There are several different types of soft story retrofit that are being implemented throughout the United States. The most common include: shear walls, steel beam and grade beams, braced frames, moment resisting frames, hollow core slabs, and supplemental truss systems.
A shear wall is a type of structural wall that resists lateral forces using materials and gravity. A shear wall is usually made of reinforced concrete, but can also be made from steel or masonry. When building a new structure, engineers must account for the fact that earthquakes and strong winds might cause walls to move laterally with respect to its foundation. To combat this, they make the wall out of material that will not bend under pressure and attach it to the foundation in such a way that it forms a stable unit. If the wall does move, it transfers that force into the ground instead of buckling like most other types of walls. This reduces stress on both foundation and frame of house, allowing them to bear more weight than they normally would be able to.
Steel beam and grade beams
A steel beam and grade beams works very much like a shear wall does in that it is used to reduce lateral movement of the building. Unlike a shear wall, however, the steel beam and grade beams are attached horizontally instead of vertically. This means that they bear most of their weight on top of the foundation rather than being solely supported by it. By attaching the beams to the foundation, you are reducing lateral movement further.
A braced frame is essentially a structure that consists of both shear walls and steel beam and grade beams. Bracing usually includes diagonal cross braces to prevent twisting or bowing in one direction due to lateral load bearing forces. Bracing is more common in multistory buildings because shear walls and steel beam and grade beams can only do so much to reduce horizontal forces.
This is a type of braced frame that is most often implemented in wood frame construction. A moment-resisting frame consists of load bearing walls on either end of a building as well as a continuous beam that connects them at the top and bottom. In between those beams, diagonal braces are connected to the beams to create stability under lateral forces common in earthquakes.
Hollow core slabs
These are floors which consist of concrete poured into an empty space below the finished floor slab. They are usually used to add more rigidity and support to a building, but they can also be used on their own for soft story retrofit.
Supplemental truss systems
Supplemental trusses are additional, smaller beams that strengthen the existing framing of a building. They work very similarly to bracing in that they prevent twisting and bowing. These types of systems are made to fit around windows, doors, and other units that would otherwise be blocked by larger bracing.
The different methods for soft story retrofit all have their strengths and weaknesses. Shear walls are typically the most commonly used type of soft story retrofit because they can be placed on top of existing walls and can be rather inexpensive due to the fact that it is usually just reinforcing a structure instead of adding new parts. The downside is that it does not work for all types of soft story retrofit, as shear walls cannot be implemented on top of wood frame construction without significantly altering the cost or aesthetics of a building.
Steel beam and grade beams work similarly to shear walls but are more often used in residential construction. They can be very expensive, but they also have the benefit of being able to span across a single story, unlike shear walls which need at least two stories to support them.
Braced frames and moment-resisting frames both work very well for soft story retrofit but are more often used in multistory construction. Braced frames can be rather inexpensive but do not typically cover as much area as moment-resisting frames, which makes them less effective at preventing lateral movement of a structure. Moment-resisting frames are even cheaper than braced frames because they only require one continuous beam instead of two. However, they are more limiting in terms of width than braced frames because the beams used in moment-resisting frames need to be much wider than the structure itself.
Hollow core slabs can be used on their own for soft story retrofit but work best when paired with other methods like shear walls or steel beams and grade beams. They can be rather inexpensive, but they are also much weaker than other methods and cannot provide much lateral support in an earthquake. Supplemental trusses add a lot of stability to a building for very little cost, but you lose living space by implementing them because the supplemental trusses need to cover the entire floor space.
In order to be effective, soft story retrofit requires design input from a structural engineer who understands the weaknesses of different types of construction. Engineers are able to determine the best way for each building to prevent damage by adding more support or bracing in certain areas.
Soft story retrofit is also important for other reasons besides earthquake safety. Without additional precautions, a soft story building is more likely to collapse and injure or kill its occupants. For example, if the first floor of a two-story wooden frame building collapses during an earthquake, those on the second floor can fall with it because there is nothing holding them up. The number of fatalities increases as the number of stories increase in height as well as the number of occupants, so it is important to retrofit soft story buildings in order to prevent loss of life.
The effectiveness of soft story retrofit may be reduced by multiple factors like age and quality of construction materials. Older buildings that have not been maintained well are more likely to collapse during an earthquake simply because their structural support has deteriorated over time. Softer materials like wood may bow or even break under stress caused by an earthquake, leaving that building with no effective structural support.
To see how much of the original (unreinforced) strength is lost during an earthquake, engineers usually test retrofit measures on simulators before implementing them in real-world buildings. These simulators are used to test how well the new construction can hold up during an earthquake like the Northridge quake in 1994. Retrofit measures that perform poorly on simulators may need to be reworked before they can be implemented, but this is only done if it will save the building from collapse.
By using simulators and understanding different types of construction, engineers are able to determine the best way for each building to prevent damage by adding more support or bracing in certain areas. This allows soft story retrofit to be an effective and cost-efficient way of preventing structural collapse during earthquakes while also reducing loss of life.
To prevent collapse during earthquakes, the U.S. Geological Survey recommends retrofitting older, weak buildings with new structural supports or braces in high-risk areas before a major seismic event. This is done to ensure the building will have increased strength to hold up against forces of nature. Retrofitting can be an effective way of reducing loss of life after earthquakes because it allows remaining residents to stay in their homes instead of being forced into temporary shelters or worse, completely abandoning their homes altogether after an earthquake strikes. Although retrofitting is considered by some as too expensive and time consuming, the cost may be less than that needed for rebuilding an entire structure following a large quake . When adding new supports or bracing there are limitations on what materials can be used based on how old the building is and what type of construction was used which can affect how effective soft story retrofit can be at reducing damage during future earthquakes.
There are many advantages of retrofitting, including the fact that it helps to avoid unnecessary damage from earthquakes and other natural disasters by reducing the number of structures that collapse. There is also fairly good evidence from past earthquake events in California indicating retrofitting can significantly increase a building’s resistance to collapse during seismic activity. However, even when effective at preventing structural failure, retrofit measures may only prevent partial damage rather than completely protect a building . Retrofit effectiveness is also limited by factors like materials used in construction and the age and type of building being retrofitted . It should be stressed that incorrect or incomplete soft story retrofit done incorrectly can lead to very dangerous situations because incorrect changes in structure actually lowers a buildings overall strength which increases its chance of collapsing if another major earthquake occurs. So although there are many benefits from conducting effective soft story retrofits on buildings, not doing so presents quality-of-life issues for residents who live in vulnerable areas as well as increasing the possibility of greater loss-of-life following an earthquake .
Soft story retrofit is a cost-effective way to keep homes safe and secure. With all the different types of ways to do a soft story retrofit, there is one perfect system for every building.
Retrofitting360 is a company that specializes in soft story retrofit. We have been in business for over 10 years and have worked with hundreds of satisfied customers. Our team of engineers will create a plan for your building’s needs to ensure the success of the project from start to finish. If you are interested in finding out more information about how our company can help Los Angeles building owners, contact us today!