girt

Figure 1.1. Simplified diagram of a post-frame building. Somecomponents such as permanent roof truss bracing and interiorfinishes are not shown.

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1.2 Evolution1.2.1 The concept of pole-type structures is notnew. Archeological evidence exists in abundance that pole buildingshave been used for human housing for thousands of years. InAmerica, pole buildings began appearing on farms in the 19thcentury (Norum, 1967). 1.2.2 Pole-type construction resurfaced in1930 when Mr. H. Howard Doane introduced the "modern pole barn" asan economical alternative to conventional barns (Knight, 1989). Mr.Doane was the founder of Doane's Agricultural Service, a firmspecializing in managing farms for absentee owners. These earlypole barns were constructed with red cedar poles that werenaturally resistant to decay, trusses spaced 2 ft oncenter, 1-inchnominal purlins and galvanized steel sheathing. In the 1940s, polebarn construction was refined by using creosotepreservative-treated sawn posts, wider truss and purlin spacings,and improved steel sheathing. Mr. Bernon G. Perkins, an employee ofDoane's, is credited for many of the refinements to Doane'soriginal pole barn. In 1949, Mr. Perkins applied for the firstpatent on the pole building concept through Doane's AgriculturalService, and the patent was issued in 1953. Rather than protectingtheir patent, they publicized the concept and encouraged its usethroughout the world. In 1995, the post-frame building concept wasrecognized as an Historic Agricultural Engineering Landmark by theAmerican Society of Agricultural Engineers. 1.2.3 In the past twodecades, post-frame construction has been further enhanced by thedevelopments of metal-plate connected wood trusses, nail- andglue-laminated posts, highstrength steel sheathing, fasteners anddiaphragm design methods. Composites such as laminated posts andstructural composite lumber offer advantages of superior strengthand stiffness, dimensional stability, and they can be obtained in avariety of sizes and pressure preservative treatments. Developmentsin metalplate connected wood truss technology allow clear spans ofover 80 feet. Design procedures were introduced in the early 1980sto more accurately account for the effect of diaphragm ac-

tion on post and foundation design (Knight, 1990). New roofpanel constructions using highstrength steel and customized screwfasteners have dramatically improved diaphragm stiffness andstrength.

1.3 Advantages1.3.1 Reliability. Outstanding structuralperformance of post-frame buildings under adverse conditions suchas hurricanes is welldocumented. Professor Gurfinkel, in his woodengineering textbook, cites superior performance of post-framebuildings over conventional construction during hurricane Camillein 1969 (Gurfinkel, 1981). Harmon et. al (1992) reported thatpost-frame buildings constructed according to engineered plansgenerally withstood hurricane Hugo (wind gusts measured at 109mph). Since post-frame buildings are relatively light weight,seismic forces do not control the design unless significantadditional dead loads are applied to the structure (Faherty andWilliamson, 1989; Taylor, 1996). 1.3.2 Economy. Significant savingscan be obtained with post-frame construction in terms of materials,labor, construction time, equipment and building maintenance. Forexample, postframe buildings require less extensive foundationsthan other building types because the wall sections between theposts are non-load bearing. Embedded post foundations commonly usedin post-frame require less concrete, heavy equipment, labor, andconstruction time than conventional perimeter foundations.Additionally, embedded post foundations are better-suited forwintertime construction. 1.3.3 Versatility. Post-frame constructionfacilitates design flexibility. Posts can be embedded into theground or surface-mounted to a concrete foundation. Steel sheathingcan be replaced with wood siding, brick veneer, and conventionalroofing materials, to satisfy the appearance and servicerequirements of the customer. One-hour fire-rated wall androof/ceiling constructions have been developed for wood framedassemblies. Exposed glued-laminated and solid-sawn timbers can besubstituted for trusses made from dimension lumber to achievedesired architectural effects.

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1.4 Industry Profile1.4.1 Post-frame construction hasexperienced tremendous growth since World War II. This growth wasfueled by the abundant supplies of steel and pressurepreservative-treated wood, together with the need for low-coststructures. In the 1950s and 1960s, the pole barn industry wascharacterized by large numbers of independent builders (Knight,1989). During this time, pole builders were expanding from theirtraditional agricultural base into other construction markets. Thisexpansion into code-enforced construction required rigorousdocumentation of engineering designs and more involvement in thebuilding code arena. 1.4.2 NFBA. Approximately 20 builders met in1969 to discuss challenges facing the postframe building industry.The group voted in favor of forming the National Frame BuildersAssociation (NFBA). The NFBA became incorporated in 1971 and thefirst national headquarters was established in Chicago, Illinois.Today, the National Frame Builders Association is headquartered inLawrence, Kansas and includes over 300 contractors and suppliers,with regional branches throughout the U.S. In addition, a CanadianDivision of NFBA was created in 1984. 1.4.3 The post-frame industryhas become one of the fastest growing segments of the totalconstruction industry. Based on light-gauge steel sales, post-frameindustry revenues are estimated to be from 2 to 2.5 billion dollarsin 1990.

ASAE: The Society for engineering in agricultural, food, andbiological systems (formerly American Society of AgriculturalEngineers). Anchor Bolts: Bolts used to anchor structural membersto a foundation. Commonly used in post-frame construction to anchorposts to the concrete foundation. ASCE: American Society of CivilEngineers. AWC: American Wood Council. The wood products divisionof the American Forest & Paper Association (AF&PA). AWPB:American Wood Preservers Bureau. Bay: The area between adjacentprimary frames in a building. In a post-frame building, a bay isthe area between adjacent post-frames. Bearing Height: Verticaldistance between a pre-defined baseline (generally the grade line)and the bearing point of a component. Bearing Point: The point atwhich a component is supported. Board: Wood member less than two(2) nominal inches in thickness and one (1) or more nominal inchesin width. Board-Foot (BF): A measure of lumber volume based onnominal dimensions. To calculate the number of board-feet in apiece of lumber, multiply nominal width in inches by nominalthickness in inches times length in feet and divide by 12. BOCA:Building Officials & Code Administrators International, Inc.The organization responsible for maintaining and publishing theNational Building Code. Bottom Chord: An inclined or horizontalmember that establishes the bottom of a truss. Bottom Plank: SeeSplashboard.

1.5 TerminologyAF&PA: American Forest & PaperAssociation (formerly National Forest Products Association). AITC:American Institute of Timber Construction. ALSC: American LumberStandard Committee. ANSI: American National Standards InstituteAPA: The Engineered Wood Association (formerly the American PlywoodAssociation)

Butt Joint: The interface at which the ends of two members meetin a square cut joint.

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Camber: A predetermined curvature designed into a structuralmember to offset the anticipated deflection when loads are applied.Check: Separation of the wood that usually extends across theannual growth rings (i.e., a split perpendicular-to-growth rings).Commonly results from stresses that build up in wood duringseasoning. Cladding: The exterior and interior coverings fastenedto the wood framing. Clear Height: Vertical distance between thefinished floor and the lowest part of a truss, rafter, or girder.Collars: Components that increase the bearing area of portions ofthe post foundation, and thus increase lateral and verticalresistance. Components and Cladding: Elements of the buildingenvelope that do not qualify as part of the main wind-forceresisting system. In postframe buildings, this generally includesindividual purlins and girts, and cladding. Diaphragm: A structuralassembly comprised of structural sheathing (e.g., plywood, metalcladding) that is fastened to wood or metal framing in such amanner the entire assembly is capable of transferring in-planeshear forces. Diaphragm Action: The transfer of load by adiaphragm. Diaphragm Design: Design of roof and ceilingdiaphragm(s), wall diaphragms (shearwalls), primary and secondaryframing members, component connections, and foundation anchoragesfor the purpose of transferring lateral (e.g., wind) loads to thefoundation structure. Dimension Lumber: Wood members from two (2)nominal inches to but not including five (5) nominal inches inthickness, and 2 or more nominal inches in width. Eave: The part ofa roof that projects over the sidewalls. In the absence of anoverhang, the eave is the line along the sidewall formed by theintersection of the wall and roof planes.

