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Concentrated load on retaining wall

Concentrated load on retaining wall

Concentrated load on retaining wall

(OP)
I have a column that comes down on a 8’-0” high retaining wall. The column is pretty large and but does not support a large load (only about 10kips). The building is being designed using IBC 2009 ASCE7-05 occupancy Cat III exp C. Using these figures I am getting a horizontal load at the base of the wall due to wind of 7 kips. The wall thickness is currently 12”. When sizing the stem of the wall and figuring the stability/bearing pressure what would others assume for a spread of this load through the wall?

Some people who I have talked to have said to use 4x the thickness of the wall (4’-0” in this instance) others have said to use a 45 degree spread (16’ for bearing pressure in this instance).

RE: Concentrated load on retaining wall

I've typically used the 45 degree method. How much length do you need to meet the allowable bearing pressure?

RE: Concentrated load on retaining wall

I use the 45 degree method unless there is a pilaster involved.

Mike McCann
MMC Engineering

RE: Concentrated load on retaining wall

(OP)
My current problem with with stability. MY FOS against overturning assuming a 26.5 degree spread (basically spreading the point load over the height of the wall) is 1.12. Assuming a 45 degree spread gives me a FOS of 1.71 and a qmax of 2.13ksf but the resultant is outside the middle 1/3 when considering wind loads (something I don't quite like but it is acceptable).

RE: Concentrated load on retaining wall

Every one around here uses the 45 degrees.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

I don't understand your stability vs OT calculation. Don't you multiply conc load say 2.4K occurring, say, 4.5 feet from left end of wall, tota wall length = say 10'

Mr = 2.4 x 4.5 ft-k against right to left (CCW) lateral

Mr= 2.4 x 5.5 ft-k against left to right (CW) lateral

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

(OP)
I don't get your calculation. My load is applied perpendicular to the wall. Therefore I have lateral soil pressure that is pushing against the wall together with wind load pushing against the wall in the same direction as the wind load. I have footing for the wall where the toe is smaller than the heal. I am checking the wall/footing for sliding, overturning and bearing pressure.

RE: Concentrated load on retaining wall

OK I have trouble with written descriptions. frequently . I get it now.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

(OP)
Really because my explanation was horrible.

RE: Concentrated load on retaining wall

Can you provide a pilaster in the wall and a rectangular footing with length perpendicular to the wall?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

I mean 7 k which may punch out the top of the wall, and that is a very gray calc.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

SteelPE:

As long as you don't overstress the soil, outside the kern is OK.

Mike McCann
MMC Engineering

RE: Concentrated load on retaining wall

Outside the kern may means FoS against OT may not be allowable - check your FoS. is it 1.5, 2 or whatever confusing number because these of new IBC codes?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

(OP)
I can put a footing where ever I want as we are in the design phase..... however, there is already a GC involved and they have seen preliminary plans w/o a footing..... but I did warn them that the plans may change slightly.

RE: Concentrated load on retaining wall

If this retaining wall is not supported at the top, I am having a hard time visualizing the mechanics of how the 7K horiz load applied at the top edge is resisted by the whatever beam action along the top edge of the wall. How do you calc that?

On the other hand, a 12" conc wall is very strong, and 7 k is not hugely large, so I have no feel for what load would actually be unsafe.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

Well, who is the Struct EOR, you or the GC? Be assertive

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

How were there ever plans shown without a footing? were they hoping on supporting the wall on hopes and dreams?

And wouldn't the point load improve your overturning? If you neglect it completely for the overturning calc and you get an acceptable FOS I wouldn't be worried about it. Or did you mean that the lateral load at the base of your column (top of your wall) is 7 kips due to wind? I might be slightly confused.

RE: Concentrated load on retaining wall

What kind of concentric dead load plf along the top of the wall, if any?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

If they object violently to the pilaster/rect ftg, or find some nugget of information to cow you into relenting, then tell them you will go back and pencil-whip your calculation, then come back 2 hrs later with a some concentrated vert rebar in the wall within 2' either side of the col, and bump out the regular wall footing to be wider for about 6'.

I need to take my wife to a Dr. appt now, so I will BBIAW about 3 hours, good luck.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

(OP)
Wow, lots of questions

There is a continuous footing under the wall that was initially sized to resist the soil loads. The vertical loads from the column are minimal(10kips). You would think that this load would help with stability and it does to a point (because it is eccentric from the center of the footing), however the column is tall and has a lot of wind load on it so I need to check the wall including the wind loads.

The wall is a cantilever wall as the building is placed into the side of a hill. There is no floor behind the wall to help resist the wind load. I am trying to figure out how far I can spread the horizontal load in the stem of the wall (the initial question). Think of a standard retaining wall with a horizontal load applied perpendicular to the wall at the top of the wall.

RE: Concentrated load on retaining wall

4 times the wall thickness in that case would make some sense to be to spread the 7 kips out over. You could beef up the top reinforcing of the wall in that area to ensure it spreads out. you could increase the footing depth to add some additional resistance to overturning.

RE: Concentrated load on retaining wall

SteelPE:

Even considering the OTM and sliding factors of safety, what I said is true.

Mike McCann
MMC Engineering

RE: Concentrated load on retaining wall

If this is a cantilevered retaining wall, it can be expected to rotate. You don't want to support a column on the top of a wall which will rotate. I would take the column down to the footing.

RE: Concentrated load on retaining wall

(OP)
Hokie that's not possible as the distance from outside face of foundation to centerline of steel is too small. That would leave only 1 inch of concrete at the column location which is something I don't like.

RE: Concentrated load on retaining wall

How you solve the dimensional issues is up to you. I was just pointing out that the top of the wall can be expected to move due to concrete deflection and/or footing rotation. Why not introduce a concrete column, isolated from the wall?

RE: Concentrated load on retaining wall

I second hokkie's recommendation..stability is more imortant than any other issues named sofar....

RE: Concentrated load on retaining wall

How can it rotate - it is part of a wall and continuous footing system. It can deflect, crack, but not rotate on its own independent of the wall and footing adjacent to it.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Concentrated load on retaining wall

(OP)
I believe the rotation Hokie is talking about is in regards to compression of the soil and deflection of the wall and footing. Plus, in order to assume certain soil conditions, this rotation is required.

RE: Concentrated load on retaining wall

Hokie:

As the wall is a retaining wall (soil assumed) and water intrusion would not be desired, I think that any column would have to be integral with the wall, as a concrete pilaster, or contained within the wall, as a steel pipe column, or it will not work.

Mike McCann
MMC Engineering

RE: Concentrated load on retaining wall

Mike,
A flexible membrane can solve that issue. At any rate, a concrete wall without a membrane is not usually watertight.

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