In-plane Seismic Design of Concrete Masonry Structures

Abstract

The research presented in this thesis consists of two parts. The first part involved the investigation of concrete masonry shear strength and the second part reports an investigation of the lateral strength of partially grout-filled nominally reinforced perforated concrete masonry walls.

Valuable information about masonry shear strength is reported following the testing of ten full scale concrete masonry walls. It was verified that horizontal shear reinforcement and axial compression load provided additional shear resistance to masonry walls. Consequently, the nominal shear strength of reinforced masonry walls could be evaluated as a sum of contributions from masonry, shear reinforcement and applied axial load. It was also established that masonry shear strength decreases inversely in relation to an increase of the wall aspect ratio.

Criteria relating to codification of the in-plane shear strength of concrete masonry walls when subjected to seismic loading are presented. Particular emphasis is placed on a computational model that is capable of representing the interaction between flexural ductility and masonry shear strength to account for the reduction in shear strength as ductility level increases. The simple method proposed here allows the strength enhancement provided by axial compression load to be separated from the masonry component of shear strength and is considered to result from strut action. In addition, minor modifications are made to facilitate adoption of the method in the updated version of the New Zealand masonry design standard, NZS 4230:2004.

Prediction of shear strength from NZS 4230:2004 and using alternative methods are compared with results from a wide range of test of masonry walls failing in shear. It was established that the shear equation in the former version of the New Zealand masonry standard (NZS 4230:1990) was overly conservative in its prediction of masonry shear strength. The current NEHRP shear expression was found to be commendable, but it does not address masonry shear strength within plastic hinge regions, therefore limiting its use when designing masonry structures in seismic regions. Finally, the new shear equation adopted by NZS 4230:2004 was found to provide significantly improved shear strength prediction with respect to its predecessor, with accuracy close to that resulted from NEHRP.

Test results obtained in the second part of this research indicated that the size of openings and the length of trimming reinforcement significantly affected the lateral strength of perforated masonry walls. The observation of diagonal cracking patterns that aligned well with the load paths by which shear force was assumed to be transferred to the foundation in the strut mechanism supported the use of strut-and-tie analysis as a viable tool to evaluate the flexural strength of walls of this type. Strength prediction using the improved strut-and-tie method and the modified plastic collapse analysis were found to closely match the experimental results of the perforated walls tested in this study. Strength prediction by the simplified strut-and-tie method was found to closely match the test results of masonry walls with a single opening, but significant underestimation of strength by this method was found for walls with double openings. Full plastic collapse analysis was found to significantly over-predict the strength of all perforated walls included in this study.

Finally, the NZS 4229:1999 detail for shrinkage control joints was shown to result in adequate structural performance. In addition, shrinkage control joints constructed in accordance with the NZS 4229:1999 prescription resulted in masonry bracing capacity substantially in excess of the tabulated values in the standard, with gradual strength and stiffness degradation. This increase in strength is due to pier double bending that is not considered by the standard.