Earthquake and Lateral Bracing

Earthquake:

Earthquake is one of the most devastating natural disasters. A major earthquake of 7.5 on the Richter-Scale has the potential to kill 88000 people and demolish 72000 buildings and cause damage to another 86000 pucca structures in Dhaka city alone. The devastation could even be worse in the port city of Chittagong because of its location along one of the fault lines. To counteract this type of damage and to provide enough safety for people due to earthquake, most engineers permit that the concept of lateral bracing assembly can be applied during construction of any structure. A braced assembly is defined by a pair of stiles, a top member and bottom member where diagonal braces and horizontal cross-braces create a triangular frame. Bracing is applied vertically between two columns. This vertical element can resist the all up coming lateral forces like earthquake so that it is called horizontal bracing system or, lateral bracing system. A lateral bracing has different shapes, it can be Cross bracing, Knee bracing, K bracing, V bracing. Three horizontal bracing systems can be used to resist earthquake forces. They are Braced frame systems, Moment-resistant systems, Shear wall systems.

According to seismic zoning map prepared by the Bangladesh University of Engineering and Technology (BUET),

43 percent areas of the country are rated as high risk, 43 percent medium risk and 16 per cent low risk. Fortunately, none of the tremors that had jolted this land for over more than century had been killer-type. It is though feared that the country runs the risk of being struck by major earthquakes anytime. This premonition has unfortunately failed to generate the necessary urge among the people and the successive governments to initiate damage-control measures.

Earthquake is a form of energy of wave motion, which originates in a limited region and then spreads out in all directions from the source of disturbance. It usually lasts for a few seconds to a minute. The effect of earthquake is the most important consideration in the construction of modern building. In Bangladesh, complete earthquake monitoring facilities are not available so that earthquake has a harmful effect more or less for any type of structure. The increase of the magnitude of the earthquake causes the damages of a structure. A modern building is a structure with more rigid at upper stories and flexible at the first storey. This design is found in that type of buildings where first storey contains garage or an open commercial area for stores and the upper floor contains office or residential apartments. This type of design creates a discontinuity of strength or stiffness. If the first storey of a building is softer then the other stories, it more damages occur due to earthquake because the first floor is generally highly loaded and thus damage causing column failure.

Lateral Bracing:

Lateral bracing is the term that can help to keep the top chord from bending horizontally. In this world, one of the primary concerns in any flexural design is the use of lateral bracing to control lateral-torsional buckling. The concept of lateral bracing is not a new idea. In many countries in this world, lateral bracing is used to provide more safety for a structure such as John Hancock Building (Chicago, Illinois), Alcoa Building (San Francisco, California), IBM Office Building (Pittsburgh, Pennsylvania), Transamerica Building (San Francisco, California), Eiffel Tower (Paris, France) etc. Now-a-days the application of lateral bracing is also increased in Bangladesh day-by-day. Crandell, J., and S. Herrenbruck (August 2006) states that “Some important rules need to follow to provide lateral bracing. Bracing angle must be at least 45 degrees and not more than 60 degrees from horizontal”.  Joseph A. Yura, Ph.D, Professor Emeritus states that “Design rules based on strength considerations only, such as a 2% rule, can result in inadequate bracing systems. Both strength and stiffness of the brace system must be checked”. In the fourth edition of Steel Structures: Design and Behavior by C.G. Salmon and J. E. Johnson, a five step procedure is given for the design of lateral bracing. "Steel Structures: Design and Behavior," shows seven types of definite lateral support. Such lateral support is attached to the compression flange and to the web-lateral support of the compression flange being most common. Guide to the IRC Wood Wall Bracing Provisions by American Planning association (APA) suggests that “To provide the bracing for a residential structure, use of the International Residential Code (IRC) is very important and necessary to resist the lateral loads that can result from wind and seismic events”.

Lateral Bracing System in Bangladesh:

Bangladesh is a country of seismic zone. In Bangladesh, many residential, commercial, industrial structures which are constructed without considering the rules and regulations of Bangladesh National Building Code (BNBC). There are different ways to ensure the enough safety of a structure. Use of the lateral bracing in a structure is an economic solution which can ensure more safety of a structure. This system may also be used to increase the strength of the structure. In case of building different types of lateral bracing can be used such as, Cross Bracing, Knee Bracing, K-Bracing, V-Bracing. Lateral bracing system depends upon the upcoming wind force and seismic load. This system is more effective when the magnitude of wind force and seismic load is very high on a building structure. Actually this lateral bracing system is not innovative concept; it is a worldwide system which is used to ensure more safety of a structure against wind force and seismic load. In case of low rise buildings where ground floor consists of garage, shop, open place; lateral bracing system is more applicable and economical and in case of high rise building lateral bracing system is needed at the every storey as well as top floor also.