Canteliver Retaining Walls reinforcement - explained

[mar2805]

Hi all!
Please see the sketch attached.
THis is aan canteliver retaining wall 10 foot high with an wall thats 8" thick.
It will be 30 feet long.

How much of the horizontal reinfrocement on the front face of the wall would you apply to control the vertical cracking due to restrain of the foundation to wall connection? Also at what distances wozld you provide contraction joints and how horizontal reinforcement would you provide in that front face?

There seems to be a lot of debate on this and also peopele seems to get confused on wich reinforcement controls what.
Im interested to hear your thoughts.


Thank you

 

[BridgeSmith]

For walls up to 12", we (the DOT) don't put reinforcing in the front face. For stem walls over 12", we add the required 'shrinkage and temperature' reinforcement to control the size of the inevitable cracking, but it doesn't extend into the footing.

A retaining wall without a toe is an odd configuration, especially for a wall of that size. With an 8" wall, you can't get the required hook length for the RF bars with them turned towards the FF. You'd have to turn them back into the heel, which comes with its own issues with getting proper anchorage.

The soil bearing pressure under the toe will be very large, also.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[mar2805]

Does you code defines the shrinkage and temperature reinforcement?

Also at what distances would you provide contraction joints and how much horizontal reinforcement would you provide in that front face of the retaining wall to control the shrinkage cracking?

 

[BridgeSmith]

The AASHTO code has a formula, based on the dimensions, for the area of steel in both directions to limit shrinkage and temperature cracking. For most retaining walls, the minimum of .11 square inches per foot, with a max spacing of 18" (12mm bars at 450mm) would apply.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[mar2805]

What aboth expansions joints?
Does the distance between them influence the amount of T&S reinforcement?

 

[BridgeSmith]

I don't think the spacing of expansion joints is covered in the AASHTO spec.

Anyway, no, the T&S reinforcement requirements are not affected by the expansion joint spacing.

 

[HTURKAK]

Probably you are at Eurozone and Eurocodes are the statutory codes , ( Your Internet country code is HR )..

The following snap from ; Manual for Detailing Reinforced Concrete Structures to EC2 ( By Jose CALAVERA )

My points are ;

- Horizontal ( distribution steel ) rebar at soil side Ast/4 is OK EC-2 defines min 1/5 of Ast

- Provide min . 0.002*Ac for The horizontal and vertical reinf. at the front face
( 20*100*0.0002 = 4 cm2, )D8 /200 would be OK

- Movement joints should be provided at 12.00 m .. ( depends on temperature , humidity etc) ..In this case the wall length 9.0 m no need to provide movement joint and CJ anymore..

- I will suggest you to look Worked Examples for the Design of Concrete Structures to Eurocode 2 ( by Tony TTHRELFALL ) and anual for Detailing Reinforced Concrete Structures to EC2 ( By Jose CALAVERA )




My opinion only based on past experience ..





Not to know is bad;
not to wish to know is worse.

NIGERIAN PROVERB

 

[mar2805]

I looked into Eurcode 2 to maybe find some more info on expansion joints for retaininig walls but there is no subject at all on this topic.
Do you mabe have any other tehnical paper that covers this? Doesnt need to be EUROCODE based.

@bridgesmith
"T&S reinforcement requirements are not affected by the expansion joint spacing."
Are you shure aboth this?

 

[mar2805]

I found an info that an 100ft long wall will contract up to 3/4inch.
Is this correct?

Just one more thing and to be 100% certain.
Expansion joint are not same as control joint.
Control joint are to control the effect due to shrikage.
Correct?

 

[hokie66]

That's correct, mar2805. Control joints are what you need in a retaining wall. In your case, I would use two, at 10' centres. As to crack control reinforcement on the front face, I would put it all on the back face.

You need to think carefully about that no toe situation. You can't get efficiency in that opening joint with 'normal' reinforcement. U-bars, both in the wall and the footing would be a way of achieving roughly 80% efficiency, I think. Suggest you research other threads on the site, and the works of Nilsen, et.al.

thread507-479389

 

[mar2805]

@hokie66
"In your case, I would use two, at 10'"
I dont understand you....are you talking aboth control joints or....?

 

[BridgeSmith]

Ok, I searched through the AASHTO spec, and while the T&S steel requirements are not affected by the spacing of expansion or contraction joints, it does have limits for both.

Contraction joints shall be provided at intervals not exceeding 30.0 ft and expansion joints at intervals not exceeding 90.0 ft for conventional retaining walls and abutments.

 

[mar2805]

"Contraction joints shall be provided at intervals not exceeding 30.0 ft"

As previously said, construction joints are to allow shrinkage cracks at the joint location. I understand. But this 10 feet distance is a very short distance. I must say Im a bit suprissed.

One question, if you provide an control/contraction joint, you basicaly cut every other horizontal bar/reinforcement in the wall at that joint location, correct?
For expansion joint, that are placed every 90 feet, you need to provide somekind of "dowel bars" (full discontinuity in concrete section) as on slabs on grade, correct?

 

[hokie66]

mar2805,

The way I would do it is not related to any code. The suggestion to place control/contraction joints at 10' is arbitrary, and coincides with the wall height. Square concrete panels make sense.

For a contraction joint, I would just provide a groove in the wall at that point. You can discontinue some of the bars if you choose.

 

[mar2805]

I dont understand how the control joint and horizonstal reinforcement (for T&S) in retainng wall are NOT related?!?!
This doesnt make any sense!
Logicaly I would say:
if I provide more horizotal reinforcement in the retaining wall, I could put two control joints more far apart, and vice versa.

 

[hokie66]

Concrete walls which are restrained by footings will always crack, and by observation these cracks occur at roughly 3 metre spacings. The amount of shrinkage reinforcement you use will affect the width of the cracks, not prevent them.

 

[mar2805]

Agree, crack cannot be prevented, but with reinforcement they (crack width)can be controled with horizontal reinforcement.
But again, this MUST be related to joint spacings.
100 feet long retainng wall will have more vertical cracks then an 30feet long retaining wall, if both are without any control joints!
100feet wall will tend to shrink more due to the fact its longer!
Like previously stated:
"I found an info that an 100ft long wall will contract up to 3/4inch"

 

[BridgeSmith]

One question, if you provide an control/contraction joint, you basicaly cut every other horizontal bar/reinforcement in the wall at that joint location, correct?

The way we do it, all the horizontal bars would be discontinuous at a contraction joint.

For expansion joint, that are placed every 90 feet, you need to provide somekind of "dowel bars" (full discontinuity in concrete section) as on slabs on grade, correct?

Typically, we would have a gap between the wall sections, with some type of compressed joint sealer material. No dowels. The wall is designed on a per-foot basis, so the stem wall sections are structural adequate independently.

I dont understand how the control joint and horizonstal reinforcement (for T&S) in retainng wall are NOT related?!?!
This doesnt make any sense!

Take it up with AASHTO; the T&S reinforcement requirements have always been based on the "least width of the component section", which for a wall can be either the height or width. I guess if the spacing of the contraction joints was less than the height, it would affect area of steel required. I didn't think about that, because we've never put them that close. It would have to be a fairly thick wall to get above the minimum, anyway.

From AASHTO LRFD, 9th Ed. ---

 

[mar2805]

"The way we do it, all the horizontal bars would be discontinuous at a contraction joint"
Do you provide also an small groove on one side of the wall? (smaller section heigth)

"The wall is designed on a per-foot basis, so the stem wall sections are structural adequate independently."
I understand you, but I would want to have some shear force transfer between adjcent section in order to prevent diferenial tilting of the walls.