fix or pin with bolts? 1

Is there any clear cut way to determine if a bolt pattern would be sufficient to use the "fixed" beam calculation for deflection instead of welding? This is a steel beam inside an enclosed steel vessel running across the tower. It sits on a footrest and is then bolted to a plate on top of the footrest.

 

[ishvaaag]

You can definitely design some base able to take some N, Mx, My, Vx and Vy forces, other thing is to believe such base will be perfectly fixed, since some rotation respect the horizontal there will be.

On some simplifications and given the geometries, modulus of subgrade reaction, and some assumptions on how the bolts and base plate behave in the union moment/rotation stiffnesses can be derived, yet practical is not.

Somewhere I have seen assumptions of the stiffness at the base of steel columns, if I remember where will let you know.

 

[orreng]

Thanks for the reply, I would appreciate, if you remember what the referance is, your posting it. A Fixed not simply supported calculation, allows for a slimmer beam to be used.

 

[Qshake]

If the beam is only bolted on the bottom flange (where sitting on the "footrest" then it is pinned as you can clearly visualize the rotation that the rest (nearly all of it) of the beam will realize. Even if this beam were bolted via a web connection it will still be treated as a pin connection.

Only if the beam is sufficiently restrained against rotation will it suffice as a fixed connection. Thus it should be bolted/welded along the web and flanges. Preferably the beam would be seated between to "clips" (angle iron) which is welded to the vertical elements and to the beam.

Now, can a connection as described above still be considered to be pinned? YES! Even if the connection is stiff, the element to which the beam is connected to must also be sufficiently stiff enough to prevent out-of-plane movement that would prevent the development of moment at that location.

 

[ishvaaag]

A main conclusion I remember out of memory of one of the studies I read is that square footings from 1.2 to 1.4 m in side can deliver a condition of almost fixity if on normal soil, say, able to stand 2 kgf/cm^2 stress.

So for normal footings with more load and size the fixity itself will be more than anything related to the stiffness of the connection itself than depending upon the supporting soil. I wouldn't have then any problem to assume as fixed a weakly loaded column on a 1.5 m of side footing as long as the baseplate joint contraption is definitely able to deliver the factored forces to the footing.

Interesting related reading (but not centered on this) is

Design of steel structures 3d edition
Gaylord, Gaylord, Stallmeyer
Mc Graw Hill
4-11 Effective length of columns in frames.

I will try to locate the reference with the footing stiffness assumptions and if I find will post.

 

[Qshake]

ISHVAAAG,

If I'm not mistaken, the beam in question is inside a vessel which means the beam has no interaction with the soil and thus any dialogue on soil/structure interaction is extraneous. The footrest being a beamseat of some nature.

 

[ishvaaag]

The wording by orr.eng also makes me think of diverse interpretations. In any case hope what posted serves someone that sees it.

Were the case just a case of when it is feasible to consider fixity to something, well, analysis and proven capacity of the end connection to meet fixity forces clearly may determine. However if the footrest is the thing to ensure final fixity then we could stay in the realm of what above posted.

 

[orreng]

Thanks to both of you. I think you have both confirmed my thoughts, that a "fixed" classification is out of the question. It is nice to be able to bounce this off of others for more views of a topic!