Shallow Foundation for a Concrete Column 10

[Bill002]

Hello,
I am designing a shallow concrete foundation for a 2ft x 2ft column. I have a concern because my moment (450 ft*k) is considerably greater than the axial load (70 ft*k). This is creating a large eccentricity at the base (e=450/70 = 6.4ft). To make this work on a shallow foundation I would need the width of the foundation to be e<b/6 so b = 38.5ft. This seems way to excessive for a 2ftx2ft column. Can somebody please tell me if there are any other methods of looking at this foundation?
Thanks!

 

[JoshPlumSE]

Include the weight of the footing and the weight of the soil or paved surface above it. It may make more sense just to dig a little deeper so that you have more stabilizing dead load.

 

[Teguci]

This is a fairly common problem which has several moderately expensive solutions:
- For a single column, use a drilled pier foundation
- For a column located near other columns, use a "pump handle" to an adjacent column. Basically, the pump handle is a big grade beam beam that transfers resisting moment to the troublesome footing.
- Provide a really big footing that everyone will question your sanity about.

 

[hokie66]

Maybe your model is incorrect. How did you get so much moment? Are you assuming a fixed end rather than pinned?

 

[Bill002]

HOOKIE66 - Trust me, I have verified the loads numerous times because it seemed too high for my understanding too. This is for a suspension bridge tower in a rural area with a wood deck. The load in the column (live+dead) is considerably low compared to the lateral force which is causing a great moment at the base.

TEGUCI, Unfortunately I don't have an option to drill or install piles. I have to resort to a shallow foundation.

JOSHPLUM, I am already 1.5 m under ground and cannot go any lower. Is there any other solution other than having a huge foundation with an extraordinary amount of concrete?

 

[msquared48]

Teguci also suggested the use of a grade beam to another area of dead load to help resist the moment. It sounds like the load is parallel to the tower foundation. Is that correct? If so, either put the grade beam between the legs of the tower, or put the whole tower assembly on a large matt footing.

If this is not possible, then it sounds like it's the bite the big footing option today.

Mike McCann
MMC Engineering

 

[hokie66]

Why so much lateral load on the tower? Don't you have a deadman to take the tension load from the cables? An elevation view of the bridge would help.

 

[ron9876]

Or put a beam between the bottoms of the tower columns.

 

[msquared48]

And is this a single or two leg tower suspension bridge?

Mike McCann
MMC Engineering

 

[Bill002]

Yes, its a two column tower. The suspension cable slides along the top of the tower so I am assuming 25% of axial load as a friction load acting parallel/longitudinally to towers. (See attached sketch).

So, putting a grade beam between the two columns will me strength laterally but not longitudinally (long with width). I am expecting this to be a combined foundation anyways.

 

[msquared48]

Hokie's suggestion of taking this longitudinal force back to deadmen at the abutment is the way to go here. It's what is commonly done.

Mike McCann
MMC Engineering

 

[Bill002]

Large dynamic movements in these type of bridges is common so unless we can provide a frictionless top on the tower there will be friction forces present.

 

[Once20036]

You said that you're 1.5m below ground already and cannot go any deeper. If you cannot go deeper because you're on rock, you could look into rock bolts to establish some tensile/uplift capacity.

 

[dhengr]

I may be all wet here, not having designed any suspension bridges in my career; but I’ll bet they didn’t apply 25% of the vertical cable forces on the towers, as lateral loads along the bridge/cable length, to the tops of the towers, on the Golden Gate or Verrazano suspension bridges. You might want to study up a little bit on the thinking behind the design and building of suspension bridges, even if the towers are only 23' high. There has to be back spans and cable anchorages, deadman or otherwise, or the system just doesn’t work so pretty good. I suspect the cables are in saddles at the tower tops which allow some cable movement, at fairly low friction level and large cable bend radius (curvature). But, no doubt there will be some tower bending and P/delta and wind, etc. in that direction. If you have a foundation problem you can’t keep rejecting every practical solution that people have offered, but then ask isn’t there another way, with out telling us why you have those constraints and moment conditions you do. But her is an alternative. Why not cast your 16'x8' ftg. or whatever and precast your towers with rocker shoes or pinned fittings on the bottom, mating to some ftg. pedestals with the other half of the rocker system? Then brace the towers until the bridge is built and let the tops of the towers move. Again, don’t forget P/delta and cable force changes to make this work. You gotta think this through, I haven’t done that for you.

 

[spats]

Am I the only engineer that designs shallow footing without requiring that the resultant be within the middle one-third? You can still get overturning safety factors of 2 or more and not exceed the bearing allowable even if one side of the footing loses contact with the ground.

See attached page from "Design Of Concrete Structures" by Winter, Urquhart, O'Rourke & Nilson. a = (Mr-Mo)/Rv. I normally use this primarily for retaining walls... the pressure results are in kips/ft. or lbs./ft., but you simply divide by footing width "b" (into the paper) and you get psf results for an isolated footing.

I think most would agree that being within the middle third is desireable, but sometimes it's impractical.

 

[JedClampett]

kidrah, are you using factored loads from your concrete analysis or have you removed the 1.7 and 1.4 factors? I can't tell you how many times I've gotten in a jackpot in foundation (and deflection) design and it turned out things were not so grim when the load factors were taken out.

 

[fancypants]

Spats,

I was thinking the same thing. I would not design this by trying to stay inside the kern.

I was going to run a couple of number for this. However, the loads stated in the post do not match what is in the later provided sketch.

 

[Bill002]

All, thanks a lot for your input. As an EIT, I am pretty new to this stuff and all your help is greatly appreciated.

Based on the comments I have changed my loads to unfactored and placed my foundation 5ft under ground to further stabilize the foundation. So my eccentricity has gone down to e=(308k*ft)/(150k) = 2.1ft. This would still keep me out of the middle third if I go with a width of 9ft (2.1 > B/6, 1.5).

I am also considering offsetting the column on my foundation by the required difference so the resultant can be positioned in the middle third (2.1-1.5 = 0.6m).

But Spats has made a legitimate point and I need to check for overturning and actually I might not even need to offset the column. Most of the resources that I have obtained does not even mention check overturning on shallow foundations. Ultimately I might even be able to decrease the width if overturning is adequately satisfied in the medium density soil.

Spats, in your formula shouldn't it just be (M resisting / M overturning) ?

 

[spats]

In order to calculate the position of the resultant force on the footing, you take the resting moment minus the overturning moment about the toe, divide by the vertical load. This is the "a" dimension in the diagrams. The formula is correct.

 

[chicopee]

If it is a suspension bridge tower, you are sure not designing the system as other have done with suspension bridges. hokie66 has the key to your problem.