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BUILDING DESIGN CRITERIA
SECTION BUILDING DESIGN CRITERIA R A LaBoube, P.E Professor of Civil Engineering, University of Missouri-Rolla, Rolla, Missouri Building designs generally are controlled by local or state building codes In addition, designs must satisfy owner requirements and specifications For buildings on sites not covered by building codes, or for conditions not included in building codes or owner specifications, designers must use their own judgment in selecting design criteria This section has been prepared to provide information that will be helpful for this purpose It summarizes the requirements of model building codes and standard specifications and calls attention to recommended practices The American Institute of Steel Construction (AISC) promulgates several standard specifications, but two are of special importance in building design One is the ‘‘Specification for Structural Steel Buildings—Allowable Stress Design (ASD) and Plastic Design.’’ The second is the ‘‘Load and Resistance Factor Design (LRFD) Specification for Structural Steel Buildings,’’ which takes into account the strength of steel in the plastic range and utilizes the concepts of first-order theory of probability and reliability The standards for both ASD and LRFD are reviewed in this section Steels used in structural applications are specified in accordance with the applicable specification of ASTM Where heavy sections are to be spliced by welding, special material notch-toughness requirements may be applicable, as well as special fabrication details (see Arts 1.13, 1.14, and 1.21) 6.1 BUILDING CODES A building code is a legal ordinance enacted by public bodies, such as city councils, regional planning commissions, states, or federal agencies, establishing regulations governing building design and construction Building codes are enacted to protect public health, safety, and welfare A building code presents minimum requirements to protect the public from harm It does not necessarily indicate the most efficient or most economical practice Building codes specify design techniques in accordance with generally accepted theory They present rules and procedures that represent the current generally accepted engineering practices A building code is a consensus document that relies on information contained in other recognized codes or standard specifications, e.g., the model building codes promulgated by 6.1 6.2 SECTION SIX building officials associations and standards of AISC, ASTM, and the American National Standards Institute (ANSI) Information generally contained in a building code addresses all aspects of building design and construction, e.g., fire protection, mechanical and electrical installations, plumbing installations, design loads and member strengths, types of construction and materials, and safeguards during construction For its purposes, a building code adopts provisions of other codes or specifications either by direct reference or with modifications 6.2 APPROVAL OF SPECIAL CONSTRUCTION Increasing use of specialized types of construction not covered by building codes has stimulated preparation of special-use permits or approvals Model codes individually and collectively have established formal review procedures that enable manufacturers to attain approval of building products These code-approval procedures entail a rigorous engineering review of all aspects of product design 6.3 STANDARD SPECIFICATIONS Standard specifications are consensus documents sponsored by professional or trade associations to protect the public and to avoid, as much as possible, misuse of a product or method and thus promote the responsible use of the product Examples of such specifications are the American Institute of Steel Construction (AISC) allowable stress design (ASD) and load and resistance factor design (LRFD) specifications; the American Iron and Steel Institute’s (AISI’s) ‘‘Specification for the Design of Cold-Formed Steel Structural Members,’’ the Steel Joist Institute’s ‘‘Standard Specifications