cranked steel bars in ACI 318

[perfectaccess]

hello,

I don't have an experience in cranked steel bars in ACI 318,
could you please refer to the conditions in ACI 318 that define he procedure and limitation ?
Also Why the ACI stop recommend this method?is it because of the earthquake behavior?

 

[KootK]

What kind of cranked bars are you considering? Column splices? Trussed longitudinal bars in beams?

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[perfectaccess]

cranked/Bend bars in Beams(Longitudinal bars)

Thanks

 

[hokie66]

If you mean trussed bars which go from top to bottom of a beam, we don't do that anymore. Too much labor in bending and placing the bars. The savings in material, if any, is offset today by the cost of labor and time.

 

[perfectaccess]

I see it is cost issue not a structural issue?but indeed some countries still do, since the cost is still lower(depend on the labor cost).

Back to our main point ,could you refer to ACI procedure for this(Clauses).

Thanks

 

[KootK]

X2 for Hokie's explanation. You can still use trussed bars but construction economics make it expensive in many markets.

See 318-11 11.4.6.5 for the shear contribution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[perfectaccess]

Thank you Kootk
But the last Closure is 11.4.6.4, besides I am also looking for moment requirement besides shear(where to bend ,how much to bend,etc,)with reference to code requirement .

 

[hokie66]

From memory, I think the place to bend the bars away from the tensile face was always at the same location along the beam where you could curtail straight bars, i.e. where those bars are no longer needed flexurally on that face. There were provisions in ACI 318-63 about trussed bars, but I don't know about later versions, except that I think now ACI considers that method obsolete.

 

[KootK]

Sorry, try 318-11 11.4.7.5.

If you have one of the older CRSI manuals, you can probably find flexural detailing procedures there.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

To me the main problem with using cranked (or truss) bars is that the original designers were trying to use the top bar portion as top negative reinforcement.

But with alternating live loads on a continuous set of spans the negative moment inflection point extends some distance past the positive moment inflection point.
So it makes it difficult to use the same bar for both positive and negative moments as the bar is needed in two separate locations at the same point along the beam.

In checking older beams for new loads we always have a lack of steel near the upturn in the bar, either positive or negative.

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[KootK]

Does anybody know, off of the top of their heads:

1) is there a standard slope to the sloping portion?

2) would one use normal straight bar extensions past inflection points? Or assume the trussed bars fully developed at the bends? Or as a standard hook development condition at the bends?

3) not all of the reinforcing is hooked typically, correct?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

KootK:

1) No - I've seen many different variations of slope

2) Generally you have two conditions:

a) trussed bars from each adjacent span extend their "top" bars past each other with no additional top straight bar used.​

b) trussed bars from each adjacent span extend their "top" bars past each other but use additional top straight bars.​

The problem always is that the bend/upturn point is such that it occurs too soon for both the bottom and top bar needs when compared to current ACI beam development rules.
I think the trussed bars are developed at the bends - but even with that they occur many times way too soon.

3) The historical practice appears to be that truss bars (bent bars) were used in conjunction with straight bottom bars and much more seldom were straight top bars added.



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[hokie66]

KootK,
Quoting from Winter et.al., based on the 1963 Code, "the ACI Code requires only that the inclined part of any such bar make an angle of at least 30 degrees with the longitudinal part". He further stated that they are usually bent at 45 degrees. This was of course a provision for diagonal tension.

I think JAE has described the practice correctly.