Earthquake construction

Earthquake construction is a branch of architectural engineering concerned with making sure structures can withstand as severe an earthquake shock as possible given the materials available.

When, the structure in question is a human habitation, the questions of surviving earthquake damage become much more serious. Examples of inhabited structures collapsing during earthquakes abound and are sadly all too frequent. Areas of the world frequently hit by fatal earthquake damage include Japan, Turkey, Algeria, and countless other regions on or near tectonic plate boundaries.

Earlier in mankind's history (during the Neolithic, for instance), mankind lived in tents, which can withstand earthquakes quite well. We moved on to more comfortable structures of timber, mud brick, limestone, wattle and daub, and even just stacked rubble.

Some of these materials can be used to form solid, earthquake resistant structures. The important part is to use them wisely and with an understanding of how earthquakes really apply stresses to structures in practice. A structure might have all the appearances of stability, yet offer nothing but danger when an earthquake occurs. The crucial fact is that for safety, earthquake resistant construction techniques are as important as using the correct materials.

For larger structures, more exotic means such as bearings and counterweights are often employed to reduce the impact of lateral ground movement on the protected structure.

The specific mode of failure in an earthquake for most structures is the lateral (sideways) shaking. It frequently collapses walls, or moves them enough that the roof displaces and falls in.

Development of earthquake construction techniques

People living in frequently shaken areas like Japan started early in developing earthquake resistant buildings based on scientific study. Other countries likewise have and continue to study intensely how to make their citizens safer by understanding the problems posed by earthquakes more accurately.

Until the last 75 years or so, the only way to run "frequent tests" was to build on a fault and hope. Even then, earthquakes may only happen at any given spot every couple of hundred years, and construction techniques may not therefore take account of earthquake concerns. Modern shake tables have helped this; large motors and computer control systems try to precisely simulate earthquake movements.

Modern materials like concrete and reinforced concrete can help, but they also must withstand the same lateral (sideways) forces. Usally these matereals will break under the amount of lateral stress put on the object.

Good earthquake construction pays careful heed to lateral forces. Proper concrete construction involves significant use of steel reinforcing bar (rebar). All the joints (where beams meet the columns), are carefully tied in with rebar. The concrete is of very high quality, and high strength. Brick infill is avoided for the walls. All structures should be properly anchored with anchor bolts.

Most countries have building codes that specify lateral strength. In the Los Angeles, California area, it is legal to build an apartment building near the San Andreas Fault with a setback limit of 50 feet. The apartments are typically designed to withstand an earthquake of 7.8 on the Richter Scale and fail in predictable ways at that level. The reasoning behind this is that an earthquake above 7.8 will bring down almost any structure and it is very difficult to design a reasonably priced structure that will withstand an earthquake above 7.8 on the Richter Scale.

In residential structures, buildings are designed to have the roof fall in the middle of a room, but stay up near the walls. People are always urged to take refuge in doorways and away from the middle of the room, and are therefore safe in these buildings. The structure of a residence may also be attached to the foundation with bolts to prevent the building from sliding off the foundation during shaking and collapsing.

Ground stabilization

Another failure mode of a structure in an earthquake involves the soil underneath the structure. In a strong enough seismic event, the soil can be shaken hard enough that it will break up, sometimes leading to the collapse of the structure sitting upon it. The most common method of protecting a structure against this failure mode is to flow cement into the soil beneath the structure. This method provides marginal support for the structure as the cement may not set up evenly.

An alternative method to infusing the ground with cement is under study. The method involves using a bacterium that secretes a viscous, sticky polymer that binds the soil together. In an experiment, the bacteria, Flavobacterium johnsoniae, was mixed with sand and was given several days to colonize the sand. The friction coefficient of the sand was measured and compared against sand without the bacteria colony. The sand colonized with the bacteria was almost twice as solid as the sand without the polymer producing bacteria. (Banagan, et al., 2005)

References

• Banagan, B.L., B.M. Wertheim, M.J.S. Roth, and L.F. Caslake (2005). "Increasing sand strength through the addition of bacteria". American Society for Microbiology meeting. June 5-9. Atlanta. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help)CS1 maint: multiple names: authors list (link) Abstract