Question on ACI ld calc

[Lion06]

ACI 318-05, section 12.2.5 gives the provision to reduce develpment length based on excess reinforcement. I just noticed the "flexural member" part of it - is that to say that you can't use this provision to reduce the development length for a pier subject to uplift and not bending?

 

[BAretired]

It seems logical to assume that the provision would apply to the pier as well.

BA

 

[TXStructural]

12.2.5 does not apply. The reinforcing you have is not for flexure.

If the pier is sized for uplift, then it requires reinforcement to maintain strength for axial tension. Once the depth is reached where tensile capacity is no longer require, reinforcement is often discontinued. The bars need to be developed by the time there is demand for them. Bars can be discontinued as you move down a pier which is restrained by side friction along its length, but those bars need to extend a full development length beyond where they are required.

 

[Lion06]

I'm actually more along the lines of APP. D for a regular pier, not a drilled pier. I'm not actually dealing with this situation right now, but say you have a braced frame column coming down onto a pier which sits atop a caisson. Say the has a single
diagonal coming UP to this column at the first floor such that this pier experiences only axial load (I.e. no flexure). It's just a fact of App. D that the concrete beakout is not going to work and you're going to have to develop the pier bars above the failure plane of the breakout cone. It seems unreasonable that minimum steel for compression (1%), that might be 400% of what you need for tension, won't allow you to reduce the development length.

I know you can hook the bars, but hooking (12) #8's in a 2'-6" pier causes a lot of congestion. The hook is 16" long which means the hooks from opposite sides of the pier are overlapping.

 

[WillisV]

SEIT - another important point to consider is that you can only reduce development length with that provision, not lap length. Not strictly applicable to your situation, but something to keep in mind.

You could consider using headed reinforcement instead of hooks. Dev. lengths in 318-08 for t-heads.

 

[Lion06]

What if it's subject to flexure and net tension?

 

[TXStructural]

Your lap splice length is for the smaller of the bars being spliced. If there is significant difference in your reinforcement, consider using dowels for the strength you need rather than continuing the pier reinforcement up as dowels. This will allow placement of the dowels to fit the column above. Using more, smaller dowels will reduce the development length.

Axially-loaded columns, not subject to cyclic/seismic loading are frequently better constructed using fewer large bars (not more small diameter bars) but this increases development and lap lengths.

You could also consider mechanical splices to eliminate the lap entirely.

 

[BAretired]

The original question was...can the development length be reduced if the bars are not stressed to their ultimate? To me, the obvious answer is yes, but I do not have access to ACI318-05, so cannot confirm that it agrees with my thinking.

If it does not, I would be interested in knowing why not.

BA

 

[TXStructural]

The easy answer is because a bar is developed or not. The Ld is not based on a single mechanism, and includes considerations other than axial bar stress and bond to concrete. Ld is not simply a function of how far it takes to develop a single bar. Read the commentary at the beginning of ACI 318 Chapter 12 for a summary.
--
MacGregor & Wight, 4th ed. says, with regard to the reduction of development development length of excess flexural reinforcement: "If room is available, it is a good practice to ignore this factor, thus ensuring that the steel is fully anchored even if a change in the use of the structure requires the bars to be fully stressed. In statically indeterminant structures, the increased stiffness resulting from the additional reinforcement can lead to higher moments at the section with excess reinforcement. In such a case, the steel is more highly stressed than would be expected from the ratio of areas. This multiplier is not applied in the design of members resisting seismic loads."

 

[BAretired]

TXStructural,

I find your explanation confusing. And, I believe you are mistaken. I suspect that Jim MacGregor would agree with me. Bars stressed below their maximum capacity require less development length than bars stressed to their maximum. How is that a problem? It seems obvious to me.

BA

 

[Lion06]

I'm thinking of a steel braced frame.
One case will have the pier in compression, so it needs to have 1% min steel. That qty of steel could be significantly more than needed when the pier sees uplift, but the bars are there.

Does any level of bending qualify the member as "flexural" with or without the presence of net tension?

 

[bones206]

In section 12.3.3, the same reduction factor is listed for bars in compression, without the "flexural member" wording. The same goes for hooked bars in 12.5.3. Doesn't seem consistent.

I never noticed the restriction to flexural members either. Although, it has always been my practice to ignore the excess reinforcement factor. Looking back to my notes from concrete design class in college, section 12.2.5 is circled with a big X drawn over it and the words "NO NO NO" written below... My professor made a point to tell us this provision was not always conservative and should be avoided.

 

[larsacious]

BAretired,

I agree with TXStructural. Consider when you calculate the cracked moment of inertia of a concrete section. If you are using extra steel to reduce development length it will increased the cracked moment of inertia and thus draw more moment to supports. If these larger bars are not developed sufficently the section could possibly fail or severe cracking will occur before the moments are redistributed. Also, for seismic detailing it is essential that ductility is achieved and one of the factors to achieve this is proper development length.

There are times when reducing the development length by using the As,req/As,prov is necessary. However, it is not a good practice and whenever possible this should be avoided.

 

[BAretired]

I was under the impression we were talking about a pier subject to uplift without bending. The extra steel is not being used with the goal of reducing development length, but if minimum steel is four times the amount required to develop the applied tension, why would you not be justified in reducing development length?

BA

 

[TXStructural]

After re-reading the thread, I think I understand. You want to know if you can lap the anchor rods less than a full development length with the pier longitudinal reinforcing which is very near the top of the pier and terminates without hooking.

In the case of lap splicing, the minimum required length is that required for the smaller diameter bar. This agrees with your desire to shorten development length.

Using App. D figure RD5.2.9, lap smaller bars as needed to meet the uplift needs and develop these up and down. If you plan to embed long anchor rods, they could extend to the same depth required to develop a rebar with equivalent capacity.

So, yes, you can reduce the required embed length, which is not the same as putting in less reinforcement. Look at 15.8 for some info about similar situations with concrete columns attaching to foundations.

As far as minimum reinforcement, part of the requirement is to ensure integrity of the mass of concrete and assure that it acts as reinforced concrete, rather than plain. Also, consider that lightly reinforced sections can have significant cracking under tension, since crack width is directly proportional to area of steel crossing the crack according to dL=PL/AE