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Our previous discussion on structural aspects of design for terrorism attack only about site planning and the design criteria for terrorism attack, now on part 2 we may focus on progressive collapse analysis and the design method.
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| Collapse due to terrorism attack? |
Progressive Collapse Analysis
Two design requirements determined by the type of weapon are ;
1) Direct effects of blast causing extensive damage to the facade and
2) Localized damage due to attack on individual elements leading to progressive collapse.
Two design requirements determined by the type of weapon are ;
1) Direct effects of blast causing extensive damage to the facade and
2) Localized damage due to attack on individual elements leading to progressive collapse.
In progressive collapse, the failure of a member in the primary load resisting system leads to redistribution of force to the adjoining members. If the adjoining member cannot resist the additional load, then even that member fails. This process continues in the structure and eventually the building collapses. Thus, the failure of a member at the local level results in the collapse of the structure at the global level, in a progressive manner, one member at a time. Of these two design requirements, the design for the latter is the most difficult.
Thus, the main concern of ensuring lateral resistance in buildings is preventing progressive collapse. Irrespective of the building function, structural system used in it, and the level of security employed in it, every building should be designed to prevent progressive collapse. Redundancy is the first measure to ensure that there are many alternate load paths and increased number of locations where plastic hinges must occur before the structure collapses.
Thus, the main concern of ensuring lateral resistance in buildings is preventing progressive collapse. Irrespective of the building function, structural system used in it, and the level of security employed in it, every building should be designed to prevent progressive collapse. Redundancy is the first measure to ensure that there are many alternate load paths and increased number of locations where plastic hinges must occur before the structure collapses.
Members where plastic hinges are likely to form should have ductile detailing incorporated in them to perform in a ductile manner, and the other non-ductile members must be designed as per capacity design concept, thus minimizing the possibility of progressive collapse.The methodology to be employed is described below in brief for ensuring that progressive collapse does not occur:
a) Perform structural analysis of the building by removing one important element in the load path, e.g., column, load-bearing wall or beam, to simulate local damage from an explosion. The method of analysis can be both linear and nonlinear, and within that both static as well as dynamic.
(b) Check if the available load path in the remaining structure is able to resist the loading. If it can, the exercise is repeated by removing another critical element in the load path. Otherwise, the structure is rendered vulnerable.
(c) If in all possible cases of removal of an important member in the load path, one at a time, the structure is able to resist the loading, the structure is said to meet the progressive collapse requirement.
Design Methods
Three methods are identified in literature (ASCE 7, 2002) for structural design of buildings to mitigate damage due to progressive collapse when blast loading initiate collapse. These are:
a) Perform structural analysis of the building by removing one important element in the load path, e.g., column, load-bearing wall or beam, to simulate local damage from an explosion. The method of analysis can be both linear and nonlinear, and within that both static as well as dynamic.
(b) Check if the available load path in the remaining structure is able to resist the loading. If it can, the exercise is repeated by removing another critical element in the load path. Otherwise, the structure is rendered vulnerable.
(c) If in all possible cases of removal of an important member in the load path, one at a time, the structure is able to resist the loading, the structure is said to meet the progressive collapse requirement.
Design Methods
Three methods are identified in literature (ASCE 7, 2002) for structural design of buildings to mitigate damage due to progressive collapse when blast loading initiate collapse. These are:
a) Indirect Method: Prescriptive guidelines that improve structural integrity are followed. These guidelines are related to selecting the structural system, locating the main lateral load resisting systems, proportioning the members, and detailing the members for ductility;
b) Alternate-Load-Path Method: The structure is designed for the forces due to the applied gravity and wind loads considering that one critical load carrying member in the load path is lost. This method is based only on loads other than the blast loading. The help of design consultants specializing in blast effect design may be required in identifying the critical member-loss cases that give the worst effect; and
c) Specific Local-Resistance Method: This is the most comprehensive method which includes both the blast loading as well as the nonlinear response of the structure. The method of analysis therefore covers both nonlinear and dynamic aspects of structural response. The location of blast loading is important in this analysis. Usually, blast is considered to occur in the lower storeys with the weapon under the building or at a stand-off distance from the building façade.
