RC WALL 3

[n3jc]

Hello, Im dealing with a manure pit (excuse my english) under stable for cows...

Im not really sure if I should use model B (see attached picture) when designing a RC wall.
Wall is loaded with horizontal soil pressures (outer side) and sometimes with water + manure (inner side).

It kinda bothers me because the wall is not connected with other walls by concrete slab (at the top) but with prefabricated concrete slabs with slots/openings. This slab lies on top of concrete corbel. The problem is that this prefabricated slab with slots doesnt offer horizontal support to the RC wall at the top so Im kinda concerned about designing RC wall with model B. Maybe a model A is better option?

On the other hand the RC wall is connected with concrete slab (left side of RC wall), which connects a wall with foundations...

What do you guys think/suggest? i think there will not be deflection at the top of the RC wall so I can use model B but im not sure. i dont want to mess this up since horizontal loads are not that small.

 

[bowlingdanish]

In this case I would use A, since I don't like the idea of a meaningful flexure crack developing at the outer base. It could lead to water/moisture intrusion and would be a nightmare to repair. I might even run both models and envelope them to add a bit more rebar in the inner face. Best not to muck about with sub structures.

 

[dik]

BowlingDanish has it... I would design for both cases with effluent for pressure as well as soil pressure... typical sh*t p*t design.

Dik

 

[wannabeSE]

I would use neither. Instead, I would design it with no support at the top (cantilever). Otherwise, the walls will require shoring while back-filling until the slab is completed

 

[jdgengineer]

The problem is that this prefabricated slab with slots doesnt offer horizontal support to the RC wall at the top

Why would you be considering either option with a pin support at top? I agree with wannabeSE you should design as cantilever from the base. If you are concerned it may have some support at the top you could envelope the solution between cantilever and A. As wannabeSE mentioned sequencing of backfilling will impact modeling assumptions.

 

[JAE]

1. It may be that the prefabricated slotted panels can indeed provide lateral support.
2. The slab-on-grade on the other side of the wall can also provide lateral support provided the construction includes temporary shoring as wannabeSE mentions.
3. If the box isn't very long or wide in plan, the walls could possibly be designed for horizontal bending instead of vertical bending.



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[dik]

Sorry guys, but, I would support the pit top by tying it into the surrounding slab.

Dik

 

[bowlingdanish]

Why would you be considering either option with a pin support at top? I agree with wannabeSE you should design as cantilever from the base. If you are concerned it may have some support at the top you could envelope the solution between cantilever and A. As wannabeSE mentioned sequencing of backfilling will impact modeling assumptions.

This is assuming the slab on grade provides restraint at the top - which should be absolutely mandatory. No point removing redundancy for whatever reason with insitu concrete. And I'll take that opinion to the grave.

 

[jdgengineer]

I wouldn't consider it redundancy if it is your primary load resisting system.

Im hesitant to justify the exterior slab to brave the wall personally.

1) sequencing would require the wall be braced until the slab is cured, the contractor will likely not like this.

2) the slab appears to still be within the failure wedge of the soil so you would need to justify it as a deep beam spanning to the return walls.

3) 10 years down the line if the slab is removed your average contractor will likely not realize it was part of the retaining wall restraint as it's somewhat uncommon

If you can justify the precast slab of say have at it. I would think there could be a way for that. Or if you can justify the wall to span horizontally to the return walls that would be great. Otherwise, I'd design it as a cantilevered wall and backfill prior to placement of slabs. This will allow the wall to deflect and engage active pressure. You could also redundantly design as a propped cantilever as well.

 

[bowlingdanish]

I'd have to respectfully disagree jdengineer. You're playing a guessing game, and increasing the cost and weight of the structure based on what you think a contractor 'would want'. The fact is in design, the sequencing is a big unknown and I really don't think it pays off to have a punt as to what the contractor will do. The lids could very well provide the restraint required to backfill, equally they could have a bunch of steel props lying around which they could use. I do agree doing it your way will result in a more robust structure though, but so would piling a 900mm secant wall.

Edit- though your point about the proximity of the footing to the wall and failure wedge is a very good one. You may have swayed me

 

[jdgengineer]

The lids could very well provide the restraint required to backfill

The problem is that this prefabricated slab with slots doesnt offer horizontal support to the RC wall at the top

To be clear, you started the thread saying the exact opposite.

And I do believe it's a component of good engineering practice to have an understanding of how the contractor will likely want to build it. Sure not everyone will be the same, but typical construction sequencing should be considered in design.

 

[bowlingdanish]

Well I'm pretty omnipresent but even for me that's a stretch.

Also my point (the one I actually said) was that in the temporary case of backfilling they might be sufficient.

And I do believe it's a component of good engineering practice to have an understanding of how the contractor will likely want to build it. Sure not everyone will be the same, but typical construction sequencing should be considered in design.

Absolutely I agree. But I prefer a note ala 'contractor to ensure stability of structure during construction', rather than altering my design on possible outcomes. If they have a fancy idea they want to use to save themselves money, fine, I'll consider it and bill them for my time.

 

[jdgengineer]

OK,maybe I am a bit confused and misunderstood the question. Is your concern with the precast slabs that they may commonly be removed during normal use of the building? If that's the case then I think considering them for temporary loading for backfilling has a ton of merit.

If that's not the case and you can rationalize them for temporary support I would think you should be able to consider them permanent support as well. The load path needs to be in tact for either condition and if the load path is there and have the strength to support for temporary backfilling and compacting I would bet they would work for permanent loading as well.

 

[JAE]

After reading all these posts, and reflecting a bit on the nature of the building - these things are usually quite long buildings so my comment number 3 above using horizontal bending probably isn't valid.

Using the slab-on-grad adjacent to the pit is questionable because the slab could be removed in the future without anyone realizing its importance to the stability of the pit walls.

In my region, these types of buildings (usually hog confinement barns) do require the removal of the slotted precast planks occasionally (not always) so relying on them isn't a good idea.

So that really just leaves a cantilevered retaining wall type structure for the pits.

And based on proportions in the sketch above, the influence line at 45 degrees from the bottom edge of the main building foundation isn't all that influential on the pit walls in any case.




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[jdgengineer]

As a side note, I suspect that the difference in the stem reinforcing at the base and bottom slab design will be pretty similar between the propped cantilever and the cantilever only options as you would be designing for at rest vs active pressure. I didn't calc it so I may be wrong but I suspect the results would be within 20-30%.

 

[BAretired]

n3jc:

Should you consider additional wall and floor thickness, concrete cover, concrete strength or other measures to combat the corrosive effects of wet manure?

BA

 

[jdgengineer]

Bowling Danish I think I owe you an apology. I was responding to the thread on my phone and thought you were the OP. I believe some of my quotes I made were from the OP and not you. I'm sorry for the confusion. It's Saturday night, hopefully there is a beer in sight.

 

[hokie66]

For what it is worth, for a pit of this type and purpose, I agree with those who would design the wall as a cantilever. I would also reinforce it on the inside face. Industrial/agricultural structures like this require a different mindset from domestic and commercial structures.

 

[n3jc]

thanks guys you have been very helpful... as always...