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Why Do We Need Corner Braces in Shoring Systems

Why Do We Need Corner Braces in Shoring Systems

(OP)
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.

RE: Why Do We Need Corner Braces in Shoring Systems

My guess is that the corner braces are less expensive than the ground anchors.

RE: Why Do We Need Corner Braces in Shoring Systems

Corner braces are usually less expensive than tieback anchors and the tieback drills can have a problem drilling so close to a corner. As long as the corner braces do not interfere with the new structure, they can be a good alternative to tiebacks.

www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

I've always just designed the waler to waler connection at the corner to act as a support and omitted the corner brace. In reality it's all welded together solid and will act that way regardless.

RE: Why Do We Need Corner Braces in Shoring Systems

(OP)

Quote (PEInc)

As long as the corner braces do not interfere with the new structure, they can be a good alternative to tiebacks.

So, if one were willing to pay extra, is it possible to not have corner braces then? My current issue is that some corner braces need to be removed prior to the casting of a foundation wall and now the shoring engineer is asking me to assess my parkade diaphragm's ability to resist considerably more than a single story worth of earth pressure. Frankly, in retrospect, I surprised that this hasn't cropped up before.

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.

RE: Why Do We Need Corner Braces in Shoring Systems

A waler without a brace does not provide lateral support. With only the two, short, perpendicular walers but without the corner brace, one wall would need to be supported by the other, perpendicular wall that also is not braced. While it may sometimes work in the field, it doesn't work on paper. It should have the corner brace.

Another consideration is that the walers should be long enough and welded enough to the sheeting wall to properly transfer the walers' axial loads to the soil behind the walls or to the waler and wall on the opposite side of the excavation. I have seen soldier beams and SSP push laterally along the wall because the walers were not long enough. The hard question to answer is, "How long is enough?"

www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

If corner braces are located below the top of the proposed foundation wall, then the forms for the concrete wall need to be boxed out around the braces. Then, after the walls are poured and have their final lateral support (floor slabs?), the braces can be removed and the wall box-outs can be patched.

If you need to have walers with your braces, then the sheeting needs to be offset from the proposed wall by about 3 to 4 feet to allow room for the waler, wall forms, waterproofing, and backfill. If the concrete wall needs to be poured directly against the sheeting wall, an alternative, SOMETIMES, is place the braced walers above the top of the proposed wall, if possible.

www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

(OP)
Are we calling the non-hypotenuse members in my bracing sketch whalers? If so, mine currently exist in the same plane as the foundation walls.

Quote (PEinc)

If you need to have walers with your braces, then the sheeting needs to be offset from the proposed wall by about 3 to 4 feet to allow room for the waler, wall forms, waterproofing, and backfill.

This is essentially placing the whalers on the exterior side of the foundation wall, right? Wouldn't that mean that, in plan, the shoring would look like a castle with "turrets" offset out at the corners? I'm having a hard time imagining that a contractor could be persuaded to do that.

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.

RE: Why Do We Need Corner Braces in Shoring Systems

KootK,
Your initial sketch seems to indicate a secant pile wall with anchors. This type wall does not typically require walers.

RE: Why Do We Need Corner Braces in Shoring Systems

I don't understand your turret question. The attached PDF shows how wales and braces could be installed with off wall line sheet piling. Soldier beams or tangent pile walls would be similar. For this job, we located the braces and wales at elevations just above the wall construction joints. Therefore, you could shore the new walls in order to take out a brace level. Because this was a structure below the ground water table, the owner did not want the braces to penetrate through the concrete walls which would have needed to be patched after brace removal.

www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

With the correct sized waler, you might be able to spread out your tiebacks and save some money. Eliminating several tieback anchors will pay for a lot of extra waler steel. Also, the sketch appears to show a waler for the tiebacks, in addition to the corner braced waler. If the tiebacks are on walers, you want to drill the ties through the unreinforced shafts, not through the shafts with the steel beams inside. Also, with the correct waler size, you can probably keep the tiebacks far enough away from the corners to eliminated the corner braces and still have enough room for the tieback drill.

www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

Quote (hokie66)

Your initial sketch seems to indicate a secant pile wall with anchors. This type wall does not typically require walers.

All the secant pile walls I have seen (and/or designed) have a waler unless lateral support is only provided at capping beam level.

RE: Why Do We Need Corner Braces in Shoring Systems

Walers can be a part of the support for the piles, but when you provide temporary ground anchors at alternate piles as indicated by KootK's sketch, walers would be superfluous. Walers do get in the way of further construction.

RE: Why Do We Need Corner Braces in Shoring Systems

Quote (hokie66)

Walers can be a part of the support for the piles, but when you provide temporary ground anchors at alternate piles as indicated by KootK's sketch, walers would be superfluous. Walers do get in the way of further construction.

I agree. I had not noticed there was an anchor every hard pile. I am used to seeing an anchor every third soft pile with a waler.

RE: Why Do We Need Corner Braces in Shoring Systems

No doubt there are lots of variations of the system.

RE: Why Do We Need Corner Braces in Shoring Systems

(OP)
My system, and that most commonly used in my area, is secant piling that has no walers other than those that are part of the corner brace (if we're considering those to be walers).

Quote (PEinc)

I don't understand your turret question.

See the sketch below. It probably makes no sense. It was just my shot in the dark at trying to sort out this statement:

Quote (PEinc)

If you need to have walers with your braces, then the sheeting needs to be offset from the proposed wall by about 3 to 4 feet to allow room for the waler, wall forms, waterproofing, and backfill.

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.

RE: Why Do We Need Corner Braces in Shoring Systems

So what is it that you are showing between the secant pile and the anchor head? That is what we were thinking was a waler beam.

RE: Why Do We Need Corner Braces in Shoring Systems

(OP)
Ah. What I'm showing between is a concerte basement wall. The anchors should have been positioned within the wall rather than in front of it.

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.

RE: Why Do We Need Corner Braces in Shoring Systems

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.


www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

(OP)

Quote (PEinc)

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

Agreed but not my call.

Quote (PEinc)

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.

RE: Why Do We Need Corner Braces in Shoring Systems

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.

www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

(OP)
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?

Quote (PEinc)

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.

X X T X T X
X
T
X
T
X

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.

RE: Why Do We Need Corner Braces in Shoring Systems

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.

www.PeirceEngineering.com

RE: Why Do We Need Corner Braces in Shoring Systems

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.

RE: Why Do We Need Corner Braces in Shoring Systems

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.

RE: Why Do We Need Corner Braces in Shoring Systems

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



www.PeirceEngineering.com

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