Behaviour of exterior frame of buildings under blast loading with special reference to theaters and halls

Abstract

Explosive and car bomb blasts have become day-to-day affairs in some of the co_untries highly affected by war and terrorism. Even peace loving countries are not free from this type of threat and fear. Oklahoma city bombing in 1995, Khobar Tower bombing in 1996, the WTCbombing in 1999 and 2001 are all incidents of blast of different kind in the world. Recent HSBC bank bombing in Turkey and Australian Embassy bombing in Indonesia are examples of ever increasing incidents of blast. Bangladesh is not totally free from this type of threat or incidents. Though of small scale, Mymensingh theatre hall bombing in 2003 made a deep imprint on many people. Therefore, the need for research on dynamic blast load has become a priority in Bangladesh as well. In order to provide protection for theaters and halls in Bangladesh and to minimize casualty one must fIrst assess the degree of vulnerability of these structures, which entails a thorough research and investigation. Practical experiment with real blast on prototype structure is not always possible. So, this study is carried out by fmite element modelling of the structure and thus applying the dynamic blast through transient dynamic analysis. This thesis focuses on the building frame analysis due to an external blast load on a popularly known theater hall type of structure, called the Osmani Auditorium. The most renowned reference of US Army, Navy and Air Force manual, TM 5-1300 and ASCE publications have been thoroughly studied and applied in this work. Basing on the actual condition of the auditorium, blast loads from various standoff distances have been considered and applied to the structure. Blast is a very short duration dynamic load of 10 to 100 milliseconds depending on the location of blast and target. As such, structure also behaves differently due to different orientation of blast and structural condition and strength. Various factors like reflection, fa\(ade condition, venting and structural properties affect the blast response of a building. The numerical modelling and transient dynamic response analysis of the Osmani Auditorium reveals that blast can create catastrophic effects due to both shear and moment failure at different external locations of a building frame. Since the structure allows a standoff distance of about 50 feet, therefore, loads are applied from 50 to 500 feet standoff distances and the structural responses are analysed basing on 4000 lbs of TNT.It is clear that shear failure occurs at a distance of 300 feet where as moments are tolerable up to a distance of 200 feet standoff. It is also evitab1e that the columns and beams of the roof are all affected if blast of 75 feet standoff is considered with 4000 lbs of charge weight, which can be easily carried, by a small truck or SUY. Based on the findings of the study some recommendations regarding the safe distance of approaching vehicles are suggested to safeguard the structure. Some indications are also given regarding retrofItting measures to increase the structural capacity against blast loads and few design criterion.