Retaining Wall Design? 1

[SteelPE]

Another simple one.

I have a question regarding retaining wall design. I have a proposed multistory structure that has one wall that is a retaining wall. Normally I would design the wall for two conditions, freestanding and propped (by the building). In this particular instance, I do not believe I will be able to develop enough resistance in the structure to resist the forces contributed by the wall (it will be cost prohibitive).

In thinking about this, if my wall is cast in place…. And then back filled prior to construction of the building (allowing for any movement to take place at the top of the wall) then the building is placed, will I end up with any loads from the wall being transferred into my structure (not counting surcharges)? To me it seems like the wall will be stressed and deflect. Then once the building is constructed on this deflected wall then there will be no additional load transfer from the wall into the building structure.

 

[slickdeals]

I think whether the building load gets transferred will depend on how close the retaining wall is relative to the building being constructed.

I am not sure about rules of thumb, but there is a distance from the base of the wall, beyond which the building surcharge will have no effect on wall loads.

 

[SteelPE]

slickdeals

I am talking about the other way around. I want to use the retaining wall as a wall for one side of the building (have 3 sides steel framed with metal panel and the other being a concrete retaining wall).

 

[steellion]

Yes, you have to design the building lateral FRS to resist the earth loads from the wall.

 

[Lion06]

Will this wall be a shearwalls or is it strictly a basement, gravity retaining wall? Is the floor framing parallel to this wall or does the floor framing bear on the wall?

You don't HAVE to dump that extra lateral load into your building, but you'll have to detail the bearing condition at the wall to allow it to move in the direction required and then design the wall and footing as a cantilevered retaining wall (with the applied axial stem load).

 

[a2mfk]

Sounds like you will have some decent P-delta effects on the top of that wall also if you backfill prior to continuing vertically with the wall... So you say you cannot use a floor slab to tie into at the top of the retaining wall to make it a simple span design, or this is forming a basement space?

 

[SteelPE]

a2mfk

This is just for a proposed structure. I told the client that you should not attache the structure to the top of the wall if it is designed as a conventional cantilever retaining wall. This wall was proposed to be 24' tall and I would expect to have large lateral forces induced into the wall if our structure is attached at 12' and 24' to the wall. I would guess that you would end up with around 12,000#/ft of wall length as a force into the LFRS.

I was wondering if the wall was constructed and back filled allowing the deflection to take place... do you still have to include the forces (from the propped wall condition) in the design of the LFRS if the deflection has take place prior to construction of the building?

 

[BAretired]

Does a retaining wall stop deflecting immediately after placing fill? If so, there are no forces transmitted to the building. Otherwise, the building will participate to some extent. The degree to which it participates depends on the relative stiffness of retaining wall and building.

If the retaining wall is designed to resist the earth pressure without help from the building, then the building must be capable of accommodating the time dependent deflection of the retaining wall.

BA

 

[SteelPE]

BAretired,

The question about the retaining wall deflection stopping I guess is part of the question. Under normal conditions (what ever that means... not clay I guess) does a retaining wall continue to deflect over time or does a majority of the deflection take place withing the first few days after back filling?

 

[msquared48]

If the retaining wall is a yielding wall (unsupported top), with the major deflections seen prior to the construction of the building, then the building will only see minor loads from the wall.

If the wall is non-yielding (supported at the top by one of the building floor diaphragms), then the building sees all of the load from the wall.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

msquared

So, if I have a diaphragm at 12' and 24' that are constructed and poured 2 months after the wall is backfilled then the diaphragm will only see lateral loads from the surcharge loads placed on top of the grade?

 

[msquared48]

Yes, primarily.

There will be a small amount of long term additionaql load due to any creep of the wall, but that should be negligable.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Jalthi]

The wall is estimated to move 0.002 times the wall height to mobilize the active earth pressure (DM 7.2). If the wall (top) movement does not reach 0.002H, you may expect additional movement as well as lateral forces in the future.

 

[BAretired]

The problem is indeterminate. The majority of deflection due to earth pressure may occur prior to constructing the building, but there likely will be some lateral deflection causing stress in both building and wall beyond that. Also, wind and seismic forces acting on the building will be shared by building elements and retaining wall according to their relative stiffness.

BA

 

[a2mfk]

I'd want a discussion with a geotech on this one (unfortunately proposed means there isn't one and you don't know the soil properties). But I tend to agree with Mike, the cantilevered wall should initially deflect and your soil will be in an active pressure state. I may be cautious about the effects of slabs and foundations constructed near the wall at a later date, they are surcharges and may exert additional pressure causing additional rotation of the wall and thus horizontal movement at the top.

I also think if possible I would design a horizontal expansion joint where your structure ties into the wall. I would think with almost any floor diaphragm and a decently stiff lateral force system, that if that wall begins to deflect that you will be putting unintended lateral loads into your diaphragm.

 

[hokie66]

For a 24' high wall, I would not try to use a cantilevered retaining wall as the wall of a building. It should be either a basement wall braced by the building both locally and globally, or a tied back wall.

 

[msquared48]

Hokie is correct here economically speaking. Normally a soldier pile wall with tiebacks would be used if adequate right of way was present. Either the toe or heel, to be economical, would likely have to extend beyond the property line, which is not allowed without a permanent easement.

That being said, maybe there could be a compromise here. Perhaps the lower 12 feet could be a yielding wall, the area with the greatest lateral pressure, and the upper 12 foot story a non-yielding basement wall. Outside of the surcharge load, this mix could reduce the total lateral pressure of a full non-yielding wall by 2/3, depending on the soil pressure diagram.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

I definately agree that a cantilever wall is not a good option. That is one thing I had mentioned to my client. I believe the cantilever wall that I had previously deisgned was around 20' In this instance the total height of the stem would be around 27-28 feet when considering frost protection.

My client is looking to take the job over from a previous owner. He showed me some preliminary drawings clearly showing a tapered cantilever wall. In my mind it can't be done without taking the forces into the building and at 12000#/ft or somehow designing the wall for horizontal slip at the connection between the diaphragm and the wall.... very tricky. I just wanted to make sure I wasn't being overly conservative.

 

[SteelPE]

I thought I would update this thread.

So we lost out on the project to another engineering company. My client must not have been happy with the way I presented my information to them. I had proposed using a buttressed retaining wall in lieu of a cantilever retaining wall. The engineering company who won the job proposed a counterfort retaining wall which the client seemed to like better (I thought they were the same thing). The other engineering company who is designing the building is not considering any loading from the retaining wall into the buildings LFRS. They are saying that the deflection of the wall will take place prior to the building being erected therefore; you do not have to consider soil loads in the design of the LFRS.

 

[SteveGregory]

Sometimes losing a job to another firm is a good thing. I think you just dodged a liability bullet.