Abstract:
Opening in cast-in-situ concrete diaphragm such as stairs, elevator cores, atriums, skylights, etc are very common in reinforced concrete buildings. Diaphragm opening adjacent to shear wall can cause high in-plane stress concentration around diaphragm opening during earthquake.
The purpose of this research is to examine the limit of extent of diaphragm opening adjacent to shear wall with respect to diaphragm discontinuity criteria of ASCE 7-10 for wall-frame structural system for some specific layouts of shear walls in a 3-storied building. The worst orientation of diaphragm opening adjacent to shear wall which have the most adverse effect on local seismic demands of diaphragm locations near diaphragm opening is to be determined with the help of in-plane stress analyses of diaphragm for some specific cases. Possible locations near diaphragm opening adjacent to shear wall where cracking and yielding of diaphragm occur during seismic loading is to be identified in this research work.
3D FEM models of dual system buildings are adopted where diaphragm is modeled with nonlinear layer shell elements. Equivalent lateral force analysis, modal response spectrum analysis, modal linear response history analysis, diaphragm design force procedure as per section 12.10.1.1 of ASCE 7-10 and pushover analysis as per ASCE 41-13 have been performed by considering unidirectional and orthogonal application of seismic loading in order to extract in-plane force/stress data from nonlinear layered shells. In-plane stress data have been analyzed statistically with standard deviation and Z-score using the concept of Gaussian distribution.
In-plane stresses in diaphragm around diaphragm opening adjacent to shear wall form pushover analyses as per ASCE 41-13 are higher than linear elastic and linear dynamic diaphragm design force procedures of ASCE 7-10. Statistical analyses shows that the building model with diaphragm opening adjacent to an interior shear wall experiences high in-plane stress around diaphragm discontinuity compared to other building models. Although open areas in diaphragm (11% or 14% of the diaphragm gross area) of models are not considered as diaphragm discontinuity according to ASCE 7-10, linear elastic diaphragm design force procedures as per ASCE 7-10 underestimates in-plane shear forces in diaphragm locations around diaphragm opening adjacent to shear wall compared to in-plane shear forces from pushover analyses according to ASCE 41-13. Hence pushover analysis should be performed when diaphragm opening is present adjacent to shear wall. Diaphragm thickness determined from regular design procedure in these locations may be suitable for gravity load transfer but may not be suitable to transfer in-plane shear stress to the seismic force resisting vertical elements. Guidelines and seismic requirements should be developed for structures with diaphragm opening adjacent to shear wall assigned to Seismic Design Categories C.
