IS 1893 Part 5 : 2016Criteria for Earthquake Resistant Design of Structures - Dams and Embankments

≠Seismic Hazard Definition and Site Characterization: IS 1893:2016 defines seismic hazard through four seismic zones (II, III, IV, V) with corresponding Zone Factors (Z = 0.10, 0.16, 0.24, 0.36) representing PGA for MCE. It classifies sites into three types (Type I, II, III) based on soil strata. ASCE 7-16 uses a more refined approach with mapped risk-targeted maximum considered earthquake (MCE_R) spectral response accelerations (Ss and S1) at short periods and 1-second period, respectively, and site classification (Site Classes A to F) determined by average shear wave velocity (Vs30) over the top 30m, which allows for a more detailed site-specific response spectrum.

≠Response Reduction Factor (R-factor) and Ductility: IS 1893:2016 provides discrete R-factors for various structural systems (e.g., R=5 for Special RC Moment Resisting Frame (SMRF), R=4 for Ordinary RC Moment Resisting Frame (OMRF)). ASCE 7-16 provides R (Response Modification Coefficient), Cd (Deflection Amplification Factor), and Ω₀ (Overstrength Factor) for a wider range of structural systems, with generally higher R-factors (e.g., R=8 for Special Reinforced Concrete Moment Frames), reflecting different assumptions about overstrength and ductility demands.

≠Design Response Spectrum: IS 1893:2016 provides a simplified design spectrum where Sa/g is constant up to a period Ta, then decreases, and then again decreases, which is less dependent on mapped ground motions directly. ASCE 7-16's design spectrum is directly derived from mapped Ss and S1 values, site coefficients (Fa and Fv), and a long-period transition period (TL), allowing for a more tailored spectrum based on the specific seismic hazard and site conditions.

≠P-delta Effects: IS 1893:2016, Clause 7.11.1, requires P-delta effects to be considered if the stability index (θ) exceeds 0.10. ASCE 7-16, Clause 12.8.7, requires P-delta effects to be considered for all structures unless specifically exempt, and provides a stability coefficient calculation; if the stability coefficient (θ) exceeds 0.10 (or 0.25 for certain cases), explicit P-delta analysis or amplification of forces is required. If θ > 0.25, the structure is deemed unstable and must be redesigned.

≠Irregularity Considerations: IS 1893:2016 defines various plan and vertical irregularities (e.g., torsional, re-entrant corner, mass, stiffness irregularities) and prescribes specific provisions such as requiring dynamic analysis or increasing design forces for torsional irregularity. ASCE 7-16 provides a more detailed classification of horizontal (e.g., torsional, diaphragm discontinuity, nonparallel systems) and vertical irregularities (e.g., soft story, mass irregularity, in-plane discontinuity in vertical elements), with specific requirements for analysis methods (e.g., 3D dynamic analysis for certain irregularities) or limitations on their use.

≈Dual Analysis Methods: Both IS 1893:2016 and ASCE 7-16 (and EN 1998-1:2004) permit the use of either the Equivalent Static Method for regular buildings within certain height limits or the more rigorous Dynamic Analysis (Response Spectrum Method) for irregular or taller buildings. The applicability criteria for these methods are similar in principle, based on building regularity and height.

≈Importance Factor: Both codes incorporate an Importance Factor (I in IS 1893, Ie in ASCE 7) to modify the design seismic forces based on the occupancy category and post-earthquake functional requirements of the building (e.g., hospitals, fire stations typically have higher factors). This ensures higher safety levels for critical facilities.

≈Torsional Effects: Both IS 1893:2016 and ASCE 7-16 mandate the consideration of torsional effects in design. This includes accounting for both natural eccentricity between the center of mass and center of rigidity, as well as an accidental torsion component to cover uncertainties in mass and stiffness distribution or rotational ground motion.

≈Minimum Number of Modes for Dynamic Analysis: Both codes specify that for dynamic analysis (response spectrum method), a sufficient number of modes must be considered such that the sum of modal masses for each principal direction is at least 90% of the total seismic mass of the building. This ensures that the significant dynamic response of the structure is captured.

≈Ductile Detailing Requirements: Both IS 1893:2016 (by reference to IS 13920) and international codes like ASCE 7-16 (by reference to ACI 318 for concrete or AISC 341 for steel) emphasize the critical importance of ductile detailing in structural members to allow for inelastic deformation without brittle failure during a major earthquake. This includes requirements for confinement, lap splice locations, and joint design.