Temporary bracing to basement wall 2

[ajk1]

The shoring engineer has submitted the attached shoring drawing for temporary bracing the earth retaining basement wall of a 2 storey underground garage of a 11 storey superstructure. He has not calculated the stress in the slab-on-grade caused by the diagonal braces to the wall which are at 3 foot centres, about 7 in a row. I calculate a 21000 pound horizontal soil load goes to each brace. The shoring engineer calculates 17,000 pounds, but he neglects any additional force due to the continuity moment in the 2 storey wall.

I find the slab-on-grade is grossly overstressed when checked by the Westergaard equation of load applied well away from any slab joints, based on a modulus of rupture of 9 root f'c.

Questions:

1. Shoring Load:Is it the practice when determining shoring loads for multi-storey basement walls to ignore the continuity moment and treat the wall as pin connected?

2. Slab-on-grade stress: Is it the practice for temporary shoring loads to not use Westergaard to determine slab-on-grade stress? If so, how is the slab checked? All the shoring engineer has done in his submitted calculation is to spread the vertical load from the diagonal wall brace at 45 degrees down thru the 5" slab and underlying 6" of granular fill and shows the pressure on the soil is ok in his calculation, but does no calculation of the stress in the slab.

3. Load Spreader to Slab-on-Grade: There seem to be 2 possible solutions: One is to place the diagonal braces at 18" centres rather than 36". That seem to me the best solution (there are not that many shores involved) and in fact the contractor's foreman has suggested that. The other solution is to place a spreader at the bottom of the diagonal braces. Any ideas on how that might be done, given that there is both a horizontal and vertical component to the load?

 

[manstrom]

Give the shoring engineer a call. Maybe you'll learn something.

At first glance, the slab on grade seems to be taking a lot of load. I would expect to see something to distribute the load better. Of course, if the shoring engineer has experience with this detail and will stamp it, you might be ok.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[KootK]

1) Not sure
2) Yes
3) Some manner of grill assembly

In my experience, the shoring guys are the renegade cowboys of structural engineering. They routinely do things that make me want to barf. And nothing ever seems to go wrong. So bless them, I say, for accepting liabity that I don't want and facilitating low cost solutions that allow my work to proceed.

I suspect that much of this stuff works out because our estimates of earth pressure often do not come to pass, or even close, out in the wild.

In the other, related post someone mentioned the idea of bracing the wall back to the P1 diaphragm instead of the SOG. Based on the shoring sketch, that idea might have legs.

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.

 

[ajk1]

I agree that the most likely explanation is that the soil pressures do not develop, at least for the duration of time the shoring is in place. I respect your advice and opinion, but I am not so sure I agree that the shoring engineer's seal on his drawing means that we, as the engineer of record for the repair, will escape liability in the event that something goes wrong. I think the courts may rule that we, as the engineers of record for the repair, are the prime engineers responsible for the proper carrying out of the work and that if there is something not in accordance with good practice and what a reasonably prudent engineer would do, we must advise our client and perhaps the regulatory authorities. I am having a conference call with the contractor and his shoring engineer later this morning, but previous meetings on site with him have been unhelpful. His answer to everything is "that's the way I do it".

How do shoring engineers check the slab on grade if they do not use Westergaard??

 

[KootK]

They check it at ULS exactly as you described in point two at the top and probably accept a slight chance that the slab on grade will crack and need to be repaired as predicted by Westergaard analysis.

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.

 

[ajk1]

Hi Manstrom: I had a conference call this morning with the shoring engineer, the contractor and others from our office, like you suggested. You were right when you said that by discussing the shoring engineer's design(?) with the shoring engineer, that I might learn something. Here is some of what I learned:

1) Shoring of walls and floors that are to be repaired is not design in the sense that structural engineer for a building would understand.
2) The shoring is based on what the shoring engineer has done in the past (perhaps for significantly different conditions, but I don't want to quibble).
3) The shoring design engineer does not check whether the rebar in the braced wall is adequate, even though he has changed the vertical span of the wall.
4) The shoring engineer does not do any calculations. If asked to submit his calc's they will be minimal and based on what he finds easiest to analyze (usually at the level of high school statics). No attempt will be made to understand or model how the structure actually behaves. This saves a lot of time. (I do not recommend this approach for structural engineers designing buildings).
5) If the project structural engineer marks up the shoring erection drawing with his comments, the truly dedicated shoring engineer will ignore the comments entirely and resubmit with no changes, and ensure that the inadequate shores are shipped to site anyway.
6) It is none of the project structural engineer's business to review and question the shoring engineer's drawings, since the shoring engineer has his signed professional engineer's seal on the drawings.
7) If there is a simple solution that would bring the design into the comfort level of a real structural engineer, kill it by telling you that it is some unbelievable high price ($20,000 for 20 shores).
8) The foregoing works and collapse of shored floors or walls does not happen.

I learned a lot.

 

[hokie66]

Classic, ajk1.