Pinned Base vs Fixed Base - Steel Frame
Pinned Base vs Fixed Base - Steel Frame
(OP)
I am designing multiple steel framed structures to support equipment and piping. I am analyzing the structures in STAAD. There is debate on how to model the steel column bases: Fixed or Pinned. A truly pinned base usually results in excessive and unrealistic deflections. I have found a good compromise is to use fixed bases in the model, with partial moment releases at the bases for Mx, My and Mz.
A 1.0 factor equals no moment restraint. I've been typically using a factor of 0.9 or 0.95, meaning that the base is almost a pin. This seems to give reasonable results. The frames still require some bracing or moment connections to control deflections. Any comments or suggestions out there?
Thanks,
Jacst3
A 1.0 factor equals no moment restraint. I've been typically using a factor of 0.9 or 0.95, meaning that the base is almost a pin. This seems to give reasonable results. The frames still require some bracing or moment connections to control deflections. Any comments or suggestions out there?
Thanks,
Jacst3






RE: Pinned Base vs Fixed Base - Steel Frame
The rule of thumb in the previous company I worked, if bolt is outside it's assumed fixed. And I didnt work for some small company, it's one of the biggest EPC in the world.
I'm pretty sure some will come here and talk about base plate bending, foundation rotation, A bolt elongation, etc..
RE: Pinned Base vs Fixed Base - Steel Frame
Thanks for your input. When you say "if the bolt is outside.." do you mean if the anchor bolt pattern at the column base is outside the footprint of the column shape then it is considered fixed?
Jacst3
RE: Pinned Base vs Fixed Base - Steel Frame
if it was a wrong assumption then 30+ str. engineers and some more overseas just made a wrong assumption on a multi-billion dollar project.
RE: Pinned Base vs Fixed Base - Steel Frame
RE: Pinned Base vs Fixed Base - Steel Frame
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Pinned Base vs Fixed Base - Steel Frame
Base plates are inherently flexible due to the oversized holes and the fact that the anchor bolts cannot be pretensioned. I would never use 100% stiffness for these in a portal frame.
In a direct cantilever then you have no choice and have to treat it as fixed, though you need to realise that there will be more deflection than calculated.
RE: Pinned Base vs Fixed Base - Steel Frame
staad has KMX KMZ KMY with units of
kip-ft/deg
so what's a good value here to assume for partial fixity
RE: Pinned Base vs Fixed Base - Steel Frame
This has been discussed before, I will dig around and post the thread numbers when I find them.
RE: Pinned Base vs Fixed Base - Steel Frame
http://www
RE: Pinned Base vs Fixed Base - Steel Frame
I completely disagree with your logic here, if the plate is not fully fixed at serviceability state then it is definately not fully fixed at ultimate.
The items that compromise fixity such as footing rotation, anchor bolt elongation and plate bending with all be more pronounced at ultimate state and therefore your factors should be the other way around if anything.
RE: Pinned Base vs Fixed Base - Steel Frame
Likewise if you are designing the baseplate at ultimate limit state I would want the baseplate and anchors to transfer the moment that arises from full fixity. Not a reduced moment because some of the base moment has been redistributed elsewhere. I would multiply the reactions at the baseplate by 1.05 for baseplate design.
RE: Pinned Base vs Fixed Base - Steel Frame
Can somebody please explain why BS5950 (UK steel code)
appears to only allow partial fixity in column bases upto 20 percent?
If i understand correctly, a partially fixed base does not induce a moment as a percentage of the fixed base moment; but rather is determined by the column stiffness itself.
Therefore can there be a situation where say taking a 70 percent partial base fixity would induce a larger moment than a fully fixed base?
But I also was of the understanding from moment distribution analysis that a fixed base had 'infinite' stiffness, in that case a partially fixed base moment could not be greater than a fully fixed base moment.
If anyone can offer any guidance into these matters it would be much appreciated.