thickness, in Orientation Spacing c to c, in a. Applications in ceilings 3/8* Across 16 1/2* Across 24 1/2* Parallel 16 5/8 Across 24 5/8 Parallel 16 b. Applications in walls 3/8 Across or parallel 16 1/2 Across or parallel 24 5/8 Across or parallel 24 * Gypsumboard for ceilings to receive a water-base-spray texture finish should be applied only across (perpendicular to) framing. For 16-in spacing of framing, hoard thickness should be increased from 3/8 to 1/2 in. For 24-in spacing of framing, board thickness should be increased from 1/2 to 5/8 in. Should not support thermal insulation. To prevent damage from structural movements or dimensional changes, control and expansion joints should be provided and floating-angle construction used, as described in Art. 11.27. Many building components may be affected by the decision to use gypsumboard construction. For example, window and door frames should have the appropriate depth for the wall thickness resulting from use of wallboard. Therefore, walls and partitions and related installations, including mechanical and electrical equipment, should be carefully planned and coordinated. Application. Wallboard preferably should be applied to studs with long dimension horizontal (Fig. 11.23), and vertical joints should be staggered. In ceilings, the long FIGURE 11.23 Single-ply application of gypsumboard to studs with long dimension horizontal. sides should span supports (often referred to as horizontal, or across, application). Board ends and edges parallel to framing or furring should be supported on those members, except for face layers of two-ply systems. Otherwise, back blocking should be used to reinforce the joints. Ceiling panels should be installed first, then the walls. Adjoining boards should be placed in contact, but not forced against each other. Tapered edges should be placed next to tapered edges, square ends in contact with square ends. (Joints formed by placing square ends next to tapered edges are difficult to conceal.) Furring. Supplementary framing, or furring, should be used when framing spacing exceeds the maximum spacing recommended by the gypsumboard manufacturer for the thickness of board to be used (Table 11.10), or when the surface of framing or base layer is too far out of alignment. At gypsumboard joints, wood furring should be at least 11/2 in wide, and metal furring 11/4 in wide, to provide adequate bearing surface and space for attachment of the gypsumboard. The furring should be aligned to receive the board, and securely fastened to framing or masonry or concrete backing. For rigidity, lumber used for furring should be at least 2
2 in when nails are used and 1
3 in when screws are used for attachment of gypsumboard. Furring on masonry or concrete may be as small as 5/8 x 11/2 in. Fastening. Gypsumboards may be held in place with various types of fasteners or adhesives, or both. Special nails, staples, and screws are required for attachment of gypsumboards, because ordinary fasteners may not hold the boards tightly in place or countersink neatly, to be easily concealed. Clips and staples may be used only to attach the base layer in multi-ply construction. To avoid damaging edges or ends, fasteners should be placed no closer to them than 3/8 in (Figs. 11.25 and 11.26). With a board held firmly in position, fastening should start at the middle of the board and proceed outward toward the edges or ends. Nails should be driven with a crown-headed hammer until a uniform depression, or dimple, not more than 1/32 in deep is formed around the nail head. Care should be taken in driving not to tear the paper or crush the gypsum core. Nails. For use in attachment of gypsumboard, nails should conform with ASTM C514. (Some fire-rated systems, however, may require special nails recommended by the gypsumboard manufacturer.) For easy concealment, nail heads should be flat or slightly concave and thin at the rim. Diameter of nail heads should be 1/4 or 5/16 in. Length of smooth-shank nails should be sufficient for penetration into wood framing of at least 7/8 in, and length of annular-grooved nails for 3/4-in

D  1.0 for a pair of stiffeners  1.8 for a single-angle stiffener  2.4 for a single-plate stiffener Bending and Shear Interaction. Plate girders should also be proportioned to satisfy Eq. (7.43) if they are designed for tension-field action, stiffeners are required, and Vu /Mu lies between 60 and 133% of Vn /Mn. M V u u  0.625  1.24 (7.44) M V n n where Mn  design flexural strength Mu  required flexural strength calculated for the factored loads but may not exceed 0.90Mn Vn  design shear strength Vu  required shear strength calculated for the factored loads but may not exceed 0.90Vn Members subject to large concentrated loads within their length or large end reactions should be proportioned so that the forces on the web or flange cannot cause local failure or the webs or flanges should be stiffened to carry the concentrated loads. Both ASD and LRFD procedures include design criteria. 7.22.1 ASD for Load-Bearing Stiffeners Webs of rolled beams and plate girders should be so proportioned that the compressive stress, ksi, at the web toe of the fillets does not exceed F  0.66F (7.45) a y where Fy  specified minimum yield stress, ksi. Web failure probably would be in the form of buckling caused by concentrated loading, either at an interior load or at the supports. The capacity of the web to transmit the forces safely should be checked. FIGURE 7.35 Web crippling in a simple beam. The critical web section is assumed to occur at the fillet. Load Distribution. Loads are resisted not only by the part of the web directly under them but also by the parts immediately adjacent. A 45 distribution usually is assumed, as indicated in Fig. 7.35 for two common conditions. The distance k is determined by the point where the fillet of the flange joins the web; it is tabulated in the beam tables of the AISC Manual of Steel Construction. Fa is applicable to the horizontal web strip of length b  k at the end support or b  2k under an interior load. Bearing stiffeners are required when Fa

For floors in places of public assembly, for live load in excess of 100 psf, and for parking garage live load, the load factor is taken as 1.0 for L. Em is the maximum seismic effect of horizontal and vertical forces. Structural capacity, or ultimate strength, is that property of a structural member that serves as a measure of is ability to support all potential loads without severe cracking or excessive deformations. To indicate when the limit on load-carrying usefulness has been reached, design specifications for the various structural materials establish allowable unit stresses or design strengths that may not be exceeded under FIGURE 5.1 Truss in equilibrium under load. Upward acting forces equal those acting downward. FIGURE 5.2 Portion of a truss is held in equilibrium by stresses in its components. maximum loading. Structural theory provides methods for calculating unit stresses and for estimating deformations. Many of these methods are presented in the rest of this section. 5.2.1 Static Equilibrium If a structure and its components are so supported that, after a very small deformation occurs, no further motion is possible, they are said to be in equilibrium. Under such circumstances, internal forces, or stresses, exactly counteract the loads. Several useful conclusions may be drawn from the state of static equilibrium: Since there is no translatory motion, the sum of the external forces must be zero; and since there is no rotation, the sum of the moments of the external forces about any point must be zero. For the same reason, if we consider any portion of the structure and the loads on it, the sum of the external and internal forces on the boundaries of that section must be zero. Also, the sum of the moments of these forces must be zero. In Fig. 5.1, for example, the sum of the forces RL and RR needed to support the roof truss is equal to be the 20-kip load on the truss (1 kip  1 kilopound  1000 lb  0.5 ton). Also, the sum of moments of the external forces is zero about any point. About the right end, for instance, it is 40
15  30
20  600  600. In Fig. 5.2 is shown the portion of the truss to the left of section AA. The internal forces at the cut members balance the external load and hold this piece of the truss