In what case should Ld formula for tension be applied in columns? 2

[foxview11]

The contractor is insisting that Ld formula for tension is not applicable in columns because by doing it so, lapping is excessive for rebars with dia 32mm size. Our design department clearly specify that lapping shall be at mid height with tension splice of 1.3xLd. So that will make the lapping length of about 3.0m. But the clear height of the floor is 4.20m. So according to contractor, it is not practical to adopt such length. They want to use Ld in compression which is shorter. Please give your opinion regarding the matter.

 

[ishvaaag]

The placement of the rebar in the section will give the tensile or compressive status for the specific bar being spliced; all hypotheses need be examined where the splice is wanted, then the one giving the worse splice length should be chosen.

In any ase, 3 m splices are unpractical and for such cases mechanical bolted/welded splices are commonly used.

 

[foxview11]

Rejoinder.Our design department clearly specify that splice of rebar in columns shall be at mid height with class B splice.My point is,the computed lapping length = 3.0m (Ldx1.3)for 32mm dia rebars, while the clear height is 4.2m only. Is it impractical to adopt that 3.0m length of splice? ACI 318 only prohibits splice for rebar size more than no.11. And also, our tender drawing specifies class B splice at mid height of column.If so, what Ld formula should I use?Is there any other practical way to adopt without compromising the safety of structure?

 

[csd72]

Get the design department to specify the lap that is actually required for this project, not just some dogmatic, too lazy to actually do the numbers, conservative oversimplification.

There will be a maximum tension and a maximum compression and the bar lap needs to be sufficient for both of them.

 

[foxview11]

I think using Ld formula for compression is not also applicable to structures design for seismic load frame. But Ld formula for tension will give longer length which will (Splice = 1.3xLd)be 3.0m as in our case with 4.2m clear height of column.

 

[foxview11]

csd72 you are right. But it's very clear in my statements that our design department specifies splice length for tension only. No restrictions as to the maximum tension & compression. Even in ACI 318, we cannot find a single provision stating the limit of splice length for tension, except for rebars size more than no.11.

Can you provide a constructive opinion in a professional way?

 

[csd72]

That was a constructive opinion!

Splice lap lengths are for full bar strength development but these are normally not fully stressed due to the steps in bar sizes.

Therefore the reference to Ld is conservative and can usually be reduced for the actual force rather than the steel area.

 

[JAE]

What csd72 is referring to is the reduction in Ld for tension development/splices using the ratio: As(required) / As(provided).

 

[Teguci]

I think if you run the number for ld you can get your lap length down to at least a 2m length as follows:
3/40 x 60000/(sqrt4000) x 1.0 x 1.0 x 1.0 /2 x 1.26 x 1.3 = 59in = 1.5m

Assumptions are 4000 psi concrete, grade 60 rebar, minimum cover to ties of 1.5in + #4 ties = 2in; cb = 2+0.5db = 2.63"; cb/db = 2

no lightweight concrete factor. How did you get 3m?

 

[foxview11]

Teguci
These are data being used for Ld:
Rebar Factor = 1.0, bar location other than top bar
Coating factor = 1.5, using epoxy coated rebar
rebar size factor: rebar size < 22.0mm = 0.8; > 22.0mm = 1.0
Concrete Factor: using normal conc = 1.0
Fy=420Mpa; F'c=28Mpa ; db=32mm
Using Ld formula in SI unit

a) For rebar > 22.0mm ===>ACI 318, chap12, sec 12.2.2
Ld = (3/5(xFyxAlphaxBettaxGammaxdb)/(sqr rootF'c))x 1/1000
= 2.286m
Ls = 1.3Ld = 2.97m
= 3.0m
My question is, Is there any further reduction factor that can be introduced without compromising the safety of structure? Is it practical to adopt 3.0 m lapping at mid height of column with a clear height of 4.2m only?

 

[foxview11]

Please be aware that i am referring to Ld of rebars size > 22mm and also splice at mid height of column. Please if you have any idea to clarify the issue.tnx a lot

 

[Teguci]

Use formula from ACI 12.2.3. Specifically the (cb+Ktr)/db factor which I get to be 2.0 or greater (not the assumed 1.5 given in the table-for columns 2.0 works up to #11 bars). You should get a required lap length of 2.25m.

