Minimum thickness for shear in concrete

[TehMightyEngineer]

Can anyone point me to a minimum thickness in ACI 318 for shear? Client wants a 3" thick edge that vehicles will drive over, I feel this is too thin regardless of the load but can't find the reference.

Maine EIT, Civil/Structural.

 

[KootK]

What's the nature of the "edge"? Cantilevered slab? Is there potential for fire truck access? Outdoors?

I don't know of a code specified minimum for shear. You could run an edge column style punching shear check on the tire contact area to get a required thickness. As a traffic surface, durability requirements may put your rebar near mid-depth which won't help shear any.

All that being said, there are precast double tees in many garages with 4" flanges. That is often made possible by sketchy assumptions regarding load sharing between adjacent tees however.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TehMightyEngineer]

See attached section cut detail.

It's actually a recessed concrete cover, but they wanted to recess it in a less than optimal way. Outdoors, level with grade, fire truck access but not highway loads or super frequent traffic. Obviously the channel will help "armor" the edges but I'm concerned that the whole thing will crack at the base of the 3" stub with repeated traffic assuming the grade is slightly lower than the top of the 3" stub.

It's probably fine but I still seem to recall 4" being the magical "you shall not pass" number for thickness.

Maine EIT, Civil/Structural.

 

[KootK]

That's altogether different from what I was imagining. Is the source of the shear load a sort of lateral impact load as a vehicle tire makes it's way onto the ledge in the future when surfaces are no longer quite flush?

I'd push for six inches so that I could get two layers of vertical rebar in there somehow. I realize that kind of overkill isn't likely to fly in the precast world however. You'll have one layer of centred vertical steel I imagine, making "d" for your shear calculation 1.5".



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[manstrom]

No minimum that I am aware of. Precast double tees use 3" and 4' flanges all the time.

 

[precast78]

I dont understand why they need a lip around it. Why not just have a lid that goes over the manhole. That is just asking for concrete spalling.

 

[TehMightyEngineer]

I agree 100% precast78 but they insisted that is has to be like this for some reason. I imagine the channel "cap" will help prevent spalling to some degree but it still seems like a poor (or at least inefficient) detail.

Kootk: Yeah, I described it poorly. Yes, we expect a lateral load as the wheel has to "climb" over the lip assuming the grade becomes lower than the top of the corner. We would like at least 4", then it meets the minimum requirement for walls per ACI.

Oh well, I guess I'll run the numbers as best we can for 3" and see if my boss approves it.



Maine EIT, Civil/Structural.

 

[TehMightyEngineer]

Well, if I offset the rebar to get a d = 2" using the minimum clear cover allowed, and include the compressive confining load from the vehicle wheel load using equation 11-4, I can get the shear strength to barely work. Flexural rebar needed to be #4 bars at 9" o.c. welded to the cap channel and developed in the wall. I imagine the precaster is going to be cursing engineers tomorrow but both I and my boss feel like we're pushing this as it is.

Maine EIT, Civil/Structural.

 

[TehMightyEngineer]

So, the precaster ensured me that this is things tons of companies do and pointed me towards this other companies website:

http://www.trenwa.com/html/ProductsHS20.html

Still seems like a crap detail to me.

Maine EIT, Civil/Structural.

 

[RHTPE]

I would agree that it's a "crap" detail.

How does one obtain reliable "interlock" between the headed studs and whatever minimal reinforcing exists in the 3" wide portion of the wall?

I can imagine a heavy vehicle moving right to left on your detail having to hit the brakes hard and snapping the 3" wide portion off.


Ralph
Structures Consulting
Northeast USA

 

[TehMightyEngineer]

We got them to change the headed stud for a 27" long #4 bar welded to the channel. This was the only way I could get the flexure and shear strength to be acceptable. We actually didn't consider breaking forces because this was going into a substation with minimal speeds but, yes, in other areas where someone could get a vehicle up to speed I bet they could do some damage to that.

Maine EIT, Civil/Structural.

 

[RHTPE]

A706 bars I hope.


Ralph
Structures Consulting
Northeast USA

 

[TehMightyEngineer]

Of course; the precaster is aware that A615 isn't generally weldable.

Maine EIT, Civil/Structural.

 

[RHTPE]

I got involved in a lawsuit over brittle A615 bars. Seems the "wheelbarrow" contractor was tack welding intersections rather than tying them. So I took a bunch of short #4 bars to a certified welder and had him do a number of sample welds - some using the AWS standard for A615, some without. The ones done without could be snapped by hand-bending. The ones done with the proper preheat did not. The best rebar fabricator that I know uses A706 exclusively for all of their prefabricated cages. One of their specialties is prefabricated cages for tunnels & precasters.


Ralph
Structures Consulting
Northeast USA

 

[TehMightyEngineer]

Yeah, my boss was really annoyed that the precaster didn't seem to care and that we really had no easy way to let the owner know that this is a poor detail. I highly suspect this is a situation of "we've done it this way for 100 years, it's just how these are done".

Anyway, back to studying for the SE exam and my whiskey.

Maine EIT, Civil/Structural.

 

[KootK]

If the nib gets overloaded laterally, it will probably just yield flexurally, lean over 1/4", and bump into the lid. Given the low traffic volume, the low speeds, the limited consequences, and the past performance history, things will probably be fine. Besides, the precaster will pretty much own this thing from a serviceability perspective.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TehMightyEngineer]

Agreed on all points; still don't like it. But, yes, any cracking will likely end up buried, the channel will span the cracks and keep it from spalling and the 1/4" will limit deflection. Get some dust and crud in the 1/4" gap and you probably don't even have an issue. I suspect this is how they've gotten away with it for "100 years".

Maine EIT, Civil/Structural.