Q&A 9 – Seismic Retrofitting
Welcome back. This week’s segment is focused around seismic retrofitting, otherwise known as earthquake retrofitting. Why do we need it? Why is it important? And what is the process? Seismic retrofitting or earthquake retrofitting is the process of attaching the building to the foundation.
Near the turn of the century to almost 1945, most of the buildings that we have in the city are made out of cinder block construction and are not structurally sound enough, or are they simply not attached to the building above. The wall that sits on top of your foundation, otherwise known as a pony wall or cripple wall, is the wood wall that’s above your foundation that should be attached to your foundation.
But in most cases it is not. At the turn of the century till about 1945, we did not have seismic requirements. We didn’t even have building permits, so there wasn’t any true regulatory affiliation or business that had to make sure that these buildings were attached to the foundations.
In the event that there’s a lateral seismic event, the building can actually shimmy, shake or jump off the foundation because there’s no true attachment between the cripple wall or pony wall, which is the wood wall above the foundation and the foundation itself. How do we counteract that? We simply attach the building to the foundation, and there is (3) step process in which we do this when you’re hiring a builder to do a seismic retrofit. Step number one is going to be to mechanically secure the pony or cripple wall to the foundation wall. We do that by using a 5/8 to 1/2 inch to 3/4 inch expansion anchor that we drill through the bottom plate into the foundation wall. That is mechanically fastened with a washer and a nut, that actually attaches the foundation wall to the cripple wall above it.
The second step there is to establish a shear component. Shear is the lateral structural support that we find in any 2×4 or 2×6 wall. We’re going to find that shear by applying a piece of half-inch CDX or OSB plywood or whatever is specified by your engineer with a specific nailing pattern. That piece of plywood is going over the 2×4 or 2×4 studs of that wall and that’s going to create torsional stability or shear within that wall.
The third component is to attach the top plate. The top plate and the upper joist framing is going to be attached with A35 clips and that is the third component in the structural engineering process. Now keep in mind, all of our calculations and all the structure engineering is being provided by licensed professional engineers; that’s a key component. We look to these engineers to properly calculate the structural loads.