CONSTRAINTS FOR SKID SUPPORTS IN STAAD PRO

[brkmech1234]

We outsourced the structural design of skid . It has 6 supports with base plate which will be attached to foundation with anchor bolts. He has considered the joint as pinned at support. I do not understand why he has considered pinned. According to me , it should be fixed as rotational moment is not possible.

 

[ishvaaag]

Using pinned bases for steel columns in structural models is quite common, more likely because the difficulty to ascertain a proper degree of fixity at each column base and leading to more safe and simple design of the structure, to some cost.

I found the last acceptance in written of the pinned assumption whilst reading some modern british recommendations for 1 story industrial buildings, where the assumption was acceptable if baseplates had just up to 4 bolts.

A proper answer to your question requires the examination of the problem, with the proposed set of loadings and structural responses linking loadings with reactions.

 

[brkmech1234]

Thank you for your valuable information. I understood that pinned joint configuration will provide the safe design for structure. I would like to understand the effect on foundation loads accuracy. I believe that this will underestimate the foundation loads as there will not be any moments considered.

 

[ishvaaag]

What is warranted is that as long as one has not performed a detailed identification of the more proper solicitations, the simplified ones won't be the best technical estimate available when studying more deeply the problem.

Respect the risk of undersizing the foundation with the pinned scheme, at least in buildings it must not be a very significant problem, since you have a notional scheme able to sustain all the loads above and pass such loads to the foundation; your columns, baseplates, and foundations will be of necessity sized to sustain such loads at the limited deformations required at both service level and limit load level, and experiencie proves the scheme has been used successfully in many occasions without problems -a result on which the generous sizing of concrete footings and conservative estimate of the working bearing loads at the foundations may have been having a say.

You may be right in that a more deeper identification of the solicitations may be necessary, particularly in cases where middle to high cycle dynamical actions are present, and then improper considerations of some actions actually present, typically tensile action on the bolts from moments, may cause unforeseen failure. This has happened lots of times with road signal structuras and of course foundations of machines.

 

[delagina]

I have designed a lot of equipment foundation. I dont recall anytime i got foundation loads from vendor drawing where they assume the support as fixed.

It's common practice to assume it as pinned.

There is no such thing as "true pin". The assumption is always, is it near pin or near fix.