Diagonal Reinforcement at the base of conrete wall

[igdelimb]

I could be able to find a clear answer to why diagonal reinforcement is used at the base of the wall (wall-mat interface) other than two places on ACI code One of them is a statement on section R6.4.3 of ACI 350-01 which states that diagonal dowels/reinforcement may be whenever a force transfer is required. And the other is the Figure 15 on ACI 315-99.

From the figure 15 on ACI 315-99, it seems like this is just an engineering practice for liquid containing structures to reduce cracking at one of the most critical regions by providing additional diagonal reinforcement.

I just would like to get your opinion on using the diagonal bars at the base of the wall.

Thanks in advance.

 

[JAE]

CRSI also has this bar indicated in their manual for retaining walls.

They indicate that these "D" bars are for shear at the base of the stem and shear at the vertical face of the heel at the back of the stem.

In both cases, these bars can be used as Vs bars for the shear check at those locations. Alternatively, you can check if Vc by itself is enough for the shear and/or rely on the horizontal footing reinforcing (for the heel) and the vertical reinforcing (for the stem) in checking for shear using shear friction methods where you would add to the flexural reinforcing the amount of steel needed for shear friction.

 

[WillisV]

JAE - you do not have to add shear friction reinforcing to the flexural reinforcing, they act independently. Read R11.7.7 of ACI-05.

 

[WillisV]

The wording I used above is confusing...what I meant is that the same reinforcing can be used to resist flexure and shear friction at the same time in most instances.

 

[igdelimb]

Thank you JAE for your response. I am looking at page 20-2,20-3,20-4 of the CRSI manual dated 1988. I think you are referring the D800, which I agree with you. Bottom bars are bent and doweled in to the stem to help with shear resistance. However the diagonal bars that I was referring on figure 15 of ACI 315-99 (page 37) may also used as Vs for shear resistance but since there are vertical dowels on each face of the wall to transfer the shear. It appears to me that additional diagonal bars are provided for another reason other than shear transfer. Like to achieve a confined area to reduce any cracking, but I am not sure whether this is the main reason behind it.

I will try to post the figure 15.

 

[igdelimb]

When you say same reinforcement can be used to resist flexure and shear friction, I assume you mean the vertical dowels bwtween the wall and the slab/mat not the diagonal bars, right?

 

[hokie66]

Diagonal restraint cracking often occurs near the end of concrete walls, and diagonal reinforcement extending upward and inward from the end of the wall is effective in controlling this type of cracking. Takes a lot of reinforcement, as the restraining force is enormous.

 

[JAE]

WillisV - got it - thanks for the correction.

 

[miecz]

I don't use the diagonal bars at the base of the wall. I rely on the vertical reinforcement for shear friction. I do, however, use diagonal bars at the corners of the tank, at the vertical joint where the walls meet. I don't rely on the reinforcing at the wall face for shear friction at the corners, as both walls end at the corners, and I envision the horizontal bars spalling the 2 inch cover, if the wall were to shear. This can't occur at the bottom of the wall, as the base mat extends beyond the joint, so the bars can't spall the base mat. I may be the only one who thinks of it this way.