Fascia: Flat surface (or covering) located at the outer end of aroof overhang or cantilever end. Flashing: Sheet metal or plasticcomponents used at major breaks and/or openings in walls and roofsto insure weather-tightness in a structure. Footing: Support basefor a post or foundation wall that distributes load over a greatersoil area. Frame Spacing: Horizontal distance between post-frames(see post-frame and post-frame building). In the absence of posts,the frame spacing is generally equated to the distance betweenadjacent trusses (or rafters). Frame spacing may vary within abuilding. Gable: Triangular portion of the endwall of a buildingdirectly under the sloping roof and above the eave line. GableRoof: Roof with one slope on each side. Each slope is of equalpitch. Gambrel Roof: Roof with two slopes on each side. The pitchof the lower slope is greater than that of the upper slope. Girder:A large, generally horizontal, beam. Commonly used in post-framebuildings to support trusses whose bearing points do not coincidewith a post. Girt: A secondary framing member that is attached(generally at a right angle) to posts. Girts laterally supportposts and transfer load between wall cladding and posts.Glued-Laminated Timber: Any member comprising an assembly oflaminations of lumber in which the grain of all laminations isapproximately parallel longitudinally, in which the laminations arebonded with adhesives. Grade Girt: See Splashboard. Grade Line(grade level): The line of intersection between the buildingexterior and the top of the soil, gravel, and/or pavement incontact with the building exterior. For post-frame building

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design, the grade line is generally assumed to be no lower thanthe lower edge of the splashboard. Header: A structural framingmember that supports the ends of structural framing members thathave been cut short by a floor, wall, ceiling, or roof opening. HipRoof: Roof which rises by inclined planes from all four sides of abuilding. IBC: International Building Code. ICBO: InternationalConference of Building Officials. The organization responsible formaintaining and publishing the Uniform Building Code. Knee Brace:Inclined structural framing member connected on one end to apost/column and on the other end to a truss/rafter. LaminatedAssembly: A structural member comprised of dimension lumberfastened together with mechanical fasteners and/or adhesive.Horizontally- and vertically-laminated assemblies are primarilydesigned to resist bending loads applied perpendicular and parallelto the wide face of the lumber, respectively. Laminated VeneerLumber (LVL) A structural composite lumber assembly manufactured bygluing together wood veneer sheets. Each veneer is orientated withits wood fibers parallel to the length of the member. Individualveneer thickness does not exceed 0.25 inches. Loads: Forces orother actions that arise on structural systems from the weight ofall permanent construction, occupants and their possessions,environmental effects, differential settlement, and restraineddimensional changes. Dead Loads: Gravity loads due to the weight ofpermanent structural and nonstructural components of the building,such as wood framing, cladding, and fixed service equipment. LiveLoads: Loads superimposed by the construction, use and occupancy ofthe building, not including wind, snow, seismic or dead loads.

Seismic Load: Lateral load acting in the horizontal direction ona structure due to the action of earthquakes. Snow Load: A loadimposed on a structure due to accumulated snow. Wind Loads: Loadscaused by the wind blowing from any direction. Lumber Grade: Theclassification of lumber in regard to strength and utility inaccordance with the grading rules of an approved (ALSC accredited)lumber grading agency. LVL: see Laminated Veneer Lumber. MainWind-Force Resisting System: An assemblage of structural elementsassigned to provide support and stability for the overallstructure. Main wind-force resisting systems in post-framebuildings include the individual postframes, diaphragms andshearwall Manufactured Component. A component that is assembled ina manufacturing facility. The wood trusses and laminated columnsused in post-frame buildings are generally manufactured components.MBMA: Metal Building Manufacturers Association. NDS: NationalDesign Specification for Wood Construction. Published by AF&PA.Mechanically Laminated Assembly: A laminated assembly in which woodlaminations have been joined together with nails, bolts and/orother mechanical fasteners. Metal Cladding: Metal exterior andinterior coverings, usually cold-formed aluminum or steel sheet,fastened to the structural framing. NFBA: National Frame BuildersAssociation. NFPA: National Fire Protection Association Nominalsize: The named size of a member, usually different than actualsize (as with lumber).

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Orientated Strand Board (OSB): Structural wood panelsmanufactured from reconstituted, mechanically oriented wood strandsbonded with resins under heat and pressure. Orientated StrandLumber (OSL): Structural composite lumber (SCL) manufactured frommechanically oriented wood strands bonded with resins under heatand pressure. Also known as laminated strand lumber (LSL) OSB: SeeOrientated Strand Board. Parallel Strand Lumber (PSL): Structuralcomposite lumber (SCL) manufactured by cutting 1/8-1/10 inch thickwood veneers into narrow wood strands, and then gluing and pressingthe strands together. Individual strands are up to 8 feet inlength. Prior to pressing, strands are oriented so that they areparallel to the length of the member. Pennyweight: A measure ofnail length, abbreviated by the letter d. Plywood: A built-up panelof laminated wood veneers. The grain orientation of adjacentveneers are typically 90 degrees to each other. Pole: A round,unsawn, naturally tapered post. Post: A rectangular membergenerally uniform in cross section along its length. Post may besawn or laminated dimension lumber. Commonly used in post-frameconstruction to transfer loads from main roof beams, trusses orrafters to the foundation. Post Embedment Depth: Vertical distancebetween the bottom of a post and the lower edge of the splashboard.Post Foundation: The embedded portion of a structural post and anyfooting and/or attached collar. Post Foundation Depth: Verticaldistance between the bottom of a post foundation and the lower edgeof the splashboard. Post-Frame: A structural building frameconsisting of a wood roof truss or rafters connected to verticaltimber columns or sidewall posts.

Post-Frame Building: A building system whose primary framingsystem is principally comprised of post-frames. Post Height: Thelength of the non-embedded portion of a post. Pressure PreservativeTreated (PPT) Wood: Wood pressure-impregnated with an approvedpreservative chemical under approved treatment and quality controlprocedures. Primary Framing: The main structural framing members ina building. The primary framing members in a post-frame buildinginclude the columns, trusses/rafters, and any girders that transferload between trusses/rafters and columns. PSL: See Parallel StrandLumber. Purlin: A secondary framing member that is attached(generally at a right angle) to rafters/ trusses. Purlins laterallysupport rafters and trusses and transfer load between exteriorcladding and rafters/trusses. Rafter: A sloping roof framingmember. Rake: The part of a roof that projects over the endwalls.In the absence of an overhang, the rake is the line along theendwall formed by the intersection of the wall and roof planes.Ridge: Highest point on the roof of a building which describes ahorizontal line running the length of the building. Ring ShankNail: See threaded nail. Roof Overhang: Roof extension beyond theendwall/sidewall of a building. Roof Slope: The angle that a roofsurface makes with the horizontal. Usually expressed in units ofvertical rise to 12 units of horizontal run. SBC: Standard BuildingCode (see SBCCI). SBCCI: Southern Building Code CongressInternational, Inc. The organization responsible for maintainingand publishing the Standard Building Code.

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Secondary Framing: Structural framing members that are used to(1) transfer load between exterior cladding and primary framingmembers, and/or (2) laterally brace primary framing members. Thesecondary framing members in a post-frame building include thegirts, purlins and any structural wood bracing. Self-DrillingScrew: A screw fastener that combines the functions of drilling andtapping (thread forming). Generally used when one or more of thecomponents to be fastened is metal with a thickness greater than0.03 inches Self-Piercing Screw: A self-tapping (thread forming)screw fastener that does not require a pre-drilled hole. Differsfrom a self-drilling screw in that no material is removed duringscrew installation. Used to connect light-gage metal, wood, gypsumwallboard and other "soft" materials. SFPA: Southern ForestProducts Association Shake: Separation of annual growth rings inwood (splitting parallel-to-growth rings). Usually considered tohave occurred in the standing tree or during felling. Shearwall: Avertical diaphragm in a structural framing system. A shearwall isany endwall, sidewall, or intermediate wall capable of transferringin-plane shear forces. Siphon Break: A small groove to arrest thecapillary action of two adjacent surfaces. Soffit: The undersidecovering of roof overhangs. Soil Pressure: Load per unit area thatthe foundation of a structure exerts on the soil. Span: Horizontaldistance between two points. Clear Span: Clear distance betweenadjacent supports of a horizontal or inclined member. Horizontaldistance between the facing surfaces of adjacent supports.Effective Span: Horizontal distance fromcenter-of-required-bearing-width tocenterof-required-bearing-width, or the "clear

span" for rafters and joists in conventional construction.Out-To-Out Span: Horizontal distance between the outer faces ofsupports. Commonly used in specifying metal-plateconnected woodtrusses. Overall Span: Total horizontal length of an installedhorizontal or inclined member. SPIB: Southern Pine InspectionBureau. Skirtboard: See Splashboard. Splashboard: A preservativetreated member located at grade that functions as the bottom girt.Also referred to as a skirtboard, splash plank, bottom plank, andgrade girt. Splash Plank: See Splashboard. Stitch (or Seam)Fasteners: Fasteners used to connect two adjacent pieces of metalcladding, and thereby adding shear continuity between sheets.Structural Composite Lumber (SCL): Reconstituted wood productscomprised of several laminations or wood strands held together withan adhesive, with fibers primarily oriented along the length of themember. Examples include LVL and PSL. Threaded Nail: A type of nailwith either annual or helical threads in the shank. Threaded nailsgenerally are made from hardened steel and have smaller diametersthan common nails of similar length. Timber: Wood members five ormore nominal inches in the least dimension. Top Chord: An inclinedor horizontal member that establishes the top of a truss. TPI:Truss Plate Institute. Truss: An engineered structural component,assembled from wood members, metal connector plates and/or othermechanical fasteners, designed to carry its own weight andsuperimposed design loads. The truss members form a

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semi-rigid structural framework and are assembled such that themembers form triangles. UBC: Uniform Building Code (see ICBO).Wane: Bark, or lack of wood from any cause, on the edge or cornerof a piece. Warp: Any variation from a true plane surface. Warpincludes bow, crook, cup, and twist, or any combination thereof.Bow: Deviation, in a direction perpendicular to the wide face, froma straight line drawn between the ends of a piece of lumber. Crook:Deviation, in a direction perpendicular to the narrow edge, from astraight line drawn between the ends of a piece of lumber. Cup:Deviation, in the wide face of a piece of lumber, from a straightline drawn from edge to edge of the piece. Twist: A curl or spiralof a piece of lumber along its length. Measured by laying lumber ona flat surface such that three corners contact the surface. Theamount of twist is equal to the distance between the flat surfaceand the corner not contacting the surface. WCLIB: West Coast LumberInspection Bureau Web: Structural member that joins the top andbottom chords of a truss. Web members form the triangular patternstypical of most trusses. WTCA: Wood Truss Council of America. WWPA:Western Wood Products Association.

