Retaining Wall Stem To Footing Construction Joint

[jgeng]

I need help understanding the different methodologies for providing shear resistance at the construction joint between the base of a retaining wall stem and the footing and how to calculate it. Some of the designs I have seen have a keyway, while others do not address the joint at all. Some cantilever design drawings I have seen appear monolithic through the joint, is this even possible? if so how is it formed and poured? Can the flexural steel up the stem be used to demonstrate shear strength? I have seen discussions about shear frictional resistance and roughening the service 1/4" per aci chapter 11, do you have to add shear steel using these procedures or can you get credit for friction in the compression area of the stem? and what about dowels?

 

[hokie66]

You will get a lot of opinions on this, and it has been discussed on the site several times before. I detail freestanding walls with the toe thicker than the heel.

 

[kslee1000]

Shear in wall/stem usually is not high enough for concern (the wall stem essentially is a two way cantilever plate, the conventional one way design is very conservative, and the compressive stress in front of wall helps to prevent shear crack develop thru wall thickness). Look for CRSI handbook, there are several good examples.

 

[cap4000]

kslee

I have done over 100 cantilevered retaining walls as one way slabs. The field research and field testing I have indicates that the lateral pressures are actually higher than all the convential books indicate. Its the applied safety factors that get used up that keeps them stable. Never heard of a 2 way slab design. Perhaps your thinking about counterfort types or buttressed walls.

jgeng

The 1/4 inch raked surfaced is ACI preferred. I like a 2x4 key for most typical appications. The ACI theory is based on a sliding suface literally going over the 1/4 inch amplitudes thereby putting the dowels into tension. On walls over 20 feet high I do both a key and roughen surface.

 

[JAE]

The typical cantilever retaining wall is a pure cantilever...NOT a two way action at all.

Shear at the bottom of the stem must be accounted for via shear friction from ACI Chapter 11 or similar concrete code depending on your country. This means that you must include vertical or diagonal bars across the joint and fully developed on either side of the joint.

Some engineers add additional vertical bars (above and beyond the vertical moment bars) to deal with the shear. Some rationalize that the bars in tension for moment still keep the concrete on either side of the joint tight so shear friction is engaged.

With shear friction, the contact area at the joint can either be 1/4" roughened surface, keyed, smooth, or monolithic and each carries with it the appropriate shear factor found in the shear friction section of ACI 318.

I don't think keyed joints have a direct reference in ACI - they are simply "traditional" means of attempting to tie a joint together. I've seen some engineers do 2 x sqrt(f'c) calculations on the key area but I don't think this is too valid.

I typically use 1/4" roughened surfaces. I've never used hokie66's method of using a thicker toe (as a stop to the lateral sliding shear) but it sounds like a good idea.

 

[cap4000]

JAE

I use a continuous toe block say 12" x 12" if necessary for additional sliding resistance.

 

[cap4000]

JAE

I think the 2 x key fell out of favor with ACI because it may crack across its width prior to any loads being applied. I always have dowels anyway so if any sliding does occur the dowels slip into tension. This shear friction method is similar to a corbel design.

 

[JAE]

cap4000, I agree tht the key isn't all that used or popular anymore. I don't use them much at all.

 

[cap4000]

JAE

Arthur Nilson's 13th Edition Design of Concrete Sructures Book still is showing a footing key. Surprising because he is an ACI Committee member.

 

[civilperson]

Bet his grad student drafter put in the key!