Footing w/ Large Moment

[SP777]

I've been asked by a client to design a footing foundation for an H-Frame (Substation Equipment (see link for a reference pic -

http://www.sabretubularstructures.com/Substation.aspx).

There are two columns and I've been given the loads at each base plate.

Axial and shear forces are not really a concern, it's the moment of 900 kN-m (factored to 1.5 SF - 600kN-m unfactored) on each base plate that is governing the design. Keeping with the pier-footing idea, I've come up with 1.5" dia. anchors embedded to 2'-6". I'm finding that to get a concrete breakout resistance with that embedment depth, I need a lot of edge distance and that is making the pier quite large. I'm getting a 10'x12' pier x 4'-6" deep (basically just a large concrete block). This would be on top of a footing 16'x14' x 1'6" thick. The concrete is all required to keep the bearing pressure in compression. This volume of concrete seems like a lot - wondering if a different foundation type is better or more suited for this

I'm still inexperienced with foundations in general (< 1 year out of school) and I've never worked with something like this so I have nothing to draw on. Couldn't find any typical foundations for this type of structure either. Just wondering if anyone has experience with something similar and could advise if this design is in the ballpark or not.

Appreciate any input, cheers

 

[hokie66]

In dealing with large base moments on columns, my first choice approach is to use bored piles with a pile cap. You will need information on uplift skin friction values. Another way is to use permanent ground anchors. Your approach of overcoming the moments by using mass is sometimes more palatable, if a bit low tech.

 

[Ron]

SP777..you have it right, but there are other ways to do this as hokie66 noted. A short drilled pier foundation will allow you to use soil resistance in addition to the mass of the "footing". I have used this approach in a lot of high base moment conditions. You can combine the two (again as described by hokie66) to achieve the required overturning resistance.

 

[azcats]

How about a drilled pier using lateral soil bearing only? Embed the anchors deep enough to engage the rebar required for the moment in the pier. Like an enlarged light pole base you'd see in a parking lot. I get something in the 4' x 12' deep range...if I did my conversions right.

 

[WARose]

Before I would make the pier that big; I would take those anchorage forces out with embedded plates, reinforcement, etc.. Anytime Appendix D starts giving you something outrageous: that what you need to do.

As far as the overall foundation goes: that kind of overturning moment screams for a pile group. Reinforced concrete caissons seem like the way to go. If that is a problem (I don’t know what your geotechnical report says) that you may want to try a combined footing (I don’t know how far apart those columns of that bent are, or what direction the moment is in).

 

[hokie66]

Good suggestion about the combined footing, WARose. Another way to do it, and probably very applicable in a bent like that.

 

[paddingtongreen]

Usually, if they design that dead end structure, they will give you a detail of the baseplate including bolt sizes. I would expect one drilled in pier for each column.

Another wrinkle is that these structures usually are set on leveling nuts and are not grouted so all of the loads are carried through the bolts.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[briancpotter]

It's also possible to overcome the moments with a wide and long footing, adding a larger resisting moment arm as well as additional weight. I'm not nuts about this solution, as you'll have net uplift on portions of the footing, but it's an available option, especially if you don't have the option of not using a spread footing.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[slickdeals]

Agree with WARose, you will be fighting a battle with Appendix D that you cannot win. See document in this link, that has some information on using reinforcement to take care of the forces, instead of relying on App. D

http://www.bechtel.com/assets/files/TechPapers/design-of-anchor-reinforcement.pdf

 

[civeng80]

Two footings connected with a strap beam may also do it.
But from the diagrams the structures seem pretty light on to me.
What is it thats causing this large moment in the footings?

 

[paddingtongreen]

civeng80, it is a dead end structure at the end of a transmission line. The cables are at high tension on the one side and light on the other as they drop down into the switchyard.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[BadgerPE]

I second the use of the paper that Slickdeals posted. I think that might be the most valuable tech report I have ever found since App D is such a pain!

 

[PUEngineer]

Helical anchors might be a good option if the majority of the force issues are tension related.

I also 'second' the paper that Slickdeals referenced. AND would add that ACI 355.3R-11 is helpful as well.

 

[civeng80]

Of course ! Thanks paddingtongreen.
Strap beam is not a good solution.
Probably go for combined bored pile and pad as suggested above.

 

[civeng80]

What about bracing the end gable ?
This would avoid moment on the pad footing.

 

[hokie66]

I think what civeng80 is suggesting is to turn the H-frame into a vertical truss, thus converting the horizontal force at the top into compressive and tensile forces on the legs. This is a much more common approach.

 

[paddingtongreen]

For my sins, I spent several years designing transmission lines and substations. For dead end structures like this, we always, with one exception for a swamp, used one large drilled in pier for each column, in the range that azcats suggested. We didn't tie them together. We did the same for angle poles (angles in the line that result in a sideways pull) which can develop similar forces.

Some outfits still build new truss-like frames but the ones we built for were using the newer tubular ones.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[civeng80]

hokie66

What I was suggesting was a simple tension brace from top of gable to an independant footing. Only applies if land is available.
But your solution is another option.

 

[Lion06]

If I have any significant force on embed plates with edge distance constraints, I go right to welded rebar on the embed plate instead of headed studs. Once you use welded rebar to the plate, you get away from Appendix D and can develop the high forces much more easily.