In what case should Ld formula for tension be applied in columns?
In what case should Ld formula for tension be applied in columns?
(OP)
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.






RE: In what case should Ld formula for tension be applied in columns?
In any ase, 3 m splices are unpractical and for such cases mechanical bolted/welded splices are commonly used.
RE: In what case should Ld formula for tension be applied in columns?
RE: In what case should Ld formula for tension be applied in columns?
There will be a maximum tension and a maximum compression and the bar lap needs to be sufficient for both of them.
RE: In what case should Ld formula for tension be applied in columns?
RE: In what case should Ld formula for tension be applied in columns?
Can you provide a constructive opinion in a professional way?
RE: In what case should Ld formula for tension be applied in columns?
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.
RE: In what case should Ld formula for tension be applied in columns?
RE: In what case should Ld formula for tension be applied in columns?
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?
RE: In what case should Ld formula for tension be applied in columns?
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?
RE: In what case should Ld formula for tension be applied in columns?
RE: In what case should Ld formula for tension be applied in columns?
RE: In what case should Ld formula for tension be applied in columns?
RE: In what case should Ld formula for tension be applied in columns?
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.
RE: In what case should Ld formula for tension be applied in columns?
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).
RE: In what case should Ld formula for tension be applied in columns?
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.
RE: In what case should Ld formula for tension be applied in columns?
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.
RE: In what case should Ld formula for tension be applied in columns?
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.)
RE: In what case should Ld formula for tension be applied in columns?
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.
RE: In what case should Ld formula for tension be applied in columns?
Hopefully, by November, the industry will have agreed to change markings back to US customary units over the next few years.
RE: In what case should Ld formula for tension be applied in columns?
RE: In what case should Ld formula for tension be applied in columns?
Yes epoxy coated rebar requires much greater lap length.