1 ENGINEER Investigation on Concrete Fins as Blast Resistors in Buildings K. Baskaran and R. M. B. Prasad Abstract: Designing structures against blast loading is becoming more and more important as the number of terrorists attacks are increasing day by day. It is necessary to protect the structures against a credible blast load to ensure the safety of the occupants. In this context, blast resisting facades are incorporated in buildings to avoid the blast pressure waves entering into the building as the highest damage is done by the pressure waves when compared with the fragments moved by an explosion. Pressure waves could damage the critical elements and it may lead to progressive collapse of the structure. This study investigated the behaviour of concrete fins that can be used to enhance the blast resisting capacity of facades. Blast pressure loads were calculated when varying the fin spacing to evaluate the capacity of fins having different section properties. Occupancy levels such as immediate occupancy, life safety and collapse prevention were used to identify the structural performance. Material nonlinearity, material strength enhancement with higher strain rates and nonlinear loading were considered in this study and analysis was done using the Sap2000 software. Weight of blast materials, standoff distance and fin spacing were considered to create different blast loads while fin sizes and there reinforcement arrangements were also varied to create different load cases. Charts were prepared for different concrete sections based on the occupancy levels, blast pressure loading and reinforcement ratio. It enables to identify the most suitable sections that are required as structural elements to be behaving in the required occupancy level for a given scale distance. Keywords: Blast pressure loads, Concrete fins, Material nonlinearity, Progressive collapse 1. Introduction Terrorist attacks are increasing globally, and it appears that there is no ending. Therefore, it is necessary to protect structures from terrorist attacks. However, the protection is not an absolute measure to issue and there should be different levels of defensive systems, which also minimize the cost of system. Protection can never offer a guarantee of safety. High level of protection will also increase the cost of the construction and could be a waste of resources. However, it is very important to take measures to protect most vulnerable structures against blasts. It is learnt that the structures, which will be constructed in the future could be designed for at least a credible blast load. There are many examples in the past that proves the above. Attack to the Central Bank of Sri Lanka on 31 January 1996 is believed to be the largest bomb blast occurred within the country. About 85 people died and more than 1500 people were injured. After observing the images of the damaged structure, it can be understood that designers had not predicted such event at the design stage. The building was severely damaged although it did not collapse. Main causes of damage are due to the blast pressure and subsequent fire occurred inside the building. This could be due to lack of defensive system that bears the blast pressure. Different types of defensive systems have been introduced to avoid the collapse of the structures due to the failure of load carrying elements and to avoid the fragments entering into the building. One such defensive technique is enhancing the load resisting capacity of facade. Most commonly, facades are constructed from brick, concrete and glass. Different techniques are used to increase the load carrying capacity of facades constructed from above materials. When there is no defensive system to protect the axially loaded elements, it could lead to progressive collapse. Brick walls are strengthened by introducing cross wall, increasing the width of the wall and adding steel plates where it fails. Glass walls are modified by adding fins while windows are modified by laminating membrane. Dr. K. Baskaran, B. Sc. Eng. (Hons) (Peradeniya), Ph.D. (Cambridge), Senior lecturer, Department of Civil Engineering, University of Moratuwa. Eng. R.M.B. Prasad, B.Sc. Eng. (Hons) (Peradeniya), M. Eng. (Moratuwa), PG. Dip. (Const. Mgt.) (Open University of Sri Lanka), C.Eng MIE (SL), Central Engineering Consultancy Bureau.