Load Tables and Weight Tables for Steel Joists and Joist Girders,’’ and the American Welding Society’s (AWS’s) ‘‘Structural Welding Code—Steel’’ (AWS D1.1) Another important class of standard specifications defines acceptable standards of quality of building materials, standard methods of testing, and required workmanship in fabrication and erection Many of these widely used specifications are developed by ASTM As need arises, ASTM specifications are revised to incorporate the latest technological advances The complete ASTM designation for a specification includes the year in which the latest revision was approved For example, A588 / A588M-97 refers to specification A588, adopted in 1997 The M indicates that it includes alternative metric units In addition to standards for product design and building materials, there are standard specifications for minimum design loads, e.g., ‘‘Minimum Design Loads for Buildings and Other Structures’’ (ASCE 7-95), American Society of Civil Engineers, and ‘‘Low-Rise Building Systems Manual,’’ Metal Building Manufacturers Association It is advisable to use the latest editions of standards, recommended practices, and building codes 6.4 BUILDING OCCUPANCY LOADS Safe yet economical building designs necessitate application of reasonable and prudent design loads Computation of design loads can require a complex analysis involving such considerations as building end use, location, and geometry BUILDING DESIGN CRITERIA 6.4.1 6.3 Building Code–Specified Loads Before initiating a design, engineers must become familiar with the load requirements of the local building code All building codes specify minimum design loads These include, when applicable, dead, live, wind, earthquake, and impact loads, as well as earth pressures Dead, floor live, and roof live loads are considered vertical loads and generally are specified as force per unit area, e.g., lb per ft2 or kPa These loads are often referred to as gravity loads In some cases, concentrated dead or live loads also must be considered Wind loads are assumed to act normal to building surfaces and are expressed as pressures, e.g., psf or kPa Depending on the direction of the wind and the geometry of the structure, wind loads may exert either a positive or negative pressure on a building surface All building codes and project specifications require that a building have sufficient strength to resist imposed loads without exceeding the design strength in any element of the structure Of equal importance to design strength is the design requirement that a building be functional as stipulated by serviceability considerations Serviceability requirements are generally given as allowable or permissible maximum deflections, either vertical or horizontal, or both 6.4.2 Dead Loads The dead load of a building includes weights of walls, permanent partitions, floors, roofs, framing, fixed service equipment, and all other permanent construction (Table 6.1) The American Society of Civil Engineers (ASCE) standard, ‘‘Minimum Design Loads for Buildings and Other Structures’’ (ASCE 7-95), gives detailed information regarding computation of dead loads for both normal and special considerations 6.4.3 Floor Live Loads Typical requirements for live loads on floors for different occupancies are summarized in Table 6.2 These minimum design loads may differ from requirements of local or state building codes or project specifications The engineer of record for the building to be constructed is responsible for determining the appropriate load requirements Temporary or movable partitions should be considered a floor live load For structures designed for live loads exceeding 80 lb per ft2, however, the effect of partitions may be ignored, if permitted by the local building code Live Load Reduction Because of the small probability that a member supporting a large floor area will be subjected to full live loading over the entire area, building codes permit a reduced live load based on the areas contributing loads to the member (influence area) Influence area is defined as the floor area over