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Concrete In Australia : December 2008
TECHNICAL Progressive collapse and earthquake resistance by Colin Gurley This paper was fi rst published in the American Society of Civil Engineers (ASCE) Practice Periodical on Structural Design and Construction in February 2008. It is reprinted here with permission. Colin Gurley is a lecturer in civil engineering at the Sydney Institute of Technology, Ultimo, NSW Structural thinking has developed in response to the accidental gas explosion at Ronan Point in England 1968, the terrorist bomb attack on the Murrah Federal Building in Oklahoma City 1995 and the attack on the World Trade Center 2001. It now seems evident that there is potential for two areas of structural engineering to share in the development of robust standards. FEMA 450 (2003), ASCE 7-02 (2002), ANSI/ AISC 341-05 (2005a), and ACI 318-05 (2004, Chapter 21) seem to cover most of the issues relating to “lost column” events. ACI 318-05 (2004, Chapter 21) does not require “special” or “intermediate” detailing, as distinct from “ordinary” ductile detailing in areas of low seismic risk even for important buildings in Category IV of ASCE 7-02 (2002, Table 1.1). Maybe it should. There is nothing to prevent the use of special ductile detailing in projects at low seismic risk where plastic hinge strengths are likely to be determined by wind loads. In earthquake engineering the assessment of dynamic performance and/or static design earthquake strength has always been separable from the ductile detailing provisions of ACI 318 (2004, Chapter 21). There is nothing to prevent the use of special ductile detailing in projects at low seismic risk where plastic hinge strengths are likely to be determined by wind loads. This would provide an improved response to terrorist attacks, or to any other unforeseeable event, in those regions. This is the approach for the design of new US federal buildings recommended by FEMA 277/ASCE (1996) The Oklahoma City Bombing where it is reckoned to increase the total cost of new buildings by 1–2%. A loadings code, such as ASCE 7-02 (2002 ), might choose to relate the minimum quality of ductile detailing to Table 1-1 “category of importance” regardless of earthquake risk. This could apply to Class IV or, perhaps, even some Class III buildings. One key issue is that of positive moments (tension bottom) at column faces. If the analysis for, say, 500 year wind finds that there is no positive moment at a column face when combined with long-term minimum gravity load then, with ordinary detailing, there may be no effective bottom rebars through the column; ACI 318 clauses 12.11.1 and 12.11.2. If a column is lost producing a “double span” then there will be no bottom rebars in the new midspan area. It makes no difference whether the lost column was removed by an earthquake or by a terrorist bomb. For concrete buildings it seems clear that the extra detailing and the cost of special detailing is minor requiring: • continuous bottom rebars through intermediate supports • closer ties to improve shear strength, confi ne concrete, and restrain compression rebars • a cost increase of 1–2% of the total cost of the building. For steel buildings, there has been a huge amount of research into earthquake resistance following the Northridge (Los Angeles) shock of 1994 culminating in ANSI/AISC 341-05 (2005a). Structural engineers now need a pause while those changes are digested and implemented. Nevertheless the issue of double span collapse mechanisms should be pretty much the same for concrete buildings and steel buildings so progress in that area will benefit both. Ronan Point Ronan Point was an apartment building of about 22 stories built, in England, in the years following World War II. Much of the housing in England and across Europe had been destroyed during the war so there was an urgent postwar need to provide high-density housing quickly. One of the then innovative schemes for rapid construction involved factory precast concrete panels for fl oors and load- bearing walls. Precast concrete construction has moved on since to the extent that California and New Zealand both now have specialised codes for the design and construction of precast buildings in regions of high earthquake risk. Often this involves using the precast elements as permanent formwork with suffi cient in-situ concrete to ensure that fl oor and wall elements are positively connected to each other in three dimensions. If there is no in-situ concrete then the structural designer must go to whatever lengths are necessary to ensure positive ductile connections between abutting elements whether floor/floor, fl oor/wall, or wall/wall. There was a British judicial enquiry which reported a few years later and found Ronan Point to be a “disproportionate collapse”. These issues were not well understood at the time when Ronan Point was built. In 1968, there was a gas explosion in the cooking stove of a corner apartment. Within 20 min all of the apartment kitchens above and below the explosion had collapsed into the street. Three people were killed and the whole building was later demolished and many similar buildings were subject to review. There was a British judicial enquiry which reported a few Concrete in Australia Vol 34 No 4 43