My application is actually off topic, but my question is general enough that I thought I would post it here. I was trying to model an aluminum truss structure using "L" 6061 extrusions, with the force from a 90 mph wind. There was a warning about "twist-bending coupling not being modeled results may not be accurate." I would say so! It was giving displacements in the region of 40Km! Does anyone know how to model a truss made from "L" extrusions? What I am using is "Mecway" freeware, a cousin of the Lisa software.

While immediately disclaiming any shred of expertise about FEA modelling of any kind, I suspect this is going to end up one of those "too broad" questions. For example, I know that modeling just the aero loads on a truss built of angles is going to be a huge task all on its own. All that vorticity, unstable flow shedding, and how the answers likely vary strongly with wind angle to the truss... And that's before you even get to structural analysis. Hopefully you'll get a useful answer, but I'm not sure your question is inside the scope of things that can be answered in a forum thread like this.

...I'm not being snarky, just curious Are you doing this for the educational experience of learning how to do FEA on L angle (a noble goal) or do you just want to build a 30' antenna mast that will work? QRG DE NM5XD

I'm looking at various options for building a 30 foot +/- mast. One option is riveted aluminum angle. And just thought the question was general enough that I put it here instead of the Hanger Flying section. After all, consider the texas parasol. 73 DE N8EE AR K

I didn't follow it through, but previously I consider the answer as I was looking for a design solution for a fuse frame built from angle similar to the Bede BD4. Then it struck me, those high voltage electricity towers that run across the countryside all made from angle. There must be millions of them, and plenty of engineers who design them, which led me to thinking that there must be software out there. Once again, I was right, simply through logical thought process.

If the extrusions are a known section, the catalogs should list the location of the shear center. Loads applied away from the shear center create moments in the angles. There is nothing in these trusses that can not be hand calculated but with buckling tools turned on, FEA should give decent results. Big problem is buckling under compression loads, as they become columns with offset loads. Now the 40 km deflection sounds more like you either collapsed the structure and it could not find a solution within the linear math limitation OR you have inadequate constraints on the model - the feet need to fixed or it just moves with the wind... Billski

I imagine it is a linear analysis. I'm not familiar with Mecway, but sometimes you can plot the displacement in terms of % (versus to scale) which can show you where the model is messed up if the warnings the programmer provided are a bit fuzzy.

Mecway is a parallel branch off from the Lisa FEA, created by one of the Lisa team programmers. It is essentially identical to Lisa, but has a number of "gee, thats nice" features that are not in Lisa. If you can use Lisa you can use Mecway.

I like grape fea....but it only gives stress, it will fail (large deflection ), but for buckling or crippling you have to know the allowable by Euler or perhaps methods found in Bruhn/Peery.