Chapter 1: INTRODUCTION TO POST-FRAME BUILDINGS
1.1 General
1.1.1 Main Characteristics. Post-frame
build-ings are structurally efficient buildbuild-ings composed of main members such as posts and trusses and secondary components such as purlins, girts, bracing and sheathing Snow and wind loads are transferred from the sheathing to the secondary members. Loads are transferred to the ground through the posts that typically are embedded in the ground or surface-mounted to a concrete
or masonry foundation. Figure 1.1 illustrates the structural components of a post-frame building.
1.1.2 Use. Post-frame construction is
well-suited for many commercial, industrial, agricul-tural and residential applications. Post-frame offers unique advantages in terms of design and construction flexibility and structural efficiency. For these reasons, post-frame construction has experienced rapid growth, particularly in non-agricultural applications.
Figure 1.1. Simplified diagram of a post-frame building. Some components such as
per-manent roof truss bracing and interior finishes are not shown.
Truss Wall girt Doorway Wall cladding Purlin Ridge cap Pressure preservative treated post Concrete footing
Pressure preservative treated splash board
1.2 Evolution
1.2.1 The concept of pole-type structures is not
new. Archeological evidence exists in abun-dance that pole buildings have been used for human housing for thousands of years. In Amer-ica, pole buildings began appearing on farms in the 19th century (Norum, 1967).
1.2.2 Pole-type construction resurfaced in 1930
when Mr. H. Howard Doane introduced the "modern pole barn" as an economical alternative to conventional barns (Knight, 1989). Mr. Doane was the founder of Doane's Agricultural Service, a firm specializing in managing farms for absen-tee owners. These early pole barns were con-structed with red cedar poles that were naturally resistant to decay, trusses spaced 2 ft on-center, 1-inch nominal purlins and galvanized steel sheathing.
In the 1940s, pole barn construction was refined by using creosote preservative-treated sawn posts, wider truss and purlin spacings, and im-proved steel sheathing. Mr. Bernon G. Perkins, an employee of Doane's, is credited for many of the refinements to Doane's original pole barn. In 1949, Mr. Perkins applied for the first patent on the pole building concept through Doane's Agri-cultural Service, and the patent was issued in 1953. Rather than protecting their patent, they publicized the concept and encouraged its use throughout the world. In 1995, the post-frame building concept was recognized as an Historic Agricultural Engineering Landmark by the American Society of Agricultural Engineers.
1.2.3 In the past two decades, post-frame
con-struction has been further enhanced by the de-velopments of metal-plate connected wood trusses, nail- and glue-laminated posts, high-strength steel sheathing, fasteners and dia-phragm design methods. Composites such as laminated posts and structural composite lumber offer advantages of superior strength and stiff-ness, dimensional stability, and they can be ob-tained in a variety of sizes and pressure pre-servative treatments. Developments in metal-plate connected wood truss technology allow clear spans of over 80 feet. Design procedures were introduced in the early 1980s to more curately account for the effect of diaphragm
ac-tion on post and foundaac-tion design (Knight, 1990). New roof panel constructions using high-strength steel and customized screw fasteners have dramatically improved diaphragm stiffness and strength.
1.3 Advantages
1.3.1 Reliability. Outstanding structural
per-formance of post-frame buildings under adverse conditions such as hurricanes is well-documented. Professor Gurfinkel, in his wood engineering textbook, cites superior perform-ance of post-frame buildings over conventional construction during hurricane Camille in 1969 (Gurfinkel, 1981). Harmon et. al (1992) reported that post-frame buildings constructed according to engineered plans generally withstood hurri-cane Hugo (wind gusts measured at 109 mph). Since post-frame buildings are relatively light weight, seismic forces do not control the design unless significant additional dead loads are ap-plied to the structure (Faherty and Williamson, 1989; Taylor, 1996).
1.3.2 Economy. Significant savings can be
ob-tained with post-frame construction in terms of materials, labor, construction time, equipment and building maintenance. For example, post-frame buildings require less extensive founda-tions than other building types because the wall sections between the posts are non-load bear-ing. Embedded post foundations commonly used in post-frame require less concrete, heavy equipment, labor, and construction time than conventional perimeter foundations. Additionally, embedded post foundations are better-suited for wintertime construction.
1.3.3 Versatility. Post-frame construction
facili-tates design flexibility. Posts can be embedded into the ground or surface-mounted to a con-crete foundation. Steel sheathing can be re-placed with wood siding, brick veneer, and con-ventional roofing materials, to satisfy the ap-pearance and service requirements of the cus-tomer. One-hour fire-rated wall and roof/ceiling constructions have been developed for wood framed assemblies. Exposed glued-laminated and solid-sawn timbers can be substituted for trusses made from dimension lumber to achieve desired architectural effects.
1.4 Industry
Profile
1.4.1 Post-frame construction has experienced
tremendous growth since World War II. This growth was fueled by the abundant supplies of steel and pressure preservative-treated wood, together with the need for low-cost structures. In the 1950s and 1960s, the pole barn industry was characterized by large numbers of inde-pendent builders (Knight, 1989). During this time, pole builders were expanding from their traditional agricultural base into other construc-tion markets. This expansion into code-enforced construction required rigorous documentation of engineering designs and more involvement in the building code arena.
1.4.2 NFBA. Approximately 20 builders met in
1969 to discuss challenges facing the post-frame building industry. The group voted in favor of forming the National Frame Builders Associa-tion (NFBA). The NFBA became incorporated in 1971 and the first national headquarters was established in Chicago, Illinois. Today, the Na-tional Frame Builders Association is headquar-tered in Lawrence, Kansas and includes over 300 contractors and suppliers, with regional branches throughout the U.S. In addition, a Ca-nadian Division of NFBA was created in 1984.
1.4.3 The post-frame industry has become one
of the fastest growing segments of the total con-struction industry. Based on light-gauge steel sales, post-frame industry revenues are esti-mated to be from 2 to 2.5 billion dollars in 1990.
1.5 Terminology
AF&PA: American Forest & Paper Association
(formerly National Forest Products Association).
AITC: American Institute of Timber
Construc-tion.
ALSC: American Lumber Standard Committee.
ANSI: American National Standards Institute
APA: The Engineered Wood Association
(for-merly the American Plywood Association)
ASAE: The Society for engineering in
agricul-tural, food, and biological systems (formerly American Society of Agricultural Engineers).
Anchor Bolts: Bolts used to anchor structural
members to a foundation. Commonly used in post-frame construction to anchor posts to the concrete foundation.
ASCE: American Society of Civil Engineers.
AWC: American Wood Council. The wood
prod-ucts division of the American Forest & Paper Association (AF&PA).
AWPB: American Wood Preservers Bureau.
Bay: The area between adjacent primary frames
in a building. In a post-frame building, a bay is the area between adjacent post-frames.
Bearing Height: Vertical distance between a
pre-defined baseline (generally the grade line) and the bearing point of a component.
Bearing Point: The point at which a component
is supported.
Board: Wood member less than two (2) nominal
inches in thickness and one (1) or more nominal inches in width.
