Straight bars in tension, development length reduction

[haynewp]

By ACI 12.2.5, you can reduce the development length of rebar when there is excess reinforcing provided for flexural members. You can also do this for hooked bars in tension by 12.5.3d. But what about straight bars in direct tension, for example when transferring uplift from anchor rods to a concrete pedestal? By ACI, if the development length is less than required for fy, does that mean the straight bars are completely ineffective?

 

[msquared48]

No. They just won't develop their full design capacity.

I would still hate to have to try to pull a #3 bar out of concrete when it was embedded only four inches.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[haynewp]

But is it permitted to use the excess reinf factor by ACI in this case?

 

[JAE]

if the development length is less than required for fy, does that mean the straight bars are completely ineffective?

I'm not sure I understand your question there. Can you elaborate?

If you are developing straight bars from a pedestal into a footing, the length of development could be reduced if you provide more rebar than is necessary for the strength calculation (As(req'd) / As(provided)). But I think you still have to provide a minimum development of 12" right?

So if you provided twice the rebar you needed, you would only need half the development length of that particular bar size.

 

[haynewp]

That is what I would think also, but ACI 12.2.5 specifically states you can do this for development lengths in "flexural members" but doesn't mention direct tension members. So are you permitted to use the same reduction ratio (As_req'd/As_provided) when the member is a tension only member?

 

[JAE]

haynewp - I understand your question - but not near my ACI codes - I'll check Monday but based on "precise" code language flexure means flexure so I would think that might be the case. But intuitively you'd think that if you only stress a bar half-way, then the needed development length is only half as well.

 

[haynewp]

I couldn't find it in there but that doesn't mean it isn't there. I know with some special tension tie members there are more strict splice requirements, but I don't know about column piers and reduced development lengths for straight bars.

 

[slickdeals]

@haynewp,

It appears you are trying to get away from Appendix D and using reinforcing bars to transfer uplift forces into the pedestal. If you look in Appendix D, their suggestion is that you use a U bar (see page ACI 318-08 page 426).

I am attaching a couple of documents for similar calculations.

 

[slickdeals]

And the other (wish you could upload multiple documents in the same post)

 

[haynewp]

Not really trying to avoid App D, if I meet App D for the base plate anchor bolts, I still need to get the force developed into the pier the column is sitting on, then from the pier into the footing. It is whether or not I have to fully develop the straight bars sticking up from the footing into the large pier or not. Or have to use hooks at both ends of those bars tying the pier to the footing.

 

[paddingtongreen]

you must develop the bars far enough to anchor the segment of the pier that anchors the bolts. In other words, there must be an elevation, above which, the bars are developed, and below which, the bolts are developed.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[haynewp]

But what if the anchor bolt forces are not high enough that the straight bars are required to be fully developed? What if large straight bars were used, like #7 bars.

 

[JAE]

haynewp - I'm not sure why the word "flexural" is found in section 12.2.5.

It seems to me that the principle of As(req'd) / As(prov) would be the same other than perhaps there is concern that with a flexural member there might be built-in redundancies while in a pure tension member you might be non-redundant and thus taking liberties with a full development would be undesirable.

 

[haynewp]

This whole thing seems pretty strict to make you completely develop a straight bar to transfer tension. If you extend up #5 bars to development them in a pier, then you are fine if it meets the tension demand. If they put in #6 bars instead and are a couple of inches short of their development length, then it does not meet code.

As a side note, for shear friction it is also required to have full development on either side of the crack.