Harmon, J.D., G.R. Grandle and C.L. Barth. 1992. Effects ofhurricane Hugo on agricultural structures. Applied Engineering inAgriculture 8(1):93-96. Knight, J.T. 1989. A brief look back. FrameBuilding Professional 1(1):38-43. Knight, J.T. 1990. Diaphragmdesign - technology driven by necessity. Frame BuildingProfessional 1(5):16,44-46. Norum, W.A. 1967. Pole buildings gomodern. Journal of the Structural Division, ASCE, Vol. 93, No.ST2,Proc. Paper 5169, April, pp.47-56. Taylor, S.E. 1996. Earthquakeconsiderations in post-frame building design. Frame Building News8(3):42-49.

1.6 ReferencesFaherty, K.F. and T.G. Williamson. 1989. WoodEngineering and Construction Handbook. McGraw-Hill PublishingCompany, New York, NY. Gurfinkel, G. 1981. Wood Engineering (2ndEd.). Kendall/Hunt Publishing Company, Dubuque, Iowa.

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Chapter 2: BUILDING CODES, DESIGN SPECIFICATIONS AND ZONINGREGULATIONS2.1 Introduction2.1.1 Definition. A building code is alegal document that helps ensure public health and welfare byestablishing minimum standards for design, construction, quality ofmaterials, use and occupancy, location and maintenance of allbuildings and structures. 2.1.2 Model Versus Active Codes. A modelcode is a code that is written for general use (i.e., a code thatis not written for use by a specific state, county, town, village,company or individual). An active code is a model or speciallywritten code that has been adopted and is enforced by a regulatoryagency such as a state or local government. It follows that in agiven jurisdiction, acceptance of a model building code isvoluntary until the model code becomes part of the active code inthe jurisdiction. 2.1.3 Active Code Variations. The content andadministration of active building codes varies not only betweenstates, but frequently between municipalities within a state. Somestates have established a hierarchy structure of state, county andtownship/village/city building codes. In this situation, morelocalized governing areas can modify the state (or county) codes,provided the changes result in more strict provisions. Despitelocal differences in content and administration, most activebuilding codes share the common trait of regulating components ofconstruction based on building occupancy and use. (SBCCI). Thesemodel building codes are commonly referred to as the UBC, BOCA andthe Southern Building Code, respectively. 2.2.2 Adoption. Moststates have adopted (and enforce) all or a major portion of one ofthe three model building codes. As shown in figure 2.1, westernstates have adopted the UBC, northeastern states the BOCA code, andstates in the southwest the Southern Building Code. 2.2.3Development. Model building codes are consensus documentscontinually studied and annually revised by building officials,industry representatives and other interested parties. 2.2.4International Building Code. On December 9, 1994, the three modelbuilding code agencies (BOCA, ICBO and SBCCI) created theInternational Code Council (ICC). The ICC was established inresponse to technical disparities among the three major modelcodes. Since its founding, the ICC has worked to create a singlemodel building code for the U.S. This code, which is entitled theInternational Building Code is now complete and will replace thethree model codes over the next couple years. With all statesadopting the same model code, it will be less difficult forbuilding designers to work in different regions of the country.

(Video) How To: Reading Construction Blueprints & Plans | #1

2.3 Building Classification2.3.1 General. Building codes givecriteria for classifying buildings based on: (1) use or occupancy,and (2) type of construction. 2.3.2 Occupancy Classifications.Occupancy classifications include assembly, business, educational,factory and industrial, high-hazard, institutional, mercantile,residential and storage. Occupancy classifications haverequirements on the number of occupants and building separation,height and area. Other limits exist, for example on lighting,ventilation, sanitation, fire

2.2 Major Model Building Codes2.2.1 Current Codes. There arecurrently three primary model building codes in the United States.These are the Uniform Building Code (UBC) published by theInternational Congress of Building Officials, the National BuildingCode published by the Building Officials and Code AdministratorsInternational (BOCA) and the Standard Building Code published bythe Southern Building Code Congress International

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Uniform Building Code (ICBO) National Building Code (BOCA)

Standard Building Code (SBCCI)

Figure 2.1. Approximate areas of model building code influence.Wisconsin and New York building codes are developed by theirrespective state code agencies and are not necessarily influencedby current model codes.

protection and exiting, depending on the specific classificationand building code. 2.3.2 Types of Construction. Classification bytype of construction is primarily based on the fire resistanceratings of the walls, partitions, structural elements, floors,ceilings, roofs and exits. Specific requirements vary somewhatbetween model building codes. There are two primary sourcedocuments for determining the fire resistance of assemblies: theFire Resistance Design Manual, published by the Gypsum Association,and the Fire Resistance Directory, published by UnderwritersLaboratories, Inc. The fire resistance of wood framed assembliescan generally be increased by using fire retardant treated (FRT)wood or larger wood members. Codes allow FRT wood to be used incer-

tain areas of noncombustible construction. The superior fireresistance of large timber members is recognized by the codes withthe inclusion of a "heavy timber" classification. To qualify forheavy timber construction, nominal dimensions of timber columnsmust be at least 6- by 8inches and primary beams shall have nominalwidth and depth of at least 6- by 10-inches. 2.3.2.1 NFBA SponsoredFire Test. In January of 1990, the National Frame BuildersAssociation had Warnick Hersey International, Inc., conduct aone-hour fire endurance test on the exterior wall shown in figure2.2. The wall met all requirements for a one-hour rating asprescribed in ASTM E119-88. The wall sustained an applied load of10,400 lbf per column throughout the test. Copies of the fire testreport can be obtained from NFBA.

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Attach metal cladding 12 in. o.c. with 1.5 in. hex head screwswith neoprene washers

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Chapter 3: STRUCTURAL LOAD AND DEFLECTION CRITERIA3.1Introduction3.1.1 Load Variations. Most structural loads exhibitsome degree of random behavior. For example, weather-related loadssuch as snow, wind and rain fluctuate over time and locations.Extensive research has been conducted to characterize this loadvariation, and to refine procedures for determining design loadswithin the context of the intended building occupancy and use.3.1.2 Codes. Calculation procedures for minimum design loads aregiven in the model building codes. Buildings shall be designed tosafely carry all loads specified by the governing building code. Inthe absence of a code, minimum design loads shall be calculatedaccording to recommended engineering practice for the region andapplication under consideration. It is impractical to describedetailed load calculation procedures in this chapter because ofdifferences between building codes and frequent revisions of thesecodes. Instead, general concepts and key references related tostructural loads and deflection criteria are presented, with anemphasis on issues that apply to post-frame buildings. approvalprocess, and then must be adopted by the model building codes.Design professionals should check the governing building code forthe latest adopted edition. For clarity of presentation, thismanual uses and will refer to ASCE 793. ASCE 7-93 is the primarytechnical source used by the model codes concerning dead, live,snow, wind, rain and seismic loads. Basically, the model codesattempt to distill the rigorous ASCE 7-93 procedures into asimpler, easy-touse format. Many specific load calculationprocedures differ between the model codes; however, most of thebasic concepts mimic ASCE 793. Background information on the windload provisions in ASCE 7-88 (which are essentially the same as inASCE 7-93) are given by Mehta et al. (1991). 3.2.2 Low RiseBuilding Systems Manual. The Low Rise Building Systems Manual,published by the Metal Building Manufacturers Association (1986),is recognized by model building codes as an excellent technicalresource document for calculating structural loads on lowrisebuildings (e.g. post-frame buildings). This document will bereferred to as MBMA-86 throughout this manual. Because wind andcrane loads frequently control the design of lowrise metalbuildings, the coverage of these loads within MBMA-86 is especiallythorough. Another attractive feature of MBMA-86 is the extensivecollection of example load calculations. 3.2.3 ASAE EP288.5Standard. Agricultural buildings generally fall into a separateclass from other types of buildings due to the lower risksinvolved. The American Society of Agricultural Engineers publishesa snow and wind load standard, EP288.5, intended for agriculturalbuildings (ASAE, 1999). The major differences between agriculturaland other types of buildings are that lower values are used forimportance and roof snow conversion factors (due to relativelylower risk factors for property and nonpublic use). If the localgoverning building code applies to agricultural buildings, then thedesign load criteria in the code must be followed.

