12.8-6) be used for the drift determination because it accounts for large displacement of structures. However, for structures in high seismic regions where S 1 is equal to or greater than 0.6g, ASCE 7-16 requires that the minimum base shear controlled by C s= 0.5 S 1/(R/I e) (Eq. The C s exemption is especially important for tall buildings that are typically drift-controlled rather than strength-controlled. However, the design earthquake forces for checking drift may be lower than the forces used for strength design due to the exemption of minimum base shear obtained from V = 0.044SDSIW ≥ 0.01W, and because the fundamental period is not restricted to the upper limit T aC u which results in lower shear forces.
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Seismic design forces, starting with ASCE 7-10 and onward, have been obtained from the code as strength level forces, and drift compliance with allowable drift limits is also computed at strength level forces.
The reason for the scaling requirement is to be consistent with the requirements for design based on the ELF procedure. Modal drift need not be scaled in all other situations. Note that the scaling is only required if the minimum seismic response coefficient controls the base shear, C s = 0.5 S 1/(R/I e) (Eq. Also, if the base shear calculated in modal analysis (V t) is less than base shear (V) as calculated by the equivalent lateral force (ELF) procedure, then the story drifts must be scaled up and drifts are multiplied by C sW/V t. The primary difference is that the periods and displacements are known for several modes of natural vibration and combined using one of the recognized combination methods to obtain the final story displacements. Scalingĭetermination of drift resulting from modal response spectrum analysis is, to some extent, similar to the equivalent lateral force determined by a statics analysis procedure. The elimination of the period upper limit, T aC u, for drift determination is intended to avoid the overestimation of displacement because of the inconsistency between forces and computed fundamental period. The fundamental period obtained from modal analysis is permitted to exceed the product of the coefficient for the upper limit on the calculated period (C u) from Table 12.8-1 and the approximate fundamental period, T a, determined in accordance with Section 12.8.2.1. The fundamental period used to obtain loading for seismic drift calculations may be either the calculated approximate fundamental period per Section 12.8.2.1 or the calculated actual period from modal analysis. The fundamental period of a structure is the main parameter affecting building displacement. All section references and equation numbering are from ASCE 7-16.
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It also addresses the effects of elastic lateral deflections of the floor plate, and how to account for inelastic drift of a structure using a deflection amplification factor, and the allowable drifts for different types of seismic forces resisting systems and the risk categories.
The article covers several factors of drift computations, including the fundamental period, scaling modal drift obtained from modal response spectrum analysis, the seismic design base shear, torsional irregularities in structures, and the significance of P-delta effects. This article provides an overview of the Provisions in ASCE 7-16, Minimum Design Loads for Buildings and Other Structures, for the determination of seismic drift.
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