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Sci Tech
Quake resistant design, construction
DESIGN OF buildings to resist earthquakes involves controlling the damage to acceptable levels at a reasonable cost.
Contrary to the common thinking that any crack in the building after an earthquake means the building is unsafe for habitation, engineers designing earthquake-resistant buildings recognise that some damage is unavoidable.
Different types of damage (mainly visualised though cracks; especially so in concrete and masonry buildings) occur in buildings during earthquakes. Some of these cracks are acceptable (in terms of both their size and location), while others are not.
For instance, in a reinforced concrete frame building with masonry filler walls between columns, the cracks between vertical columns and masonry filler walls are acceptable, but diagonal cracks running through the columns are not.
In general, qualified technical professionals are knowledgeable of the causes and severity of damage in earthquake-resistant buildings.
Earthquake-resistant design is therefore concerned about ensuring that the damages in the buildings during earthquakes are of the acceptable variety, and also that they occur at the right places and in right amounts.
This approach of earthquake-resistant design is much like the use of electrical fuses in houses: to protect the entire electrical wiring and appliances in the house, you sacrifice some small parts of the electrical circuit, called fuses; these fuses are easily replaced after the electrical over-current.
Likewise, to save the building from collapsing due to the quake, you need to allow some pre-determined parts to undergo the acceptable type and level of damage.
Acceptable damage: Ductility
So, the task now is to identify acceptable forms of damage and desirable building behaviour during earthquakes.
To do this, let us first understand how different materials behave. Consider white chalk used to write on blackboards and steel pins with solid heads used to hold sheets of paper together.
Yes... a chalk breaks easily! On the contrary, a steel pin allows it to be bent back-and-forth. Engineers define the property that allows steel pins to bend back-and-forth by large amounts, as ductility; chalk is a brittle material.
Earthquake-resistant buildings, particularly their main elements, need to be built with ductility in them. Such buildings have the ability to sway back-and-forth during an earthquake, and to withstand earthquake effects with some damage, but without collapse.
Ductility is one of the most important factors affecting the building performance. Thus, earthquake-resistant design strives to predetermine the locations where the damage takes place and then to provide good detailing at these locations to ensure ductile behaviour of the building.
Sponsored by Building Materials and Technology Promotion Council New Delhi, India
C. V. R. Murty
IIT, Kanpur
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