Board-Foot (BF): A measure of lumber volume
based on nominal dimensions. To calculate the number of board-feet in a piece of lumber, multi-ply nominal width in inches by nominal thickness in inches times length in feet and divide by 12.
BOCA: Building Officials & Code Administrators
International, Inc. The organization responsible for maintaining and publishing the National Building Code.
Bottom Chord: An inclined or horizontal
mem-ber that establishes the bottom of a truss.
Bottom Plank: See Splashboard.
Butt Joint: The interface at which the ends of
Camber: A predetermined curvature designed
into a structural member to offset the anticipated deflection when loads are applied.
Check: Separation of the wood that usually
ex-tends across the annual growth rings (i.e., a split perpendicular-to-growth rings). Commonly re-sults from stresses that build up in wood during seasoning.
Cladding: The exterior and interior coverings
fastened to the wood framing.
Clear Height: Vertical distance between the
finished floor and the lowest part of a truss, raf-ter, or girder.
Collars: Components that increase the bearing
area of portions of the post foundation, and thus increase lateral and vertical resistance.
Components and Cladding: Elements of the
building envelope that do not qualify as part of the main wind-force resisting system. In post-frame buildings, this generally includes individ-ual purlins and girts, and cladding.
Diaphragm: A structural assembly comprised of
structural sheathing (e.g., plywood, metal clad-ding) that is fastened to wood or metal framing in such a manner the entire assembly is capable of transferring in-plane shear forces.
Diaphragm Action: The transfer of load by a
diaphragm.
Diaphragm Design: Design of roof and ceiling
diaphragm(s), wall diaphragms (shearwalls), primary and secondary framing members, com-ponent connections, and foundation anchorages for the purpose of transferring lateral (e.g., wind) loads to the foundation structure.
Dimension Lumber: Wood members from two
(2) nominal inches to but not including five (5) nominal inches in thickness, and 2 or more nominal inches in width.
Eave: The part of a roof that projects over the
sidewalls. In the absence of an overhang, the eave is the line along the sidewall formed by the intersection of the wall and roof planes.
Fascia: Flat surface (or covering) located at the
outer end of a roof overhang or cantilever end.
Flashing: Sheet metal or plastic components
used at major breaks and/or openings in walls and roofs to insure weather-tightness in a struc-ture.
Footing: Support base for a post or foundation
wall that distributes load over a greater soil 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 be-tween adjacent trusses (or rafters). Frame spac-ing may vary within a buildspac-ing.
Gable: Triangular portion of the endwall of a
building directly under the sloping roof and above the eave line.
Gable Roof: Roof with one slope on each side.
Each slope is of equal pitch.
Gambrel Roof: Roof with two slopes on each
side. The pitch of the lower slope is greater than that of the upper slope.
Girder: A large, generally horizontal, beam.
Commonly used in post-frame buildings to sup-port trusses whose bearing points do not coin-cide with a post.
Girt: A secondary framing member that is
at-tached (generally at a right angle) to posts. Girts laterally support posts and transfer load be-tween wall cladding and posts.
Glued-Laminated Timber: Any member
com-prising an assembly of laminations of lumber in which the grain of all laminations is approxi-mately parallel longitudinally, in which the lami-nations are bonded with adhesives.
Grade Girt: See Splashboard.
Grade Line (grade level): The line of
intersec-tion between the building exterior and the top of the soil, gravel, and/or pavement in contact with the building exterior. For post-frame building
design, the grade line is generally assumed to be no lower than the lower edge of the splash-board.
Header: A structural framing member that
sup-ports the ends of structural framing members that have been cut short by a floor, wall, ceiling, or roof opening.
Hip Roof: Roof which rises by inclined planes
from all four sides of a building.
IBC: International Building Code.
ICBO: International Conference of Building
Offi-cials. The organization responsible for maintain-ing and publishmaintain-ing the Uniform Buildmaintain-ing Code.
Knee Brace: Inclined structural framing member
connected on one end to a post/column and on the other end to a truss/rafter.
Laminated Assembly: A structural member
comprised of dimension lumber fastened to-gether with mechanical fasteners and/or adhe-sive. Horizontally- and vertically-laminated as-semblies are primarily designed to resist bend-ing loads applied perpendicular and parallel to the wide face of the lumber, respectively.
Laminated Veneer Lumber (LVL) A structural
composite lumber assembly manufactured by gluing together wood veneer sheets. Each ve-neer is orientated with its wood fibers parallel to the length of the member. Individual veneer thickness does not exceed 0.25 inches.
Loads: Forces or other actions that arise on
structural systems from the weight of all perma-nent construction, occupants and their posses-sions, environmental effects, differential settle-ment, and restrained dimensional changes.
Dead Loads: Gravity loads due to the
weight of permanent structural and non-structural components of the building, such as wood framing, cladding, and fixed service equipment.
Live Loads: Loads superimposed by the
construction, use and occupancy of the building, not including wind, snow, seismic or dead loads.
Seismic Load: Lateral load acting in the
horizontal direction on a structure due to the action of earthquakes.
Snow Load: A load imposed on a structure
due to accumulated snow.
Wind Loads: Loads caused by the wind
blowing from any direction.
Lumber Grade: The classification of lumber in
regard to strength and utility in accordance with the grading rules of an approved (ALSC accred-ited) lumber grading agency.
LVL: see Laminated Veneer Lumber.
Main Wind-Force Resisting System: An
as-semblage of structural elements assigned to provide support and stability for the overall structure. Main wind-force resisting systems in frame buildings include the individual post-frames, diaphragms and shearwall
Manufactured Component. A component that
is assembled in a manufacturing facility. The wood trusses and laminated columns used in post-frame buildings are generally manufactured components.
MBMA: Metal Building Manufacturers
Associa-tion.
NDS®: National Design Specification® for
Wood Construction. Published by AF&PA.
Mechanically Laminated Assembly: A
lami-nated assembly in which wood laminations have been joined together with nails, bolts and/or other mechanical fasteners.
Metal Cladding: Metal exterior and interior
cov-erings, usually cold-formed aluminum or steel sheet, fastened to the structural framing.
NFBA: National Frame Builders Association.
NFPA: National Fire Protection Association
Nominal size: The named size of a member,
usually different than actual size (as with lum-ber).
Orientated Strand Board (OSB): Structural
wood panels manufactured from reconstituted, mechanically oriented wood strands bonded with resins under heat and pressure.
Orientated Strand Lumber (OSL): Structural
composite lumber (SCL) manufactured from mechanically oriented wood strands bonded with resins under heat and pressure. Also known as laminated strand lumber (LSL)
OSB: See Orientated Strand Board.
Parallel Strand Lumber (PSL): Structural
com-posite lumber (SCL) manufactured by cutting 1/8-1/10 inch thick wood veneers into narrow wood strands, and then gluing and pressing the strands together. Individual strands are up to 8 feet in length. Prior to pressing, strands are ori-ented so that they are parallel to the length of the member.