which the influence surface for structural effects on a member is significantly different from zero Thus the influence area for an interior column comprises the four surrounding bays (four times the conventional tributary area), and the influence area for a corner column is the adjoining corner bay (also four times the tributary area, or area next to the column and enclosed by the bay center lines) Similarly, the influence area for a girder is two times the tributary area and equals the panel area for a two-way slab The standard, ‘‘Minimum Design Loads for Buildings and Other Structures’’ (ASCE 795), American Society of Civil Engineers, permits a reduced live load L (lb per ft2) computed from Eq (6.1) for design of members with an influence area of 400 ft2 or more: L ⫽ Lo(0.25 ⫹ 15 / 兹AI) (6.1) TABLE 6.1 Minimum Design Dead Loads Component Load, lb / ft2 Ceilings Acoustical fiber tile 0.55 Gypsum board (per 1⁄8-in thickness) Mechanical duct allowance Plaster on tile or concrete Plaster on wood lath Suspended steel channel system Suspended metal lath and cement plaster 15 Suspended metal lath and gypsum plaster 10 Wood furring suspension system 2.5 Coverings, roof, and wall Asbestos-cement shingles Asphalt shingles Cement tile Clay tile (for mortar add 10 lb): Book tile, 2-in Book tile, 3-in Ludowici Component Load, lb / ft2 Waterproofing membranes: Bituminous, gravel-covered Bituminous, smooth surface Liquid applied Single-ply, sheet Wood sheathing (per inch thickness) Wood shingles 5.5 1.5 1.0 0.7 3 Floor fill Cinder concrete, per inch Lightweight concrete, per inch Sand, per inch Stone concrete, per inch 8 12 Floors and floor finishes 16 12 20 10 Asphalt block (2-in), 1⁄2-in mortar Cement finish (1-in) on stone-concrete fill Ceramic or quarry tile (3⁄4-in) on 1⁄2-in mortar bed Ceramic or quarry tile (3⁄4-in) on 1-in mortar bed 30 32 Component Load, lb / ft2 Frame partitions Movable steel partitions Wood or steel studs, 1⁄2-in gypsum board each side Wood studs, ⫻ 4; unplastered Wood studs, ⫻ 4, plastered one side Wood studs, ⫻ 4, plastered two sides 12 20 Frame walls Exterior stud walls: ⫻ @ 16 in, 5⁄8-in gypsum, insulated, ⁄8-in siding ⫻ @ 16 in, 5⁄8-in gypsum, insulated, ⁄8-in siding Exterior stud walls with brick veneer Windows, glass, frame and sash 11 12 48 Masonry walls* 16 23 Clay brick wythes: in in 39 79 6.4 TABLE 6.1 Minimum Design Dead Loads Component Continued Load, lb / ft2 Coverings, roof, and wall (cont.) Clay tile (cont.) Roman 12 Spanish 19 Composition: Three-ply ready roofing Four-ply felt and gravel 5.5 Five-ply felt and gravel Copper or tin Deck, metal, 20 ga 2.5 Deck, metal, 18 ga Decking, 2-in wood (Douglas fir) Decking, 3-in wood (Douglas fir) Fiberboard, 1⁄2-in 0.75 Gypsum sheathing, 1⁄2-in Insulation, roof boards (per inch thickness): Cellular 0.7 Fibrous glass 1.1 Fiberboard 1.5 Perlite 0.8 Polystyrene foam 0.2 Urethane foam with skin 0.5 Plywood (per 1⁄8-in thickness) 0.4 0.75 Rigid insulation, 1⁄2-in Skylight, metal frame, 3⁄8-in wire glass Slate, 3⁄16-in 10 Slate, 1⁄4-in Load, lb / ft2 Component Floors and floor finishes (cont.) Masonry walls (cont.) Concrete fill finish (per inch thicknes) Hardwood flooring, 7⁄8-in Linoleum or asphalt tile, 1⁄4-in Marble and mortar on stone-concrete fill Slate (per inch thickness) Solid flat tile on 1-in mortar base Subflooring, 3⁄4-in Terrazzo (11⁄2-in) directly on slab Terrazzo (1-in) on stone-concrete fill Terrazzo (1-in), 2-in stone concrete Wood block (3-in) on mastic, no fill Wood block (3-in) on 1⁄2-in mortar base 12 33 15 23 19 32 32 10 16 Floors, wood-joist (no plaster) double wood floor Joist sizes, in 12-in spacing, lb / ft2 16-in spacing, lb / ft2 24-in spacing, lb / ft2 ⫻6 6 6 5 6 2 2 ⫻8 ⫻ 10 ⫻ 12 Load, lb / ft2 Component Clay brick wythes: (cont.) 12 in 16 in Hollow concrete masonry unit wythes: Wythe thickness (in) Unit percent solid Light weight units (105 pcf): No grout 48 o.c 40 o.c 32 o.c Grout 24 o.c spacing 16 o.c Full grout Normal Weight Units (135 pcf): No grout 48 o.c 40 o.c 32 o.c Grout 24 o.c spacing 16 o.c Solid concrete masonry unit