Hi.
See attached picture. This is a combination between a box-beam and a girder-beam. Is it possible with Dietrichs static software to calculate the strength of this structure as a beam???
Calculation of structure in static-program.
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When analyzing frames, I normally create centerlines (1-03-3) in DiCam, and, then save those as a DXF, which I then import into Visual Analysis. Visual Analysis is one of the common engineering softwares in North America for analysis and design of timber structures.
This method would work great for exporting the geometry of your box beam, but, a DXF does not include information about each member (Size, grade, etc) so that needs to be re-entered in Visual Analysis.
Accurately taking into account the composite action from the built up plywood and the multi-layers of the top and bottom chords is not a trivial matter, though. This comes down to types of nails/screws used, how much they deform, the shear stiffness of the plywood, and a bunch of other things. I've not seen a software (of any shape or form) that could analyze this adequately without substantial effort. There are ways to do it "by hand", but, it will wear out the buttons of your calculator pretty quick.
Joe
Wish list item! Export centerlines and member data to a standard staad file format so any analysis software can use it... -
I have actually calculated the box-beam part of the beam, without killing my calculator (I programmed a Excel program to do it). These calculations are based on a box beam with top and bottom flange of wood, and side flanges of x layers of OSB, glued to the bottom and top flanges. It is basically the same calculation as for a I-beam. And yes - even though I am a clever engineer (
) it took me a while to read and understand the formulas behind and to put it into the Excel sheet. But it would be even worse with mechanical connections as nails/screws and the strength and stiffness of the beam would not be as good. I also know how to calculate the girder beam part, with a software (QSE), but I don't have that software at hand. For me the very interesting part would be to calculate the combinated beam to see how good the design actually is. I think it might be possible with Robobat software or similar FEM analysis software. -
If the OSB is glued on rather than nailed / screwed, it does make the beam a lot easier to analyze, just like an I-beam as you had done.
A quick look at your calculations (or maybe I am not following them) shows that the box beam is adequate for the load as is? (Not included the girder portion of the beam.) If this is the case, the girder / diagonal framing will only further to stiffen this and make the beam perform better. Or, conversely, you could argue that you could take the diagonal framing out all together, and use the standard box beam.
Analyzing the girder portion could also be done in Excel or calculator using a matrix structural analysis technique. Essentially, the same way a finite element analysis or structural analysis software (including Visual Analysis or Robobat) would do it, except, you have to do most of the work (rather than the software.) The nice thing is though, is, it is free (assuming your time isn't worth anything...)
Once you analyzed the girder, you could determine an equivalent moment of inertia (I) and combine this "I" with the "I" component from the OSB.
An interesting project to analyze, and, a chance for me to talk engineerese with someone, which, is even cooler yet!
Joe
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You're right - my calculations show that the strength of the beam is ok, but deformation is a little bit too much (normally it is ok to evaluate deformation for dead load alone and for variable load alone) - so the stiffness is just ok. But it's a long beam and wood will settle over time and I wanted to add some extra security for strength and stiffness. Of course it is some extra work to make the girder part, but not much. Sorry that the calculation is in Danish. I haven't yet had the time to translate this excel program.
The nice thing is though, is, it is free (assuming your time isn't worth anything...) Doing the calculation in Excel, I don't think I will do that - first of all because I don't have the time, secondly my time is absolutely not free of charge. A few weeks after I started in this job, my boss still wondered how I managed to convince him to give me that salary... :lol:
I didn't think about that, if I analyze the girder structure I could combine the moments of inertia with the OSB from the box beam part - pretty good idea.
But still - I would love to make that calculation with all parts together in a FEM analysis!!
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It's the problem with most fun projects; you don't have the time, or, more likely, the budget to engineer it all.
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