modification of base plates
modification of base plates
(OP)
I researched the topic of base plates in this site and found some interesting discussions but not what I am lookiing for. I am reviewing a steel structure that has already been fabricated and erected. The structure supports a tank and at this time the tank is empty. The end user wants to use this tank, per its original design, but needs verification of the support steel design. Everything checks out with the exception to the base plates. The existing plate is 3/8" thick, with (4) 3/4"dia bolts, set at 1.5" from the corners. The column is a W4x13 w/ an axial load of 25kip. The structure sits on a concrete pad. I calculate the allowable bending stress (Fb) of the plate to be 35.91ksi the actual fb is 48.326ksi. Does this look right? and if so, how does one remedy the base plate being to thin? There is one catch (there always is) the tank is in an extremely rural location (read desert) and the only repair methods available is welding plates to the structure, no crane to alleviate loads, no large machinery. (I thought about welding triangular plates to the side of the colmn down to the base plate for a larger bearing area.)






RE: modification of base plates
Usually, hand calculations are based on a "rigid" plate assumption which results in a triangular distribution of the bearing stresses. That's not necessarily what really happens to the plate.
There are hand calc methods that attempt to distribute this load in other ways (yield line theory?). But, I'm not too familiar with them. Instead, I've always just used an FEM solution (i.e. RISABase) whenever the hand calc method gives me results that I don't trust. The FEM solution won't always give me results that I like better, but it will give me a good rational basis for accepting or rejecting the Rigid Plate assumption hand calcs.
I think welding stiffeners to the base plate should work fine to reinforce that plate. It's just a question of whether or not a retrofit is truly necessary.
Josh
Disclaimer: I'm an SE that works for RISA Technologies. Though, I'm trying not to overly advocate here.
Any FEM solution with a good plate element can be made to work for this situation. The only reason I mention RISABase is that it is the one I am familiar with...that and it is specifically designed for base plate analysis.
RE: modification of base plates
RE: modification of base plates
Josh: I currently learning how to use RISA. So, I'll try the plate thing later. Thanks for the advise.
RE: modification of base plates
DaveAtkins
RE: modification of base plates
what he has suggested is the quickest way forward on this one and is a very valid approach.
RE: modification of base plates
RE: modification of base plates
"creep & relaxation"? I not familiar with these terms used for steel design. Are you talking about settlement of the supporting soil?
RE: modification of base plates
RE: modification of base plates
RE: modification of base plates
BA
RE: modification of base plates
I have always stated with 1/2" base plate as the minimum. add another 1/8 - 1/4" for tears & wears, which are common place in the industrial settings. Also, future addition in column load without checking base plate is a frequent phenomenon.
RE: modification of base plates
Existing plates are sized correctly. No need for modifications assuming at least 3,000 psi concrete footing.
RE: modification of base plates
The authors are Richard M. Drake and Sharon J. Elkin. I just checked and the paper can be downloaded for $10 from the AISC website.
I don't think the procedure ever really caught on. It's not a bad concept, but I don't think any labratory testing was done to verify the behavior. Even so, you may investigate using it as an alternate procdure to justify an existing design.
Josh