3.2 Technical References on Structural Load Determination3.2.1ANSI/ASCE 7 Standard. The National Bureau of Standards published areport titled Minimum Live Load Allowable for Use in Design ofBuildings in 1924. The report was expanded and published as ASAStandard A58.1-1945. This standard has undergone several revisionsto become the current ASCE Standard ANSI/ASCE 7 Minimum DesignLoads for Buildings and Other Structures. At the time this designmanual was written, the most recent revision of ASCE 7 was 1999(ASCE, 1999); however, the edition most commonly used is ASCE 7-93.The ASCE 7 standard is periodically revised and balloted throughthe ANSI consensus

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Table 3.1. Approximate Weights of Construction Materials (fromHoyle and Woeste, 1989) Weight Material Material (lb/ft2) CeilingsAcoustical fiber tile Gypsum board (see Walls) Mechanical ductallowance Suspended steel channel system Wood purlins (see Wood,Seasoned) Light gauge steel (see Roofs) Floors Hardwood, 1-in.nominal Plywood (see Roofs) Linoleum, 1/4-in. Vinyl tile, 1/8-in.Roofs Corrugated Aluminum 14 gauge 16 gauge 18 gauge 20 gaugeBuilt-Up 3-ply 3-ply with gravel 5-ply 5-ply with gravel CorrugatedGalvanized steel 16 gauge 18 gauge 20 gauge 22 gauge 24 gauge 26gauge 29 gauge Insulation, per inch thickness Rigid fiberboard,wood base Rigid fiberboard, mineral base Expanded polystyreneFiberglass, rigid Fiberglass, batt Lumber (see Wood, Seasoned)Roofs (continued) Plywood (per inch thickness) Roll roofingShingles Asphalt Clay tile Book tile, 2-in. Book tile, 3-inLudowici Roman Slate, in. Wood Walls Wood paneling, 1-in. Glass,plate, 1/4-in. Gypsum board (per 1/8-in. thickMasonry, per 4-in.thickness Brick Concrete block Cinder concrete block StonePorcelain-enameled steel Stucco, 7/8-in. Windows, glass, frame, andsash Wood, Seasoned Cedar Douglas-fir Hemlock Maple, red OakPoplar, yellow Pine, lodgepole Pine, ponderosa Pine, Southern Pine,white Redwood Spruce

Weight (lb/ft2)

1.0 4.0 2.0

3.0 1.0 2.0 9.0-14.0 12.0 20.0 10.0 12.0 10.0 3.0

4.0 1.0 1.4

1.1 0.9 0.7 0.6 1.5 5.5 2.5 6.5 2.9 2.4 1.8 1.5 1.3 1.0 0.8 1.52.1 0.2 1.5 0.1

2.5 3.3 0.55 38.0 20.0 20.0 55.0 3.0 10.0 8.0 Density lb/ft332.0 34.0 31.0 37.0 45.0 29.0 29.0 28.0 35.0 27.0 28.0 29.0

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3.3 Minimum Design LoadsTechnical Note

Sections 3.4, 3.5, 3.6, 3.7, and 3.8 give general loadrequirements, sources of load data and references for makingdetailed load calculations. Detailed calculation procedures are notprovided due to differences between the model codes and thefrequency of code revisions.

Horizontal Uniform Dead Load Calculation Many structuralanalysis programs (e.g. Purdue Plane Structures Analyzer) requirethat the dead load associated with a sloping surface be representedas a uniform load, wDL, acting on a horizontal plane as shown infigure 3.1. For a given horizontal distance, bH, a sloping roofsurface contains more material and is heavier than a flat one.Thus, wDL increases as roof slope increases. Load wDL is obtainedby multiplying the unit weight of the roof assembly, wR, by theslope length, bS, and dividing the resulting product by thehorizontal length, bH. Numerically, this is equivalent to dividingwR by the cosine of the roof slope. Example: For a roof at a 4:12slope, with materials weighing 4 lbm for each square foot of roofsurface area, the equivalent load, wDL, to apply to the horizontalplane would be: wDL = (4 lbm/ft2)/(cos 18.4) = 4.21 lbm/ft2

3.4 Dead Loads3.4.1 Definition. Dead loads are the gravity loadsdue to the combined weights of all permanent structural andnonstructural components of the building, such as sheathing,trusses, purlins, girts and fixed service equipment. These loadsare constant in magnitude and location throughout the life of thebuilding. 3.4.2 Code Application. Minimum design dead loads shallbe determined according to the governing building code. In theabsence of a building code, dead load data can be found in ASCE7-93, or actual weights of materials and equipment can be used.3.4.3 Special Considerations. Design dead loads that exceed theweights of construction materials and permanent fixtures arepermitted, except for when checking building stability under windloading. Using inflated design dead loads may lead to conservativedesigns for gravity load conditions; however, it would not be aconservative assumption for designing anchorage to counteractuplift, overturning and sliding due to wind loads. In the cases ofwind uplift and overturning, the dead load used in design must notexceed the actual dead load of the construction. 3.4.4 Weights ofConstruction Materials. Table 3.1 lists approximate weights ofmaterials. commonly used in post-frame construction.

wDL

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Roof assembly with weight wR per unit area Rafter or truss topchord

3.5 Live Loads3.5.1 Definition. Live loads are defined as theloads superimposed by the construction, maintenance, use andoccupancy of the building, and therefore do not include wind, snow,seismic or dead loads.

Figure 3.1. Roof dead load represented by an equivalent uniformload acting on a horizontal plane. 3.5.2 Code Application. Designlive loads shall be determined so as to provide for the servicerequirements of the building, but should never be lower than theminimum live load specified in

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the governing building code. In the absence of a governingbuilding code, the minimum live loads found in ASCE 7-93 arerecommended. The minimum roof live load recommended foragricultural buildings in ASAE Standard EP288.5 is 12 psf. Someagricultural buildings do not necessarily pose a "low risk", andthe ASAE higher minimum live load reflects the possibility ofhighvalue agricultural constructions now common in the UnitedStates 3.5.3 Reductions. In some cases, reductions are allowed foruniform loads to account for the low likelihood of the loadssimultaneously occurring over the entire tributary area.

pf R Ce Ct I Cs Pg

= = = = = = =

roof snow load in psf, roof snow factor that relates roof loadto ground snowpack, snow exposure factor, roof temperature factor,importance factor, roof slope factor, and ground snow load in psf(50-yr mean recurrence).

3.6 Snow Loads3.6.1 Code Application. Minimum design snow loadsshall be determined by the provisions of the governing buildingcode. The presentation of snow loads varies among the model codes,but they all follow the basic concepts presented in ASCE 7-93. Inthe absence of a building code, procedures given in ASCE 7-93 arerecommended. For low-risk agricultural buildings, snow loadcalculation procedures given in ASAE EP288.5 are permitted. 3.6.2Ground Snow Load Maps. ASCE 7-93 presents ground snow load mapsthat correspond to a mean recurrence interval of 50 years. Thesemaps do not give snow load values for areas that are subject toextreme variations in snowfall, such as western mountain regions.In some regions, the best and only reliable source for ground snowloads is local climatic records. 3.6.3 Roof Snow Loads. Roof snowloads are influenced by a number of factors besides ground snowload. These factors include roof slope, temperature and coefficientof friction of the roof surface, and wind exposure. Snow loads arealso adjusted by an importance factor to account for risk toproperty and people. The basic form of the snow load calculationfound in ASCE 7-93 is: pf where: = R Ce Ct I Cs Pg (3-1)

The roof snow factor, R, varies from 0.6 for Alaska to 0.7 forthe contiguous United States. The snow exposure factor in the modelcodes accounts for the combined effects of R and Ce given inEquation 3-1. The thermal factor defined in ASCE 7-93 varies from1.0 for heated structures to 1.2 for unheated structures. Thethermal factor is not included in the model building codes. Theimportance factors range from 0.8 to 1.2 depending on the specificbuilding code. Roof slope factors vary linearly from 0 to 1 as roofslope increases from 15 to 70 degrees. 3.6.5 SpecialConsiderations. Several factors, such as multiple gables, roofdiscontinuities, and drifting can cause snow to accumulate unevenlyon roofs. These factors must be considered in the design. Specificrecommendations and calculation procedures are given in the modelcodes and ASCE 7-93.

3.7 Wind Loads3.7.1 Controlling Factors. Wind loads areinfluenced by wind speed, building orientation and geometry,building openings and exposure. Wind loading on structures is acomplex phenomenon and is being actively researched. 3.7.2 CodeApplication. Minimum design wind loads shall be determined by theprovisions of the governing building code. In the absence of abuilding code, procedures given in ASCE 7-93 or MBMA-86 arerecommended. For low-risk agricultural buildings, wind loadcalculation procedures given in ASAE EP288.5 are permitted. 3.7.3Design Wind Speed. ASCE 7-93 gives a map showing basic wind speedsthroughout the United States that correspond to a mean recurrenceinterval of 50 years. Local weather rec-

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ords should be used in regions that have unusual wind events.Detailed procedures and illustrations for calculating wind loads onlow-rise buildings are given in MBMA-86.Technical Note

Wind Speed Wind speeds are derived from data which reflect bothmagnitude and duration. Wind speeds can be reported as peak gusts,or can be averaged over some time interval. The time interval maybe fixed, as with mean hourly speeds, or variable, as withfastest-mile wind speeds. Fastest-mile wind speeds are used inANSI/ASCE 793 to calculate design loads, and are defined on thebasis of the period of time that one mile of wind takes to pass ananemometer at a standard elevation of 10 meters. The U.S. NationalWeather Service no longer collects fastest-mile wind speed data;instead, they record 3-second gust speeds. The 1995 and laterrevisions of ASCE-7 base wind loads on 3-second gust windspeeds.

model codes publish fewer exposure categories. Importancefactors vary from 0.95 for agricultural buildings (25-yearrecurrence interval) to 1.07 for buildings that represent a highhazard to property and people in the event of failure (100yearrecurrence interval). Wind pressure is related to the square of itsspeed, therefore the terms V and I are squared in equation 3-2. Themodel building codes simplify the calculation in equation 3-2 bypublishing tables of effective wind velocity pressures, Pb, for abase wind speed and various heights. 3.7.5 Pressure Coefficients.Wind loads are calculated for each part of the building bymultiplying the effective wind pressure by a pressure coefficient.The pressure coefficient, which may be different for each planarportion of the building, accounts for building orientation,geometry and load sharing. It also accounts for localized pressuresat eaves, overhangs, corners, etc. Wind pressures, qi, for the ithbuilding surface are calculated by: qi = Cpi qz where: (3-3)

3.7.4 Effective Wind Velocity Pressure. The first step indetermining wind loads is to calculate the effective wind velocitypressure. The most severe exposure factors that will apply duringthe service life of the structure should be used. Wind velocitypressure is a function of the wind speed, exposure and importance.The equation for calculating wind velocity pressure, qz , is givenby: qz = where: 0.00256 Kz (I V)2 (3-2)

Cpi = qz =

ith pressure coefficient, and wind velocity pressure.

