P1: MRM/SPH PB262A-04
P2: MRM/UKS PB262/Seismic.cls
QC: MRM/UKS January 3, 2003
Comm. C5.]
T1: MRM 8:37
Char Count=
PART I – STORY DRIFT
79
loads for strength design to calculate the seismic drift. The use of these factors should be consistent with those specified in the Applicable Building Code with due consideration of the limitations and modifications necessary to account for building category, building height, vertical or horizontal irregularities, and site characteristics.
C5.
STORY DRIFT For non-seismic applications, story drift limits, like deflection limits, are commonly used in design to assure the serviceability of the structure. They vary because they depend upon the structural usage and contents. As an example, for wind loads such serviceability limit states are regarded as a matter of engineering judgment rather than absolute design limits (Fisher and West, 1990) and no specific design requirements are given in the LRFD Specification or these Provisions. The situation is somewhat different when considering seismic effects. Research has shown that story drift limits, although primarily related to serviceability, also improve frame stability (P- ) and seismic performance because of the resulting additional strength and stiffness. Although some building codes, load standards and resource documents contain specific seismic drift limits, there are major differences among them as to how the limit is specified and applied. Nevertheless, drift control is important to both the serviceability and the stability of the structure. As a minimum, the designer should use the drift limits specified in the Applicable Building Code. The analytical model used to estimate building drift should accurately account for the stiffness of the frame elements and connections and other structural and nonstructural elements that materially affect the drift. Recent research on steel Moment Frame connections indicates that in most cases Panel Zone deformations have little effect on analytical estimates of drift and need not be explicitly modeled (FEMA, 2000f ). In cases where nonlinear element deformation demands are of interest, Panel Zone shear behavior should be represented in the analytical model whenever it significantly affects the state of deformation at a beam-to-column connection. Mathematical models for the behavior of the Panel Zone in terms of shear force-shear distortion relationships have been proposed by many researchers. FEMA 355C presents a good discussion of how to incorporate panel zone deformations in to the analytical model (FEMA, 2000f). Adjustment of connection stiffness is usually not required for connections traditionally considered as fixed, although FEMA 350 (FEMA, 2000a) contains recommendations for adjusting calculated drift for frames with Reduced Beam Sections. Nonlinear models should contain nonlinear elements where plastic hinging is expected to properly capture the inelastic deformation of the frame. The story drift limits in ASCE 7 (ASCE, 2002) and the 2000 NEHRP Provisions (FEMA, 2000g) are to be compared to an amplified story drift that approximates the difference in deflection between the top and bottom of the story under consideration during a large earthquake. The amplified story drift is determined by multiplying the elastic drift caused by the horizontal component of the earthquake load E by a deflection amplification factor Cd , which is dependent upon the type of building system used; see Table C-I-4.2. Seismic Provisions for Structural Steel Buildings, May 21, 2002 American Institute of Steel Construction