"U" Bars to match vertical bars

[ajk1]

Some engineers put notes on the structural drawings saying, "Add 'U' bars same size and spacing as vertical bars". This would presumably apply to elements such as reinforced concrete balustrade/guard in parking structures. I cannot see the point of such U bars. Can anyone see what their function would be in an element such as a balustrade? I think they are undesirable because they are just more rebar to corrode (at least in a parking garage) where water collects on the top of the balustrade. Also if the width of the "U" bars is not precisely fabricated the side cover for corrosion protection may not be achieved.

 

[KootK]

The connection between the guard and the slab is a moment connection, possibly a rather high demand moment connection. The u-bars creates a rather optimal reinforcement arrangement for resisting moment. Inwards turned bars would be my next favorite. My least favorite would be the ubiquitous, contractor preferred, outward turned bars. U-bars, if fabricated too wide, can be twisted a bit in order to meet cover.

 

[hokie66]

I agree with KootK about the moment resistance, but the concern about corrosion at the top is well founded. The main issue is plastic settlement around these bars. Re-vibration can alleviate that problem, but most contractors can't be relied on to do it correctly.

Not sure what KootK means about inward and outward bars at the top of the balustrade, but that sounds like side cover, while the plastic settlement issue leads to cracking above the bars across the top.

 

[Trenno]

I think KootK is talking about the barrier/slab connection, whereas OP is talking about the top of the barrier (rebar termination detail).

 

[hokie66]

Yes, Trenno, I see that now. Thanks. I was focusing on the top of the wall, because I thought that was what ajk1's question was about.

 

[ajk1]

I should have been clearer...I meant "U-bars" installed at the top of the balustrade barrier wall. Are we all agreed that such bars serve no purpose at the top of a barrier wall? (hokie66's point about settlement cracks is a good point which I had not thought of).

 

[KootK]

I agree.

 

[BridgeSmith]

They may not serve a structural purpose (I don't know, since I'm not familiar with the design or detailing), but possibly a convenience purpose? For our concrete barriers, using a single bent bar, with both legs tied to the dowels out of the slab makes them self-supporting, so there are no external ties to the formwork. Some contractors even slip-form the barriers.

 

[BadgerPE]

I have seen these installed on an AASHTO project that had guardrails mounted on the top. I would think U-bars would be beneficial for anchorage to the top vs. single vertical bars.

 

[MotorCity]

Perhaps they are part of an attempt to create closed stirrups for shear?

 

[KootK]

Someplace, I have a rather comprehensive rebar detailing manual from the UK. In it, they showed a lot of these "cap bars" at the tops of basement walls. I always found that curious as it's not typical practice where I've worked. I suppose that it feels good to have some rebar for crack control on all exposed surfaces but, then, I'm not aware of any problems arising from not having the cap bars. And, as ajk points out, they create another vector for corrosion.

I find it fascinating that these things are so regional at times. You never know when something that you always thought was necessary turns out to be redundant. I started out in the midwest of the US where all concrete openings got 2-#4 diagonals in the corners for crack control. Now that I'm in the Canadian prairies, we just just don't do that. When I first brought it up here, I was told "the trim vertical and horizontal bars cross the same crack and their vector components perpendicular to a potential crack provide pretty much the same restraint". And sure enough, omitting the diagonals seems to create no adverse consequences at all.

 

[TehMightyEngineer]

Agree with hotrod and Badger, we use these kinds of cap bars for both spacing/support of the rebar and for curbs/guardrail anchorage.

Interesting anecdote regarding shear. I worked on the failure analysis of a very large concrete water tank where the wall had a shiplap style shear connection to the roof slab. The wall was something like 2 feet thick with two layers of heavy rebar but relied on the roof slab to provide resistance to the internal lateral force from the water.

The wall cracked over time at this joint. Due to water pressure infiltrating into the crack it pressurized and split the two layers of the wall vertically. This breached the tank and the resulting rush of water blew out the entire corner and side of the tank. Threw a shipping container across the yard, thankfully an adjacent building kept the tank wall pieces from traveling far. Happened in the middle of the night so no injuries.

I'd love to share pictures but those photos are under a NDA, see the attached sketch.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[hokie66]

Not a good recommendation for keyed joints, TME.

 

[TehMightyEngineer]

No it's not! The tank was something like 40 years old so not exactly something I'd worry about but this was a failure mode I'd never even heard of, let alone considered, until I saw that tank. Probably somewhat of a specialized failure given the usage and loading. Normal structures likely wouldn't be susceptible to it.

I wonder if I should see if I can get the NDA lifted; it would be a fantastic case study paper and it's been long enough that I doubt it would reflect poorly on the facility owner.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[charliealphabravo]

Maybe that tank wall would have been fine if they had placed "U" bars at the top of the wall to match the vertical bars

 

[TehMightyEngineer]

That's what we did in our repair of the wall.

In short, if you're loading up the top of a wall (guard rail anchorage, roof slab for big tank, etc.) I use a U-bar.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[ajk1]

To TehMightyEngineer (Structural)- if the rebar was as shown in your sketch, it did not meet building code requirements to extend a certain percentage of the positive rebar (in this case the exterior face rebar) at least 150 mm into the support (i.e. in this case above the horizontal joint). Anyway, that is a particular application, and my question related to a typical detail our engineers are developing for building walls, not water tank walls, although I did not say this in my original post.

To BadgerPE (Structural)- would it be too much trouble for you to post a sketch? I am not understanding the configuration that you describe.

TehMightyEngineer (Structural) - how does "loading up the top of the wall" apply to a normal building wall, i.e. basement wall, shear wall, or parking garage balustrade (guard) wall? Do you mean applying lateral load to the top of the wall?

To KootK (Structural)- I agree completely with what you say. From 1964 to 2006 I worked at a firm that definitely did not put "U" bars at the top of walls or balustrade guards and never had any resulting problems that I know of. Then about 10 years ago the firm came under new ownership, hired a very opinionated engineer who was known to design heavy and he always specified such bars. This spread like the plague thru the office, and it is hard for me to try to get rid of. So I started wondering if I was missing something.

The only place I see such bars being of any real use in the types of buildings that we design is elements subject to torsion (as Motorcity suggests). In such relatively rare cases, this should in my opinion be designed by the engineer and shown on section on the drawings, not left to boiler plate type a typical detail.

 

[TehMightyEngineer]

ajk: Well, it technically wasn't a building.

More accurately, this was the top of a wall and the ACI 318 section you're referring to states that it must extend into the support by 6 inches (150 mm) and specifically refers to beam and not walls. The reinforcement did extend 6 inches if I recall. Like I said, this was a huge tank, 6 inches was nothing. Also, as this was a ~40 year old tank I don't even know if that section of ACI 318 was in there or if ACI 318 was even applicable.

ajk: By "loading up a wall" I meant applied lateral, vertical, or fixed end moments. Essentially forces applied such that the reinforcement at the end is engaged in transferring these forces into the rest of the wall.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[ajk1]

Thanks.

 

[msquared48]

OP:

For years I have used inverted U bars at the intersection of PT planks to detention tank external concrete walls. They serve as confinement steel to keep the top wall steel together when subjected to tension forces from the "L" shaped bars tying the planks to the walls.

Don't know if the connection above is similar ro yours or not.

Mike McCann, PE, SE (WA)