When looking at structural design or analysis for response to blast loading, there are two basic analytic approaches that are commonly implemented: Dynamic Single-Degree-of-Freedom Analysis and Dynamic Finite Element Analysis.
Dynamic Analysis
Dynamic analysis is used when dealing with blast loads. This type of analysis assumes that the load is applied instantaneously (or nearly so), generally with a finite duration (in the range of milliseconds). Strength Increase Factors (SIFs) and Dynamic Increase Factors (DIFs) are applied to the static material values to account for:
Dynamic analysis typically yields a displacement response that reaches a maximum value, then decreases with time (as opposed to a static analysis that reaches a steady state solution). The response may oscillate multiple times depending on the blast load and loaded component characteristics.
Dynamic Single-Degree-of-Freedom (SDOF) Analysis
SDOF analysis is a simplified analytic technique, which is the most common dynamic approach used when investigating blast resistance. In this approach, a structural component is represented as an SDOF model and is assigned equivalent properties of a spring-mass system (e.g., mass, stiffness, and resistance) to approximate component bending response. This simplified system is typically used to calculate the displacement of the center point of the element (e.g., beam, column, wall). The figure below shows graphically how the system is being simplified.
The results of the SDOF analysis are generally defined in terms of support rotation and ductility. The calculated maximum rotation and ductility are then compared to pre-determined response limits to translate the analytic results to damage levels (e.g., superficial, moderate, heavy, and hazardous) and then to overall Levels of Protection (e.g., Low, Medium, and High). There are numerous industry reference documents that provide this correlation of analytic results to damage levels. Some widely-used documents include:
SDOF analysis allows for rapid analysis of individual building components, which means that it is well-suited to blast vulnerability assessments of existing buildings and for use during initial and final design phases. It allows multiple variations of calculations to be run in order to converge on a cost-effective and constructible solution that meets the applicable protective design requirements.
Finite Element Analysis
FEA is another approach to blast resistant design. Dynamic FEA is a high fidelity technique in which the structural component or system is broken down into a large (but finite) number of discrete elements. The analysis entails simultaneous equations being solved at different time steps, thereby allowing various parameters (e.g., deformation, stress) along the component (as opposed to a single deflection value in an SDOF analysis) to be determined and visualized throughout the response cycle.
The figure below shows an example of FEA output.
FEA is generally not called for when analyzing simple building elements/systems for far-field blast loads (i.e., blasts that are far enough away to develop uniform loading over the face of the element/system being analyzed). FEA can be a useful tool when looking at close-in explosions and/or complex configurations that act as multi-degree-of-freedom systems. If FEA analysis is used, it is most appropriate once the design is well-developed, and not expected to vary significantly as the design process goes forward. This is because the FEA approach is often time-consuming (and, in following, costly). FEA assumes a far more detailed understanding of how a specific building is constructed than is commonly available for blast assessments or during the schematic and early design development phases of a project.
Blast loads are defined by their “time history,” which is the trace of over-pressure throughout the load duration. An idealized blast time history is shown in the figure below (after UFC 3-340-01):
The pressure (relative to ambient) is on the vertical axis and time is on the horizontal axis. The blast duration is generally taken to be the amount of time it takes for the blast load to initially decay to ambient pressure with respect to its time of onset (typically assumed to be the time of peak pressure). Another term that you will hear is ‘impulse’. The impulse can be thought of as the amount of energy delivered during the blast load, and is calculated as the area under the pressure vs. time curve. When performing a simplified dynamic analysis, there are two common simplifications made when using blast time histories.
When speaking about the magnitude of blast forces that a building is subjected to, that a design should resist, or that a product needs to protect against, both the peak pressure and duration (or impulse) must be used. This is because a load of 10 psi (for example) that lasts for 2 msecs can result in significantly less damage than the same 10 psi load that lasts for 20 msecs.
Time matters.
New York, New York (Oct. 8, 2015 ) – Stone Security Engineering, P.C., the woman-owned small business specializing in protecting people and property from accidental and manmade hazards, announced today that Khaled El-Domiaty, P.E. and Arturo Montalva, P.E. are now Principals and co-owners of the company.
“I am thrilled that Arturo and Khaled are now co-owners of the business. It is important to me that we all are deeply invested in the success of Stone Security Engineering because that will translate to the quality and professionalism we bring to our clients. We are looking forward to continuing our record of excellence and creativity in all that we do.” Hollice Stone, President and Founder
Mr. El-Domiaty received his Masters in Civil Engineering from the University of Missouri-Rolla in 2002. He joined Stone Security Engineering in September 2014 as an Associate Principal/Vice President and Director of DC office Operations. Mr. El-Domiaty brings a remarkable background in design, analysis, research, testing, and leading-edge solutions. As a key member in the safety and security design fields for more than 14 years, he has played an instrumental role in developing, managing, and delivering a wide array of projects. Mr. El-Domiaty has provided risk assessment services to property owners (government, commercial or industrial) to mitigate potential risks associated with blast, fragmentation, progressive collapse, ATFP, fire and toxic hazards. Additionally, he has performed forensic investigation for litigation support, training, R&D, guideline development, structural design, and remedial and retrofit design for a variety of structures. Mr. El-Domiaty is a licensed professional engineer in DC, VA, MD and NY.
Mr. Montalva studied Industrial/Mechanical Engineering at the Universidad Politécnica de Valencia in Spain. In 2010, he joined Stone Security Engineering as Project Manager. He has been featured in a number of publications including, Structures Magazine and Structural Engineers World Congress. Mr. Montalva is a specialist in the mitigation of seismic, blast and progressive collapse hazards, with 16 years professional experience in the engineering industry. He has in-depth expertise in linear and non-linear structural dynamics and finite element analysis. Past projects include US Federal Buildings and Courthouses, high threat environment facilities, historic building renovations, development of mandated federal design criteria and numerous US Department of Defense and General Service Administration facilities. Mr. Montalva has utilized his knowledge in numerical methods to develop in-house analytical tools to better support our clients’ needs and to help lead the firm into the future.
Stone Security Engineering remains a small woman-owned business, even with our expanded ownership structure.
Stone Security Engineering, P.C., is an internationally recognized specialty engineering consulting business with offices In New York City and Washington, DC with focus on blast resistance, security and safety engineering and design, predicting and mitigating hazards from explosions, fires and toxins; assessing security and blast vulnerability; research and development, testing and training. Our engineers have participated in multi-hazard vulnerability, threat, and risk assessments for more than 200 facilities around the world and abnormal loading design for more than 300 buildings and structures. The company’s web site (www.StoneSecurityEngineering.com) contains more information.
