Why Do We Need Corner Braces in Shoring Systems 3

[KootK]

Local shoring contractors in my area always seem to use corner braces in conjunction with their tie-back shoring systems. Why are corner braces used instead of continuing the tie-backs right into the corner? Is it because the machine used for tie-back installation cannot get physically close to the corner? Are there systems available that do not require corner braces?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[PEinc]

Without using walers, you are installing a lot of tiebacks. If you are using thru-beam tieback connections, perhaps you can use eliminate the corner braces if you install corner ties as shown on my sketch. If the drilled shafts are wide enough, you may be able to fit the tieback drill and drill the two tiebacks that are immediately next to the corner.


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[KootK]

Without using walers, you are installing a lot of tiebacks.

Agreed but not my call.

perhaps you can use eliminate the corner braces if you install corner ties as shown on my sketch. If the drilled shafts are wide enough

Perhaps. It seems to me that the drilled shaft in the very corner should not need a tie back. It ought to be able to lean on the adjacent drilled shaft walls as shear walls/diaphragms.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[PEinc]

If the corner shaft does not get a tieback, then the two adjacent shafts need tiebacks and, again, the drill may not be able to drill that close to the perpendicular wall. I would not leave three shafts in the corner without tiebacks.

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[KootK]

So based on prior sketches, is 4' the clearance that we think is needed to install tie-backs then? I don't know what the tie-back installer thing looks like really. Got a pic of that by chance?

I would not leave three shafts in the corner without tiebacks.

Why not? With the scheme that I've crudely shown below, the corner pile should be every bit as restrained as a column with a tie-back. More so really.

I do realize that you're rather good a this. I simply want to understand the parts of this that are still confusing me.

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I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[PEinc]

Tieback drills come in various sizes with varying degrees of articulation for the drill mast and drilling controls. Therefore, how much room is enough depends on the particular drill.
I would not trust three, adjacent, drilled shafts in a corner without tiebacks or a braced waler.
The crawler track width of a tieback drill could easily be 8 feet, therefore, maybe 4 feet on either side of the tieback being drilled off the end of the drill. However, many drills can also drill off to the side at up to 90 degrees from the end of the drill. Usually, the 90 degrees is to the left side of the drill. Therefore, in a rectangular sheeted excavation, two opposite corner tiebacks may be able to be drilled while the other two opposite tiebacks may have insufficient room for the drill mast and operator's controls. If you can keep the tiebacks a few feet from the corner or a perpendicular wall, the drilling may be easier.

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[Riggly]

I am late on this,but interesting thread. Going back to the original question:
1. Corner braces are much easier and cheaper to install that tiebacks.
2. There is also an equipment constraint, which will depend on the size of the rig. That is, the rig is unlikely to drill very close perpendicular to the corner walls.
3. I think the need for corner bracing is dependent on the loading and structural stiffness. If the corner is stiff enough for the given loading, I do not see the need for bracing. In the case of the secant wall, the corners will be more rigid than the other sections of the wall. In the case of sheet pile which while be essentially a pin connection, the walers will need to have a moment connection at the corner, or a brace will be required to maintain the stiffness. If no bracing, a heavier section is likely required for the waler using moment connection. For the typical excavation support system, I do not see design of moment connection a viable alternative.

 

[bdbd]

First of all, I didn't know it is called corner braces. We call it struts, steel struts.

Second of all, in corners, it is harder to anchor. Also, "corner braces" are always better, since they simply cannot deform, we are certain that walls will not deflect. (They may break though, due to high shear, so check your reinforcements carefully.)

I have seen some of the contractors do not like the corner braces. This is due to mainly they do not have skilled welders. With a two good skilled welders and a crane, it is easiest job.

Considerations:

*Buckling of corner brace.
*Self weight deflection of brace.
*Capacity of welds should be greater than the shear that will carry. Take the weld thickness at most 1cm. If this is not enough you have to extend its ends with a bigger pipe to increase the weld area.
*Pile reinforcement.
*Capacity of small anchors that will hold the plate (that braces will push on) on the wall.

 

[PEinc]

Couldn't use upper tier tiebacks in the corner due to underground utilities.

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