Precast concrete is an excellent building material that has recently seen more use in the United States. Its extensive use in earthquake prone areas like eastern Europe and central Asia, has tested its ability to stand up to seismic activity.
What is Precast Concrete?
Precast concrete is another way to build using concrete. Unlike cast-in-place concrete, where the concrete forms are built and the concrete is poured on site, precast concrete is made indoors and shipped to the project location. The controlled atmosphere during production means the temperature and humidity can be kept at ideal temperatures – making a better, stronger product.
How Does it Do During an Earthquake?
Concrete is used in projects like bridges and power plants, because it can handle high compressive forces. Where we see concrete failure is with tensile loads. Adding steel reinforcement, which can handle large tensile forces, makes the concrete capable of resisting loads in any orientation. Because precast concrete products are made to such a high standard of production, they will respond better to an earthquake than cast-in-place concrete.
During an earthquake it’s important that the building can move and sway some with the shocks. If it’s too rigid and brittle, the building will collapse. Precast concrete moves more than other building materials – equaling less damage.
Precast Seismic Structural Systems
U.S. researchers noticed that very little work had been done to investigate precast concrete in seismic regions. In the late 1990s, they developed the PRESSS (Precast Seismic Structural Systems) program. This program sought to prove the viability of using precast concrete in seismic regions and determined the seismic performance levels for adequately designed precast concrete setups.
The program went through three phases – Phase I, Phase II, and Phase III. In Phase I, researchers developed the basic concepts for successfully using precast concrete in seismic regions. Phase II tested the individual concepts developed in Phase I. In Phase III a building was designed, built, and tested using the concepts developed in the previous two phases. At the end of the program, researchers presented five seismic framing systems. The use of these framing systems greatly improves the ability of precast concrete systems to resist damage from earthquakes.
The weakest points on precast concrete are at the joints and connections, so extra consideration should be given to these points during design and construction. The PRESS program recommends fiber-reinforced, high performance joints to help dissipate the energy during seismic loading. Energy dissipation can also be improved by using pre-stressed strands to connect members or by centering tension bars on beams.
During the 1906 earthquake in San Francisco, there was a lot of damage due to the earthquake itself, but up to 90% of the damage came from a secondary issue – fire. Gas lines were damaged during the quake – starting a fire that burned through around 25,000 buildings. Water lines were also damaged by the earthquake, meaning firefighters had no way to fight the fires. Concrete does not burn and provides no fuel for the fire to keep burning – essentially eliminating this concern.