Available at: http://digitalcommons.calpoly.edu/theses/143
Date of Award
MS in Architecture
Peter T. Laursen
Unreinforced masonry (URM) structures have historically been regarded as structurally unsound in response to seismic events. The tendency for URM walls to collapse out-of-plane in a brittle manner is continually cause for concern. Retrofit of these walls is necessary in order to prevent severe damage and injury to occupants.
This paper is concerned with the retrofit of unreinforced masonry (URM) walls in response to out-of-plane loading. A retrofit design was developed and verified through structural testing. The selected retrofit technique incorporates vertical coring of URM walls to allow for the insertion of a single post-tensioning (PT) tendon. Tendons are spaced at a regular interval and anchored at the top of the wall parapet and at the lower diaphragm level. Tensioning of the tendons imparts a compressive stress to the wall that effectively increases the wall cracking moment strength, ultimate moment strength and displacement capacity. Additionally, the insertion of PT tendons allows the wall to behave in a ductile manner in response to out-of-plane ground motion.
Extensive research was conducted in order to accurately portray the material properties and construction methods of unreinforced masonry walls in San Luis Obispo, California. Various mortar mix designs were generated and tested so that a mix design could be selected to best reflect the target URM structures. Seismic parameters were generated to reflect a URM structure in San Luis Obispo.
An unreinforced masonry wall was constructed by a professional mason using the established mortar mix proportions and salvaged bricks from the 1920 era. Having a pin-pin unsupported height of 11 feet, the wall constructed for testing was reflective of the configuration of URM walls in many downtown San Luis Obispo structures. The wall was loaded in the out-of-plane direction by 4 equal point loads mimicking a uniformly distributed load. The testing program consisted of cycling the wall through target internal moments and target displacements.
It was verified through testing that post-tensioning tendons can be successfully introduced in URM walls to resist out-of-plane bending. Testing showed that the addition of PT tendons significantly increased the wall’s cracking moment capacity, giving it the elastic strength to resist twice the forces imposed by the design-level ground motion. PT tendons also increased the nominal strength of the wall, allowing the wall to achieve large displacements without collapse. It was also found that PT tendons provided a restoring force to the wall returning it to almost no residual displacement after each displacement cycle. Thus, the URM wall retrofitted with PT tendons demonstrated significant integrity as a structural system, providing for improved strength and ductility with no residual displacement.