Determination of Story Drifts per ASCE 7-16 with ideCAD
How does ideCAD determine story drifts according to ASCE 7-16?
For any column or wall in the earthquake direction, storey drifts expressing the displacement difference between two floors, ∆_{i }^{(X)}, is automatically obtained by Section 12.8.6.
The condition given in 12.8.2 and also the minimum equivalent lateral force condition defined in Section 12.9.1.4.2 are taken into account automatically.
Symbols
C_{d }= Deflection amplification factor in Table 12.2-1
δ_{xe} = Deflection at the location required by this section determined by an elastic analysis
I_{e} = Importance Factor
Δ_{i }= Story drift
δ_{i }= Total displacement
Eq. (12.8-15) is used to estimate inelastic deflections (δ_{x}), which are then used to calculate design story drifts, Δ. These story drifts must be less than the allowable story drifts, Δ_{a}, of Table 12.12-1. For structures without torsional irregularity, computations are performed using deflections of the centers of mass of the floors bounding the story. If the eccentricity between the centers of mass of two adjacent floors, or a floor and a roof, is more than 5% of the width of the diaphragm extents, it is permitted to compute the deflection for the bottom of the story at the point on the floor that is vertically aligned with the location of the center of mass of the top floor or roof. This situation can arise when a building has story offsets, and the diaphragm extents of the top of the story are smaller than the extent of the bottom of the story. For structures assigned to Seismic Design Category C, D, E, or F that are torsionally irregular, the standard requires that deflections be computed along the edges of the diaphragm extents using two vertically aligned points.
If the structure remained elastic during an earthquake, the force developed would be V_{E}, and the corresponding displacement would be δ_{E}. V_{E} does not include R, which accounts primarily for ductility and system overstrength. According to the equal displacement approximation rule of seismic response, the maximum displacement of an inelastic system is approximately equal to that of an elastic system with the same initial stiffness. This condition has been observed for structures idealized with bilinear inelastic response and a fundamental period, T, greater than T_{s} (see Section 11.4.6). For shorter-period structures, the peak displacement of an inelastic system tends to exceed that of the corresponding elastic system. Because the forces are reduced by R, the resulting displacements are representative of an elastic system and need to be amplified to account for the inelastic response.
First of all, the displacements of all columns and walls on each floor are obtained for each mode. The horizontal displacement differences between two consecutive floors are then calculated using the displacements obtained for each mode and combined with an earthquake code-compliant method(CQC or SRSS).
The deflection amplification factor, C_{d}, in Eq. (12.8-15) amplifies the displacements computed from an elastic analysis using prescribed forces to represent the expected inelastic displacement for the design-level earthquake. It is typically less than R (Section C12.1.1).
The R and I_{e} values used in the account are available in the ASCE 7-16 Options table in the Analysis Settings report.
Determination of Story Drift Limit Value
The displacements induced in a structure include inelastic effects, and structural damage resulting from a design-level earthquake is the value of Δ_{a} stated in Table 12.12-1.
Structure | Risk Category | ||
---|---|---|---|
I or II | III | IV | |
Structures, other than masonry shear wall structures, four stories or less above the base as defined in ASCE Section 11.2, with interior walls, partitions, ceilings, and exterior wall systems that have been designed to accommodate the story drifts | 0.025h_{sx}^{c} | 0.020h_{sx} | 0.015h_{sx} |
Masonry cantilever shear wall structures | 0.010h_{sx} | 0.010h_{sx} | 0.010h_{sx} |
Other masonry shear wall structures | 0.007h_{sx} | 0.007h_{sx} | 0.007h_{sx} |
All other structures | 0.020h_{sx} | 0.015h_{sx} | 0.010h_{sx} |
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