Not all retrofits are created equal - see how we build BETTER retrofit solutions.
As a retrofit contractor that specializes in soft-story buildings, I see lots of engineering plans that have been designed by other companies. Every day property owners send me their drawings with the intention of getting a construction quote, they want to know just how much their soft-story retrofit will cost to build. Sometimes I have to be the bearer of bad news - I can't tell you how frustrated they get when they find out that their plans will not work as designed. Their plans have a series of shortcomings, they won't do enough to protect the structure in the event of a major earthquake. The most common issue we see time and time again is that not enough attention is being focused on the existing wood-frame members; many of these buildings are old and have not been maintained properly which causes damage to the existing wood frame. In this latest blog article, I will discuss one aspect of soft-story retrofit design that is being ignored ... the existing wood frame.
So just what do I mean? Most of the engineering plans have strictly focused on the size and strength of the steel being used in the proposed solution (OMF/SMF moment frame, or OCCS/SCCS cantilever column). Seeing a huge steel column/beam being installed in your building can give you a sense of security, even if it's not designed properly. Installing steel is the easy part, designing it to perform properly is hard. Unfortunately, I've seen lots of ill-designed retrofit plans that are flawed - and I fear they will do more harm than good. The flaws are in the absence of reinforcing the existing wood frame, specifically the floor joists. The heavy steel that is installed is a stiff element, that can crush the aging wood frame. If you do not strengthen the existing wood members, they will buckle when severe seismic forces push them into the stiff steel. It takes a careful eye and tons of experience in wood-frame construction to design a new structure that transfers seismic loads correctly.
Would it shock you if I told you that 95% of plans do not address this issue? I've seen projects where nothing is done to the existing wood frame. Why does this matter? The building will not perform correctly - in the event of a large earthquake, the building could collapse! That's right, all the money you spent on your seismic retrofit goes down the toilet. The "value-engineering" solution you paid for did not protect your building. The building will suffer massive amounts of damage that could have been avoided, mostly caused by the fact that the seismic loads did not transfer correctly. It is a systemic design flaw created by a void in the Santa Monica and Los Angeles City soft-story retrofit ordinances (more about that in another article).
New structural hardware installed.
Developing a comprehensive retrofit solution requires the reinforcement of the existing wood frame - this includes adding structural connections like Simpson Strong-Tie LTP4/LTP5 and A34/A35 plates. Creating a rigid structure that transfers seismic loads correctly will ensure the building can perform during a significant earthquake. Here is one way to fix your engineering plans. Once the stucco is peeled back and the building is opened up, a trained eye must assess where the building is structurally weak. The picture above was taken at our seismic retrofit project in Los Angeles, the area in yellow is where we put in more small steel connectors like Simpson Strong-Tie LTP 4 connectors over the new blocking. Other contractors do install this type of hardware, but in my opinion, they don't use nearly enough of them. The yellow squares represent the area that must be blocked with 4" x 12" lumber. This will create a blocking system that will help the subfloor resist torsion and will tie the floor system to the existing dragline beam (the opening where the cars enter the garage).
New Special Moment Frame & framing system.
Systems Working Together
In the picture above we look at the wood blocking (in green) that we installed above the Steel Moment Frame. This "solid" connection allows the seismic forces to transfer between the existing building and the new special moment frame. The next step is to run that blocking along the entire length of the buildings. This creates more contact points between the old and the new structures, allowing them to work together, instead of against each other. A larger footprint increases the likelihood that the seismic forces will transfer correctly back to the steel moment frame. This is what our team tries to design, a superior seismic solution that is strong enough and smart enough to protect the soft-story structure in the event of a significant earthquake.
New lumber is needed to strengthen the building.
This requires a tremendous amount of lumber and the knowledge to know where to put it! Here is a picture of half the lumber required to make this load transfer possible. Retrofit Pros follows the philosophy, "building doesn't fall, building doesn't break", we build better & smarter solutions that perform when it matters most. We are loyal to the building - we asses each one based on their current condition and strengthen them accordingly. That does not mean that the buildings will be 'earthquake proof' but it represents our obsession with making the building stronger and safer. If you keep head straight and focus on the building, you will not fall victim to the follies of "value engineering" which simply translates into 'do less and make it cheaper." This mentality ultimately leads to poor retrofit design! What do you think about this article? Post your comments/questions below.
New and stronger framing.