Pennyweight: A measure of nail length,
abbre-viated by the letter d.
Plywood: A built-up panel of laminated wood
veneers. The grain orientation of adjacent ve-neers are typically 90 degrees to each other.
Pole: A round, unsawn, naturally tapered post.
Post: A rectangular member generally uniform
in cross section along its length. Post may be sawn or laminated dimension lumber. Com-monly used in post-frame construction to trans-fer loads from main roof beams, trusses or raf-ters to the foundation.
Post Embedment Depth: Vertical distance
be-tween the bottom of a post and the lower edge of the splashboard.
Post Foundation: The embedded portion of a
structural post and any footing and/or attached collar.
Post Foundation Depth: Vertical distance
be-tween the bottom of a post foundation and the lower edge of the splashboard.
Post-Frame: A structural building frame
consist-ing of a wood roof truss or rafters connected to vertical timber columns or sidewall posts.
Post-Frame Building: A building system whose
primary framing system is principally comprised of post-frames.
Post Height: The length of the non-embedded
portion of a post.
Pressure Preservative Treated (PPT) Wood:
Wood pressure-impregnated with an approved preservative chemical under approved treatment and quality control procedures.
Primary Framing: The main structural framing
members in a building. The primary framing members in a post-frame building include the columns, trusses/rafters, and any girders that transfer load between trusses/rafters and col-umns.
PSL: See Parallel Strand Lumber.
Purlin: A secondary framing member that is
attached (generally at a right angle) to rafters/ trusses. Purlins laterally support rafters and trusses and transfer load between exterior clad-ding and rafters/trusses.
Rafter: A sloping roof framing member.
Rake: The part of a roof that projects over the
endwalls. In the absence of an overhang, the rake is the line along the endwall formed by the intersection of the wall and roof planes.
Ridge: Highest point on the roof of a building
which describes a horizontal line running the length of the building.
Ring Shank Nail: See threaded nail.
Roof Overhang: Roof extension beyond the
endwall/sidewall of a building.
Roof Slope: The angle that a roof surface
makes with the horizontal. Usually expressed in units of vertical rise to 12 units of horizontal run.
SBC: Standard Building Code (see SBCCI).
SBCCI: Southern Building Code Congress
In-ternational, Inc. The organization responsible for maintaining and publishing the Standard Build-ing Code.
Secondary Framing: Structural framing
mem-bers that are used to (1) transfer load between exterior cladding and primary framing members, and/or (2) laterally brace primary framing mem-bers. The secondary framing members in a post-frame building include the girts, purlins and any structural wood bracing.
Self-Drilling Screw: A screw fastener that
com-bines the functions of drilling and tapping (thread forming). Generally used when one or more of the components to be fastened is metal with a thickness greater than 0.03 inches
Self-Piercing Screw: A self-tapping (thread
forming) screw fastener that does not require a pre-drilled hole. Differs from a self-drilling screw in that no material is removed during screw in-stallation. Used to connect light-gage metal, wood, gypsum wallboard and other "soft" mate-rials.
SFPA: Southern Forest Products Association
Shake: Separation of annual growth rings in
wood (splitting parallel-to-growth rings). Usually considered to have occurred in the standing tree or during felling.
Shearwall: A vertical diaphragm in a structural
framing system. A shearwall is any endwall, sidewall, or intermediate wall capable of trans-ferring in-plane shear forces.
Siphon Break: A small groove to arrest the
cap-illary action of two adjacent surfaces.
Soffit: The underside covering of roof
over-hangs.
Soil Pressure: Load per unit area that the
foun-dation of a structure exerts on the soil.
Span: Horizontal distance between two points.
Clear Span: Clear distance between
adja-cent supports of a horizontal or inclined member. Horizontal distance between the facing surfaces of adjacent supports.
Effective Span: Horizontal distance from
of-required-bearing-width to center-of-required-bearing-width, or the "clear
span" for rafters and joists in conventional construction.
Out-To-Out Span: Horizontal distance
be-tween the outer faces of supports. Com-monly used in specifying metal-plate-connected wood trusses.
Overall Span: Total horizontal length of an
installed horizontal or inclined member.
SPIB: Southern Pine Inspection Bureau.
Skirtboard: See Splashboard.
Splashboard: A preservative treated member
located at grade that functions as the bottom girt. Also referred to as a skirtboard, splash plank, bottom plank, and grade girt.
Splash Plank: See Splashboard.
Stitch (or Seam) Fasteners: Fasteners used to
connect two adjacent pieces of metal cladding, and thereby adding shear continuity between sheets.
Structural Composite Lumber (SCL):
Recon-stituted wood products comprised of several laminations or wood strands held together with an adhesive, with fibers primarily oriented along the length of the member. Examples include LVL and PSL.
Threaded Nail: A type of nail with either annual
or helical threads in the shank. Threaded nails generally are made from hardened steel and have smaller diameters than common nails of similar length.
Timber: Wood members five or more nominal
inches in the least dimension.
Top Chord: An inclined or horizontal member
that establishes the top of a truss.
TPI: Truss Plate Institute.
Truss: An engineered structural component,
assembled from wood members, metal connec-tor plates and/or other mechanical fasteners, designed to carry its own weight and superim-posed design loads. The truss members form a
semi-rigid structural framework and are assem-bled such that the members form triangles.
UBC: Uniform Building Code (see ICBO).
Wane: Bark, or lack of wood from any cause, on
the edge or corner of a piece.
Warp: Any variation from a true plane surface.
Warp includes bow, crook, cup, and twist, or any combination thereof.
Bow: Deviation, in a direction perpendicular
to the wide face, from a straight line drawn between the ends of a piece of lumber.
Crook: Deviation, in a direction
perpendicu-lar to the narrow edge, from a straight line drawn between the ends of a piece of lum-ber.
Cup: Deviation, in the wide face of a piece
of lumber, from a straight line drawn from edge to edge of the piece.
Twist: A curl or spiral of a piece of lumber
along its length. Measured by laying lumber on a flat surface such that three corners contact the surface. The amount of twist is equal to the distance between the flat face and the corner not contacting the sur-face.
WCLIB: West Coast Lumber Inspection Bureau
Web: Structural member that joins the top and
bottom chords of a truss. Web members form the triangular patterns typical of most trusses.
WTCA: Wood Truss Council of America.
WWPA: Western Wood Products Association.
1.6 References
Faherty, K.F. and T.G. Williamson. 1989. Wood Engineering and Construction Handbook. McGraw-Hill Publishing Company, New York, NY.
Gurfinkel, G. 1981. Wood Engineering (2nd Ed.). Kendall/Hunt Publishing Company, Dubuque, Iowa.
Harmon, J.D., G.R. Grandle and C.L. Barth. 1992. Effects of hurricane Hugo on agricultural structures. Applied Engineering in Agriculture 8(1):93-96.
Knight, J.T. 1989. A brief look back. Frame Building Professional 1(1):38-43.
Knight, J.T. 1990. Diaphragm design - technol-ogy driven by necessity. Frame Building Profes-sional 1(5):16,44-46.
