Isolated Footing at Crane Column

[enigma2]

I'm working on an industrial building that has a series of cranes. In one of the crane bays, I have a tube column supporting the runway beam for the top running cranes. At a distance of 16" center to center of this tube column, I have a W14 building column supporting the roof. The columns are tied together at three points with horizontal members.
The building column is part of a moment frame that for lack of better terminology stradles the crane bay. Both columns have fixed bases. I am struggling to know how to design the isolated footing that will support both columns. The worst case load condition gives me an axial load and an applied moment at the building column. I also have an axial load and moment in the tube column due to the crane run. This is tricky, because this is an intermittent load. It only hits its peak load if the crane is fully loaded and the crane is positioned just right. I'm sure I can justify a reduction of this load.

At any rate--I have seen textbook examples of isolated footings designed with axial load and applied moment and I have seen examples of combined footings designed with axial loads at each column. However, I am struggling to wrap my brain around a combined footing where each column has an axial load and a moment. My office doesn't have any good analysis software that can address this, so I need to do it by hand.

Can anyone give me some insight into how to approach this? Or can you reference an example that might help? I'm still a young engineer and this is my first time designing a structure for cranes and foundations for cranes.

One thought I do have is that since the two columns are tied together, I could look at them as one section, find its centroid and manipulate the loads so that the axial load is applied at the centroid and likewise for the moment. Then, I could design it like the textbook example as a column on an isolated footing with one axial load and one applied moment. Is this a reasonable approach?

Thank you.

 

[Lion06]

If you are assuming the foundation is rigid (as you would if doing this by hand, I'm sure), then the location of the applied moment doesn't matter. It doesn't matter if it's in the middle or at the end, or both. Add the base moments, then determine the additional moments that the footing will see based on the location of the axial loads with respect to the centroid of the footing. Now, you will have a Total P (at the footing centroid) and a total M. Use that to check the bearing pressure. Use that bearing pressure (which will be trapezoidal at best, traingular at worst - though, if you are designing it new, keep it trapezoidal) to analyze a beam with two fixed supports (where the columns are located) to get the shears and moments in the footing, then just design. Surely you have software that can analyze a beam with one span and two cants, rights?

 

[Lion06]

I should clarify. In step 1, when checking the bearing pressures, you will probably want to use the service loads. When designing the concrete, do it again with factored loads to get the factored soil pressure acting on your "beam".

 

[csd72]

enigma,

16" spacing is not much, I would find the effective point of loading of each column load (calculating the eccentricity as M/P) then I would take an equivalent load as the effective centroid of these (i.e. (P1e1 + p2e2)/(P1 +P2) = effective eccentricity

This method is fine when your footing is effectively rigid compared to the loads.

By the way, if your columns were much further apart (say 16 feet or more) then you may have to look at them as a beam on an elastic foundation. But I dont think this is necessary in your case.

 

[hokie66]

Firstly, it is very unusual to try to fix the base of a crane column. The crane loads parallel to the rail should be resisted by bracing, and the crane loads normal to the rail taken back to the building columns. Your base connection may look fixed, but it should be assumed as pinned in designing all other elements, including the footings.

For designing the footing, then you just have a moment from the building column and two point loads. Usually the wind loading will control the footing size for a fixed base, neglecting the crane, as during a design wind event, the crane would not be operating. You will have to check other load cases such as crane and snow loading.

 

[par060]

I agree with csd72 ...you are probably bearing your coluns on a pier so by the time the reactions get to the footing the forces will be such that useing an equivilant load is reasonable. AISC has a design guide for industrial buildings...and tech 13 is a well used standard to get load combinations from.