wythes (incl concrete brick): Wythe thickness, Lightweight units (105 pcf): Normal weight units (135 pcf): 6.5 * Weights of masonry include mortar but not plaster For plaster, add lb / ft2 for each face plastered Values given represent averages In some cases there is a considerable range of weight for the same construction 冧 115 155 70 55 52 10 50 12 48 22 27 31 33 34 37 42 57 35 40 43 45 49 56 77 42 49 53 56 61 70 98 49 58 63 66 72 84 119 29 35 33 36 38 41 47 64 45 50 53 55 59 66 87 54 61 65 68 73 82 110 63 72 77 80 86 98 133 32 49 67 10 84 12 102 41 63 86 108 131 冧 6.6 SECTION SIX TABLE 6.2 Minimum Design Live Loads a Uniformly distributed design live loads Occupancy or use Armories and drill rooms Assembly areas and theaters Fixed sets (fastened to floor) Lobbies Movable seats Platforms (assembly) Stage floors Balconies (exterior) On one- and two-family residences only, and not exceeding 100 ft2 Bowling alleys, poolrooms, and similar recreational areas Corridors First floor Other floors, same as occupancy served except as indicated Dance halls and ballrooms Decks (patio and roof) Same as area served, or for the type of occupancy accommodated Dining rooms and restaurants Fire escapes On single-family dwellings only Garages (see Table 6.2b also) Passenger cars only For trucks and buses use AASHTOa lane loads (see Table 6.2b also) Grandstandsc (see Stadium) Gymnasiums, main floors and balconiesc Hospitals (see Table 6.2b also) Operating room, laboratories Private rooms Wards Corridors above first floor Libraries (see Table 6.2b also) Reading rooms Stack roomsd Corridors above first floor Live load, lb / ft2 150 60 100 100 100 150 100 60 75 100 100 100 100 40 50 100 60 40 40 80 60 150 80 Occupancy or use Manufacturing (see Table 6.2b also) Light Heavy Marquees and canopies Office buildingsb (see Table 6.2b also) Lobbies Offices Penal institutions Cell blocks Corridors Residential Dwellings (one- and twofamily) Uninhabitable attics without storage Uninhabitable attics with storage Habitable attics and sleeping areas All other areas Hotels and multifamily buildings Private rooms and corridors serving them Public rooms, corridors, and lobbies serving them Schools (see Table 6.2b also) Classrooms Corridors above first floor Sidewalks, vehicular driveways, and yards, subject to truckinga (see Table 6.2b also) Stadium and arenasc Bleachers Fixed seats (fastened to floor) Stairs and exitways (see Table 6.2b also) Storage warehouses Light Heavy Stores Retail First floor Upper floors Wholesale, all floors Walkways and elevated platforms (other than exitways) Yards and terraces (pedestrians) Live load, lb / ft2 125 250 75 100 50 40 100 10 20 30 40 40 100 40 80 250 100 100 60 100 125 250 100 75 125 60 100 6.7 BUILDING DESIGN CRITERIA TABLE 6.2 Minimum Design Live Loads (Continued ) b Concentrated live loadse Location Load, lb Elevator machine room grating (on 4-in2 area) Finish, light floor-plate construction (on 1-in2 area) Garages: Passenger cars: Manual parking (on 20-in2 area) Mechanical parking (no slab), per wheel Trucks, buses (on 20-in2 area) per wheel Hospitals Libraries Manufacturing Light Heavy Office floors (on area 2.5 ft square) Roof-truss panel point over garage, manufacturing, or storage floors Schools Scuttles, skylight ribs, and accessible ceilings (on area 2.5 ft square) Sidewalks (on area 2.5 ft square) Stair treads (on 4-in2 area at center of tread) 300 200 2,000 1,500 16,000 1000 1000 2000 3000 2,000 2,000 1000 200 8,000 300 c Minimum design loads for materials Material Aluminum, cast Bituminous products: Asphalt Petroleum, gasoline Pitch Tar Brass, cast Bronze, to 14% tin Cement, portland, loose Cement, portland, set Cinders, dry, in bulk Coal, bituminous or lignite, piled Coal, bituminous or lignite, piled Coal, peat, dry, piled Charcoal Copper Earth (not submerged): Clay, dry Clay, damp Clay and gravel, dry Silt, moist, loose Silt, moist, packed Load, lb / ft3 165 81 42 69 75 534 509 90 183 45 47 47 23 12 556 63 110 100 78 96 Material Earth (not submerged) (Continued ): Sand and gravel, dry, loose Sand and