Norum, W.A. 1967. Pole buildings go modern. Journal of the Structural Division, ASCE, Vol. 93, No.ST2, Proc. Paper 5169, April, pp.47-56. Taylor, S.E. 1996. Earthquake considerations in post-frame building design. Frame Building News 8(3):42-49.
Chapter 2: BUILDING CODES, DESIGN SPECIFICATIONS
AND ZONING REGULATIONS
2.1 Introduction
2.1.1 Definition. A building code is a legal
document that helps ensure public health and welfare by establishing minimum standards for design, construction, quality of materials, use and occupancy, location and maintenance of all buildings and structures.
2.1.2 Model Versus Active Codes. A model
code is a code that is written for general use (i.e., a code that is not written for use by a spe-cific state, county, town, village, company or individual). An active code is a model or spe-cially written code that has been adopted and is enforced by a regulatory agency such as a state or local government. It follows that in a given jurisdiction, acceptance of a model building code is voluntary until the model code becomes part of the active code in the jurisdiction.
2.1.3 Active Code Variations. The content and
administration of active building codes varies not only between states, but frequently between municipalities within a state. Some states have established a hierarchy structure of state, county and township/village/city building codes. In this situation, more localized governing areas can modify the state (or county) codes, provided the changes result in more strict provisions.
Despite local differences in content and admini-stration, most active building codes share the common trait of regulating components of con-struction based on building occupancy and use.
2.2 Major Model Building Codes
2.2.1 Current Codes. There are currently threeprimary model building codes in the United States. These are the Uniform Building Code (UBC) published by the International Congress of Building Officials, the National Building Code published by the Building Officials and Code Administrators International (BOCA) and the Standard Building Code published by the Southern Building Code Congress International
(SBCCI). These model building codes are com-monly referred to as the UBC, BOCA and the Southern Building Code, respectively.
2.2.2 Adoption. Most states have adopted (and
enforce) all or a major portion of one of the three model building codes. As shown in figure 2.1, western states have adopted the UBC, north-eastern states the BOCA code, and states in the southwest the Southern Building Code.
2.2.3 Development. Model building codes are
consensus documents continually studied and annually revised by building officials, industry representatives and other interested parties.
2.2.4 International Building Code. On
De-cember 9, 1994, the three model building code agencies (BOCA, ICBO and SBCCI) created the International Code Council (ICC). The ICC was established in response to technical disparities among the three major model codes. Since its founding, the ICC has worked to create a single model building code for the U.S. This code, which is entitled the International Building Code is now complete and will replace the three model codes over the next couple years. With all states adopting the same model code, it will be less difficult for building designers to work in different regions of the country.
2.3 Building
Classification
2.3.1 General. Building codes give criteria for
classifying buildings based on: (1) use or occu-pancy, and (2) type of construction.
2.3.2 Occupancy Classifications. Occupancy
classifications include assembly, business, edu-cational, factory and industrial, high-hazard, in-stitutional, mercantile, residential and storage. Occupancy classifications have requirements on the number of occupants and building separa-tion, height and area. Other limits exist, for ex-ample on lighting, ventilation, sanitation, fire
Figure 2.1. Approximate areas of model building code influence. Wisconsin and New
York building codes are developed by their respective state code agencies and are not necessarily influenced by current model codes.
protection and exiting, depending on the specific classification and building code.
2.3.2 Types of Construction. Classification by
type of construction is primarily based on the fire resistance ratings of the walls, partitions, struc-tural elements, floors, ceilings, roofs and exits. Specific requirements vary somewhat between model building codes.
There are two primary source documents for determining the fire resistance of assemblies: the Fire Resistance Design Manual, published by the Gypsum Association, and the Fire Resis-tance Directory, published by Underwriters Laboratories, Inc.
The fire resistance of wood framed assemblies can generally be increased by using fire retar-dant treated (FRT) wood or larger wood mem-bers. Codes allow FRT wood to be used in
cer-tain areas of noncombustible construction. The superior fire resistance of large timber members is recognized by the codes with the inclusion of a "heavy timber" classification. To qualify for heavy timber construction, nominal dimensions of timber columns must be at least 6- by 8-inches and primary beams shall have nominal width and depth of at least 6- by 10-inches.
2.3.2.1 NFBA Sponsored Fire Test. In
January of 1990, the National Frame Build-ers Association had Warnick HBuild-ersey Inter-national, Inc., conduct a one-hour fire en-durance test on the exterior wall shown in figure 2.2. The wall met all requirements for a one-hour rating as prescribed in ASTM E-119-88. The wall sustained an applied load of 10,400 lbf per column throughout the test. Copies of the fire test report can be obtained from NFBA.
Uniform Building Code (ICBO)
National Building Code (BOCA)
Standard Building Code (SBCCI)
Figure 2.2. Construction details for exterior wall that obtained a one-hour fire endurance
rating during a January 1990 test conducted for the National Frame Builders Association by Warnock Hersey International, Inc. Details of the test are available from NFBA upon request.
2.4 Specifications and Standards
2.4.1 General. Design of buildings is covered inthe model building codes either by direct provi-sions or by reference to approved engineering specifications and standards. Engineering speci-fications and standards provide criteria and data needed for load calculation, design, testing and material selection. They are based on the best available information and engineering judgment.
2.4.2 Wood Design Specifications. The tech
nical literature for wood design and construction is somewhat fragmented. New design specifica-tions and standards are continually under devel-opment, and existing documents are periodically revised. Keeping abreast of this literature re-quires a determined effort on the part of the de-sign professional. To assist in this effort, Table 2.1 gives a partial list of engineering design specifications, standards and other technical references specifically related to post-frame construction. The reader is encouraged to main-tain communication with the organizations isted in Table 2.1 concerning new and revised publi-cations.
A A
Nominal 2- by 4-inch nailers, 24 in. o.c.
3- by 24- by 48-inch mineral wool, attach with
3 in. square cap nails (3 per 48 in. width) Fire side nailers, nominal 2- by 4-inches
24 in. o.c.
Gold Bond 5/8 in. Fireshield G Type X, attached with 1-7/8 in. cement coated nails
(0.0195 in. shank, 1/4 head, 7 in. o.c.)
Metal cladding 29 gage
Nominal 2- by 2-inch blocking between nailers
(nailed to nominal 2- by 6-inch edge blocks) 4-1/16- by 5-1/4-inch glue-laminated column 10 ft
Nail-laminated column fabricated from 3 nominal 2- by 6-inch No. 2 KD19 SP members Nominal 2- by 4-inch blocking attached to column
Section B-B Section A-A Unexposed nominal 2- by 4-inch nailers 24 in. o.c. B B 1 ft 8 ft 1 ft
Attach metal cladding 12 in. o.c. with 1.5 in. hex head screws with neoprene washers
Of the documents listed in Table 2.1, the primary engineering design specification cited by the model building codes for wood construction is the National Design Specification® for Wood Construction (NDS®), published by the American Forest & Paper Association (AF&PA). The NDS was first issued in 1944 and in 1992 it became a consensus standard through the American Na-tional Standards Institute (ANSI).