The wind velocity pressure is based on the wall height for thewindward wall and on the mean roof height for the leeward wall androof. Wind pressures act normal to the building surfaces. Inwardpressures are denoted with positive signs, while outward pressures(suction) are denoted with negative signs.Technical Note

Kz = I V = =

velocity pressure exposure coefficient, importance factor, andbasic wind speed in mph (50-year mean recurrence interval).

Components of Wind Load Many structural analysis programsrequire uniform loads to be entered in terms of their horizontaland vertical components. Wind loads act normal to buildingsurfaces, so an adjustment is needed for sloping members such asroof trusses. The roof wind load, w, shown in figure 3.2a isequivalent to the horizontal and vertical components shown infigure 3.2b. The relationship depicted in figure 3.2 can be provenas follows:

The velocity pressure exposure coefficient is a function ofheight above ground and exposure category. Exposure categoriesaccount for the effects of ground surface irregularities caused bynatural topography, vegetation, location and building constructionfeatures. ASCE 7-93 lists four wind exposure categories, whereasthe

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1. Convert the uniform wind load, w, to its resultant vectorforce. R = w (span)/(cos ) 2. Multiply resultant force, R, by costo obtain its vertical component. Fy = R (cos ) = w (span) 3.Divide the vertical component, Fy, by the span to obtain thehorizontally projected uplift pressure, whoriz. whoriz = Fy /(span)= w (span)/(span) = w The vertically projected uniform load can beproven similarly. A common mistake is to multiply the normalpressure by sine and cosine of the roof slope to obtain the twocomponents.

these members have relatively large tributary areas, localizedwind effects tend to be averaged out over the tributary areas.Pressure coefficients for main members reflect this averagingeffect. 3.7.7 Components and Cladding. Wind pressures are higher onsmall areas due to localized gust effects. This observation hasbeen verified by wind tunnel studies (MBMA, 1986), as well as siteinspections of wind-induced building failures (Harmon, et al.,1992). For this reason, components and cladding have higherpressure coefficients than main frames. Components and claddinginclude members such as purlins, girts, curtain walls, sheathing,roofing and siding. 3.7.8 Openings. Wind loads are significantlyaffected by openings in the structure. ASCE 793 and the modelbuilding codes specify internal wind pressure coefficients (oradjustments to external pressure coefficients) for structures withdifferent amounts and types of openings. Each model code hasslightly different definitions and wind load coefficients for open,closed and partially open buildings. In general, "openings" referto permanent or other openings that are likely to be breachedduring high winds. For example, if window glazings are likely to bebroken during a windstorm, the windows are considered openings.However, if doors and windows and their supports are designed toresist design wind loads, they need not be considered openings. Itshould be noted that internal wind pressures act against allinterior surfaces and therefore do not contribute to sidesway loadson a building.

(a) w

(b)

3.8 Seismic Loads3.8.1 Cause. Earthquakes produce lateral forceson buildings through the sudden movement of the buildingsfoundation. Building response to seismic loading is a complexphenomenon and there is considerable controversy as to how totranslate knowledge gained through research into practical designcodes and standards. 3.8.2 Code Application. Seismic loads shall bedetermined by the provisions of the governing building code. In theabsence of a building code, procedures given in ASCE 7-93 arerecommended. Sweeping changes were made in the

Figure 3.2. Illustration of wind load acting normal to inclinedsurface and equivalent horizontal and vertical load components. Acommon mistake is to multiply the normal load by sin() and cos()for the vertical and horizontal components, respectively.

3.7.6 Main Frames. Different pressure coefficients are used tocalculate wind loads on main frames as compared to components andcladding. Main frames include primary structural systems such asrigid and braced frames, braced trusses, posts, poles and girders.Since

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1993 revision of ASCE 7 with respect to seismic loads. Theseismic provisions in ASCE 7-93 were based on work by the BuildingSeismic Safety Council under sponsorship of the Federal EmergencyManagement Agency. 3.8.3 Lateral Force. Basic concept of seismicload determination for low-rise buildings is to calculate anequivalent lateral force at the ground line as follows: V = Cs Wwhere: V = total lateral force, or shear, at the building basetotal dead load, plus other applicable loads specified in the codeor ASCE 7-93. For most single-story post-frame buildings, the onlyother minimum applicable load is a portion (20% minimum) of theflat roof snow load. If the flat roof snow load is less than 30psf, the applicable load to be included in W is permitted to betaken as zero. seismic design coefficient 1.2 Av S/(T2/3 R)coefficient representing effective peak velocity-relatedacceleration coefficient for the soil profile characteristicsresponse modification factor fundamental period of the building(3-4)

3.9.2 Load Combinations. Except when applicable codes provideotherwise, the following load combinations shall be considered (asa minimum) and the combination which results in the mostconservative design for each building element shall be used. Notethat different load combinations may control the design ofdifferent components of the structure. Case 1: Snow) Case 2: Case3: Case 4: Case 5: Dead + Floor Live + Roof Live (or Dead + FloorLive + Wind (or Seismic) Dead + Floor Live + Wind + Snow Dead +Floor Live + Wind + Snow Dead + Floor Live + Snow + Seismic

W =

3.9.3 Floor Live Loads. Most post-frame buildings are singlestory and therefore would not have floor live loads acting on thepost-frames. When a concrete floor is used in a single storybuilding, consideration must be given to anticipated live andequipment loading. 3.9.4 Reductions. Reductions in some of the loadterms in Cases 1 through 5 are permitted, depending on governingbuilding code or reference document. With some exceptions, themodel building codes permit allowable stresses used in allowablestress design to be increased one-third when considering wind orseismic forces either acting alone or when combined with verticalloads. The allowable stress increase for wind loading can be tracedback to the New York City Building Code of 1904 (Ellifritt, 1977),and appears to be based on judgment rather than engineering theory.It should be noted that ASCE 7-93 does not include the one-thirdincrease factor, but instead specifies load combination factorsthat are intended to account for the low probability of maximumlive, seismic, snow and wind loads occurring simultaneously. Thecommentary of ASCE 7-93 implies the stress increase for wind andseismic found in codes is not appropriate if the combined loadeffects are also reduced by the load combination factors publishedin ASCE 7-93. Finally, the National Design Specification (NDS) forWood Construction (NF&PA, 199) addresses the issue of loadcombination versus load duration factors by stating, The loadduration factors, CD, in Table 2.3.2 and Appendix B are independentof load combination factors, and both shall be permitted to be usedin design calculations.

Cs = = Av = S R T = = =

3.8.4 Seismic loads rarely control post-frame building designbecause of the relatively low building dead weight as compared withother types of construction (Taylor, 1996; Faherty and Williamson,1989). For post-frame buildings, lateral loads from wind usuallyare much greater than those from seismic forces.

3.9 Load Combinations for Allowable Stress Design3.9.1 CodeApplication. Every building element shall be designed to resist themost critical load combinations specified in the governing buildingcode.

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3.10 Load Duration Factors for WoodIt is well documented thatwood has the property of being able to carry substantially greaterloads for short durations than for long durations of loading. Thisproperty is accounted for in design through the application of loadduration factors to all allowable design values except modulus ofelasticity and compression perpendicular to grain. Additionalrestrictions and details on load duration adjustments can be foundin Chapter 2 and Appendix B of the NDS (AF&PA, 1997). 3.10.1Snow Load. The cumulative duration of maximum snow load over thelife of a structure is generally assumed to be two months. Itshould be emphasized that the two-month period does not necessarilymean that the design snow load from any one event would last twomonths. Rather, it means that the total time that the roof supportsthe full design snow load over the life of the structure is twomonths. If the cumulative full design load is two months, anallowable stress increase of 15 percent is allowed (AF&PA,1997). However, in some situations, such as unheated or heavilyinsulated buildings in cold climates, longer snow load durationsmay occur and the stress increase may not be justified. 3.10.2 WindLoad. The cumulative duration of maximum wind (and seismic) loadsover the life of a structure is generally assumed to be 10 minutes(AF&PA, 1997), if design wind loads are based on ASCE 7-93, andthe corresponding load duration factor is 1.6. Other load durationadjustments may be appropriate when design wind loads are based onearlier versions of ASCE 7-93 or other standards (with differentwind gust duration assumptions).