gravel, dry, packed Sand and gravel, wet Gold, solid Gravel, dry Gypsum, loose Ice Iron, cast Lead Lime, hydrated, loose Lime, hydrated, compacted Magnesium alloys Mortar, hardened: Cement Lime Riprap (not submerged): Limestone Sandstone Sand, clean and dry Load, lb / ft2 100 120 120 1205 104 70 57.2 450 710 32 45 112 130 110 83 90 90 6.8 SECTION SIX TABLE 6.2 Minimum Design Live Loads (Continued ) c Minimum Design loads for materials (Continued ) Material Sand, river, dry Silver Steel Stone, ashlar: Basalt, granite, gneiss Limestone, marble, quartz Load, lb / ft3 106 656 490 165 160 Material Stone, ashlar (Continued ): Sandstone Shale, slate Tin, cast Water, fresh Water, sea Load, lb / ft2 140 155 459 62.4 64 a American Association of State Highway and Transportation Officials lane loads should also be considered where appropriate File and computer rooms should be designed for heavier loads; depending on anticipated installations See also corridors c For detailed recommendations, see American National Standard for Assembly Seating, Tents, and Air-Supported Structures ANSI / NFPA 102 d For the weight of books and shelves, assume a density of 65 pcf, convert it to a uniformly distributed area load, and use the result if it exceeds 150 lb / ft2 e Use instead of uniformly distributed live load, except for roof trusses, if concentrated loads produce greater stresses or deflections Add impact factor for machinery and moving loads: 100% for elevators, 20% for light machines, 50% for reciprocating machines, 33% for floor or balcony hangers For craneways, add a vertical force equal to 25% of the maximum wheel load; a lateral force equal to 10% of the weight of trolley and lifted load, at the top of each rail; and a longitudinal force equal to 10% of maximum wheel loads, acting at top of rail where Lo ⫽ unreduced live load, lb per ft2 AI ⫽ influence area, ft2 The reduced live load should not be less than 0.5Lo for members supporting one floor nor 0.4Lo for all other loading situations If live loads exceed 100 lb per ft2, and for garages for passenger cars only, design live loads may be reduced 20% for members supporting more than one floor For members supporting garage floors, one-way slabs, roofs, or areas used for public assembly, no reduction is permitted if the design live load is 100 lb per ft2 or less 6.4.4 Concentrated Loads Some building codes require that members be designed to support a specified concentrated live load in addition to the uniform live load The concentrated live load may be assumed to be uniformly distributed over an area of 2.5 ft2 and located to produce the maximum stresses in the members Table 6.2b lists some typical loads that may be specified in building codes 6.4.5 Pattern Loading This is an arrangement of live loads that produces maximum possible stresses at a point in a continuous beam The member carries full dead and live loads, but full live load may occur only in alternating spans or some combination of spans In a high-rise building frame, maximum positive moments are produced by a checkerboard pattern of live load, i.e., by BUILDING DESIGN CRITERIA 6.9 full live load on alternate spans horizontally and alternate bays vertically Maximum negative moments at a joint occur, for most practical purposes, with full live loads only on the spans adjoining the joint Thus pattern loading may produce critical moments in certain members and should be investigated 6.5 ROOF LOADS In northern areas, roof loads are determined by the expected maximum snow loads However, in southern areas, where snow accumulation is not a problem, minimum roof live loads are specified to accommodate the weight of workers, equipment, and materials during maintenance and repair 6.5.1 Roof Live Loads Some building codes specify that design of flat, curved, or pitched roofs should take into account the effects of occupancy and rain loads and be designed for minimum live loads, such as those given in Table 6.3 Other codes require that structural members in flat, pitched, or curved roofs be designed for a live load Lr (lb per ft2 of horizontal