2.5 Zoning
Regulations
2.5.1 General. Zoning laws are established to-control construction activities and regulate land use, in terms of types of occupancy, building
height, and density of population and activity. Zoning laws may also dictate building appear-ance and location on property, parking signs, drainage, handicap accessibility, flood control and landscaping. Typically land is zoned for residential, commercial, industrial or agricultural uses.
2.5.2 Development and Enforcement. Zoning
laws are developed by municipalities. They (and building codes) are principally enforced by the granting of building permits and inspection of construction work in progress. Certificates of occupancy are issued when completed buildings satisfy all regulations.
Table 2.1. Partial list of technical references related to post-frame building design and construction
Organization & Address Publications
AF&PA
American Forest & Paper Association 1111 19th Street, N.W., Suite 800 Washington, D.C. 20036
http://www.awc.org/
Allowable stress design (ASD) manual for engineered wood construction
National design specification® (NDS®) for wood construction NDS commentary
Design values for wood construction (NDS supplement) Load and resistance factor design (LRFD) manual for
engi-neered wood construction
Wood frame construction manual (WFCM) for one-and two-family dwellings
Span tables for joists and rafters
AITC
American Inst. of Timber Construction 7012 S. Revere Parkway, Suite 140 Englewood, CO 80112
Timber construction manual
ANSI
American National Standards Institute 11 West 42nd Street
New York, NY 10036 http://www.ansi.org/
ANSI/AF&PA National design specification for wood construc-tion (see AF&PA)
Table 2.1. Partial list of technical references related to post-frame building design and construction
Organization & Address Publications
APA
The Engineered Wood Association P.O. Box 11700
7011 South 19th Street Tacoma, WA 98411 http://www.apawood.org/
APA design/construction guide; residential and commercial Plywood design specification (PDS)
Diaphragms and shear walls
Performance standard for APA EWS I-joists Panel handbook & grade glossary
ASAE
2950 Niles Road
St. Joseph, MI 49085-9659 http://asae.org/
ASAE EP288 Agricultural building snow and wind loads
ASAE EP484.2 Diaphragm design of metal-clad, wood-frame rectangular buildings
ASAE EP486 Post and pole foundation design
ASAE EP558 Load tests for metal-clad, wood-frame dia-phragms
ANSI/ASAE EP559 Design requirements and bending proper-ties for mechanically laminated columns
ASCE
American Society of Civil Engineers 1801 Alexander Bell Drive
Reston, Virginia 20191-4400 http://www.asce.org/
ASCE Standard 7 Minimum Design Loads for Buildings and Other Structures
Standard for load and resistance factor design (LRFD) for engi-neered wood construction
Guide to the use of the wind load provisions of ASCE 7-95
AWPA
American Wood Preservers Assoc. P.O. Box 5690
Granbury, TX 76049
Standard C2 lumber, timbers, bridge ties and mine ties - pre-servative treatment by pressure processes
Standard C15 wood for commercial-residential construction - preservative treatment by pressure processes
Standard C16 wood used on farms - preservative treatment by pressure processes
Standard C23 round poles and posts used in building construc-tion - preservative treatment by pressure processes
Standard M4 standard for the care of preservative-treated wood products
AWPI
American Wood Preservers Institute 2750 Prosperity Avenue, Suite 550 Fairfax, Virginia 22031-4312 http://www.awpi.org/
Answers to often-asked questions about treated wood Management of used treated wood products booklet
Gypsum Association
810 First St., NE, #510 Washington DC, 20002 http://www.gypsum.org/
Fire resistance design manual GA-600 Design data - gypsum board GA-530
Table 2.1. Partial list of technical references related to post-frame building design and construction
Organization & Address Publications
ICC
International Code Council http://www.intlcode.org/ BOCA International, Inc. 4051 West Flossmoor Road Country Club Hills, IL 50478-5794 http://www.bocai.org/
ICBO
5360 Workman Mill Road Whittier, CA 90601-2298 http://www.icbo.org/ SBCCI, Inc. 900 Montclair Road Birmingham, AL 35213-1206 http://www.sbcci.org/
International building code
International energy conservation code International zoning code
International property maintenance code commentary International property maintenance code
International fuel gas code
International mechanical code commentary International mechanical code
International mechanical code supplement International private sewage disposal code International one and two family dwelling code International plumbing code commentary International plumbing code
MBMA
Metal Building Manufacturers Assoc. 1300 Sumner Ave
Cleveland, OH 44115-2851 http://www.mbma.com/
Low rise building systems manual Metal building systems
NFBA
National Frame Builders Association 4840 W. 15th St., Suite 1000
Lawrence, KS 66049-3876 http://www.postframe.org/
Post wall assembly fire test
NFPA
National Fire Protection Association 1 Batterymarch Park
Quincy, MA 02269-9101 http://www.nfpa.org/
NFPA 1: Fire prevention code NFPA 13: Installation of sprinkler NFPA 70: National electrical code NFPA 72: National fire alarm code NFPA 101: Life safety code
SPIB
Southern Pine Inspection Bureau 4709 Scenic Highway
Pensacola, Fl. 32504-9094 http://www.SPIB.org/
Grading rules
Standard for mechanically graded lumber Kiln drying southern pine
Table 2.1. Partial list of technical references related to post-frame building design and construction
Organization & Address Publications
SFPA & Southern Pine Council
Southern Forest Products Association P. O. Box 641700
Kenner, LA 70064-1700 http://www.southernpine.com/ http://www.SFPA.org/
Southern pine use guide
Southern pine joists & rafters: construction guide Southern pine joists & rafters: maximum spans Post-frame construction guide
Southern pine headers and beams Pressure-treated southern pine
Permanent wood foundations: design & construction guide
TPI
Truss Plate Institute
583 D'Onofrio Drive, Suite 200 Madison, WI 53719
ANSI/TPI 1-1995 National design standard for metal plate con-nected wood truss construction
HIB-91 Summary sheet: handling, installing & bracing metal plate connected wood trusses
HIB-98 Post frame summary sheet: recommendations for han-dling, installing & temporary bracing metal plate connected wood trusses used in post-frame construction
HET-80 Handling & erecting wood trusses: commentary and recommendations
DSB-89 Recommended design specifications for temporary bracing of metal plate connected wood trusses
UL
Underwriters Laboratories, Inc. 333 Pfingsten Road
Northbrook, IL 60062-2096 http://www.ul.com/
Fire resistance directory
WTCA
Wood Truss Council of America One WTCA Center
6425 Normandy Lane Madison, WI 53711 http://www.woodtruss.com/
Metal plate connected wood truss handbook
Commentary for permanent bracing of metal plate connected wood trusses
Standard responsibilities in the design process involving metal plate connected wood trusses
WWPA
Western Wood Products Association 522 SW Fifth Ave., Suite 500
Portland, Oregon 97204-2122 http://www.wwpa.org/
Western woods use book Western lumber span tables Western lumber grading rules
Chapter 3: STRUCTURAL LOAD AND DEFLECTION CRITERIA
3.1 Introduction
3.1.1 Load Variations. Most structural loads
exhibit some degree of random behavior. For example, weather-related loads such as snow, wind and rain fluctuate over time and locations. Extensive research has been conducted to characterize this load variation, and to refine procedures for determining design loads within the context of the intended building occupancy and use.
