Development Length of bars in compression

[JoshPlumSE]

This is really more of a question about RISAFoot rather than RISA-3D. But, since there isn't a forum for that program I thought I would post it here.

For years RISAFoot has reported the required straight development length for the pedestal bars compared to the available development length based on the thickness of the footing.

For tension these bars are typically hooked and will, therefore, have enough development length. For compression, however, the thickness of the footing limits how much available development length there is. The result is a failed development check in most cases.

Some comments / questions:

1) The most popular footing design example we have is in the PCA notes. This is the section where they require the full development length for bars in compression. They even imply that the designer needs to increase the thickness of the footing if this development length isn't met.

2) Their rationale for this is ACI code section 12.5.5 which says, "Hooks shall not be considered effective in developing bars in compression.".

3) What's not said in this example is WHY you need to develop these bars in compression. What type of failure are we going to see in these hooked bars if they are not developed in compression? For compression, I would personally be more worried about concrete bearing failure, soil failure or punching shear.

Is anyone out there actually using this compression development length as a requirement? Talking myself through this, I'm guessing that folks are really using the 12.2.5 reduction "based on excess reinforcment" to reduce the required development length?

This is something that has always bothered me about the program. It just never agreed with my past design experience. Therefore, I thought that this forum might be a good place to broach the subject. Any suggestions or insight about the PROPER way to develop these bars would be appreciated.

 

[ishvaaag]

I practice in Spain and in one case we were directed to ensure whole strength development with the straight lengths in footings by one institution charged with reviewing projects in order to insurance. We made then use of welded stirrups that can be here -Spain- accounted to pass to the concrete the compression force (in shear-friction like manner), hence serving to reduce the length required for development, and hence the footing thickness.

 

[baw1]

I have used compression development length in columns for cases where the column will not see tension.
I would agree to be more worried about concrete compressive stress. In a footing the column/pedestal steel would be more confined by surrrounding concrete and I would be less worried about steel buckling, as you might be in the column.

 

[haynewp]

If the straight portion of the hooked compression bars bringing axial load across the concrete interface is not adequately developed, I would think the bottom of the footing could blow out beneath the bar locations. Punching through of the compression bars and a bearing failure at the top of the footing/column interface should then be possible.

If the footing is plenty thick but the bars are not developed into it sufficiently, then I would expect that the concrete directly beneath where the bars terminate could crush since there is not adequate bond length above which would allow a small slip of the bars. Is that what you are asking?

I have gone through the provisions for reducing the development length required because of excess reinf. before also. I don't see anything wrong with using that.

 

[JoshPlumSE]

Haynewp -

Yes, I think you have it basically correct. The only way these pedestal bars work is if you take the reduction in the development length allowed because of excess reinforcment.

In practice, I have never actually seen anyone do that calculation.... But, that's one of the reason why it will end up being a valuable addition to the program.

Josh

 

[Stazz]

I know I'm a little late to this party, but wouldn't the reduction result in a development length of 0?

First off, all of the pedastals I have designed only require minimum steel per ACI - 10.9.1 (1% of Ag) but I usually use less per 10.8.4 (0.5% of Ag) which allows you to reduce the Ag to the amount that is only required for strength but limited up to 1/2 Ag. Now usually the Ag in my pedastals are governed by my baseplate which causes the pedastal to be overdesigned with a FOS of over 5.

So when using the development length reduction factor, the numerator (As required) would be 0 since no steel is required for strength therefore putting the factor at 0 and your development length at 0.

Is this right?

 

[JoshPlumSE]

Stazz -

Check out the related thread that I started over in the ACI group.

http://www.eng-tips.com/viewthread.cfm?qid=228427

My final take on it was that if you need the bars for the capacity of the column / pedestal above, then you have to develop them. Of course, the key is that you can reduce the compression development by As(req'd)/As(prov).... Which should dramatically reduce this length.

The only problem then becomes how to calculate As required for a column type section. The calculation is easy for a singly reinforced section, but is not easy for a section with reinforcing on all faces.

 

[Stazz]

I would think As req is the amount of steel you need in excess of the concrete (typically none).

Whats the difference between a singly reinforced section
and a section with steel on all faces? Are you thinking of a non uniform stress distribution if theres moment in the column?


Also, I still don't understand the concern. Do we need to develop this steel even if its not needed for strength?

Actually, looking at it in a composite analysis, I can visualize/understand that even though the steel isn't needed, it still will take alot of the axial load because it has greater stiffness than the concrete, so this load will need to be released into the footing and if there isn't enough development length then there will be too much bearing stress directly under the head of the reinforcement bar which will cause the concrete to crush. I can see that concern.

 

[JoshPlumSE]

Stazz -

If you design the pedestal as a singly reinforced section where no compression steel is assumed to exist, then you do not need to develop those bars at all.

However, if you calculate the capacity of the pedestal based on including the contribution of the reinforcement from all 4 faces (as RISAFoot does) then the assumption is that the reinforcment has to be fully developed for you to reach that capacity. Right? This full development would be based on the required compression steel development length in the code.

The code then allows you to reduce this development based on As_required / As Provided. This As_required is then the challenging thing to calculate. We could estimate this ratio as the code check ratio, but how accurate is that really?

 

[ishvaaag]

I would appreciate opinions on if the approach of passing the rebar-force in shear friction (through properly welded stirrups rebar) to the concrete would be tolerable in american practice. Here (Spain) surely is, for transverse welded bars (such in welded rebar grids) are used as a device to shorten development length, and in fact is in full accord with what the generally accepted theory of shear-friction is. To some extent, of course, at some point a typical punching failure may occur, reinforcement compressive force development likely not being then the controlling issue.