Tech Notes
The current practice in blast resistant design of building structures for conventional blast loads is, in general, to design the building envelope (infill and curtain walls) to resist the design blast load and dissipate energy through inelastic response while keeping inhabitants safe from the blast overpressures and debris. These façade components are usually anchored to the building structure at the perimeter beams or floors and roof diaphragms. The dynamic reactions of the façade components are transferred to the building lateral load resisting frames by the floor diaphragms. Ultimately, the lateral load resisting frames must transfer the dynamic reactions from the curtain and infill walls through the connections to the foundation while satisfying performance requirements. For conventional design blast loads, standard practice usually is to assume that the dynamic reactions, transferred to the building structural frame by the façade components, will produce only a moderate lateral response of the building frame. This is due to the relatively large natural period of the structure (compared to the façade components) and inertial mass. However, for larger than conventional threats, specially reinforced façade systems are required which may result in transfer of large lateral forces into the building frame structure. This may result in large force and deformation demands for the lateral load resisting frames.
The study presented herein is part of the “Steel Frame Structure Performance in Blast Environments” research program being carried out by Protection Engineering Consultants for the U.S. Department of State (DOS). The objective of this study is to investigate the implications of the blast resistant design of façade systems on the lateral response of steel frame structures under a specified design blast load environment. One of the guiding principles of this study was to make use of conventional structural analysis tools and methodologies that line up with established consensus criteria so that they can be widely usable and sustainable. The lateral response of different types of midrise typical steel frame structures, including moment frames and braced frames, was investigated through nonlinear dynamic analysis using the SAP2000 structural analysis software. Plastic hinge definitions and analysis procedures followed FEMA and UFC guidelines for nonlinear dynamic analysis of structures. The performance of these building frames was evaluated and compared globally and locally through assessment of overall frame response, and strength and plastic deformation demands frame members and connections.
Read more in the full PEC TechNote by Aldo McKay, Marlon Bazan, Kirk Marchand and Matthew Gomez…
Laminated glass interlayers provide significant protection and energy dissipating capacity when windows are subjected to blast loads. Current protection criteria (DoD, GSA) prescribe protective glass assemblies and analytical approaches for validating protection using HazL (DoD) and WinGARD (GSA) glass response and hazard models. The laminated glass membrane models in these “standard” codes (HazL and WinGARD) used to predict glass laminate membrane response have been shown to yield both overly conservative as well as non-conservative results. Protection Engineering Consultants (PEC) has developed an improved and simple laminate membrane model that greatly increases the accuracy of hazard predictions. Static and dynamic laminated glass test data is available from tests in the last two to three years that will improve and validate the new variable modulus-based membrane model. The more accurate membrane model will provide cost savings through more efficient designs that meet protective glazing specifications and higher levels of confidence in protective system response.
PEC has applied its direct experience with laminate glass static and dynamic tests, and has leveraged our relationships with US and UK agencies and laboratories to gather and use static and dynamic test data to improve and validate the glass laminate membrane model. PEC engineers have collected and evaluated recent test data on laminated glass specimens. PEC’s variable modulus membrane model is being updated using this data; both for stiffness adjustments as a function of in-plane strain and for strain rate effects (dynamic increase factors) on membrane response. Following incorporation of all static and dynamic test data into the membrane model, a version of the improved model is being implemented into analysis tools such as Single Degree of Freedom Blast Effects Design Spreadsheet (SBEDS). The SBEDS platform is well-utilized by designers to perform SDOF dynamic analysis for other engineering materials (steel, reinforced concrete, masonry, etc…) and is well suited for incorporation of an additional material.
Read more in the full PEC TechNote by Kirk Marchand and Carrie Davis…