For buildings with re-entrant corners, analysis shall be done using
- AEquivalent static analysis with rigid diaphragm
- BTwo dimensional analysis only
- CThree dimensional dynamic analysis with flexible floor diaphragmCorrect
- DCan be done using a manual method as per IS 456
Solution
Concept
Dynamic analysis for plan-irregular re-entrant buildings
Formula
IS 1893 (Part 1): 2016 Clause 7.7.5.4
Step-by-step solution
1. Re-entrant corners cause stress concentrations and significant torsional coupling between lateral modes. 2. A rigid diaphragm assumption fails to capture in-plane diaphragm deformation and stress amplification at notch re-entrant corners. 3. Code provisions in IS 1893 dictate a 3D structural model with dynamic analysis incorporating flexible floor diaphragm behavior. 4. Hence, 3D dynamic analysis with flexible floor diaphragm is mandated. Answer: C
Explanation
Buildings with re-entrant corners exhibit significant plan irregularity and torsional coupling during earthquake shaking. Under IS 1893:2016 Clause 7.7.5.4, such structures must be evaluated using a three-dimensional dynamic analysis that accounts for floor diaphragm flexibility.
Why the other options are wrong
A) Rigid diaphragm idealization assumes infinite in-plane stiffness, missing corner deformations. B) Two-dimensional analysis cannot capture spatial torsional coupling. D) Manual IS 456 methods cannot assess complex seismic dynamic response.
Shortcut
Plan irregularity (re-entrant corners) = 3D dynamic analysis with flexible diaphragm.
Common mistake
Relying on 2D analysis or equivalent static methods which ignore re-entrant corner stress concentrations.
Correct answer: C) Three dimensional dynamic analysis with flexible floor diaphragm