Strength of concrete cantilever 1

[ACE58]

A new truck maintenance building has a 183' long fiberglass trench drain with variable depth of 6" to 13" below fin. floor. The top of the trench drain is a attached to steel angles w/ Nelson studs that were cast into the thickened concrete floor. Due to improper bracing & over-vibration of the concrete during installation, the trench body is twisted out of vertical alignment by varying amounts along its length. In the worst instance, there is now a 1 1/2"H x 7"V triangular "lip" of concrete (rather than a vertical face) supporting the trench frame. The contractor won't warrantee the installation for more than 1 year & the owner wants it replaced because he expects this lip to crack off in the future (as do I). Besides finite element analysis programs (our company doesn't have that capability), what design procedures/standards/formulas are applicable to produce hard data (rather than empirical knowledge) that this installation will fail after repeated loadings? I've asked other engineers I know & they're stumped as well. Any advice would be appreciated.

 

[ToadJones]

Might be over-thinking this one.
I'd say, if it looks screwed up and both you and the owner agree...then fix it.
All the FEA in the world can't justify botched workmanship.

 

[ToadJones]

I supposed you could analyze the small concrete cantilever for the Cracking Moment. Should take you about 5 minutes to get some hard data that way.

 

[ACE58]

Unfortunately, construction isn't free. We estimste that it will cost approximately $3200 to remove & replace a 9' section; we're looking at replacing 8 or 9 sections. Neither the owner nor the contractor want to absorb that cost.
The crack will most likely develop at floor level, parallel with the trench. The thickened slab extends below the trench & encases it. How do you propose modeling this for beam depth, length & load application, as well as fatigue?

 

[JAE]

Can you post a sketch? Hard to visualize the condition.

Also, I'd agree with ToadJones - the contractor screwed up and didn't construct the drain properly. There isn't (or shouldn't be) any cost to the Owner for this.

"Being Stupid should hurt"

 

[BAretired]

What about a sketch?

BA

 

[ACE58]

Sorry it took so long to reply - out of the office. A sketch of the trench drain is attached. The solid lines indicate proper installation & the dashed lines indicate what was installed (worst case). The contractor is willing to fight this; he says that his "experts" say that the drain doesn't need to be replaced - the concrete won't fail.

 

[asixth]

Are there U-bars which pass around the trench and is the headed reinforcement developed within these bars?

 

[ToadJones]

Where are you concern with the concrete cracking>?

 

[Zambo]

Good sketch - is it to scale and is that the worse case? Does it drain correctly? should be easy to test with a reasonable water supply.

If there is rebar in there and the cover is correct I doubt it will crack. But I guess you need proof. Although the contractor's contractual warranty is only for 12 months I expect he is legally liable for a longer period for defective work.

You have to check the contract but it is possible that at the end of the 12 month warranty the warranty period can be extended for the defects picked up in the final inspection.

 

[BAretired]

I don't see a concrete cantilever. The whole trench has been twisted out of shape by as much as 1.5" and the concrete has filled in on each side until it meets the sloping wall of the trench.

I don't believe that strength is affected by the twist during pouring. If the drain functions properly, the only issue is whether or not the work is deemed acceptable. The contractor did not conform to the intended geometry shown on the drawings, so he did not meet normal criteria for acceptance.

Perhaps a financial adjustment could be made which would be accepted by all parties. This would be based on the unarguable fact that the contractor did not deliver what was specified.

BA

 

[apsix]

You note that the drain is up to 1.5" out of vertical.
What is the lengthand spacing of the Nelson stud?
Is there any other rebar which would intersect the failure plane?

I think that the Nelson studs should prevent the 'cantilever' from breaking off, depending on length and spacing.

 

[BAretired]

Where is the cantilever?

BA

 

[apsix]

As the drain wall is not vertical, on one side the concrete overhangs the drain.
I believe this is what's is meant by 'cantilever'. That is using a broad definition of the word.

 

[BAretired]

So far as I can see, there is no cantilever. There is simply a mass of concrete surrounding a trench which, in some locations, has rotated from its intended position. If there is a cantilever, please illustrate in a sketch. Thank you because I do not see it.

BA

 

[Zambo]

BAretired - is this starting to annoy you? As apsix suggests (but I guess you posted about the same time) the description is using a "broad definition" of the word cantilever. In fact apsix is being too polite I would say it is a "wrong definition" for the word - there is no cantilever.

I also don't see this failing unless there are further workmanship issues that cannot be seen on a sketch.

 

[apsix]

The wall is at 12deg to vertical and should probably be called a slight overhang.
If it was at 80deg to the vertical say, cantilever would be a correct term.
The risk of failure increases as the deviation from vertical increases.
The question is whether the risk has increased significantly at 12deg. I would say no, if the potential failure plane is crossed by studs/rebar. I don't know if that's not the case.

 

[ACE58]

The scale of the sketch was 1 1/2" = 1'-0". The function of the drain isn't in question. What the condition is termed is irrelevant in my eyes - the analysis or evidence of future failure is the concern. The question is if dynamic loads from repeated heavy truck traffic will cause this concrete cantilever/corbel/overhang/lip to fail. The construction documents contain a trench section which indicates #4 @ 2'-0"c/c each way located 4" below the floor surface. There is no dimension indicating the distance to the closest bar (parallel or perpendicular), but it scales (I know, never scale sections - use dimensions only) as 6" away. Who knows where they were actually installed. Product data indicates the Nelson studs are at 1'-6"c/c. No length of stud is given.

 

[ToadJones]

even if this "overhang" or "cantilever" were to crack, it is confined in such a way that the crack probably would not open up. I don't know if there is any "engineering" to be had here as far as checking this situation.
It is just a bad installation job.
...it reminds me of when I build my house and install my sump well in the basement, but left the lid off. The concrete finishers I hired to do the basement slab poured the concrete around the sump well and "smooshed" it out-of-round and the lid would not sit afterwards. The floor looks absolutely beautiful though

 

[miecz]

ACE58

Your specification should have a Quality Assurance section that puts a tolerance on formed surfaces, and some verbiage about rejecting out of tolerance work.