Unreinforced masonry buildings are more vulnerable against a seismic activity. There are many such buildings in Los Angeles, with moderate to high risk of seismic activity; and it not only poses threat to the residents in the building, but also to others nearby.
Such buildings posses little or no ductility, and so, retrofitting unreinforced masonry buildings could be very costly. However, is proper attention was given, earthquake retrofitting of such structure would not be that aggravating. Here are the main areas that need to be attended aptly while retrofitting unreinforced masonry buildings.
Parapets pose a great threat not only to the surrounding but also to the building itself, which is a major risk to the overall safety. Bracing the parapet is a key step while evaluating the soft story retrofit works needed for the building.
Walls of unreinforced masonry buildings should be tied to a horizontal diaphragm, such as the roof or floor, in order to raise its resiliency to loading and catastrophic failures. The most commonly used method for wall anchoring involves bolting large bearing plates to the exterior, along with epoxy adhesive or grout. This method provides both shear and tensile resistance to the wall anchor.
After the walls are anchored to diaphragms, there is a chance for un-braced length of the brittle walls to buckle out-of-plane, in case of lateral seismic loading. Additional bracings should be placed between diaphragms when height to thickness ratio of the wall goes beyond the standard limits. This will increase out-of-plane strength of the wall assembly.
Fiber reinforced polymer materials can be used in both exterior and interior faces of wall, which will result in increased in-plane shear strength while adding less weight to the frame.
When the walls lack enough shear strength to resist a seismic quiver, reinforced shotcrete method could be utilized to increase the in-plane shear strength in such buildings. This method takes a lot of space and adds weight to the structure, creating an increased seismic load on the structure.
A good proportion of the seismic load on retrofitted buildings can be resisted by wood diaphragms. Therefore, it is necessary to increase its strength in unreinforced masonry buildings. The strengthening may require replacement or fastening of the wooden diaphragm.
Posted on Tuesday, January 23rd, 2018.