March 2014 10 design issues for structural engineers STRUCTURAL DESIGN By Eric L. Sammarco, P.E., M. ASCE, Clif A. Jones, P.E., M. ASCE, Eric B. Williamson, Ph.D., P.E., M. ASCE and Harold O. Sprague, P.E., F. ASCE Acknowledging Differences and Leveraging Synergies Design for Blast and Seismic O ver the past few decades, signii- cant advances have been made in the areas of earthquake engineer- ing and seismic design. A growing database of strong motion records, reined ground motion attenuation relationships, and proba- bilistic seismic hazard methodologies have led to an improved design basis for seismic events. In addition, multi-scale component-level and system-level research have given rise to innovative energy dissipation and kinematic isolation con- cepts, enhanced structural detailing provisions, and performance-based design methodologies. Perhaps most importantly, many of these techno- logical advances are currently being implemented in practice and taught in colleges and universities. he protection of buildings against airblast due to explosions has been a national interest for many years. For at least half a century, the U.S. Government has invested in physical testing, research, and develop- ment eforts focused on wartime defense scenarios involving both nuclear weapons and high-explosive detonations. Moreover, the heavy industrial sector has long been concerned with damage and injury mitigation from accidental explosions occurring in petrochemical facilities. With the rise in international and domestic terrorism, the vulnerability and state-of-security of the nation’s buildings and infrastructure have become national concerns. As a result, interest in blast efects and protective design has increased among the general structural engineering community. Recent public domain research has led to a number of signiicant technological advances related to blast threat miti- gation and anti-terrorist/force protection (ATFP) design. Blast-resistant design guidance is avail- able in specialized building design standards, handbooks, and guidance documents such as ASCE 59-11 Blast Protection of Buildings, UFC 4-010-01 DoD Minimum Anti-Terrorist Standards for Buildings, the compilation text entitled Handbook for Blast-Resistant Design of Buildings, and FEMA 427 Primer for Design of Commercial Buildings to Mitigate Terrorist Attacks. However, unlike earthquake engineering, the integration of fundamental blast-resistant analysis/design principles with the general structural engineer- ing community and major college curricula has been slow at best. Consequently, understanding that blast and seismic are both dynamic phenom- ena, many structural engineers are left drawing from their seismic knowledge when faced with a blast-resistant analysis and/or design scenario. Extrapolating in this manner is ill-advised because an adequate seismic design does not necessar- ily imply adequacy from a blast-resistant design perspective. Recognizing the risk presented by unqualiied engineers performing blast-resistant design, the United Kingdom has taken a proactive approach by initiating the Register of Security Engineers and Specialists (RSES) through the Institution of Civil Engineers. he Register aims to ensure that registrants have achieved a recog- nized competence standard, accepted a code of ethics, and are committed to continuing profes- sional development. While no such specialized register or certiication currently exists in the U.S., it is important for structural engineers to be mindful of the fact that design adequacy for one load case does not guarantee design adequacy for the other. As the title of this article suggests, there are important diferences between seismic-resis- tant design and blast-resistant design, despite the dynamic nature of both. By acknowledg- ing the diferences and leveraging the synergies between the two design methodologies, structural engineers can improve the overall eiciency, efec- tiveness, and robustness of their building designs. his is not a new topic; however, past treatments have typically been cursory and fragmented. his article aims to provide a relatively comprehen- sive overview of the blast versus seismic topic by addressing demand, system response, component response, and design synergies in a practical way that will hopefully beneit the structural engineer- ing community. Differences from a Demand Perspective Aside from the dynamic nature of both types of loads, earthquake ground motion charac- teristics are markedly diferent from those of a blast-induced overpressure history. Figure 1 ( page 12 ) shows a comparison between a normal- ized ground acceleration record from the 1989 Loma Prieta earthquake and a normalized free- ield overpressure history from a high-explosive (HE) detonation. he duration of an unconined blast pulse from a high-explosive detonation is generally on the order of microseconds to mil- liseconds, whereas the strong motion duration of a typical earthquake record is generally on the order of several seconds and can last over a minute. In Figure 1, note the cyclic nature of the ground motion acceleration record, which includes mul- tiple peaks. Conversely, for the blast overpressure history, note the nearly instantaneous rise to peak overpressure followed by a rapid decay to a sub- atmospheric “negative pressure” condition. Blast overpressure is often reported as a gauge pressure relative to ambient atmospheric conditions. hus, a negative gauge pressure condition represents sub-atmospheric pressure, resulting in a tempo- rary suction efect. Earthquake demand input is kinematic in nature, where near-ield seismic waves excite continued on page 12