Design for abnormal loads
Definition Progressive Collapse:
v Progressive collapse occours when a key member or members of a structure fail.
v The isolated failure of this key member or section then initiates a sequences of events, causing failure of the entire structure.
Progressive collapse basics
Progressive collapse can be defined as collapse of all or a large part of a structure by
failure or damage of a relatively small part of it. The general services Administration (GSA,2003b) offers a somewhat more specific description of the phenomenon: 'Progressive
collapse is a situation where local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse.
It has also been suggested that the degree of 'Progressivity' in a collapse be defined as the ration of total collapsed area or volume to the area or volume damaged or destroyed directly by the triggering event.
Codes and standards
The American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures shall be designed to sustain local damage with the structural system as a whole remaining stable and not being damaged to an extent disproportionate to the original local damage. This shall be achieved through an arrangement of the structural elements that provides stability to the entire structural System by transferring loads from any locally damaged region to adjacent regions capable of resisting those lads without collapse. This
shall be accomplished by providing sufficient continuity, redundancy, or energy-dissipating capacity (ductility), or a combination thereof, in the members of the structure. 'Clearly, the
focus in the ASCE standard is on redundancy and alternate load paths over all other means of avoiding susceptibility to disproportionate collapse. But the degree of redundancy is not specified, and the requirements are entirely threat-independent.
The American Concrete Institute Building Code Requirements for Structural Concrete
(ACI,2002) include extensive 'Requirements for structural integrity' in the chapter on reinforcing steel details. Though the Commentary states that it 'is the intent of this section...
to improve... 6 red undancy' there is a explicit mention of redundancy or alternate load paths in the Code. The Code provisions include a general statement that 'In the detailing of
reinforcement and connections, members of a structure shall be effectively tied together to improve integrity of the overall strucute' and many specific prescriptive requirements for
continuity of reinforcing steel and interconnection of components. There are additional requirements for the tying together of precast structural components. None of the ACI provisions are thereat-specific in any way.
GSA PBS Facilities Standards 2003
The 2003 edition of the GSA's Facilities Standards for the Public Buildings Service (GSA,2003a) retained the 'Progressive Collaps' heading from the 200 edition.
GSA Progressive Collapse Guidelines 2003 The GSA Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects (GSA, 2003b) begins with a process for determining whether a building is exempt from progressive collapse considerations. Exemption is based on the type and size of the structure (for instance, any building of over ten stories is nonexempt) and is unrelated to the level of threat. Typical non-exempt buildings in steel or concrete have to be shown by analysis collapse. Considerable detail is provided regarding the features of the analysis and the acceptance criteria.
GSA Progressive Collapse Guidelines 2003
The GSA Progressive Collapse Analysis and Design Guideliness for New Federal SCEOffice Buildings and Major Modernization Projects (GSA, 2003b) begins with a process for determining whether a building is exempt from progressive collapse considerations. Exemption is based on the type and size of the structure (for instance, any buildings of over ten stories is nonexempt) and is unrelated to the level of threat. Typical non-exempt buildings in steel or concrete have to be shown by analysis to be able to tolerate removal of one column or one 30-ft length of bearing wall without collapse. Considerable detail is providing regarding the features of the analysis and the acceptance criteria.
Methods Of Avoiding Disproportionate Collapse
There are, in general, three alternative approaches to designing structures to reduce their susceptibility to disproportionate collapse:
v Redundancy or alternate load paths
v Local Resistance
v Interconnection or continuity.
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