A soft-story structure with a wood frame may pose a higher risk to the residents and residential building stock during an earthquake. Majority of them are the wood-framed structures, made before the current building codes, and whose ground floors can collapse when it is shaken very hard enough. Most of the cities have implemented compulsory soft story retrofit to decrease structural deficiencies by economical & feasible methods.
Resistance your building needs can be determined by 3 important factors and they are how much does your building weighs, is located close to the fault line, and the highest hypothetical magnitude, which fault line will experience. Methods used in the soft-story retrofits will determine the real cost of this project. Let us check more about soft story retrofitting buildings in this post.
What’s a Soft Story?
“Soft-story” generally refers to a single level of the building that is more flexible and weak in the lateral load resistance compared to the stories above and floors or foundation below (around 70% and higher reduction from a floor to next as per the modern and International Building Code. This condition will happen in any conventional construction type & is associated with the large openings in walls and tall story height than its adjacent stories. The soft stories will present a serious risk in an event of the earthquake, in human safety & financial liability.
1. Four-Sided Moment Frames
The prefabricated element, which has special moment connections on all 4 corners was founded by Mitek. This innovative addition of the beam with moment connections replaces your need for the engineered and steel-reinforced beam and provides the benefits of the fixed base connection. The field installation will be simpler and inspections will be reduced.
2. Special Moment Frames
These frames are an ideal solution for soft-story retrofits and they generally come in different sizes and are delivered flat shipped and assembled. The better frames will be the bolted system and this frame generally comes in the sections and gets bolted together, so making it simple to repair after the earthquake. The frame systems are made with a special link, which absorbs any kind of damage & shock from the seismic event and ones that can be reused.
3. Plywood Shear Walls
Plywood shear walls help to strengthen the whole structure to prevent any movement during seismic events. Even though a simple solution, shearing the structure with the plywood strengthens the whole building, it prevents collapse of a structure during its lateral forces experienced at the time of major earthquakes. During the earthquake, walls may move laterally, so if seismic movement is good enough structural connections will separate. These reinforced shear walls will prevent walls from separating & causing the collapse. You can check out how lateral forces can affect the wall during an earthquake. These are the same forces that can impact your houses and rental properties.
4. Steel Cantilever System
For the smaller units that have narrow spaces, it’s good to use the Steel Cantilever system. It’s a highly optimized soft-story retrofitting option but not appropriate for all the structures. For the building owners who can select this method, they need to ensure that their current wood frame is properly checked by the company. The cantilever beams are highly affordable than making use of the special movements frames and shear walls and it helps to reinforce your building foundation not to break. So, for the small apartment unit, the cost for this to get retrofitted can be over $35,000 & $65,000 – $75,000 for the bigger buildings with 2 and more stories.
5. Shear Walls
When the structural loads aren’t very high enough to need the moment frame, then prefab shear walls will be your right choice. The products provide design flexibility that will meet the ordinance standards with the narrow walls and larger openings. Simple installation can save you valuable time on this job.
For any building, their weakest parts are reinforced structurally to resist the seismic movement during the earthquake. In a lot of cases, structural steel connectors & I-beams can be used. In many situations, glue-lam beams & steel connection hardware will be used to strengthen any weak sections of the structure.