Boundary Conditions for A Structure With Bolted Attachments
Boundary Conditions for A Structure With Bolted Attachments
(OP)
Let's say I am designing a structure that is bolted to something else. I am interested in finding the stresses in my structure, but I don't care about the stresses in the something else it is attached to. Let's say for example I am designing a cantilever beam that has a flange at the end where it is bolted to a wall that can be assumed to be infinitely rigid. What I have always done in this case is to fix all the degrees of freedom on the circular edge of the bolting holes. Always done it this way, but recently I have had two customers come back with comments on a structural analysis report I submitted to them stating that fixed DOF is incorrect and that the boundary conditions should be pinned instead. Are my customers correct? Does it make a big difference? and is there some reference out there that some one could point me to that discusses this? Thanks.





RE: Boundary Conditions for A Structure With Bolted Attachments
RE: Boundary Conditions for A Structure With Bolted Attachments
The main question is are you (or your clinet) interested and talking about local stresses (i.e. around your bolt holes) or a global behaviour / stresses. Let's assume a beam supported at both ends and subject to lateral loading. The end reactions (on a 'macro' scale) could either be forces only (i.e. global pinned connection) and forces and moments (built in / fixed connections). If you modelled the beam with line elements than you would restrain either translations only (for the pinned) or translations and rotations (for the fixed connection). However, if you model the beam using shell elements, then even if you pin (i.e. restrain tranlations only) all the nodes at the end section you are actually modelling a (global) fixed end connection.
RE: Boundary Conditions for A Structure With Bolted Attachments
I don't like the idea of fixing a nodal moment at a bolt hole. I don't think you really get that unless you weld, and I think it just results in ficticously high stresses that you have to explain away as a modeling artifact. I usually pin it for analysis. The ability to sustain a moment comes from the multiplicity of bolts. Per St Venant, it make no difference to the rest of the model once you get away from the BC. The Steel Construction Manual identifies two types of bolted joints: slip-critical and bearing-type. If the bolts are properly torqued, then the joint is slip critical, and as long as the joint is maintained (i.e., no slip), then the proper boundary would probably be to restrain (X,Y,Z only, no rotations) the contact area around the bolts. If the bolts aren't torqued, you have a bearing-type connection, and that is exactly equivalent to a pinned joint. Typically what I do when analyzing a structure is to use a code such as the Steel Construction Manual, to analyze the bolted connection. I pin the bolts, sum the reactions (forces and moments) around the centroid of the bolt group, and take those reaction forces to a spreadsheet and the SCM.
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: Boundary Conditions for A Structure With Bolted Attachments
pinning a bolt is better, but still a little "lazy" in that the support structure and the fastener itself will always have a finite stiffness (as opposed to the infinite stiffness of a constraint).
best is to support the fastener with strings. i usually use three rods to support the three directions. I'll use an area of 1, 0.1, 0.01 ... something to let the structure have a little "give". sure you can use fastener stiffness calc (Huth seems to be preferred these days) and use a CBUSH type of string element.
Quando Omni Flunkus Moritati
RE: Boundary Conditions for A Structure With Bolted Attachments
RE: Boundary Conditions for A Structure With Bolted Attachments
Quando Omni Flunkus Moritati
RE: Boundary Conditions for A Structure With Bolted Attachments
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: Boundary Conditions for A Structure With Bolted Attachments
Quando Omni Flunkus Moritati
RE: Boundary Conditions for A Structure With Bolted Attachments