Identifying buildings requiring retrofit

But how is a building identified as a possible candidate for retrofit in the first place? The most obvious reason is that it has suffered damage during an earthquake. If the damage is severe enough to jeopardize building safety in a subsequent event then demolition or retrofit and repair are the only two options. However, the most common reason for retrofit is central or local government regulation. Many countries, states or cities require both new and certain existing buildings to comply with accepted seismic safety standards. For example, Los Angeles, San Francisco and Wellington have, for some years, taken pro-active steps to improve their communities ' seismic resilience by reducing vulnerability to seismic hazards. San Francisco has published a brochure for building owners of unreinforced masonry buildings advising them of the steps to be taken towards retrofit.5 Retrofitting is also often required when an under-strength building undergoes a change of use. Increasingly, state or city laws require old buildings and even those more recent yet seismically vulnerable, but usually excluding houses, to meet certain minimum standards.

Old buildings are targeted for several obvious reasons. In the light of today's knowledge past codes of practice to which buildings were originally designed are out-dated. The concept of ductility, that structural characteristic most likely to ensure earthquake survivability, was tentatively first introduced into codes around the 1960s. Capacity Design was first codified in the mid-1970s. Until then, designers lacked knowledge and guidelines on how to prevent a building from collapsing when seismic forces exceeded its design strength. As in all professions, new knowledge is continually introduced, leading to improved practices.

▲ 12.4 A damaged reinforced concrete building, 1995 Kobe, Japan earthquake.

(Reproduced with permission from Adam Crewe).

▲ 12.4 A damaged reinforced concrete building, 1995 Kobe, Japan earthquake.

(Reproduced with permission from Adam Crewe).

Another reason for focusing attention on older buildings is because some types have performed poorly in recent earthquakes. The 1995 Kobe earthquake highlighted, perhaps more dramatically than the preceding Californian 1989 Loma Prieta and 1994 Northridge earthquakes, the vulnerability of pre-mid 1970s non-ductile reinforced concrete buildings (Fig. 12.4). The Northridge earthquake was notorious for exposing the deficiencies of steel moment frame buildings. These lessons, along with others learnt from research computer simulations and laboratory test programs, have led to improvements in current codes. Design professionals and also politicians have been alerted to the seismic vulnerability of a significant portion of their communities' building stock.

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