flexibility of corrugated metal siding, girt deflections presentno serviceability problems, and consequently, girt size isgenerally only stress dependent. 3.11.3 Time Dependent Deflection.In certain situations, it may be necessary to limit deflectionunder long term loading. Published modulus of elasticity, E, valuesfor wood are intended for the calculation of immediate deflectionunder load. Under sustained loading, wood members exhibitadditional time-dependent deformation (i.e. creep). It is customarypractice to increase calculated deflection from long-term loadingby a factor of 1.5 for glued-laminated timber and seasoned lumber,or 2 for unseasoned lumber (see Appendix F, AF&PA, 1997). Thus,total deflection is equal to the immediate deflection due tolong-term loading times the creep deflection factor, plus thedeflection due to the short-term or normal component of load. Forapplications where deflection is critical, the published value of E(which represents the average) may be reduced as deemed appropriateby the designer. The size of the reduction depends on thecoefficient of variation of E. Typical values of E variability areavailable for different wood products (see Appendix F, AF&PA,1997). 3.11.4 Shear Deflection. Shear deflection is usuallynegligible in the design of steel beams; however, shear deflectioncan be significant in wood beams. Approximately 3.4 percent of thetotal beam deflection is due to shear for wood beams of usualspan-to-depth proportions (i.e. 15:1 to 25:1). For this reason, thepublished value of E in the Supplement to the National DesignSpecification is 3.4 percent less than the true flexural value(AF&PA, 1993). This correction compensates for the omission ofthe shear term in handbook beam deflection equations. Forspan-to-depth ratios over 25, the predicted deflection using thepublished E value will exceed the actual deflection. Similarly, forspan-todepth ratios less than 15, predicted deflections will besignificantly less than actual. This could lead to unconservativedesigns (with respect to serviceability) for post-frame memberssuch as door headers. Practical information on the effects of sheardeformation on beam design is given in Appendix D of Hoyle andWoeste (1989) for rectangular wood beams and Triche (1990) for woodI-beams.

3.11 Deflection3.11.1 Code Application. Post-frame buildingcomponents must meet deflection limits specified in the governingbuilding code. 3.11.2 Exception to Code Requirements. Girtssupporting corrugated metal siding are typically not subjected todeflection limitations unless their deflection compromises theintegrity of an interior wall finish. Because of the inherent

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3.12 ReferencesAmerican Forest & Paper Association(AF&PA). 1997. ANSI/AF&PA NDS-1997 - National DesignSpecification for Wood Construction. AF&PA, Washington, D.C.American Forest & Paper Association (AF&PA). 1993.Commentary to the National Design Specification for WoodConstruction. AF&PA, Washington, D.C. ASAE. 1999. ASAE EP288.5:Agricultural building snow and wind loads. ASAE Standards 1999,46th edition, ASAE, St. Joseph, MI. American Society of CivilEngineers (ASCE). 1993. Minimum design loads for buildings andother structures. ANSI/ASCE 7-93, ASCE, New York, NY. AmericanSociety of Civil Engineers (ASCE). 1999. Minimum design loads forbuildings and other structures. ANSI/ASCE 7-99, ASCE, New York, NY.Ellifritt, D.S. 1977. The mysterious 1/3 stress increase. AmericanInstitute of Steel Construction Engineering Journal (4):138-140.Faherty, K.F. and T.G. Williamson. 1989. Wood Engineering andConstruction Handbook. McGraw-Hill, New York, NY. Hoyle, R.J. andF.E. Woeste. 1989. Wood Technology in the Design of Structures.Ames, IA: Iowa State University Press. Mehta, K.C., R.D. Marshalland D.C. Perry. 1991. Guide to the Use of the Wind Load Provisionsof ASCE 7-88 (formerly ANSI A58.1). American Society of CivilEngineers, New York, NY. Metal Building Manufacturers Association(MBMA). 1986. Low rise building systems manual. MBMA, Cleveland,OH. Taylor, S.E. 1996. Earthquake considerations in post-framebuilding design. Frame Building News 8(3):42-49.

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Chapter 4: STRUCTURAL DESIGN OVERVIEW4.1 Introduction4.1.1General. The aim of this chapter is to give a broad overview ofpost-frame building design, and then highlight unique aspects ofpost-frame that require special design considerations. Postframe isa special case of light-frame wood construction. Light-frameconstruction is accepted by all model building codes, and thedesign procedures are well documented. The design rules that applyto light-frame wood construction also apply to post-frame. However,there are some aspects of post-frame that are not as familiar tobuilding designers, such as diaphragm design, interaction betweenpost-frames and diaphragms, and post foundation design. Hence,Chapters 5, 6, 7 and 8 focus on these topics in more detail. 4.1.2Primary Framing. Primary framing is the main structural framing ina building. In a postframe building, this includes the columns,trusses (or rafters), and any girders that transfer load betweentrusses and columns. Each truss and the post(s) to which it isattached form an individual "post-frame". Post-frames collect andtransfer load from roof purlins and wall girts to the foundation.In the context of wind loading in standards and building codes,post-frames are an integral part of the main wind-force resistingsystem. Specific sections dedicated to primary framing include:Section 4.2 Posts, Section 4.3 Trusses, Section 4.4 Girders, andSection 4.5 Knee braces. 4.1.3 Secondary Framing. Secondary framingincludes any framing member used to (1) transfer load betweencladding and primary framing members, and/or (2) laterally braceprimary framing members. The secondary framing members in apost-frame building include the girts, purlins and any structuralwood bracing such as permanent truss bracing. Specific sectionsdedicated to secondary framing include: Section 4.6 Roof Purlins,Section 4.7 Wall Girts, and Section 4.8 Large Doors. 4.1.4Diaphragms and Shearwalls. When cladding is fastened to the woodframe of a post-frame building, large shearwalls and roof andceiling diaphragms are formed that can add considerable rigidity tothe building. In many post-frame buildings, diaphragms andshearwalls are carefully designed and become an integral part ofthe main wind-force resisting system. Roof and Ceiling Diaphragmsare covered in Section 4.9 and Shearwalls in Section 4.10. 4.1.5Limitations. The structural design of buildings involves makingmany judgments, such as determining design loads, structuralanalogs and analyses, and selecting materials that can safelyresist the calculated forces. New research or testing could justifya change of design procedure for the industry or for an individualdesigner. The considerations presented here are not exhaustive,since many issues in a specific building design will require uniquetreatment.

4.2 Posts4.2.1 General. The function of the wood post is tocarry axial and bending loads to the foundation. Posts are embeddedin the ground or attached to either a conventional masonry orconcrete wall or a concrete slab on grade. Posts can be solid sawn,mechanically laminated, glued-laminated or wood composite. Anyportion of a post that is embedded or exposed to weather must bepressure-treated with preservative chemicals to resist decay andinsect damage. 4.2.2 Controlling Load Combinations. The loadcombination that usually controls post design is dead plus windplus one-half snow; however, local codes may stipulate differentload combinations. It is possible for any one of the combinationsto be critical; therefore, they all should be considered for aspecific building design. For example, maximum gravity load willgovern truss-to-post bearing and post foundation bearing; whereaswind minus dead load will govern the truss-to-post connection (foruplift). 4.2.3 Force Calculations. The diaphragm analysis methodpresented in Chapter 5 is the most accurate method to determinedesign

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moments, and axial and shear forces in posts. Historically, somedesigners calculated the maximum post moment for embedded posts byusing the simple structural analog of a propped cantilever (i.e.fixed reaction at the post bottom and pin reaction at the top). Theimplicit assumption of this analog is that the roof diaphragm andshearwalls are infinitely stiff. This model may be adequate forbuildings with extremely stiff roof diaphragms and forconservatively estimating shear forces in the roof diaphragm;however, it may underestimate the maximum post moment for manypost-frame buildings. The analysis procedures described in Chapter5 are more reliable since they account for the flexible behavior ofthe roof diaphragm. If posts are embedded, generally two bendingmoments must be calculated - one at the groundline and the otherabove ground. Groundline bending moment and shear values are usedin embedded post foundation design calculations. Forsurface-attached posts, the bottom reaction can be modeled as apin, and generally only one bending moment is calculated. 4.2.4Combined Stress Analysis. Forces involved in post design subjectthe posts to combined stress (bending and axial) and must bechecked for adequacy using the appropriate interaction equationfrom the NDS (AF&PA, 1997). In theory, every post lengthincrement must satisfy the interaction equation, but in practice, aminimum of two locations are checked: the point of maximuminteraction near the ground level (column stability factor, Cp,equal to 1.0) and the upper section of the posts where the maximummoment occurs in conjunction with column action (Cp where: va = Vs= W = DT = allowable shear capacity of shearwall, lbf/ft (N/m)force induced in shearwall, lbf (N) building width, ft (m) totalwidth of door and window openings in the shearwall, ft (m) Vs / (WDT) (5-19)

The allowable shear capacity of end and intermediate shearwalls,va, is obtained from validated structural models, or from tests asoutlined in ASAE EP558 (see Section 6.5). The total force in theshear wall, Vs, is obtained from computer output (e.g. figure 5.8),or equation 57 or equation 5-12 if applicable. The total width ofdoor and window openings, DT, generally varies with height as shownin figure 5.12. At locations where DT is the greatest (section b-bin figure 5.12) additional reinforcing may be required to ensurethat the allowable shear stress is not exceeded. The structuralframing over a door or window opening will act as a drag struttransferring

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shear across the opening. The header over the opening shall bedesigned to carry the force in tension and/or compression acrossthe opening.

