Protective design against disproportionate collapse of RC and steel framed buildings

Abstract

To avoid disproportionate collapses in steel framed buildings the British standards provide the tying force method. This work investigates the reliability of this method for redistributing loads away from damaged columns and the results indicate that the factor of safety against collapse is less than 0.2 if low ductility nominally pinned connections are used. This is shown to be as a result of industry standard nominally pinned joints possessing the ductility required to redistribute column loads using catenary action. A simple method has been used to estimate this factor of safety; it is unlikely that a more complex analysis would change the overall conclusion. This work then move on to estimate the likelihood of column fracture due to blast loading from high explosives. The column response to this loading is then analysed using both single degree of freedom (SDOF) and multi-degree of freedom solutions to the equations of motion. Response charts were developed considering both systems and then compared. The deflection and bending moment obtained using SDOF system are comparable with those from MDOF models. However, the SDOF system is shown to significantly underestimate shear force when the ratio of the duration of blast load to the natural vibration period is less than 0.1 (Tc/T»<0A). It is concluded that the design shear force should be increased by 50% if SDOF system predictions of behaviour are used. A method for simplifying the blast loading on columns with clearing is developed. The method provides a simplification of the complex blast load profile in case of clearing whereby the same reflected pressure is adopted but the time duration load is adjusted to give an equivalent impulse. The simplification is assessed using a parametric study which showed that the results provide a conservatism of less than 10%. This technique for estimating column response to blast loading is compared against the real life behaviour as observed from the forensic investigation of the Murrah Building. The blast analysis predicts two columns to fail in shear and an existing method presented by McVay predicts a third column failure in brisance. This failure modes correspond to those found in the forensic investigation. The Murrah building was subsequently reanalysed by replacing the transfer girders with a conventional beam column arrangement and the blast analysis suggests that a substantial progressive failure would still have occurred because of widespread column shear and brisance failures. Thus is was concluded that even if the Murrah Building had been as per GSA guidelines for federal buildings which were revised following the Murrah event, it could not have survived. Subsequently, the methodology to estimate the safe stand off distance for a particular RC column and a charge weight is proposed and the design charts are prepared through nonlinear regression analysis. The safe scaled distance of 2.0 m/kg1/3 is estimated for the Murrah Building. The recommendation suggested by US DoD (4.46 m/kg1/3) is much higher than the actual value.

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