In this article, we take a look at blast analysis with a focus on SBEDS (Single Degree of Freedom Blast Effect Design Spreadsheet) vs ELS (Extreme Loading for Structures). Our goal is to explain why ELS should be considered over SBEDS for blast analysis.
Firstly, let’s take a brief look at each method of blast analysis.
What is SBEDS?
Without going into too much detail, here is a rough outline of SBEDS. SBEDS was originally developed by the US Army as a tool for the design and analysis of structural components subjected to dynamic loads using single-degree-of-freedom methodology using Excel.
The Excel sheet is set up so that the user only needs to enter the parameters related to material properties and geometry to get the final SDOF properties. Masonry, reinforced concrete, steel, cold-formed steel, and wood components are included in the SDOF results. One-way and two-way support conditions can also be selected. A flexure resistance function is used with compression membrane and/or tension membrane contributions where applicable and selected by the user.
Issues with using SBEDS
SBEDS along with SDOF Analysis method has a lot of problems that limit the user from working in an efficient way that produces a cost-effective design for members subjected to blast loads. The following are some of the problems that face the Engineers working with the tool.
- SBEDS depends on SDOF Analysis with over-estimate the design by 25~30%.
- SDOF analysis is not suitable for some structural components. For example, Roof trusses cannot be designed with SBEDS because of the complications associated with estimating the period of the roof trusses.
- P-Delta effect is not considered with all the structural components.
- Pressures are only generated for hemispherical surface burst.
- Most of the analyzed components are only working with uniform loads.
- SDOF analysis required experience, especially in generating the resistance function for built-up sections or, for sections that are not available in the database.
- SBEDS cannot design or analyze CFS panels or Wood Panels
- SBEDS V5.1 is not available for the public.
- The version of SBEDS that is available for the public is based on Excel© 2007 and can’t work on higher versions.
What is ELS?
Extreme Loading for Structures (ELS) is a structural-analysis software based on the applied element method (AEM). The software uses the applied element method to automatically track and propagate cracks, separation of elements, element collision, and the collapse of structures under extreme loads.
4 Reasons ELS is better than SBEDS for blast analysis.
- The results from SBEDS is always over-engineered by 20-30%. This means the material cost for the job could be between 20-30% more expensive than it should.ELS, on the other hand, is refined, detailed and accurate, potentially saving you thousands of dollars in materials.
- SBEDS only works on one component at a time. For example, SBEDS is capable of evaluating a stud within a wall, but cannot evaluate the wall as a structure which is made up of components like bridging, connections to the floor and ceiling, bracing, etc. SBEDS will evaluate a brick as if the entire wall is one giant brick, rather than taking into account other properties such as mortar and the staggering of bricks.By contrast, when ELS evaluates the effects of a blast on a wall, it takes into account every variable. This leads to an accurate depiction of exactly how the wall will react, where the debris will land, whether or not the wall will fail, and how all of this will affect the rest of the structure.
- SBEDS is restricted to calculating only certain range blasts. If the blast is closer than three feet from the component, SBEDS cannot perform an analysis. This restriction exists because the charts used in the development of the blast pressure used inside SBEDS are limited to this standoff distance. This is strange because SBEDS was developed as a military tool. It is unrealistic to think that every blast will be three feet or more from a structure.
- For the analysis output, ELS gives you an exact visual representation of what the result of a blast will be. This includes displacements, stresses, strains, bending, shear and normal force diagrams.SBEDS only produces numbers within excel which then need be interpreted by the engineer. The most obvious downside to this method is it increases the probability of inaccuracy due to human error.
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