California is known to be an active earthquake region. Stress along the San Andreas Fault, which extends roughly 810 miles through California, has been building up with little relief since 1895. Many geologists have predicted that the “Next Big One” is 97% likely to happen at any time. Therefore, California residents should be aware of whether their properties are at risk of earthquake damages and what solutions are available to strengthen their structural features.
Buildings at Risk from Seismic Events
Soft Story Building
Currently in California the most common type of building that needs seismic retrofitting is called soft-story. These multi-story buildings, usually wood framed, are exemplified by apartment dwellings with tuck under parking. This style of building is a common design in both San Francisco and Los Angeles. Due to multiple seismic retrofit ordinances in the State of California, cities like San Francisco and Los Angeles have already begun engineering and construction to become compliant. Other cities, like Santa Monica and West Hollywood, are quickly following suit. We expect many more cities to jump in line to become compliant with new seismic safety standards. Common retrofitting solutions for soft-story buildings are steel special moment frames, cantilevered column solutions, or shear wall paneling.
Non-Ductile Concrete Building
Another type of seismic retrofitting involves non-ductile concrete buildings. Often seen in multi-story buildings built pre-1976 before seismic codes were tightened, these buildings are considered reinforced concrete in design, but often don’t have enough steel reinforcement to handle a major seismic event. While there are several combinations of solutions for non-ductile concrete buildings, steel beam installations and polymer fiber wrap solutions are often the preferred option.
Unreinforced Masonry (URM) Structure
This type of construction often seen on older historic buildings has no metal or steel reinforcements at all and is literally often only brick and mortar. A common solution is to center core or drill through the bricks vertically and thread rebar or steel through and pour back concrete to secure hole. URM construction is another type of seismic retrofit that Santa Monica is most recently preparing for.
Common Seismic Retrofit Solutions
- A moment frame is a system of columns and beams that are connected to one another with fully and/or partially restrained connections.
- A special moment frame is expected to withstand significant inelastic deformation as a result of lateral forces. They are used typically in mid/high-seismic regions and are significantly more expensive than other retrofit options.
- A cantilever is a horizontal beam that is unsupported at its end.
- A cantilever column is the vertical component that supports the horizontal cantilever beams.
- Retrofitting companies often use cantilever columns to strengthen soft-story buildings.
- A shear wall is a vertical-oriented wide beam that transfers lateral forces from exterior walls, floors, and roofs to the ground foundation.
- Shear walls are commonly used for soft-story retrofitting.
Other Elements Within Seismic Retrofit Projects
- Destructive testing is the process of determining design weaknesses.
- For seismic retrofit projects, the city would like to see the quality and connections of load-bearing beams that are at-risk of failure in the event of an earthquake prior to approving permitting for the project.
- This type of testing requires exposure of these connections.
- Non-destructive testing is the process of evaluating materials or components for discontinuities or differences in characteristics without causing damage to the structure.
- Ground penetrating radar uses radio waves to scan concrete.
- Concrete x-ray uses x-rays or gamma rays to image the interior of a concrete target to identify embedded objects.
Fiber-reinforced polymer (FRP) composites consist of fibers bonded with polymers which allow for the carrying of loads and transfer of shear forces. This material is used to strengthen structures in the same way that steel would, but an added benefit is that these composite materials add significantly less weight. FRP systems can be used to strengthen reinforced concrete and unreinforced masonry walls, if deemed the best solution for your building’s retrofit needs. This strengthening solution has been utilized in thousands of retrofit and strengthening projects and has proved to perform very well in the event of earthquakes. Penhall is proud to be a certified applicator of Tyfo Fibrwrap.