3.1.2 Codes. Calculation procedures for
mini-mum design loads are given in the model build-ing codes. Buildbuild-ings shall be designed to safely carry all loads specified by the governing build-ing code. In the absence of a code, minimum design loads shall be calculated according to recommended engineering practice for the re-gion and application under consideration. It is impractical to describe detailed load calcula-tion procedures in this chapter because of dif-ferences between building codes and frequent revisions of these codes. Instead, general con-cepts and key references related to structural loads and deflection criteria are presented, with an emphasis on issues that apply to post-frame buildings.
3.2 Technical References on
Structural Load Determination
3.2.1 ANSI/ASCE 7 Standard. The National
Bureau of Standards published a report titled Minimum Live Load Allowable for Use in Design of Buildings in 1924. The report was expanded and published as ASA Standard A58.1-1945. This standard has undergone several revisions to become the current ASCE Standard ANSI/ASCE 7 Minimum Design Loads for Build-ings and Other Structures. At the time this de-sign manual was written, the most recent revi-sion of ASCE 7 was 1999 (ASCE, 1999); how-ever, the edition most commonly used is ASCE 7-93. The ASCE 7 standard is periodically re-vised and balloted through the ANSI consensus
approval process, and then must be adopted by the model building codes. Design professionals should check the governing building code for the latest adopted edition. For clarity of presenta-tion, this manual uses and will refer to ASCE 7-93.
ASCE 7-93 is the primary technical source used by the model codes concerning dead, live, snow, wind, rain and seismic loads. Basically, the model codes attempt to distill the rigorous ASCE 7-93 procedures into a simpler, easy-to-use format. Many specific load calculation pro-cedures differ between the model codes; how-ever, most of the basic concepts mimic ASCE 7-93. Background information on the wind load provisions in ASCE 7-88 (which are essentially the same as in ASCE 7-93) are given by Mehta et al. (1991).
3.2.2 Low Rise Building Systems Manual.
The Low Rise Building Systems Manual, pub-lished by the Metal Building Manufacturers As-sociation (1986), is recognized by model build-ing codes as an excellent technical resource document for calculating structural loads on low-rise buildings (e.g. post-frame buildings). This document will be referred to as MBMA-86 throughout this manual. Because wind and crane loads frequently control the design of low-rise metal buildings, the coverage of these loads within MBMA-86 is especially thorough. Another attractive feature of MBMA-86 is the extensive collection of example load calculations.
3.2.3 ASAE EP288.5 Standard. Agricultural
buildings generally fall into a separate class from other types of buildings due to the lower risks involved. The American Society of Agricul-tural Engineers publishes a snow and wind load standard, EP288.5, intended for agricultural buildings (ASAE, 1999). The major differences between agricultural and other types of buildings are that lower values are used for importance and roof snow conversion factors (due to rela-tively lower risk factors for property and non-public use). If the local governing building code applies to agricultural buildings, then the design load criteria in the code must be followed.
Table 3.1. Approximate Weights of Construction Materials (from Hoyle and Woeste, 1989)
Material Weight
(lb/ft2) Material
Weight (lb/ft2)
Ceilings Roofs (continued)
Acoustical fiber tile 1.0 Plywood (per inch thickness) 3.0
Gypsum board (see Walls) Roll roofing 1.0
Mechanical duct allowance 4.0 Shingles
Suspended steel channel system 2.0 Asphalt 2.0
Wood purlins (see Wood, Seasoned) Clay tile 9.0-14.0
Light gauge steel (see Roofs) Book tile, 2-in. 12.0
Book tile, 3-in 20.0
Floors Ludowici 10.0
Hardwood, 1-in. nominal 4.0 Roman 12.0
Plywood (see Roofs) Slate, ¼ in. 10.0
Linoleum, 1/4-in. 1.0 Wood 3.0
Vinyl tile, 1/8-in. 1.4
Walls
Roofs Wood paneling, 1-in. 2.5
Corrugated Aluminum Glass, plate, 1/4-in. 3.3
14 gauge 1.1 Gypsum board (per 1/8-in. thick- 0.55
16 gauge 0.9 Masonry, per 4-in. thickness
18 gauge 0.7 Brick 38.0
20 gauge 0.6 Concrete block 20.0
Built-Up Cinder concrete block 20.0
3-ply 1.5 Stone 55.0
3-ply with gravel 5.5 Porcelain-enameled steel 3.0
5-ply 2.5 Stucco, 7/8-in. 10.0
5-ply with gravel 6.5 Windows, glass, frame, and sash 8.0 Corrugated Galvanized steel
16 gauge 2.9 Wood, Seasoned Density
3
18 gauge 2.4 lb/ft3
20 gauge 1.8 Cedar 32.0
22 gauge 1.5 Douglas-fir 34.0
24 gauge 1.3 Hemlock 31.0
26 gauge 1.0 Maple, red 37.0
29 gauge 0.8 Oak 45.0
Insulation, per inch thickness Poplar, yellow 29.0
Rigid fiberboard, wood base 1.5 Pine, lodgepole 29.0
Rigid fiberboard, mineral base 2.1 Pine, ponderosa 28.0
Expanded polystyrene 0.2 Pine, Southern 35.0
Fiberglass, rigid 1.5 Pine, white 27.0
Fiberglass, batt 0.1 Redwood 28.0
Lumber (see Wood, Seasoned) Spruce 29.0
3.3 Minimum Design Loads
Sections 3.4, 3.5, 3.6, 3.7, and 3.8 give general load requirements, sources of load data and references for making detailed load calculations. Detailed calculation procedures are not provided due to differences between the model codes and the frequency of code revisions.
3.4 Dead Loads
3.4.1 Definition. Dead loads are the gravity
loads due to the combined weights of all perma-nent structural and nonstructural compoperma-nents of the building, such as sheathing, trusses, purlins, girts and fixed service equipment. These loads are constant in magnitude and location through-out the life of the building.
3.4.2 Code Application. Minimum design dead
loads shall be determined according to the gov-erning building code. In the absence of a build-ing code, dead load data can be found in ASCE 7-93, or actual weights of materials and equip-ment can be used.
3.4.3 Special Considerations. Design dead
loads that exceed the weights of construction materials and permanent fixtures are permitted, except for when checking building stability under wind loading. Using inflated design dead loads may lead to conservative designs for gravity load conditions; however, it would not be a con-servative assumption for designing anchorage to counteract uplift, overturning and sliding due to wind loads. In the cases of wind uplift and over-turning, the dead load used in design must not exceed the actual dead load of the construction.
3.4.4 Weights of Construction Materials.
Ta-ble 3.1 lists approximate weights of materials. commonly used in post-frame construction.
