Temp wall bracing for early backfill 1

[Dzastr]

Rookie PE. Here is my situation:
I have a retaining wall that will be temporarily braced to allow for backfill prior to placement of the concrete deck that will go above the wall. Once the deck is placed it will act as a diaphragm and the bracing can be removed. I need to calculate the spacing of my pipe bracing.

Our soils report provides an Equivalent Fluid Pressure of 40, 60, and 350* psf/ft for active, at-rest, and passive conditions respectively. With the wall being 9' tall, my pipe bracing will connect to the wall at an h=7'. At a 1:1 slope that gives me a brace length of 9'11". At this length my pipe braces can withstand 6000lbs.

Is my calculation process below correct?

The force of the soil will be [40(psf/ft)*(9'^2)]/2 = 1,620 lbs/ft and it will be acting at an h=3' (1/3 of the way up from the bottom)
This gives me 3'*1620(lbs/ft) = 4,860 ft-lbs/ft
The horizontal force of my wall brace will be 6000lbs*COS(45) = 4242.6lbs and it will be acting at 7' yielding 29,700 ft-lbs.

(29,700 ft-lbs) / (4,860 ft-lbs) = 6' spacing

Follow-up question, how could I include a surcharge of say 300 psf?

Thanks in advance, please let me know if i can provide any clarification. Really appreciate this, trying to increase competency here.

 

[PEinc]

First, you should not post the same question in multiple forums.

Second, you need to find the reaction needed to resist the overturning moment. You don't start with the capacity of the pipe brace. The wall strength may be the weak link.
Solve for the reaction needed. 1620 plf x 3' = 4860 ft-lbs/lf
Assuming the brace is 2' down from the top of the 9' high wall, 4860 / 7' = 695 plf horizontal.
At 45" angle, load to brace is 695 plf / cos45 = 982 plf of wall. This does not include any safety factor or surcharge.
If the 10' long pipe can support 6000" maximum, the max. pipe spacing without any added safety factor or surcharge load is 6000# / 982 plf = 6.11'
Now, can the concrete wall span horizontally between the pipe braces without cracking? You need to check the ability of the wall to span horizontally between brace locations. If you used a bigger pipe, the pipe spacing could increase but the wall probably would not be strong enough. If the wall is not strong enough, you may need to design and install a horizontal wale to support the wall between braces.
You need to consider if you should design for active at rest pressures. At rest could be about 40 to 50% greater than active.
You need a safety factor.
You need to add a surcharge behind the wall. This surcharge could be a rectangular load of q x ka or a Boussinesq surcharge from a strip or area load where q = your 300 psf vertical surcharge.
Add the surcharge load to the overturning calculation to get the needed brace load.
You also need to consider how the braces will be attached to the wall. You need to design anchor bolts, a bearing plate, and the welded connection of the pipe to the plate.
The same design needs to be done at the bottom end of the pipe brace.
What is the bottom end of the pipe brace connected to? A finished concrete slab or a concrete footing that will be buried below the new slab? If connected to the slab, you need to check is the slab can take the pipe load without breaking.
When the wall is braced and backfilled, there needs to be sufficient support (backfill) at the bottom of the wall to prevent it from kicking out.
You really should get help for this from a more senior PE in your company. There's more to this than just sizing the pipe brace.

http://www.PeirceEngineering.com

 

[oldestguy]

Appears t be a granular backfill, so the horizontal pressure at top of wall from the surcharge would be about 100 psf. That decreases with depth as if the surcharge was a footing, with a spread of load. What will be that width of a continuous "footing"?

Taking the backfill pressure as triangular and a resultant at 1/3 h is about as far as you can go toward a low factor of safety and using active, not at rest. Will the wall move? If not active is not the thing then. Also, a more realistic approach would be to take that total loading at 1/2 the height.

As an alternative to pipe bracing, what about timber? Old power poles or old RR ties? RR ties make good temporary footings for stuff like this.

 

[PEinc]

We have braced walls up to about 15' high with wood braces. Above that height, the wood gets big fast and the connections become difficult so we use steel braces - usually pipes or beams.
We have most often braced for active pressures without any problems. However, on paper, the amount of wall deflection needed to cause active pressure may be more than the EOR, architect, or owner may want to see.

http://www.PeirceEngineering.com

 

[Dzastr]

PEinc and Oldestguy,sorry about posting to multiple forums. improper form on my part.

I can't tell you how much I appreciate your time in providing a thorough answer. Reading your comments was both exciting and enlightening. That said...

The walls final condition will be at rest since it will be braced from the top and bottom, and will be tied into a mat foundation that covers the entire floor of the project. As such I should be using the at rest condition (as opposed to the active condition, which I mistakenly used), correct? We'd like the wall to stay vertical during the construction phase, so that it is vertical after completion.

In considering the surcharge load, I noticed last night that the soils report states

Loads applied within a 1:1 projection from the surcharging
structure on the stem of the wall shall be considered as lateral surcharge. For
lateral surcharge conditions, we recommend utilizing a horizontal load equal to
50 percent of the vertical load, as a minimum​

"

Would this mean that if assuming a 300psf surcharge taking place on top of the horizontal backfill, I could assume that I had a distributed load of 150psf (at a minimum) acting on the face of the wall?

If the above is true, my calc would look something like this:

150psf*9'*4.5'+[(60*(9'^2))/2]*3'=13,365 lbs-ft/lf (overturning moment)

assuming the brace is at 7' --> (13,365 lbs-ft/lf)/(7') = 1,909plf

with 6,000lbs@ 10' pipe length I get a spacing of 6,000lbs/1,909plf = 3.14'



A few other notes on your comments:

The issue of wall strength for brace spacing is definitely something I was ignorant too, however this bracing system will be submitted to the EOR and I am assuming he would determine if he was comfortable with the strength of his wall vs. brace spacing. I am the concrete subcontractor on the project, we want to make sure our system doesn't fail, the engineer might require us to change something with the design of the wall (added steel, thickened wall, etc.), or might say his wall requires a max spacing of X' but I will look to him for direction on that.

The entire subterranean parking structure is being built on an 18" thick mat slab, this is what the bracing will be secured to at the bottom and the wall itself uses a section thickened to 2' for it's foundation, therefore there is not really any concern of the wall kicking out at the bottom, nor any issue with the slab breaking due to the load.

Our system is based around the post shores that we use for our concrete deck shoring. We own plenty, so no need to use lumber or other material. We also do not weld a plate to the pipe bracing for the connection to the wall, we bolt a ledger to the wall and to the floor (sketch attached), and use a diagonal cut 4x4 to fill the void between the ledger and the pipe brace.

Fully understand the need to look into the anchor bolts for shear and pullout. I feel confident in my understanding of that situation.


Your comments would be greatly appreciated. Thanks again.

 

[Dzastr]

Also, i realize i have not touched on factor of safety yet.

 

[Dzastr]

Also, worth noting that the wall is 60' long, and will have wingwalls on both ends, one side will have them in both directions, which will aid in preventing overturn.

 

[PEinc]

Using 50% of the vertical surcharge load as the uniform horizontal surcharge is about equal to q x ko where ko = 0.5.
Don't assume that the EOR for the wall will check his wall for your bracing. He will want you to do that.
Since you are the concrete sub, you could add resteel, if needed, to act as a horizontal wale internal to the concrete wall. That could allow you to spread your braces out.
Again, consider the connections.
Consider also the locations of the braces with respect to wall's vertical expansion and contraction joints where the resteel may not run through the joints.

http://www.PeirceEngineering.com