Throughout history, earthquakes have taken many lives and destroyed property in all parts of the world. While the smallest quakes are often not felt by humans, the largest ones have destroyed entire cities, causing billions of dollars worth of damage. However, modern technology has now made it possible to build structures that are resistant to such forces of nature. The characteristics of these earthquake-resistant buildings have had a tremendous impact on greatly reducing the loss of life and the damages incurred.
General Design Aspects
The more symmetrical the structure is about both axes, the better. Asymmetrically designed buildings are subject to substantial torsional forces during earthquakes, making them considerably dangerous. In addition, simple designs with rectangular shapes tend to hold up better in earthquakes than more complicated designs with protruding sections. In some cases, a large building performs better in an earthquake if it is separated into several appropriately spaced blocks, in order to maintain this symmetry and regularity in each block. The spacing between these blocks is carefully calculated to prevent contact or hammering in a severe seismic event.
An appropriate foundation is also critical in the design of any earthquake-resistant building. The type of soil on which the structure is built is classified as either firm, soft or weak, according to its bearing capacity. Soft soil is avoided whenever possible, although methods do exist to provide special strengthening, if necessary. Weak soils are too dangerous to build on top and must either be compacted to bring their quality up to firm or soft, or avoided entirely. The foundation itself must be well tied together, as well as tied securely to the walls.
Ductility refers to the ability of a material or structure to deform and yield, dampen vibration and absorb energy. Materials such as steel and wrought iron are considered ductile, thus making them more suitable for use in the construction of an earthquake-resistant structure. Materials that are brittle (non-ductile), such as concrete, adobe or cast iron may break suddenly when subjected to stress. In order for ductile materials to have the proper effect within the body of a structure, there must be a sufficient quantity of them placed in areas of high tensile stress. In addition, any materials used must be of high quality and care must be taken that they are protected from the elements, insects and any other action that could potentially weaken them, so that their strength will last.
Deformability refers to the ability to deform a substantial amount without collapsing. In order to achieve this, a structure must be properly proportioned and constructed so as to avoid excessive concentrations of stress. The structure must have a proper aspect ratio and sufficient resisting elements, such as braces, shear walls and wall ties connecting to floors and roofs, in order to ensure material and geometrical stability. Proper connections must be maintained in areas such as beam seats to prevent beams from falling and to permit adequate deformation during the motions produced by an earthquake.
The ability to withstand substantial damage without collapsing is referred to as a structure's damageability. The structural framing system must be designed to furnish sufficient lateral resistance, such as with diagonal bracing or very rigid jointed beams. Redundancy, by providing additional means of support for critical structural members, greatly improves the level of damageability. In the event that certain components fail, the additional support would serve to hold the surrounding components together, preventing a total collapse of the structure. Care must be taken to avoid reliance on centrally located support columns and walls to hold up large portions of the structure.
A recent approach to earthquake-resistance called Base Isolation involves reducing the vibrations in a structure by isolating it from the motions in the ground. This can be accomplished by reducing friction between the structure itself and its foundation or by using some type of flexible connection in that area. One method by which this is done is through the use of special bearings. When this method is utilized, the sideways movement occurs mostly in the bearings themselves, reducing the effect on the building. Another method is to use two layers of high-quality plastic beneath the structure, that will slide over each other, reducing friction.