Allowable bearing pressurea Maximum allowable bearing pressureb Massive crystalline bedrock 4,000 psf (200 kPa) 12,000 psf (600 kPa) Sedimentary and foliated rock 2,000 psf (100 kPa) 6,000 psf (300 kPa) Gravel and sandy gravel (GW, GP) 2,000 psf (100 kPa) 6,000 psf (300 kPa) Nonplastic soil: sands, silts, and NP silt (GM, SW, SP, SM)c 1,500 psf (75 kPa) 4,500 psf (220 kPa) Plastic soil: silts and clays (ML, MH, SC, CL, CH)c 1,000 psf (50 kPa) 3,000 psf (150 kPa)d aMinimum footing width and embedment depth equals 1 ft (0.3 m). bAn increase of 20% of the allowable bearing pressure is allowed for each additional foot (0.3 m) of width or depth up to the maximum allowable bearing pressures listed in Column 3. An exception is plastic soil, see note d. c Group symbols from Table 6.8. dNo increase in the allowable bearing pressure is allowed for an increase in width of the footing. For dense or stiff soils, allowable bearing values in this table are generally conservative. For very loose or very soft soils, the allowable bearing values may be too high. Source: Data from Uniform Building Code (1997) There are many charts, graphs, and figures that present bearing capacity factors developed by different engineers and researchers based on varying assumptions. For example, Fig. 6.28 presents bearing capacity factors Nc, N, and Nq , which automatically incorporate allowance for punching and local shear failure. Another example is Fig. 6.29, which presents bearing capacity factors that have not been adjusted for punching or local shear failure. Figure 6.29 also presents the bearing capacity equations for square, rectangular, and circular footings. The equations for granular soil (i.e., cohesionless soil, c  0) and for a total stress analysis for cohesive soil (i.e.,   0 and c  su) are also shown in Fig. 6.29. Other Footing Loads. In addition to the vertical load acting on the footing, it may also be subjected to a lateral load. A common procedure is to treat lateral loads separately and resist the lateral loads by using the soil pressure acting on the sides of the footing (passive pressure) and the frictional resistance along the bottom

Seismic Design Category. The Seismic Design Category is based on the seismic group and the design spectral response acceleration coefficients, SDS and SD1, which will be explained later. The Seismic Design Category for a structure can be determined in accordance with Tables 5.10 and 5.11. Seismic Design Categories are used to determine the permissible structural systems, the limitations on height and irregularity of the structural components that must be designed for seismic resistance and the types of lateral force analysis that must be performed. Seismic Use Groups I and II structures located on sites with mapped maximum considered earthquake spectral response acceleration at 1-second period S1, equal to or greater than 0.75g, shall be assigned to Seismic Design Category E. Seismic Use Group III structures located on such sites shall be assigned to Seismic Design Category F. A structure assigned to Seismic Design Category E or F shall not be sited where there is the potential for an active fault to cause rupture of the ground surface at the structure. Building Irregularity. Building with irregular shapes, changes in mass from floor to floor, variable stiffness with height, and unusual setbacks do not perform well during earthquakes. Thus, for each type of these irregularities, additional design requirements shall be followed to maintain seismic-resisting capacity. IBC 2000 requires that all buildings be classified as regular or irregular based on the plan and vertical configuration. See Tables 5.12 and 5.13 for classification and corresponding

Trusses serve the same purposes as girders but consists of slender horizontal, vertical, and inclined components with large open spaces between them. The spaces are triangular in shape. Light beams similarly formed are called openweb joists (Fig. 1.6d). Floor and roof decks or the beams that support them are usually seated on loadbearing walls or carried by columns, which carry the load downward. (The horizontal members also may be suspended on hangers, which transmit the load to FIGURE 1.3 Structural-steel skeleton framing for a multistory building. (Courtesy of the American Institute of Steel Construction.) other horizontal members at a higher level.) The system comprising decks, beams, and bearing walls is known as load-bearing construction (Fig. 1.1). The system composed of decks, beams, and columns is known as skeleton framing (Fig. 1.3). Both types of systems must be designed to transmit to the foundations vertical (gravity) loads, vertical components of inclined loads, horizontal (lateral) loads, and horizontal components of inclined loads. Vertical walls and columns have the appropriate alignments for carrying vertical loads downward. But acting alone, these structural members are inadequate for resisting lateral forces. One way to provide lateral stability is to incorporate in the system diagonal members, called bracing (Fig. 1.3). Bracing, columns, and beams then work together to carry the lateral loads downward. Another way is to rigidly connect beams to columns to prevent a change in the angle between the beams and columns, thus making them work together as a rigid frame to resist lateral movement. Still another way is to provide long walls, known as shear walls, in two perpendicular directions. Lateral forces on the building can be resolved into forces in each of these directions. The walls then act like vertical beams cantilevers) in transmitting the forces to the foundations. (See also Art. 3.2.4.) Because of the importance of the structural system, the structural members should be protected against damage, especially from fire. For fire protection, bracing FIGURE 1.4 Roofs composed of plane surfaces: (a) flat roof; (b) shed roof; (c) pitched roof; (d) hipped roof; (e) gambrel roof; () mansard roof; (g) monitored roof; (h) sawtooth roof. (Reprinted with permission from F. S. Merritt and J. Ambrose, Building Engineering and Systems Design, 2d ed., Van Nostrand Reinhold, New York.) may be encased in fire-resistant floors, roofs, or walls. Similarly, columns may be encased in walls, and beams may be encased in floors. Or a fire-resistant material, such as concrete, mineral fiber, or plaster, may be used to box in the structural members (Fig. 1.6c). See also Secs. 7 to 11. Systems for Enclosing Buildings. Buildings are enclosed for privacy, to exclude wind, rain, and snow from the interior, and to control interior temperature and humidity. A single-enclosure type of system is one that extends continuously from the ground to enclose the floor. Simple examples are cone-like tepees and dome igloos. A multiple-enclosure type of system consists of a horizontal or inclined top covering, called a roof (Fig. 1.1), and vertical or inclined side enclosures called