COMBINED FOOTINGS - two-way shear

[penpe]

Two way shear is in question - as two columns for a refinery train fuel-loading rack are only 4'-1" apart, and are supported by the same block of concrete. Our foundations software warns checking two-way shear for columns that share a shear perimeter is not a feature of the software, ("outside of the scope of this calculation"). I have searched my textbook and ACI-318, and have not yet found how to check shear capacity in this case. Any ideas?

 

[KootK]

I would:

1) Check each column individually using the typical procedures.

2) Check a failure perimeter encompassing both columns. You'll likely need to include a moment on the failure perimeter resulting from the aggregate axial column load acting over an eccentricity measure from the failure perimeter centroid.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

How thick is your foundation?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[penpe]

For constructability the decision was handed down that it will be a "block" for each pair of columns. The thickness will be 42", depth dictated by frost depth. The footprint will be 6' x 10'.

 

[KootK]

With that ratio of column spacing to thickness, I think that a combined punching check as proposed makes sense. Obviously, if the foundation is very thin relative to the column spacing, it becomes a different kind of problem.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WARose]

Are these RC columns? If so, you'd do it like ACI 352 does. (Kind of the same thing, except that it's inverted. There is a sample calculation for a slab-column transfer.)

I guess one thing to decide is will it be a combined shear perimeter or two perimeters. I guess that depends on the "d" of your "block" of concrete.

http://civilwares.free.fr/ACI/MCP04/3521r_89.pdf

 

[jayrod12]

I seriously can't see punching shear being an issue with a 42"thick footing with those plan dimensions. I would expect any level of load causing punching shear issues on something that thick to be exceeding the bearing capacity long ago unless it was bearing directly on rock, in which case punching shear and moment wouldn't matter.

 

[BAretired]

Two way shear is not the same as punching shear. With two way shear, the 6' width and 10' width are checked separately. If the column loads are approximately equal and with a 4' spacing of columns, the results would be the same for each direction because the cantilever is 3' from center of column to edge of footing.

BA

 

[BAretired]

If 42" depth is required by frost but not required structurally, it may be worthwhile to consider a thinner footing with a pedestal, or in this case, two pedestals. For a square footing with side dimension A, a pedestal A/3 by A/3 reduces the concrete considerably, not to mention the reinforcement.

BA

 

[jayrod12]

BA in the canadian code, two way shear and punching shear are synonymous. What you described is just two different instances of one way shear.

 

[BAretired]

Thanks jayrod12, you are correct. It appears I have been using the wrong definition for two way shear for many years.

BA