In this post, we follow up on our November webinar, Earthquake Retrofit Solutions and Resources Webinar, by answering some of the interesting questions raised by attendees.
During the webinar, we discussed seismic retrofitting strategies and solutions designed to enhance the earthquake resistance of existing structures. We walked through practical solutions and resources for both homeowners and contractors looking to improve building safety in seismic zones.
As with our previous webinars, we ended with a Q&A session for the attendees. Our regional product specialist, Jonathan Freeman, and our territory manager, Carlos Zarate, answered as many questions as they could in the time allowed. Now we are back to recap some of the commonly asked questions and their answers, but if you’d like to see the full list, click here .
Q: If a design meets Florida hurricane requirements, is that similar to earthquake requirements?
A: No, the demand loads created by hurricanes are different than those created by earthquakes. While they both result in lateral and uplift loads, they can be quite different and need to be resisted by connectors and fasteners in a specific way. Make sure to design your structures to resist the loads that are necessary in your location.
Q: If there is already exterior plywood sheathing on the cripple wall, is it required on the interior of the cripple wall?
A: When retrofitting your home, visible plywood beyond the studs on the exterior may be sufficient, depending on the load requirements and the condition of the existing plywood. In many older homes, however, there’s often just siding attached to the exterior. If you’re uncertain about if that’s sufficient, it’s best to consult a professional, as additional reinforcement on the interior may be necessary. In some cases, the exterior alone could suffice.
Q: How can you get HOA / condo associations to retrofit?
A: The first step is open communication. If you’re concerned about the seismic resilience of your building, start by discussing it with the building owner or manager. Ask whether the building is under a retrofit mandate or if there are any known structural concerns. This is especially important for buildings constructed before the 1960s, and even more so for those built in the 1930s or 1940s. Confirming whether the building has already been retrofitted or if a retrofit is needed can help start the conversation and promote safety awareness within the HOA or condo association.
Q: When you have the room to install, which is better? Titen® bolts or anchor bolts glued with epoxy?
A: This comes down to personal preference. Most folks prefer to use the Titen HD® over epoxy due to the speed of installation and thus completion. However, in certain cases, epoxy may be the optimal option depending on specific site conditions/factors.
Q: I thought there was already a major mandate to reinforce soft stories after the Northridge earthquake? Why are there so many still not retrofitted after 30 years?
A: The mandate following the Northridge earthquake was for specific building types and occupancies, so many smaller buildings were not required to retrofit and may still lack reinforcement.
If you live in a single-family home built before the 1950s on a raised foundation (not a slab), it’s a good idea to check whether a retrofit is needed. One simple way to get an initial sense is to look under the house. If you see only ½” anchor bolts along the perimeter, you may want to have it inspected further. Some contractors offer free inspections to help homeowners assess whether a retrofit is recommended. The most thorough approach, however, is to consult a structural engineer familiar with your area’s seismic requirements and building codes.
Q: For Yield-Link moment frames in line with wood framed shear walls (R=6.5), what is the recommendation for shear to the walls and steel frame (i.e., deformation compatibility)?
A: According to Section 12.2.3 of ASCE 7-16, when a moment frame is combined with other lateral systems in the horizontal direction, the R-value used for design in the direction under consideration shall not be greater than the least value of R for any system in that direction (i.e., when combining a wood shearwall with R = 6.5 and a steel SMF with R = 8.0, R = 6.5 shall be used for the design of the SMF).
However, there is an exception if the following three conditions are all met:
- Risk category I or II building
- The building is two stories or less above grade
- The use of light-frame construction or flexible diaphragms
If the above three conditions are met, then lateral-resisting elements are permitted to be designed using the least value of R found in each independent line of resistance. For example, if a wood shearwall with R = 6.5 is used at the interior wall of a garage and a steel SMF is used at the front of the garage parallel to the interior shearwall, then the SMF can be designed using an R-value of 8.
For vertical combinations of lateral systems, according to ASCE 7-16 Section 12.2.3.1, where the lower system has a lower R-value compared to the upper system, a higher R-value can be used for the upper system. In other words, when combining an OMF (R = 3.5) at the first level and a wood shearwall (R = 6.5) at the upper level, the design of the shearwall above can use an R = 6.5. However, the lower system shall be designed using the lower R-value (i.e., R = 3.5 for the OMF). In addition, force transferred from the upper system to the lower system shall be increased by multiplying by the ratio of the higher R-value to the lower R-value (in the OMF and shearwall example, this ratio would be 6.5/ 3.5).
When the upper system has an R-value lower than that of the lower system, the R-value of the upper system shall be used for both systems (i.e., when a SMF [R = 8] is used at the lower level and a wood shearwall is used at the upper level, R = 6.5 shall be used for the design of both systems). When it comes to retrofits with moment frames, the International Existing Building Code (IEBC) allows the use of moment frames with a higher R-value at the base regardless of the existing lateral system at the top of the frames. Check with your local building official for applicable ordinance or additional requirements.
https://training.strongtie.com/courseware/Webinars/VCSRET24/WebinarQ&A_0000038078.pdf
