2000

where h is the story height for every story.

5. All frames not complying with the criteria in (3) or (4) are considered to be sway frames.

EC3 (1992a,b) also provides some guidelines to distinguish between sway and nonsway frames. It states that a frame may be classified as nonsway for a given load case if Pcr/P > 10 for that load case, where Pcr is the elastic critical buckling value for sway buckling and P is the design value of the total vertical load. When the system buckling load factor is ten times more than the design load factor, the frame is said to be stiff enough to resist lateral load and it is unlikely to be sensitive to sidesway deflections. AISC LRFD (1993) does not give specific guidance on frame classification. However, for frames to be classified as nonsway in AISC LRFD format, the moment amplification factor, B2, has to be small (a possible range is B2 < 1.10) so that sway deflection would have negligible influence on the final value obtained from the beam-column capacity check.

22.2.6 Classification of Multistory Buildings

The selection of appropriate structural systems for tall buildings must satisfy both the strength and stiffness requirements. The structural system must be adequate to resist lateral and gravity loads that cause horizontal shear deformation and overturning deformation. Other important issues that must be considered in planning the structural schemes and layout are the requirements for architectural details, building services, vertical transportation, and fire safety, among others. The efficiency of a structural system is measured in terms of their ability to resist higher lateral load, which increases with the height of the frame (Iyengar et al. 1992). A building can be considered as tall when the effect of lateral loads is reflected in the design. Lateral deflections of tall buildings should be limited to prevent damage to both structural and nonstructural elements. The accelerations at the top of the building during frequent windstorms should be kept within acceptable limits to minimize discomfort to the occupants (see Section 22.5).

Figure 22.5 shows a chart which defines, in general, the limits to which a particular framing system can be used efficiently for multistory building projects. The various structural systems in Figure 22.5 can be broadly classified into two main types: (1) medium-height buildings with shear-type deformation predominant and (2) high-rise cantilever structures such as framed tubes, diagonal tubes, and braced trusses. This classification of system forms is based primarily on their relative effectiveness in resisting lateral loads. At one end of the spectrum in Figure 22.5 is the moment resisting frames, which are efficient for buildings of 20 to 30 stories, and at the other end is the tubular systems with high cantilever

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