 

[csd72]

JAE covered very succinctly what I was trying to get at.

 

[foxview11]

Teguci:
Exactly, we can use formula from 12.2.3 to calculate the factor (cb+Ktr)/db but ACI 318 limits only up to 2.5. Well tnx Teguci for your technical insights to clarify the matter. But my concern is that our design department use the factor 1.5 being used to calculate Ld as per equation 12.2.2 in the ACI-318 so the computed Ls = 1.3Ld for 32mm rebar will be 3.0m which is too long for a clear height of 4.2m for a particular column. The question is, Is it technically applicable to adopt 3.0m for lapping at mid height of column with 4.20m clear height only? If so or otherwise, please give opinion to clarify the matter. tnx a lot.

 

[Teguci]

If you can find the room to place the full length of the class B lap splice, then you are good to go. I am not aware of any spacing limitations regarding the length of the lap splice unless you are detailing a "special" column (in which case your lap splice must be half the column length).

For critical laps, I usually go through the effort of reducing their lengths. For typical wall and slab detailing I let the contractor provide the extra length (which has saved a lot of work in the past).

 

[foxview11]

csd72:
yes you have a good point which is theoritical but, my point is that using Ld formula as per ACI 318, 12.2.2 equation gives a longer lapping at mid height of column. Is there any idea to introduce further reduction factor to the computed Ld, which is proven to be safe and established by any technical studies and experiments of a renowned institution? ACI 318 is not clear in this matter.

According to ishvaag, having 3.0m lapping in any case, is impractical. But practicality in construction does not warrant the stability and quality of structure.

Hopefully, at this point in time, things will be cleared out from any doubts.

 

[hokie66]

I don't want to enter the argument over lap lengths, although I agree with csd72 that the lap lengths required should be explicitly shown on the drawings rather than left to interpretation of code clauses.

What struck my eye is that you are using epoxy coated rebar, and only 28 MPa concrete, which seems a mismatch of materials. Epoxy coated rebars, I assume, are used for durability, although the effectiveness of epoxy remains contentious.

With such large bars, I assume that the reinforcement percentage in the columns is high. Why not reduce the amount of reinforcement by using higher strength concrete? This would also have the benefit of increasing durability. Where I am, column concrete is almost never specified below 32 or 40 MPa, with 50 to 65 MPa being common and more economical than more reinforcement.

 

[TXStructural]

Epoxy-coated reinforcement (ECR) is extremely effective in reducing incidence and rate of corrosion in exposed concrete. I'm curious what kind of structure is using ECR where column laps are required. Also, you might be able to use uncoated reinforcement if you increase cover depth or concrete quality.

As to the practicality of long splices, the underlying mechanisms of development require the length they require, so either go to smaller bar diameters or another method of splicing.

Where it is not practical or economical to splice are each level, column cages are routinely constructed using 40-60 foot (up to 18 meter) long bars, with mechanical splices/couplers at the end(s). These are hoisted into position and guyed to keep them erect until the concrete is placed to the top level. Be aware that the assembled cage is only as resistant to bending as a single bar.

I assume you are in/around Europe from the use of 32 mm bars? (In the US, we do not use the soft metric sizing for engineering or construction documents, and these sizes will likely be dropped from all references and bar markings in the coming year or two.)

 

[hokie66]

TX,
I didn't know you used any metric sizing in the US. 32 mm bars are used in a lot of places in the world. In Australia, we use 10, 12, 16, 20 24, 28, 32, 36. If we need corrosion protection on the bars themselves, we galvanize them.

 

[TXStructural]

The US government mandated the use of SI/metric units for certain projects (initially highway construction IIRC) about 20 years ago, then abandoned them. Because of the nature of the machinery used to produce rebar, changing the bar markings is not trivial, and so lingers as "soft metric". Our bars are US customary sizes just labeled with the nearest approximate SI size, and are not the same as your true SI sizes. The most recent bridge manual has entirely dropped the SI facade.

Hopefully, by November, the industry will have agreed to change markings back to US customary units over the next few years.