Geology And Geophysics

An Overview on Common Seismic Retrofitting Techniques

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"An Overview on Common Seismic Retrofitting Techniques"
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There are many buildings and residences that need to be seismically retrofitted, especially those that were built prior to 1975 and are situated on earthquake faults. For example, certain locations such as Los Angeles, The San Francisco-Bay Area, Kansas, etc., that have had prior experiences with large earthquakes; owners of businesses, apartment buildings, and residential areas should take extra precautions to assure that their properties are as seismically retrofitted as possible.

Although there is no way to totally secure a building from an earthquake, and other than prior experience; no way to tell if the location in which you reside will have a major earthquake (for example Kansas); it is imperative that you do whatever you can to make sure your property is structurally sound. There are steps one can do to use seismic retrofitting techniques:

1) Hire a structural engineer to do an earthquake assessment. Make sure this person is the right one to conduct the study before you put out a lot of money. There have been cases where selecting the cheapest bid caused the owner additional money because the engineer was not as experienced as they should have been.

2) Before choosing an engineer, do contact the Seismic Retrofitters Association for recommendations. They will let you know whether the person is a member.

3) Ask the engineer how many seismic retrofitting jobs they have completed. They should have performed between three and ten. If not, do get another bid.

4) Search the Internet and ask other owners who they used and what was done. Have some prior knowledge as to what seismic retrofitting includes.

5) Ask questions because the more secure your buildings are, the lower your earthquake insurance will be.

6) After completion, you should feel confident; knowing that you have done all you can humanly do to protect your property, employees, equipment, etc.; however, should an earthquake occur; at least you have taken the necessary steps to be seismically retrofitted, knowledgeable, and insured.

The following are some recommendations for seismic retrofitting:

1) Beam joint connections should be securely designed; when they fall, it can lead to catastrophic collapse of frame buildings.

2) Floor diaphragm - Wooden buildings constructed on deep spans of wood called "joists" will collapse if the blocking is inadequate. There should be a subfloor underneath that is covered diagonally with wood planking or plywood. Can also add extra nails especially on the outer edges.

3) Although single or two-story wood frame structures built on slab foundations are usually safe, if they were built before 1950, they are not built as well and may not be sufficiently bolted. A sideways earthquake can slide the building totally off the foundation or slab. The corners should be well reinforced to make sure they are well connected to each other. Some people use plywood (not common) that is rot-resistant.

4) Structures built in shallow pits, especially in tropical conditions, during an earthquake might tip knocking the building to the ground.

5) Concrete should be reinforced so columns do not burst. Steel plates can be used to surround the columns to hold up a large building. Some people will fill the space between the jacket and column with concrete or grouting. Sometimes soil or the structure will require additional wood; or concrete might need to be added at or below the ground.

6) Brick buildings - Sometimes the upper and lower levels are not securely fastened and upper stories and roof structures may become detached because bolts are not adequate between foundations. Special clamps may have to be added or external jackets included to reinforce concrete.

7) Soil failure can also occur, especially on a slope or flat area that has been saturated by water, sand, or mud. (Probably as in Haiti). Soil might be better at the beginning of the wet season than the beginning of the dry season. Whether it slides or not can be dependent on the season. Water needs to be drained by inserting horizontal tubing.

8) Wood frame structures - Interestingly enough, are some of the best because materials are less brittle than masonry and low mass; however, they have to be properly connected to the foundation. In older wood-frame structures, metal clips should be used to bolt walls to the foundation. For two-story buildings, walls have to be securely erected on the lower story's upper diaphragm.

9) Pipes and Cables - If affected by earthquake, a device may be used to close off the valve which will not respond to leaks; or you can use a metal ball at the edge of the orifice. If a shock occurs, the ball will roll into the orifice, sealing it to prevent gas flow.

10) Tunnels - Should be covered with wire mesh and pinned down with metal rods - can also use this for highways (similar to prevent snow avalanches).

11) Bridges - Adding high-strength restraints that are clamped to the beams and designed to slide under extreme stress will limit the motion. Suspension bridges may go side-to-side during an earthquake. They require a diagonal reinforcement.

Earthquake safety, as shared earlier, cannot be predicted because it depends on the level, soil conditions, materials, reinforcement, etc. Seismic retrofitting is simply adding one or more forms of enhancement that will help to keep the building, workers, and equipment safe from seismic activity that occurs suddenly or over a period of time. It could be as simple as using straps to secure equipment, using anchors to secure roofing, modifying or reinforcing, or demolishing the entire building and/or rebuilding per the earthquake codes.


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