September 11, 2013
The Right Tool for the Job
by Hinman Team
As an avid home improvement enthusiast, I quickly learned that availing yourself with the most appropriate tool greatly expedites any task and boy, do they have a tool for everything…
I think this also translates well into the field of engineering – specifically blast engineering – there are a number of ways to calculate a blast load and the effect of this on your building – but which is the most appropriate for you? Well, it depends….let me explain your options:
First, I want to be clear that regardless of the method used, all blast engineering computations entail large time dependent loads and the response is measured in terms of deformation, which is typically out of the comfort zone of most structural engineering firms. You want to work with an engineer who understands these concepts and is able to employ them competently.
Typically, there are three options for assessing the damage due to an explosion:
1) Perform single-degree-of –freedom (SDOF) analysis on an element by element basis
2) Perform high-fidelity finite element analysis of a portion of the building
3) Perform an explosive test
Let’s talk about these in more detail:
The first is the quickest and involves considering structural elements individually– i.e. column, beam, slab. Because blast is so localized and does not usually cause a global building response this is an effective approach which is a standard in the industry. This approach is inappropriate when the system being analyzed is more complex and the response of one element depends on the response of another, for example; a flexible curtain wall (cable-supported or similar); a complex framing system where beams are supported by other beams or transfer girders; truss systems; or it entails progressive collapse.
The second is finite element (FE) analysis. These analyses are a lot more time consuming and are only as accurate as the information that goes into them. They look great and seem more believable. An engineer experienced in FE will keep it as simple as possible and start with a simpler analysis to understand what is important to model and what can be simplified. All too often these days I see extremely complicated FE analyses that model everything, which is unnecessary and often results in errors. I always start out simple and progress to more complex method if the simple methods are too conservative or un-conservative or inappropriate.
The final approach is to employ explosive testing of the portion of the building under consideration. The prior two methods should be performed before you get to this point so that you know what to expect and are not surprised at the test and don’t waste the test on unexpected results. We have done this for complex curtain wall systems; cable catchment systems and for innovative blast-resistant products. A good test will have simple and FE predictions beforehand and then again after using information learned from the test. Once the behavior is well understood a calibrated SDOF approach may be used for future predictions – this is especially useful for vendors with blast products.
Now the time and cost between these different approaches is pretty significant. FE analysis is roughly 10 times the cost of SDOF. Physical testing can vary from $50-$500k depending on the type and size.
At Hinman we saw that there was a big gap between SDOF and FE methods and developed our own in-house software to bridge this called BAM – it is more accurate than SDOF methods but is fast running and similar to the cost of SDOF methods. It is customizable to meet client needs and feedback. We currently have over 100 modules that have been developed that range from load determination through to analysis of structural and facade elements.
So which is the best tool to use? BAM, or course J!
Actually, it depends on several things:
1) How quickly you want an answer
2) Your budget
3) The nature and complexity of the building or façade system
Performing finite element analysis with or without testing can be a very expensive and time consuming proposition. SDOF or BAM is very fast and economical. Generally we find our clients prefer the latter. Though we have the capability of doing sophisticated analyses and testing, we generally do not recommend it unless the structure is complex or a new material is used whose behavior is not well understood.
In short, more sophisticated is not necessarily better. Pretty moving images on a screen can be mesmerizing and watching structures get blown up can be entertaining, but that does not necessarily mean it is a good blast design.