projection) computed from Lr ⫽ 20R1 R2 ⱖ 12 (6.2) where R1 ⫽ reduction factor for size of tributary area ⫽ for At ⱕ 200 ⫽ 1.2 ⫺ 0.001At for 200 ⬍ At ⬍ 600 ⫽ 0.6 for At ⱖ 600 Al ⫽ tributary area, or area contributing load to the structural member, ft2 (Sec 6.4.3) R2 ⫽ reduction factor for slope of roof ⫽ for Fⱕ TABLE 6.3 Roof Live Loads (lb per ft2) of Horizontal Projection* Tributary loaded area, ft2, for any structural member Roof slope Flat or rise less than 4:12 Arch or dome with rise less than 1⁄8 of span Rise 4:12 to less than 12:12 Arch or dome with rise 1⁄8 span to less than 3⁄8 span Rise 12:12 or greater Arch or dome with rise 3⁄8 of span or greater to 200 201 to 600 Over 600 20 16 12 16 14 12 12 12 12 * As specified in ‘‘Low-Rise Building Systems Manual,’’ Metal Building Manufacturers Association, Cleveland, Ohio 6.10 SECTION SIX ⫽ ⫽ F⫽ ⫽ 6.5.2 1.2 ⫺ 0.05F for ⬍ F ⬍ 12 0.6 for F ⱖ 12 rate of rise for a pitched roof, in / ft rise-to-span ratio multiplied by 32 for an arch or dome Snow Loads Determination of design snow loads for roofs is often based on the maximum ground snow load in a 50-year mean recurrence period (2% probability of being exceeded in any year) This load or data for computing it from an extreme-value statistical analysis of weather records of snow on the ground may be obtained from the local building code or the National Weather Service Maps showing ground snow loads for various regions are presented in model building codes and standards, such as ‘‘Minimum Design Loads for Buildings and Other Structures’’ (ASCE 7-95), American Society of Civil Engineers The map scales, however, may be too small for use for some regions, especially where the amount of local variation is extreme or high country is involved Some building codes and ASCE 7-95 specify an equation that takes into account the consequences of a structural failure in view of the end use of the building to be constructed and the wind exposure of the roof: pƒ ⫽ 0.7CeCt Ipg where Ce Ct I pƒ pg ⫽ ⫽ ⫽ ⫽ ⫽ (6.3) wind exposure factor (Table 6.4) thermal effects factor (Table 6.6) importance factor for end use (Table 6.7) roof snow load, lb per ft2 ground snow load for 50-year recurrence period, lb per ft2 The ‘‘Low-Rise Building systems Manual,’’ Metal Building Manufacturers Association, Cleveland, Ohio, based on a modified form of ASCE 7, recommends that the design of roof snow load be determined from pƒ ⫽ IsCpg (6.4) where Is is an importance factor and C reflects the roof type In their provisions for roof design, codes and standards also allow for the effect of roof slopes, snow drifts, and unbalanced snow loads The structural members should be investigated for the maximum possible stress that the loads might induce 6.6 WIND LOADS Wind loads are randomly applied dynamic loads The intensity of the wind pressure on the surface of a structure depends on wind velocity, air density, orientation of the structure, area of contact surface, and shape of the structure Because of the complexity involved in defining both the dynamic wind load and the behavior of an indeterminate steel structure when subjected to wind loads, the design criteria adopted by building codes and standards have been based on the application of an equivalent static wind pressure This equivalent static design wind pressure p (psf) is defined in a general sense by p ⫽ qGCp where q ⫽ velocity pressure, psf G ⫽ gust response factor to account for fluctuations in wind speed (6.5) ... 49 56 77 42 49 53 56 61 70 98 49 58 63 66 72 84 119 29 35 33 36 38 41 47 64 45 50 53 55 59 66 87 54 61 65 68 73 82 110 63 72 77 80 86 98 133 32 49 67 10 84 12 102 41 63 86 108 131 冧 6. 6 SECTION. .. ⫺50 ⫹ 46 ⫺58 ⫹53 ? ?67 ? ?60 ⫺ 76 ? ?68 ⫺ 86 50 ⫹ 16 ⫺19 ⫹18 ⫺21 ⫹22 ⫺ 26 ⫹ 26 ⫺31 ⫹31 ⫺37 ⫹37 ⫺44 ⫹42 ⫺51 ⫹49 ⫺58 ⫹55 ? ?66 ? ?63 ⫺75 500 ⫹14 ⫺15 ⫹15 ⫺17 ⫹19 ⫺21 ⫹23 ⫺25 ⫹27 ⫺30 ⫹32 ⫺35 ⫹37 ⫺40 ⫹43 ⫺ 46 ⫹49... ⫺110 ⫹ 36 ⫺125 100 ⫹8 ⫺18 ⫹9 ⫺20 ⫹11 ⫺25 ⫹14 ⫺30 ⫹ 16 ⫺ 36 ⫹19 ⫺42 ⫹22 ⫺49 ⫹ 26 ⫺57 ⫹29 ? ?64 ⫹33 ⫺73 10 ⫹17 ⫺18 ⫹19 ⫺20 ⫹24 ⫺25 ⫹29 ⫺30 ⫹34 ⫺ 36 ⫹40 ⫺42 ⫹ 46 ⫺49 ⫹53 ⫺57 ? ?60 ? ?64 ? ?68 ⫺73 50 ⫹ 16 ⫺17 ⫹18