PreCast Concrete in Earthquake Regions
How do concrete precast behave in earthquakes?
Based on experience in past earthquakes in Eastern European and in Central Asian countries where these systems have been widely used, it can be concluded that their seismic performance has been fairly satisfactory. when it comes to earthquake performance, the fact is that "bad news" is more widely publicized than "good news."
For example, the poor performance of precast frame systems of Seria 111 in the 1988 Spitak (Armenia) (M7.5) earthquake is well known. However, few engineers are aware of the good seismic performance (no damage) of several large-panel buildings under construction at the same site; these large-panel buildings were of a similar serial as the large-panel buildings.
The precast prestressed slab-column system has undergone extensive laboratory testing that predicted excellent resistance under simulated seismic loading. These building have been subjected to several moderate earthquakes without experiencing significant damage.
Due to their large wall density and box-like structure, large-panel(precast concrete panels) buildings are very stiff and are characterized with a rather small fundamental period. For example, a 9-story building in Kazakhstan has a fundamental period of 0.35 to 0.4 sec. In general, large-panel buildings performed very well in the past earthquakes in the former Soviet Union, including the 1988 Armenia earthquake and the 1976 Gazly earthquakes.
It should be noted, however, that large-panel buildings in the area affected by the 1976 Gazly earthquakes were not designed with seismic provisions. Most such buildings performed well in the first earthquake (M 7.0), but more damage was observed in the second earthquake that occurred the same year (M 7.3), as some buildings had been already weakened by the first earthquake.
Large-panel buildings performed well in the 1977 Vrancea (Romania) earthquake (M 7.2) and in subsequent earthquakes in 1986 and 1990
According to WHE reports, no major efforts have been reported regarding seismic strengthening of precast concrete buildings. However, seismic strengthening of precast frame buildings was done in Uzbekistan (WHE Report 66). The techniques used include the installation of steel straps at the column locations and reinforcing the joints with steel plates to provide additional lateral confinement of the columns.
By safe construction, 80% to 90% of loss can be reduced.
By constructing houses and public buildings safe, 80% to 90% of loss can be reduced. Because
Disaster risk = Hazard * Vulnerability.
- If Vulnerability is reduced, risk of disaster will decrease.
- Prediction of earthquake only saves human losses not economic losses due to collapse.
- Most of earthquake occurs in ocean.
- Earthquake of Magnitude 6 evolves energy that is equal to 1 Hiroshima atomic bomb.
- Magnitude 7 ==> 100* Energy evolved = 1 Hiroshima atomic bomb.
- Magnitude 8 ==> 100* 100* Hiroshima atomic bomb.
- Earth is not consistent but its made up of plates.
- Each moves independently.
- Plates are called tectonic plates.
- Earthquake occur due to movement of tectonic plates and their head on collisio
- Deep in earth crust. 5 to 6 - 70 to 80 km depth.
- More depth, less will be disaster as more is absorbed inside.
- Projection of focus on earth surface is called epicentre.
- Equivalent Energy is transferred through waves causing vibrations in structure.
- Foundation also moves to and fro. If foundation is made stable, structure will be safe. For example if beams are provided on foundation and structure moves on the beams.
- Walls and Columns are the most important by Earthquake point of view.
- About 1500 tones of steel can be saved in a 60 storey building due to only one level difference of seismic zone.
- 90% of economy losses and 90% of human losses are due to the collapse of buildings.
- Earthquake resistant building is one which does not collapse even if it has irreparable cracks.
Design span of buildings
- Nuclear power plants-> 2500 years.
- Dams and Bridges-> 500 years.
- Houses-> 50 years.
- Not even small cracks should occur in Nuclear power plants because implications are very high i:e, life safety levels are not common in each type of structure.
- 2% of total cost can save a building from earthquake.
Dhajji: Timber frames and bracing. To make earthquake resistant mortar take 1 bag of cement + 3 wheel barrows of sand. The mortar should be used within 1 hr.
- Rectangular or square shaped buildings are good in earthquake resistance.
- Building openings should be well distributed and less.
- Provide beam at plinth + lintel level.