3.5 Live
Loads
3.5.1 Definition. Live loads are defined as the
loads superimposed by the construction, main-tenance, use and occupancy of the building, and therefore do not include wind, snow, seismic or dead loads.
Technical Note
Horizontal Uniform Dead Load Calculation Many structural analysis programs (e.g. Purdue Plane Structures Analyzer) require that the dead load associated with a sloping surface be repre-sented as a uniform load, wDL, acting on a
hori-zontal plane as shown in figure 3.1. For a given horizontal distance, bH, a sloping roof surface
contains more material and is heavier than a flat one. Thus, wDL increases as roof slope
in-creases.
Load wDL is obtained by multiplying the unit
weight of the roof assembly, wR, by the slope
length, bS, and dividing the resulting product by
the horizontal length, bH. Numerically, this is
equivalent to dividing wR by the cosine of the
roof slope.
Example: For a roof at a 4:12 slope, with materi-als weighing 4 lbm for each square foot of roof surface area, the equivalent load, wDL, to apply
to the horizontal plane would be: wDL = (4 lbm/ft
2)/(cos 18.4°) = 4.21 lbm/ft2
Figure 3.1. Roof dead load represented by an
equivalent uniform load acting on a horizontal plane.
3.5.2 Code Application. Design live loads shall
be determined so as to provide for the service requirements of the building, but should never be lower than the minimum live load specified in
wDL θ bH bS Rafter or truss top chord Roof assembly with weight wR
the governing building code. In the absence of a governing building code, the minimum live loads found in ASCE 7-93 are recommended. The minimum roof live load recommended for agri-cultural buildings in ASAE Standard EP288.5 is 12 psf. Some agricultural buildings do not nec-essarily pose a "low risk", and the ASAE higher minimum live load reflects the possibility of high-value agricultural constructions now common in the United States
3.5.3 Reductions. In some cases, reductions
are allowed for uniform loads to account for the low likelihood of the loads simultaneously occur-ring over the entire tributary area.
3.6 Snow
Loads
3.6.1 Code Application. Minimum design snow
loads shall be determined by the provisions of the governing building code. The presentation of snow loads varies among the model codes, but they all follow the basic concepts presented in ASCE 7-93. In the absence of a building code, procedures given in ASCE 7-93 are recom-mended. For low-risk agricultural buildings, snow load calculation procedures given in ASAE EP288.5 are permitted.
3.6.2 Ground Snow Load Maps. ASCE 7-93
presents ground snow load maps that corre-spond to a mean recurrence interval of 50 years. These maps do not give snow load values for areas that are subject to extreme variations in snowfall, such as western mountain regions. In some regions, the best and only reliable source for ground snow loads is local climatic records.
3.6.3 Roof Snow Loads. Roof snow loads are
influenced by a number of factors besides ground snow load. These factors include roof slope, temperature and coefficient of friction of the roof surface, and wind exposure. Snow loads are also adjusted by an importance factor to account for risk to property and people. The basic form of the snow load calculation found in ASCE 7-93 is:
pf = R Ce Ct I Cs Pg (3-1)
where:
pf = roof snow load in psf,
R = roof snow factor that relates roof load to ground snowpack,
Ce = snow exposure factor,
Ct = roof temperature factor,
I = importance factor, Cs = roof slope factor, and
Pg = ground snow load in psf (50-yr
mean recurrence).
The roof snow factor, R, varies from 0.6 for Alaska to 0.7 for the contiguous United States. The snow exposure factor in the model codes accounts for the combined effects of R and Ce
given in Equation 3-1. The thermal factor de-fined in ASCE 7-93 varies from 1.0 for heated structures to 1.2 for unheated structures. The thermal factor is not included in the model build-ing codes. The importance factors range from 0.8 to 1.2 depending on the specific building code. Roof slope factors vary linearly from 0 to 1 as roof slope increases from 15 to 70 degrees.
3.6.5 Special Considerations. Several factors,
such as multiple gables, roof discontinuities, and drifting can cause snow to accumulate unevenly on roofs. These factors must be considered in the design. Specific recommendations and cal-culation procedures are given in the model codes and ASCE 7-93.
3.7 Wind
Loads
3.7.1 Controlling Factors. Wind loads are
in-fluenced by wind speed, building orientation and geometry, building openings and exposure. Wind loading on structures is a complex phe-nomenon and is being actively researched.
3.7.2 Code Application. Minimum design wind
loads shall be determined by the provisions of the governing building code. In the absence of a building code, procedures given in ASCE 7-93 or MBMA-86 are recommended. For low-risk agricultural buildings, wind load calculation pro-cedures given in ASAE EP288.5 are permitted.
3.7.3 Design Wind Speed. ASCE 7-93 gives a
map showing basic wind speeds throughout the United States that correspond to a mean recur-rence interval of 50 years. Local weather
rec-ords should be used in regions that have un-usual wind events. Detailed procedures and il-lustrations for calculating wind loads on low-rise buildings are given in MBMA-86.
Technical Note
Wind Speed
Wind speeds are derived from data which reflect both magnitude and duration. Wind speeds can be reported as peak gusts, or can be averaged over some time interval. The time interval may be fixed, as with mean hourly speeds, or vari-able, as with “fastest-mile” wind speeds. Fast-est-mile wind speeds are used in ANSI/ASCE 7-93 to calculate design loads, and are defined on the basis of the period of time that one mile of wind takes to pass an anemometer at a stan-dard elevation of 10 meters. The U.S. National Weather Service no longer collects fastest-mile wind speed data; instead, they record 3-second gust speeds. The 1995 and later revisions of ASCE-7 base wind loads on 3-second gust wind speeds.
3.7.4 Effective Wind Velocity Pressure. The
first step in determining wind loads is to calcu-late the effective wind velocity pressure. The most severe exposure factors that will apply dur-ing the service life of the structure should be used. Wind velocity pressure is a function of the wind speed, exposure and importance. The equation for calculating wind velocity pressure, qz , is given by:
qz = 0.00256 Kz (I V) 2
(3-2) where:
Kz = velocity pressure exposure
coeffi-cient,
I = importance factor, and
V = basic wind speed in mph (50-year mean recurrence interval).
The velocity pressure exposure coefficient is a function of height above ground and exposure category. Exposure categories account for the effects of ground surface irregularities caused by natural topography, vegetation, location and building construction features. ASCE 7-93 lists four wind exposure categories, whereas the
model codes publish fewer exposure categories. Importance factors vary from 0.95 for agricul-tural buildings (25-year recurrence interval) to 1.07 for buildings that represent a high hazard to property and people in the event of failure (100-year recurrence interval). Wind pressure is re-lated to the square of its speed, therefore the terms V and I are squared in equation 3-2. The model building codes simplify the calculation in equation 3-2 by publishing tables of effective wind velocity pressures, Pb, for a base wind
speed and various heights.