5.7.6 Shearwall Overturning. Diaphragm loading producesoverturning moment in shearwalls. This moment induces verticalforces in shearwall-to-foundation connections that must be added tovertical forces resulting from tributary loads. In the case ofembedded posts, increases in uplift forces may require an increasein embedment depth, and increases in downward force may require anincrease in footing size (see Chapter 8).

5.8 Rigid Roof Designb b

Figure 5.12. Shearwall showing variations in opening width, DT,with height. Shearwall strength can easily be increased when theapplied load exceeds shearwall capacity. For example, the densityof stitch screws can be increased and additional fasteners can beadded in panel flats (on both sides of each major rib is the mosteffective). If only one side of the wall has been sheathed, addwood paneling or metal cladding to the other side. Metal diagonalbraces can also be added beneath any wood paneling or corrugatedmetal siding. 5.7.5 Shearwall Connections. Connections that fasten(1) roof and ceiling diaphragms to a shearwall, and (2) shearwallsto the foundation system, must be designed to carry the appropriateamount of shear load. The design of these connections may be provedby tests of a typical connection detail or by an appropriatecalculation method. At end shearwalls it is not uncommon to use thetruss top chord to transfer load from roof cladding to endwallcladding. Sidewall steel is fastened directly to the truss chord,as is the roof steel when purlins are inset. In buildings withtop-running purlins, roof cladding can not be fastened directly tothe truss. In such cases, blocking equal in depth to the purlins isplaced between the purlins and fastened to the truss. Roof claddingis then attached directly to this blocking.

5.8.1 General. When diaphragm stiffness is considerably greaterthan the stiffness of interior post frames, the designer may wantto assume that the diaphragm and shearwalls are infinitely stiff.Under this assumption, 100% of the applied eave load, R, istransferred by the diaphragm to shearwalls, and none of the appliedeave load is resisted by the frames. Because all eave load isassumed to be transferred to shearwalls, no special analysis toolsor design tables are required to determine load distributionbetween diaphragms and post-frames. This simplifies the entirediaphragm design process. This simplified procedure is referred toas rigid roof design (Bender and others, 1991). 5.8.2 Calculation.When (1) the shearwalls and roof/ceiling diaphragm assembly areassumed to be infinitely rigid, (2) the only applied loads withhorizontal components are due to wind, and (3) wind pressure isuniformly distributed on each wall and roof surface, then themaximum shear force in the diaphragm assembly is given as: Vh =where: Vh = L hwr hlr hww hlw = = = = = maximum diaphragm elementshear force, lbf (N) building length, ft (m) windward roof height,ft (m) leeward roof height, ft (m) windward wall height, ft (m)leeward wall height, ft (m) L (hwr qwr hlr qlr + hww fw qww hlw flqlw) / 2

(5-20)

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qwr = qlr = qww = qlw = fw fl = =

design windward roof pressure, lbf/ft2 (N/m2) design leewardroof pressure, lbf/ft2 (N/m2) design windward wall pressure,lbf/ft2 (N/m2) design leeward wall pressure lbf/ft2 (N/m2)frame-base fixity factor, windward post frame-base fixity factor,leeward post

Output from a DAFI analysis of a building with relatively highdiaphragm and shearwall stiffness values is presented in figure5.9. This output shows less than 3% of the total horizontal eaveload being resisted by the interior frames. Although rigid roofdesign expedites calculation of maximum diaphragm shear forces, thedesign procedure does not provide estimates of sidesway restrainingforce for interior post-frame design.

Inward acting wind pressures have positive signs, outward actingpressures are negative (figure 5.8). As previously noted,frame-base fixity factors, fw and fl, determine how much of thetotal wall load is transferred to the eave, and how much istransferred directly to the ground. The greater the resistance torotation at the base of a wall, the more load will be attracteddirectly to the base of the wall. For substantial fixity againstrotation at the groundline, set the frame-base fixity factor(s)equal to 3/8. For all other cases, set the frame-base fixityfactor(s) equal to 1/2. For symmetrical base restraint and framegeometry, equation 5-20 reduces to: Vh = L [hr (qwr qlr) + hw f(qww qlw)] / 2 where: hr = hw = f = roof height, ft (m) wallheight, ft (m) frame-base fixity factor for both leeward andwindward posts (5-21)

5.9 ReferencesAnderson, G.A., D.S. Bundy and N.F. Meador. 1989.The force distribution method: procedure and application to theanalysis of buildings with diaphragm action. Transactions of theASAE 32(5):1781-1786. ASAE. 1999a. EP484.2 Diaphragm design ofmetal-clad wood-frame rectangular buildings. ASAE Standards, 46thEd., ASAE, St. Joseph, MI. ASAE. 1999b. EP558.1 Load tests formetalclad wood-frame diaphragms. ASAE Standards, 46th Ed., ASAE,St. Joseph, MI. Bender, D. A., T. D. Skaggs and F. E. Woeste. 1991.Rigid roof design for post-frame buildings. Applied Engineering inAgriculture 7(6):755-760. Bohnhoff, D. R., P. A. Boor, and G. A.Anderson. 1999. Thoughts on metal-clad wood-frame diaphragm actionand a full-scale building test. ASAE Paper No. 994202, ASAE, St.Joseph, MI. Bohnhoff, D. R. 1992. Expanding diaphragm analysis forpost-frame buildings. Applied Engineering in Agriculture8(4):509-517. Gebremedhin, K.G. 1987a. SOLVER: An interactivestructures analyzer for microcomputers. (Version 2). NortheastRegional Agricultural Engineering Service. Cornell University,Ithaca, NY. Gebremedhin, K.G. 1987b. METCLAD: Diaphragm design ofmetal-clad post-frame buildings using microcomputers. NortheastRegional Agricultural Engineering Service. Cornell University,Ithaca, NY.

5.8.3 Application. The Vh value calculated using equation 5-20(or 5-21) is always a conservative estimate of the actual maximumshear force (due to wind) in a diaphragm assembly. This estimatebecomes increasingly conservative as the amount of load resisted byinterior post-frames increases. It follows that equations 5-20 and5-21 are most accurate when diaphragm stiffness is considerablygreater than interior post-frame stiffness. This tends to be thecase in buildings that are relatively wide and/or high, and inbuildings where individual posts offer no resistance to rotation(i.e., the posts are more-or less pin-connected at both the floorand eave lines).

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Post-Frame Building Design Manual

McGuire, P.M. 1998. One equation for compatible eavedeflections. Frame Building News 10(4):39-44. Meader, N.F. 1997.Mathematical models for lateral resistance of post foundations.Trans of ASAE, 40(1):191-201. Niu, K.T. and K.G. Gebremedhin. 1997.Evaluation of interaction of wood framing and metalcladding in roofdiaphragms. Transactions of the ASAE 40(2):465-476. Pollock, D. G.,D. A. Bender and K. G. Gebremedhin. 1996. Designing for chordforces in post-frame roof diaphragms. Frame Building News8(5):40-44. Purdue Research Foundation. 1986. Purdue planestructures analyzer. (Version 3.0). Department of Forestry andNatural Resources. Purdue University, West Layfette, IN. Williams,G. D. 1999. Modeling metal-clad wood-framed diaphragm assemblies.Ph.D. diss., University of Wisconsin-Madison, Madison, WI. Wright,B.W. 1992. Modeling timber-framed, metal-clad diaphragmperformance. Ph.D. diss. The Pennsylvania State University,University Park, PA.

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Chapter 6: METAL-CLAD WOOD-FRAME DIAPHRAGM PROPERTIES

6.1 Introduction6.1.1 General. One of the first steps indiaphragm design is to establish in-plane shear strength andstiffness values for each identified diaphragm section. In mostpost-frame buildings, these diaphragm sections consist ofcorrugated metal panels that have been screwed or nailed to woodframing. Behavior of these metalclad wood-frame (MCWF) diaphragmsis complex, and consequently, has been the subject of considerableresearch during the past 20 years. In addition to improving overalldesign, this research has led to improved methods for predictingmetal-clad wood-frame diaphragm strength and stiffness. 6.1.2Predicting Diaphragm Behavior. There are essentially threeprocedures for predicting the strength and stiffness of a buildingdiaphragm. First, an exact replica of the building diaphragm(a.k.a. a full-size diaphragm) can be built and tested to failure.Second, a smaller, representative section of the building diaphragmcan be built and laboratory tested. The strength and stiffness ofthis test assembly are then extrapolated to obtain strength andstiffness values for the building diaphragm. Lastly, diaphragmbehavior can be predicted using finite element analysis software.The latter requires that the strength and stiffness properties ofindividual component (e.g., wood framing, mechanical connections,cladding) be known. Of the three procedures for predictingmetal-clad wood-frame diaphragm properties, only the second oneextrapolation of diaphragm test assembly data - is commonly used.This is because testing full-size diaphragms is simply notpractical (a new test would have to be conducted every time overalldimensions changed), and finite element analysis of MCWF diaphragmsis, for practical purposes, still in a developmental stage. Thelater can be attributed to the fact that the large number ofvariables affecting diaphragm structural properties, as well as thenonlinear behavior of some variables, has thus far precluded thedevelopment of a quick and reasonably accurate closed-formapproxi-

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mation of diaphragm strength and stiffness. 6.1.3 ASAE EP558 andEP484. Construction specifications and testing procedures fordiaphragm test assemblies are given in ASAE EP558 Load Test forMetal-Clad Wood-Frame Diaphragms (ASAE, 1999b). EP558 also givesequations for calculating diaphragm test assembly strength andstiffness. These calculations along with constructionspecifications and testing procedures from EP558 are outlined inSection 6.3: Diaphragm Assembly Tests. For additional details andfurther explanation of testing procedures, readers are referred tothe ASAE EP558 Commentary (ASAE, 1999b). ASAE EP484, which wasintroduced in detail in Chapter 5, contains the equations forextrapolating diaphragm test assembly properties for use inbuilding design. These calculations are presented in Section 6.4:Building Diaphragm Properties.

