Experienced designers are aware of certain practical limitations on the size of individual members. Flexural members which have marginal or too shallow a depth can cause deflections that can damage other building elements, as well as cause vibrations under moving loads that disturb a building’s occupants. Almost all building code leave stiffness design criteria to the designer. Experienced designers have found that to specify limits for all possible variations loads, occupancies, and types of construction is impracticable.
This section outlines various criteria, originally based on experience but up-dated on the basis of testing, which the designer can incorporate to develop a serviceable design. The ASD specification (Table 7.1) restricts the maximum live-load deflection of beams and girders supporting plaster ceilings to 1⁄360 of the span. This requirement is not applicable to less rigid construction details. The AISC LRFD specification contains no numerical limits for serviceability criteria. Table 7.10 may be used to set limits on deflections of flexural members frequently encountered in building design.
Minimum Depth-Span Ratios. Also, as a guide, Table 7.10 lists suggested minimum depth-span ratios for various loading conditions and yield strengths of steel up to Fy 50.0 ksi. These may be useful for estimating or making an initial design selection. Since maximum deflection is a straight-line function of maximum bend ing stress ƒb and therefore is nearly proportional to Fy, a beam of steel with Fy 100.0 ksi would have to be twice the depth of a beam of steel with Fy 50.0 ksi when each is stressed to allowable values and has the same maximum deflection.
Vibration of large floor areas that are usually free of physical dampeners, such as partitions, may occur in buildings such as shopping centers and department stores, where pedestrian traffic is heavy. The minimum depth-span ratios in Table 7.10 suggested for ‘‘heavy pedestrian traffic’’ are intended to provide an acceptable solution.
One rule of thumb that may be used to determine beam depth quickly is to choose a depth, in, not less than 1.5% of Fy times the span, ft. Thus, for A36 steel depth, in, should be at least half the span, ft.
Ponding. Beams for flat roofs may require a special investigation to assure stability against water accumulation, commonly called ponding, unless there is adequate provision for drainage during heavy rainfall. The AISC specification gives these criteria for stable roofs:
Drift. AISC Design Guide No. 3 (Table 7.9) suggests that the lateral deflection of a building frame (drift) be limited to a value which does not damage other structural or architectural components when subject to a 10-year recurrence interval wind pressure. The 10-year wind pressure can be reasonably estimated at 75% of the 50-year wind pressure.