Effect of Earthquakes on Structures

Architect — Tue, 15 Jun 2010 00:16:26 +0000

Violent Ground Motion During Earthquakes

The seismic waves travel for great distances before finally losing most of their energy. At some time after their generation, these seismic waves will reach the earth’s surface, and set it in motion, which we surprisingly refer to as earthquake ground motion. When this earthquake ground motion occurs beneath a building and when it is strong enough, it sets the building in motion, starting with the buildings foundation, and transfers the motion throughout the rest of building in a very complex way. These motions in turn induce forces which can produce damage.

Haiti Earthquake Damage 2010

Real earthquake ground motion at a particular building site is vastly more complicated than the simple wave form. Here it’s useful to compare the surface of ground under an earthquake to the surface of a small body of water, like a pond. You can set the surface of a pond in motion – by throwing stones into it. The first few stones create a series of circular waves, which soon being to collide with one another. After a while, the collisions, which we term interference patterns, are being to predominate over the pattern of circular waves. Soon the entire surface of water is covered by ripples, and you can no longer make out the original wave forms. During an earthquake, the ground vibrates in a similar manner, as waves of different frequencies and amplitude interact with one another.

Building Frequency and Period

The characteristics of earthquake ground motions which have the greatest importance for buildings are the duration, amplitude (of displacement, velocity and acceleration) and frequency of ground motion.

Frequency

Frequency is defined as the number of complete cycles of vibration made by the wave per second.

Here we can consider a complete vibration to be the same as the distance between one crest of the wave and the next, in other words one full wavelength. Surface ground motion at the building site, then, is actually a complex superposition of vibration of different frequencies. We should also mention that at any given site some frequencies usually predominate.

The response of building to the ground motion is as complex as the ground motion itself, yet typically quite different. It also begins to vibrate in a complex manner, and because it is now a vibratory system, it also posses a frequency content. However, the buildings vibrations tend to center around one particular frequency, which is known as its natural or fundamental frequency. In general…

The shorter a building is, the higher its natural frequency. The taller the building is, the lower its natural frequency

Period

The natural period is the time it takes for the building to make one complete vibration.

The relationship between frequency F and period T is thus given as

T = 1 / F

This means that a short building with a high natural frequency also has a short natural period. Conversely, a very tall building with a low frequency has a long period.

Building Construction Materials for Earthquake Resistance

Architect — Tue, 15 Jun 2010 00:06:46 +0000

In India, most non-urban buildings are made in masonry. In the plains, masonry is generally made of burnt clay bricks and cement mortar. However in hilly areas, stone masonry with mud mortar is more prevalent. But now a day we are very familiar with R.C.C. buildings, and a variety of new composite constructions materials.

Brittle and Ductile Building Materials

Construction Materials

I. Masonry

Masonry is made up of burnt clay bricks and cement or mud mortar. Masonry can carry loads that cause compression (i.e. pressing together) but can hardly take load that causes tension (i.e. pulling apart). Masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus infill walls act like sacrificial fuses in buildings: they develop cracks under severe ground shaking but they share the load of the beams and columns until cracking.

II. Concrete

Concrete is another material that has been popularly used in building construction particularly over the last four decades. Cement concrete is made of crushed stone pieces (called aggregate), sand, cement and water mixed in appropriate proportions. Concrete is much stronger than masonry under compressive loads, but again its behavior in tension is poor. The properties of concrete critically depend on the amount of water used in making concrete, too much and too little water both can cause havoc.

III. Steel

Steel is used in masonry and concrete buildings as reinforcement bars of diameter ranging from 6mm to 40mm. reinforcing steel can carry both tensile and compressive loads. Moreover steel is a ductile material. This important property of ductility enables steel bars to undergo large elongation before breaking. Concrete is used with steel reinforcement bars. This composite material is called as reinforced cement concrete. The amount and location of steel in a member should be such that the failure of the member is by steel reaching its strength in tension before concrete reaches its strength in compression. This type of failure is ductile failure, and is preferred over a failure where concrete fails first in compression. Therefore, providing more steel in R.C. buildings can be harmful even!!