Things learned from the 1971 Sylmar, California earthquake regarding the need to start retrofitting of commercial and industrial buildings.
Prior to the February 9, 1971, 6.6-magnitude earthquake there hadn't been a large earthquake in Southern California in quite a few years. This earthquake had so much force that it caused the earth to displace three feet horizontally and vertically. There were a series of strong aftershocks, four reaching 5.0-magnitude or greater. Everyone was caught off guard when the Sylmar earthquake hit Southern California. There were a lot of lessons to be learned from this earthquake. Structural engineers from all over the country flocked to Southern California to see first hand what type of damages came about from the earthquake. They found weaknesses in building designs that would have a far-reaching impact on how we were going to be designing buildings in the future. Even though there was a number of deaths from this earthquake, we were very fortunate that it occurred early in the morning 6:01 A. M. and it was not in a heavily populated area. Had it occurred later in the day and in a more populated area the death toll would have been much greater. At the time of the earthquake, there wasn't much thought about retrofitting of the older buildings. The engineering community performed an in depth study of the damages and started an upgrade of the building codes.
As the construction industry progresses and learns about damage from earthquakes, the buildings are built better and stronger. The engineer's design is based on their experience and assumptions on how buildings will react during an earthquake. After each earthquake they learn from the previous design methods and add additional strengthening of the weak links in the buildings.
After the 1971 earthquake in Sylmar California upon inspection of the damages, the industry saw that the exterior walls separated from the roof structures. This was because of splitting of the wood ledger (the wood member that was attached to the wall and the roof sheathing was nailed to the ledger). The grain in the wood ledger ran the same direction as the wall and the nailing was installed into this wood that simply split and separated. This resulted in the walls falling away from the roof structure and portions of roof structure falling. If large enough portions separated, then the walls fell down. This was a serious problem because once the walls were no longer attached to the roof structure they were more vulnerable to collapse because of no support. Another observation was that a large number of the un-re-enforced brick building received major damage.
A major reevaluation of the seismic connections in buildings was required. As a result the building codes were strengthened on new construction projects. The City of Long Beach and the City of Los Angeles moved to also set up and require a new program be put in place to strengthen their un-re-enforced buildings.
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