Paper for the Fourth International Conference on Forensic Engineering 2008 From Failure to Understanding

Fire in buildings does occur and doubt often arises as to the severity of fire damage to these buildings. It is paramount to ascertain their structural condition using effective test methods as to safeguard the safety of inhabitants in these buildings. Over-prudence as to cordon off these buildings from returning residents or declaring them as unsafe for inhabitation after fire will put unfortunate residents to further disturbance and aggravate their grievances. Negligence will posed further danger and hazard to the residents.

The task of structural assessment/appraisal can either be undertaken by using mostly visual inspection together with perhaps Schmidt rebound hammer or cored concrete samples strength tests. Visual inspection is highly subjective on the part of the forensic investigators whilst a laboratory/field test at the material level is helpful but they are no substitute for in-situ tests. Structural integrity does not just dependent on sound structural materials but also on how these materials was put together as components or structures.

This paper reports an approach using in-situ modal forced vibration tests on building components especially identical components (i.e. a number of identical Reinforced Concrete RC beams in this case) and by comparing their experimentally and in-situ determined modal characteristics (i.e. resonant frequencies), one can ascertain whether these building components have suffered from fire-damage or not.

Comparing the modal characteristics of seemingly identical building components (identical in terms of dimensional, materials and reinforcing bars arrangement and other structural details etc.) in RC buildings is shown both an effective and efficient method. Most buildings are designed with as much identical structural characteristics as possible to ease the construction of the various building components at the same floor level as well as between floors within the whole building. This design strategy unintentionally aids their forensic investigation as suspected structural degradation such as exposure to fires can be determined using the advocated methodology.

Determining modal characteristics from supposedly identical building components exposed to fire (BCEF) and building components not exposed to fires (BCNEF) provides an effective strategy as the modal characteristics of both BCNEF and BCEF provide useful baselines of these components before/after exposures to fires respectively. The larger number in the data bases they are, the more effective and confident will be as to the validity of these data bases providing base-lines for ascertaining the plausibility of damages due to fire exposure. Forced vibration tests to determine resonant frequencies of building components such as RC beams is both easy and speedy with the help of modern instrument.

This paper reports an experience from the application of this test strategy on a real life reinforced concrete RC building having exposed to fire. Such experience shows that with mobile test equipment, tests to determine these resonant frequencies took only minutes and as far as resonant frequencies are concerned, it doesn't matter very much as to the test locations of the building components as resonant frequencies are considered ‘global characteristics’ which should be the same irrespective of locality of measurement on the building components. If data are supplemented by other tests at the material levels such as ultrasonic pulse velocity UPV tests then higher levels of confidence in the forensic investigation can be achieved.

• Introduction

• Methodology

• The Tested Structure

• The Forensic Test Programme

• Test Results

• Discussions

• Conclusions

• References