Unreinforced Masonry buildings in moderate to high seismic areas can be a disaster in waiting. These types of structures have little or no ductility capacity (reference the recent “Building Drift – Do You Check It?” blog post for a discussion on ductility) required for structures to prevent loss of life in a seismic event. Many of these buildings are in densely populated areas, have historical meaning, and can be costly to retrofit. Fortunately, there are tools available for engineers to assess and design the needed retrofits to mitigate the potential loss of life and increase the seismic resiliency of these buildings.
ASCE 31-03, Seismic Evaluation of Existing Buildings, and ASCE 41-06, Seismic Rehabilitation of Existing Buildings, are two reference standards that are referenced in the 2012 International Existing Building Code (IEBC). (It should be noted that both of these reference standards are currently being combined into one document – ASCE 41-13.) Although ASCE 31 and 41 provide assistance to engineers in determining minimum seismic retrofits for these brittle structures, it is recommended that design of the retrofits be performed by a qualified engineer with experience in working with these types of brittle structures.
Currently the 2012 IEBC has been adopted in 39 states in the U.S. and several other areas (see reference map below).
For the past couple of years, I have focused on providing education in the New Madrid and Charleston Fault areas promoting the mitigation and seismic rehabilitation of URM buildings in moderate to high seismic hazard regions. Although a large majority of the URM building inventory along the western coast has been retrofitted, there is much work to do in these other regions. Some key areas to consider when evaluating URM buildings that are discussed in the workshops we offer:
1) Parapet Bracing:
A lot of damage to URM buildings can be attributed to parapet failures when a seismic event occurs. The damage can occur not only to the building itself, but to nearby buildings, and poses a major risk to life safety. Bracing the parapets is a must when evaluating the needed retrofitting of a URM building.
2) & 3) Wall Anchorage:
The URM walls must be tied to the horizontal diaphragms (roof and floors) to increase their resiliency to out of plane loading and catastrophic failures. There are many ways to anchor the walls to the diaphragms. Through bolting with large bearing plates on the exterior, combined with epoxy adhesive or grout in the URM wythes, will provide tensile and shear resistance for the wall anchor and is one of the most widely used methods.
One caveat in determining the optimal location for these anchors is usually dependent on the premise that many URM buildings utilize beam pockets to support the joist. The wall anchor will perform better when it is away from the beam pocket. When deciding on the anchor location, the midpoint between framing is more ideal, but requires additional framing elements to attach to the joist.
4) Out-of-Plane Wall Bracing:
Once the walls have been anchored to the diaphragms, the unbraced length of these brittle walls may cause them to buckle out-of-plane during lateral loading. Where the height-to-thickness ratio of the walls exceeds the limits given in the IEBC, additional bracing should be utilized between diaphragms to increase the out-of-plane strength of the URM wall assembly. This can be done by attaching steel columns to the walls. Another option is to add Fiber Reinforced Polymer (FRP) material to both the interior and exterior face of the wall. The use of FRP material will also greatly increase the in-plane shear strength without adding weight to the building. There will be more discussion on this topic in the next section.
5) Lateral Shear Resistance:
Where unreinforced masonry walls lack sufficient shear strength to resist seismic loading, a traditional reinforced shotcrete method has been utilized to increase the in-plane shear strength of URM structures. Although effective, this method takes up space and adds a lot of weight to the structure which increases the seismic load on the structure. This higher seismic force will impact all of the other key elements that were listed and should be designed prior to evaluating the other areas. An option for increasing the in-plane shear strength of the URM wall assembly without increasing the structure weight and adding to the seismic demand is to utilize FRP material. Special detailing is required when attaching an impermeable element, such as FRP, to a porous URM wall.
In areas where there is little wall space (such as retail frontage with large openings), one should consider the installation of a special moment frame. These frames can be designed to resist large forces and have minimal impact on open space. Go here for more information.
6) Diaphragm strengthening
With most of the seismic load on the retrofitted URM structure being resisted by the wood diaphragms, it is may be necessary to increase the diaphragm strength. This may require additional fastening or replacement of the diaphragm altogether.
These are just a few areas that should be considered by the qualified engineer and the building owner when evaluating the strengthening of a URM building. Implementing all or some of these seismic retrofits can go a long way to increase the resiliency of these often historic structures and increase life safety for the public that inhabits them.
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