Experimental Modal Analysis of a Prestressed Concrete Double-Tee Joist Roof Subject to Blast

Abstract

Prestressed concrete has received increased attention as a structural system for blast resistance and protection. However, prestressed concrete members not intentionally designed to resist blast loads, as well as connections to such members, may be especially susceptible to blast initiated damage. Full-scale testing of a prestressed double-tee joist roof in an industrial building was conducted both prior to and after internal detonation of a modestly sized explosive charge located approximately six feet below the base elevation of the double-tee joists. The testing included experimental modal analysis of the roof using a pair of long stroke electrodynamic shakers and a distributed network of 60 accelerometers. A large set of modal parameter estimates are extracted from the measurement data using a combined stochastic-deterministic subspace identification algorithm. Comparisons are made to a numerical model of the roof, developed using properties obtained from supplemental nondestructive evaluation and local historic design handbooks, to assess the plausibility of the modes. Differences in the natural frequencies and mode shapes are highlighted to qualitatively draw conclusions on plausible damage to the roof system alongside physical observations from the site.