Seismic Vulnerability Assessment of Low-Rise Buildings in Regions with Infrequent Earthquakes

Excerpt from the ACI Structural Journal. For a copy of the article in its entirety, see ACI Journal, V. 94, No. 1, January – February 1997.

This paper presents a simplified method of ranking reinforced concrete, low-rise, monolithic buildings according to their vulnerability to seismic damage. Thc ranking process requires only the dimensions of the structure. The process is tested using a group of buildings that suffered various levels of damage during the Erzincan earthquake of 1992. Thc ranking procedure reflected the observed damage satisfactorily.

Keywords: buildings; earthquake-resistant structures; earthquakes; evaluation; failure; inspection; reinforced concrete; school buildings. .

The goal of conventional methods for evaluation of seismic vulnerability is to select buildings with a high probability of survival. This paper contains an alternative approach. A simple method is presented to help identify buildings with a high probability of severe damage.

In regions of frequent earthquake occurrence, it is proper and feasible to calibrate seismic safety assessment procedures conservatively in deference to extreme cases of damaged structures. Contradictions posed by buildings that survive earthquakes even though they would be rated hazardous by a ranking procedure calibrated exclusively on damaged structures are often ignored. As long as the number of buildings classified as hazardous is not overwhelming, this "upper-bound approach" does not stop the development of a policy for earthquake risk reduction.

In regions of infrequent earthquake occurrence where buildings with poorly delineated or weak structural systems are likely to represent a large portion of the building inventory, the upper-bound approach may actually be unconservative. If nearly all buildings are deemed hazardous, the likely policy is inaction.

In regions where earthquakes occur in intervals measured in centuries, there is a need for a simple evaluation method that focuses on selection of buildings with high vulnerability rather than those with a high probability of survival. Because seismic risk evaluation methods are based on concepts that are not all well understood, a procedure designed to identify buildings with a high probability of survival cannot be adapted conveniently to identify buildings with a high probability of failure simply by relaxing some of its requirements.

Undeniably, there is no better vehicle for identifying a vulnerable building than the considered judgment of an experienced professional. But this is an expensive vehicle, especially in regions of infrequent earthquakes. There is a need to provide reasonably objective criteria to be used for initial filtering of the building inventory. These criteria need to be at a very low level of sophistication in deference to the principle of proportionality. The required levy of calculation has to be proportional to the quality of input.

The readily accessible data for an existing building are the dimensions and arrangement of its structural elements and the floor area. The challenge is to determine whether these properties alone may be used to determine the seismic vulnerability of a building inventory at a given location.

In a paper related to damage caused by the Tokachi-Oki earthquake of 1968, Shiga, Shibata, and Takahashi presented a format (referred to as the SST Format in following text) for evaluating the seismic safety of low-rise monolithic construction in reinforced concrete. They defined the critical attribute for seismic vulnerability to be the weight of the structure divided by the sum of the cross-sectional areas of the walls and the columns.

The SST format is very attractive. The required data are easily acquired. The needed calculation is not time consuming. The result is crisp. But the application of the SST Format in general is questionable because it was derived explicitly in relation to a group of buildings with well-reinforced walls dominating lateral resistance.

Recalibrating or testing the SST format on the basis of theory or experiment is not productive because the procedure needs to be tested on the basis of responses that defy calculation and organized experiment. The procedure has to be tested against observed phenomena in a collection of buildings with dimensional and material properties based on random decisions in construction.

An opportunity for recalibrating the SST format was provided by the Erzincan earthquake of 1992. After the earthquake, the Ministry of Housing and Natural Disasters of the Turkish Republic sponsored the Middle East Technical University (METU), Ankara, to document the damage to 46 institutional building units in Erzincan. The METU team also developed floor plans of the buildings inspected. The body of information assembled by engineers from METU will be referred to at METU data.