3.7.5 Pressure Coefficients. Wind loads are
calculated for each part of the building by multi-plying the effective wind pressure by a pressure coefficient. The pressure coefficient, which may be different for each planar portion of the build-ing, accounts for building orientation, geometry and load sharing. It also accounts for localized pressures at eaves, overhangs, corners, etc. Wind pressures, qi, for the i
th
building surface are calculated by:
qi = Cpi qz (3-3)
where: Cpi = i
th
pressure coefficient, and qz = wind velocity pressure.
The wind velocity pressure is based on the wall height for the windward wall and on the mean roof height for the leeward wall and roof. Wind pressures act normal to the building surfaces. Inward pressures are denoted with positive signs, while outward pressures (suction) are denoted with negative signs.
Technical Note
Components of Wind Load
Many structural analysis programs require uni-form loads to be entered in terms of their hori-zontal and vertical components. Wind loads act normal to building surfaces, so an adjustment is needed for sloping members such as roof trusses. The roof wind load, w, shown in figure 3.2a is equivalent to the horizontal and vertical components shown in figure 3.2b. The relation-ship depicted in figure 3.2 can be proven as fol-lows:
1. Convert the uniform wind load, w, to its re-sultant vector force.
R = w (span)/(cos θ)
2. Multiply resultant force, R, by cos θ to obtain its vertical component.
Fy = R (cos θ) = w (span)
3. Divide the vertical component, Fy, by the
span to obtain the horizontally projected up-lift pressure, whoriz.
whoriz = Fy /(span) = w (span)/(span) = w
The vertically projected uniform load can be proven similarly. A common mistake is to multi-ply the normal pressure by sine and cosine of the roof slope to obtain the two components.
Figure 3.2. Illustration of wind load acting
nor-mal to inclined surface and equivalent horizontal and vertical load components. A common mis-take is to multiply the normal load by sin(θ) and cos(θ) for the vertical and horizontal compo-nents, respectively.
3.7.6 Main Frames. Different pressure
coeffi-cients are used to calculate wind loads on main frames as compared to components and clad-ding. Main frames include primary structural sys-tems such as rigid and braced frames, braced trusses, posts, poles and girders. Since
these members have relatively large tributary areas, localized wind effects tend to be aver-aged out over the tributary areas. Pressure coef-ficients for main members reflect this averaging effect.
3.7.7 Components and Cladding. Wind
pres-sures are higher on small areas due to localized gust effects. This observation has been verified by wind tunnel studies (MBMA, 1986), as well as site inspections of wind-induced building failures (Harmon, et al., 1992). For this reason, compo-nents and cladding have higher pressure coeffi-cients than main frames. Components and clad-ding include members such as purlins, girts, cur-tain walls, sheathing, roofing and siding.
3.7.8 Openings. Wind loads are significantly
affected by openings in the structure. ASCE 7-93 and the model building codes specify internal wind pressure coefficients (or adjustments to external pressure coefficients) for structures with different amounts and types of openings. Each model code has slightly different definitions and wind load coefficients for open, closed and par-tially open buildings. In general, "openings" refer to permanent or other openings that are likely to be breached during high winds. For example, if window glazings are likely to be broken during a windstorm, the windows are considered open-ings. However, if doors and windows and their supports are designed to resist design wind loads, they need not be considered openings. It should be noted that internal wind pressures act against all interior surfaces and therefore do not contribute to sidesway loads on a building.
3.8 Seismic
Loads
3.8.1 Cause. Earthquakes produce lateral
forces on buildings through the sudden move-ment of the building’s foundation. Building re-sponse to seismic loading is a complex phe-nomenon and there is considerable controversy as to how to translate knowledge gained through research into practical design codes and stan-dards.
3.8.2 Code Application. Seismic loads shall be
determined by the provisions of the governing building code. In the absence of a building code, procedures given in ASCE 7-93 are recom-mended. Sweeping changes were made in the (a) θ w (b) w w θ
1993 revision of ASCE 7 with respect to seismic loads. The seismic provisions in ASCE 7-93 were based on work by the Building Seismic Safety Council under sponsorship of the Federal Emergency Management Agency.
3.8.3 Lateral Force. Basic concept of seismic
load determination for low-rise buildings is to calculate an equivalent lateral force at the ground line as follows:
V = Cs W (3-4)
where:
V = total lateral force, or shear, at the building base
W = total dead load, plus other applica-ble loads specified in the code or ASCE 7-93. For most single-story post-frame buildings, the only other minimum applicable load is a por-tion (20% minimum) of the flat roof snow load. If the flat roof snow load is less than 30 psf, the applicable load to be included in W is permitted to be taken as zero.
Cs = seismic design coefficient
= 1.2 Av S/(T2/3 R)
Av = coefficient representing effective
peak velocity-related acceleration S = coefficient for the soil profile
charac-teristics
R = response modification factor T = fundamental period of the building
3.8.4 Seismic loads rarely control post-frame
building design because of the relatively low building dead weight as compared with other types of construction (Taylor, 1996; Faherty and Williamson, 1989). For post-frame buildings, lateral loads from wind usually are much greater than those from seismic forces.
3.9 Load Combinations for
Allowable Stress Design
3.9.1 Code Application. Every building
ele-ment shall be designed to resist the most critical load combinations specified in the governing building code.
3.9.2 Load Combinations. Except when
appli-cable codes provide otherwise, the following load combinations shall be considered (as a minimum) and the combination which results in the most conservative design for each building element shall be used. Note that different load combinations may control the design of different components of the structure.
Case 1: Dead + Floor Live + Roof Live (or Snow)
Case 2: Dead + Floor Live + Wind (or Seismic) Case 3: Dead + Floor Live + Wind + ½ Snow Case 4: Dead + Floor Live + ½ Wind + Snow Case 5: Dead + Floor Live + Snow + Seismic
3.9.3 Floor Live Loads. Most post-frame
build-ings are single story and therefore would not have floor live loads acting on the post-frames. When a concrete floor is used in a single story building, consideration must be given to antici-pated live and equipment loading.
3.9.4 Reductions. Reductions in some of the
load terms in Cases 1 through 5 are permitted, depending on governing building code or refer-ence document. With some exceptions, the model building codes permit allowable stresses used in allowable stress design to be increased one-third when considering wind or seismic forces either acting alone or when combined with vertical loads. The allowable stress in-crease for wind loading can be traced back to the New York City Building Code of 1904 (Elli-fritt, 1977), and appears to be based on judg-ment rather than engineering theory. It should be noted that ASCE 7-93 does not include the one-third increase factor, but instead specifies load combination factors that are intended to account for the low probability of maximum live, seismic, snow and wind loads occurring simul-taneously. The commentary of ASCE 7-93 im-plies the stress increase for wind and seismic found in codes is not appropriate if the com-bined load effects are also reduced by the load combination factors published in ASCE 7-93. Finally, the National Design Specification (NDS) for Wood Construction (NF&PA, 199) addresses the issue of load combination versus load dura-tion factors by stating, “The load duradura-tion fac-tors, CD, in Table 2.3.2 and Appendix B are
in-dependent of load combination factors, and both shall be permitted to be used in design calcula-tions.”
