Progressive Earthquake Design Philosophy

CHAPTER-1

INTRODUCTION

GENERAL

The term “progressive collapse” has been used to describe the spread of an initial local failure in a manner similar to a chain reaction that leads to partial or total collapse of a building. The fundamental characteristic of progressive collapse is that the final state of failure is disproportionately greater than the failure that originated the collapse. Progressive collapse as “the spread of an initial local failure from element to element resulting, finally in the collapse of an entire structure or a disproportionately large part of it”. The disproportionality refers to the condition in which failure of one member causes a major collapse, with an amount disproportionate to the original event. Thus, “progressive

Under moderate but infrequent shaking, the main members may withstand repairable damage, while the other parts of the building may be damaged such that they may even have to be substituted after the earthquake; and Under strong but rare shaking, the main members may withstand severe (even irreparable) damage, but the building should not collapse. Thus, after minor shaking, the building will be completely functioning within a short time and the repair costs will be small. And, after restrained shaking, the building will be functioning once the repair and strengthening of the damaged main members is accomplished. But, after a strong earthquake, the building may become dysfunctional for further use, but will stand so that people can be displaced and property recuperated

The alternate load path method has been selected by interventions including the U.S. General Services Administration (GSA 2003) as the preferred approach for provided that resistance to progressive collapse. In the DOD UFC 4-023-04 (DOD 2005), this method is used when the required vertical tie abilities cannot be obtained in low level of protection (LLOP) structures designed by the indirect method. It is also used in structures chosen as requiring a medium level of protection (MLOP) or high level of protection (HLOP) when design by the indirect method is not possible. The alternate load path method offers a recognised check of the ability of the structural system to resist the removal of precise elements, such as a column at the building perimeter. The method does not require explanation of the risk causing loss of the element, and is, therefore, a risk independent approach. Depending on the analytical method used to appliance the alternate load path method, the results may not provide an accurate representation of actual recital in the event of a destructive event