StructuralEIT:
I think you're fine with neglecting the warping torsional stresses. The Saint Venant shear stresses will be more than adequate for sizing the weld, in addition to the bending shear stresses. If you have access to a simple FEA program, a quick shell model of the beam will confirm...
I'm a little confused as to why you're modifying to site class D before entering the analysis. Is this to account for the fact that you're only including the top 100 feet in your analysis, and there appear to be at least another hundred feet below that elevation to bedrock? If so, you might...
Are you modeling with pin joints, or fixed joints? If with pin joints, there should be no differences. With fixed joints, the member size will affect the amount of "secondary" stresses caused by bending of the truss member. This in turn has a small effect on the axial load.
Castigliano
Bridgebuster's post is a good starting point for pony truss info. If you want to learn more, look in the "Guide to stability Design Criteria for Metal Structures," the latest by Galambos (4th edition, I think). This has quite a bit of information and explanation on pony truss behavior and...
Generally, if there is nothing framing into the panel point, there is no bracing. The exception being Pony trusses, which use a frame consisting of the floorbeam (in a bridge truss) and the truss verticals to brace the top flange.
Castigliano
In my experience, AASHTO is strictly a US code, the Canadians have their own specification, and it is a very good one! Each state is master of it's own domain, so they typically will alter the AASTHO spec to suit their needs, but generally follow its guidance. You should be aware that the...
By definition, isn't the moment at the plastic hinge equal to the plastic moment capacity of the pile? Obviously, you need to adjust for the axial load in calculating Mp. Are you really asking where the plastic hinge occurs? Or are you looking for the max moment, and not necessarily expecting...
If you are only loading a truss at the joints, your assumption of connection fixity is relatively unimportant. The deflections of the truss will tend to induce frame-action moments in the truss members, but because the members are so stiff axially, the moments tend to be small enough to...
There is a gentleman (a non-engineer, I believe), who has been building suspension bridges in disadvantaged regions across the world, primarily with surplus industrial and petroleum supplies. For the life of me I can remember his name, but there have been at least two articles on him in...
Slightly off-topic, but I just received an e'mail solicitation (by way of eng-tips.com) for Norwich University which uses the I-35W bridge collapse as the basis for it's pitch of a Masters of Engineering. Does anyone else find this to be in bad taste? I mean, to say that if people take their...
I've used LUSAS quite a bit, and what has helped me out greatly is following the sample problems contained in the help function, and if I get really stuck, contacting the support desk at LUSAS. After all, you do pay for the support in the yearly licensing fees!
Castigliano
I'd be a little hesitant to take any action till I understood why the program was giving that message. There are hand methods you could use to verify that there is an instability; I seem to recall that some of the structural engineering handbooks address this issue. In my experience, when the...
Several reasons could be causing the program to behave as you've described. As mentioned above, unrestrained rigid body modes are most often the cause. For the models that give you some number of modes before giving the error message, you may have asked for more modes than there are DOF's...