6.2 Design Variables6.2.1 General. Many variables affect theshear stiffness and strength of a structural diaphragm, including:overall geometry, cladding characteristics, wood properties,fastener type and location, and blocking. A short description ofeach of these variables follows. 6.2.2. Geometry. Geometricvariables include: spacing between secondary framing members (e.g.purlins), spacing between primary framing members (e.g.,trusses/rafters), and overall dimensions. With respect to overalldimensions, diaphragm depth is measured parallel to primary frames,diaphragm length is meas

FAQs

How to build a pole barn step by step? ›

Building Your Pole Barn

Dig the holes three to five feet deep, mix your concrete and even out the height of the poles. Next, add the roof support beams, install the trusses and add additional boards for support. Lastly, add the siding, install the roof and add doors & windows – and you've built a pole barn!

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How far apart should post frame girt be? ›

Girts are perpendicular to post and may be installed upright on the face of the post, or turned flat (bookshelf) between the post. Spacing is usually no further than 2' on-center.

What is the best foundation for a pole barn? ›

If you are looking for the best foundation for post frame buildings, precast concrete column footings are the way to go.

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What size footing do I need for a pole barn? ›

This pre-formed base should be at least 6 inches thick and 12-16 inches in diameter. It is then packed with crushed rock or drainable soil to ensure proper drainage away from the post to prevent rotting.

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When building a pole barn Do you pour concrete first? ›

You should not have your concrete slab poured before building your pole barn. After the poles are set and skirt board is placed around the perimeter of the poles, you will have a form to pour your concrete foundation. Concrete is normally poured through the large door opening.

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How deep do you set posts for a pole barn? ›

To set up a pole building, you want your foundation posts measured correctly. Dig holes at least 3 feet (91.44 cm) deep for each post with a shovel, posthole digger or backhoe. Set each post into a hole and then fill them with concrete.

How far do you put the purlins on a pole building? ›

Purlins are usually spaced 24 inches on center in low snow loads, but they are put closer together if snow loads are higher. The most common method of installing purlins in a pole barn is to lay them flat. This is typically done with 2x4s. The purlins are nailed to the trusses with two 20D galvanized ring shank nails.

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How far apart should 6x6 posts be? ›

For Building a Deck Railing, Deck Frame, or Fence

The maximum spacing of 4x4 deck posts should be 6 feet on center, while the maximum spacing of 6x6 deck posts should be 8 feet on center.

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How do I keep my pole barn posts from rotting? ›

Keeping water away from your pole barn posts is the most effective way to prevent rot. While preparing the site for your pole barn, take care in grading the soil away from your barn to discourage water from pooling around your pole barn posts.

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Do I need a vapor barrier in my pole barn walls? ›

The warm, humid air inside your building can cause condensation to form on cooler, drier surfaces. This is called vapor transmission and can lead to severe damage if it is allowed to build over time. Vapor transmission can move from interior to exterior and vice versa, so you need to protect all areas of your home.

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How thick should cement be for pole barn? ›

Lightweight vehicles - 4” thick minimum. Livestock barns with manure storage - 5” thick. Motorhomes or dump trucks - 5”- 6” thick. Heavy duty agriculture equipment - 6” thick.

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Do foundation piers need rebar? ›

Plain concrete deck foundations without rebar are acceptable under the minimum standards of construction established in the International Residential Code. However, placing reinforcing steel within footings is a relatively easy and inexpensive practice that can provide increased performance.

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How far apart should 4x4 posts be on a pole barn? ›

The standard distance between pole barn columns is 8 feet. However, depending on who you choose for your post-frame builder, column spacing may vary between 6 feet through 10 feet. Very rarely do we see columns that are spaced 12 feet apart.

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What size rebar should I use for a footing? ›

Footers must be twice as wide as wall minimum with 1/2 inch or 5/8 inch rebar in footing with 2 runs, placed in the bottom half of the footing, at least 6 inches apart and not less than 3 inches from the bottom and the sides of the footing supported on chairs.

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Should I spray foam my pole barn? ›

Additionally, spray foam is a water and vapor barrier so it will help to control condensation in pole barns. Since most people will put heaters in this space during the winter, it can create additional moisture inside the pole barn. Spray foam insulation will prevent a condensation problem from happening.

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Do Pole barns need a footer? ›

Soil is not usually able to resist applied vertical loads through a post alone. Pole barn posts should therefore be set on footings to provide additional support. Footings must be large enough in area to prevent the pole barn from settling under the weight of the building, snow, and minimum live load requirements.

How many inches should a post be in the ground? ›

The depth of the hole should be 1/3-1/2 the post height above ground (i.e., a 6-foot tall fence would require a hole depth of at least 2 feet). Add about 6 inches of QUIKRETE All-Purpose Gravel into the bottom of the hole.

What is the best insulation for post frame building? ›

EcoFoil Bubble Insulation is the recommended product for use in post frame buildings. It's designed to be installed in the walls and roof to help keep your building cooler in the summer and warmer in the winter.

Is it cheaper to build your own pole barn? ›

With pole barns becoming popular in the home-building industry, many people wonder if they are more cost-efficient than stick-built custom homes. Pole buildings typically cost less per square foot, but there are other factors that will contribute to the cost of your custom barndominium.

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Can I build a pole barn by myself? ›

Pole barns, built using a post-frame construction method, are exceptionally durable and can be put together in days or weeks, rather than the months it would take for a traditional stick-built garage. The added benefit is that they're a reasonable do-it-yourself project if you're comfortable with construction.

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What is the opening at the top of a barn called? ›

Better ventilation from a cupola also keeps moisture down inside the barn and improves air quality, which is ideal if there are farm animals spending the winter inside it. If a cupola has windows, it can also provide some natural lighting inside the barn as well.

What are standard sizes for pole buildings? ›

The most common sizes we get asked for are a 30′ x 40′ pole barn, 40′ x 60′ pole barn, or a 40′ x 40′ pole barn. It is important to keep it mind that it is typically most cost-effective to build in 8-, 9- or 10-feet increments because of the standard spacing options for pole barn posts.

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Do pole barns have footings? ›

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How much does a 40x60 stick-built shop cost? ›

How much does a 40x60 steel building cost? Steel buildings are priced by square feet. In 2022, the cost of steel per square foot ranges from $25-30 per square foot for non-complex designs. Since a 40×60 steel building has a total of 2,400 sq ft, the price of your building kit can range from $60,000-72,000.

What is the cheapest siding for a pole barn? ›

Pole Barn Vinyl Siding

For those who don't want a metal pole barn but are looking for a more economical option than cedar board and batten, stucco or Hardie board siding, vinyl siding offers much more in the way of durability, low maintenance and efficiency.

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How much is a 30x30 pole barn kit? ›

30×30 Pole Barn Prices

The cost of a 30 x pole barn kit varies depending on the materials used and the company you purchase it from. Expect to pay between $6,000 and $12,000 for a basic kit, not including shipping or any other additional costs.

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Should pole barn posts be set in concrete? ›

Also, be aware that simply setting posts in concrete will not prevent rot. The wood won't be in contact with the ground, but moisture is absorbed by the concrete and pulled up into the wood. Over time, rotting will occur.

How far apart should pole barn trusses be? ›

Pole barn construction spaces the trusses 8 feet apart - or even up to 12 feet apart depending on the building design. With traditional stick-frame construction, the trusses are usually spaced 2 feet apart.

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How far can you space 6x6 posts for pole barn? ›

The standard distance between pole barn columns is 8 feet.

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Should I drywall my pole barn? ›

Perhaps you're building a pole barn to use as living quarters, so finishing touches are an absolute necessity. When it comes to attaining a completed appearance, drywall can help to achieve this. The best part is that you can incorporate drywall in your pole barn's interior.

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How do you stable a pole in the ground? ›

Place your post in the hole so that it's straight and level with the other posts in the fence or structure. Get an assistant to hold it steady or secure it with stakes and screws to keep it in place. Add crushed stone or soil around the sides of the post to fill the rest of the hole.

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What is the best way to square up a pole barn? ›

To square up the building lines measure from left front corner to right rear corner. Then measure from right front corner to left rear corner. The building is square when these two measurements are equal length.

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Post-Frame Building Design Manual - [PDF Document] (2023)

FAQs

How to build a pole barn step by step? ›

How thick should cement be for pole barn? ›

How many inches should a post be in the ground? ›

What is the best insulation for post frame building? ›

Is it cheaper to build your own pole barn? ›

How far can you space 6x6 posts for pole barn? ›

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