Development of Hooked bars in compression

[JoshPlumSE]

This question relates mostly to footing and foundations. Specifically, development the pedestal bars (or dowels) that hook into the Footing or Foundation Slab.

I'm looking at a footing design in the PCA notes. In this example 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. This seems somewhat overkill based on my past experience.

The rationale given in the PCA notes is ACI code section 12.5.5 which says, "Hooks shall not be considered effective in developing bars in compression.""

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? I know that I've never worried about it much.

Walking through the logic, perhaps the issue is really that if you run all the numbers then section 12.2.5 and its reduction ""based on excess reinforcment"" reduces the development length of these bars to levels that don't realistically control. That sounds like a more reasonable path forward than the one given in the PCA notes.

What do you guys (and gals!) think? Is my take on this reasonable?

 

[JAE]

JoshPlum,

I would agree with you that many engineers probably do not do this check.

The commentary for 15.8.2.1 indicates that it is to provide "a degree of structural integrity during the construction stage and during the life of teh structure."

As(req'd)/As(prov) is the way to go here. Usually, most engineers that I know simply take the wall or column vertical bars and use the same size/spacing into the footing. Of course smaller bars would reduce this development length further.

Section 12.3.1 also provides an absolute minimum of 8" so your footing generally would have to be 8" plus 3" cover = 11" thick in any case. Sort of kills the 10" footing usage doesn't it?

 

[dik]

If column f'c is greater than footing f'c then there might be an issue, also area of footing is much greater than area of column (usually) and the higher compressive strength based on confinement can be used.

Dik

 

[rapt]

For the column capacity to be achieved at any cross-section over its depth the bars have to be developed at that cross-section

So if you are relying on the ultimate axial capacity of the column just above the footing the bars have to be developed there.

 

[hokie66]

I don't agree that you necessarily have to develop the bars in a column into the footing. As Dik said, you can use the bearing surface provisions which allow you to increase the concrete stress in the footing.

 

[JoshPlumSE]

Rapt -

I agree that any bars that are in TENSION have to be developed.... otherwise the bond stress between the bars and the concrete will break down. That development is required to keep the steel and the concrete acting together.

But, what type of failure are we expecting when the bars and the concrete go into compression? If the strain is the same in the bars and concrete (which is what we assume for compresion)then we're not as worried. I don't necessarily disagree with you.... Though I wouldn't mind a code section where you are getting this requirement for compression bars.

But, my question is actually more along the lines of:
1) WHY do we have to develop these bars in compression. What are we protecting against? I want a theoretical understanding of the concept as well as a practical knowledge of the governing code section.

2) There just isn't enough room in most footings for this development. Therefore, I'm curious what code provisions people are using to justify this.

 

[miecz]

If you cut a section just above your footing, and you are depending on the reinforcing at that section, then it must be developed in compression. Otherwise, the steel may slip and lose compression, which would be transfered to the concrete, which in turn is overstressed.

 

[JAE]

JoshPlum:

1) miecz seems to be correct in why develop the bars. And my reference above to the ACI commentary on this seems to also imply that general structural integrity and "good practice" is what it's all about. Probably, there are not technical or test result-oriented reasons for it.

2) I think a lot of engineers blow off the development by perceiving that the concrete is simply bearing on other concrete at the interface and thus there is no need.

Finally, I don't ever recall hearing of a failure or failure mechanism where someone didn't develop their compression bars into a footing....have you?

 

[JoshPlumSE]

Development of compression bars is required.... Period.

I can see folks debating about the nature of this development and whether it needs to be the same as the tension development length. Yada, yada, yads.

However, conceptually you just have to give some development for that bar if you're going to use it at that location. Otherwise you could terminate it half an inch past the pedestal / footing interface.

The practical question is whether the hook development that you provide for the bar when it is in tension is sufficient for when that bar is in compression.... Because that is what folks are actually providing.

I think the key is the As_required / As_provided will allow a good reduction. In practicality, I think this is what could be used to justify the amount of development length normally provided.

The only problem then becomes calculating As_required. For a Pedestal with reinforcement on all four sides that isn't as straight forward as it sounds.

 

[jike]

I thought I read someplace in the code where hooks cannot be used to develop compression.

 

[JAE]

Hooks can't develop in compression - what you do is count on the vertical dowel length exclusive of the hook to see if you have enough length.

The minimum compression length for any bar is 8" and this raises the question as to how you could ever use a 10" thick footing and develop the bar with 3" bottom cover.

 

[ash060]

At the interface of the column and footing there is just bearing. Usually that works without a problem unless the concrete strengths between the footing and the column are very different.

The code does require a minimum amount of dowel bars, but standard practice is to provided matching dowels to the column reinforcing, so there is a great excess of reinforcing. Which will lower your development lengths.

Check PCA Notes on ACI 318-05 examples 22.4 and 22.5 for the requirements for development.

 

[miecz]

I am surprised that PCA Notes Example 22.5 calculates the required area of dowel bars according to the bearing strength, ACI Code Ref. 10.17.1. For this example with no bending moment, I would have used formula 10-2 for column strength. Using the formula for bearing satisfies the development for the section cut at the interface of the column and the footing, but it doesn't satisfy the required development for the reinforcing in the section just above the interface of the column and the footing.

 

[JoshPlumSE]

Just an FYI for everyone about what type of foundation started this thread. The types of pedestals that I'm referring to support steel structures. The base plate and anchor bolts connect the steel to a relatively short concrete pedestal.

The only purpose for the pedestal is connect the above grade column to the below grade footing. Typical height for these pedestals would be 2 to 5 ft. The amount of moment in these pedestals is typically very small compared to the 0.5% requirement for the pedestal steel.

When you're doweling into a 36 inch mat slab, then you're in pretty good shape for development. But, when you're going into an 12~18" footing then the straight bar development length (un-reduced) may not be provided.

The whole question stemmed from whether this requirement was overkill. My beief is that it's really not overkill when you look at the reductions in development lengths allowed for sections with reserve capacity.... though I still suspect that there aren't many engineers who ever consider this compression development length in their footing design.

 

[kslee1000]

I adimit never thought of this issue because the pedestal (subjects to axial compression only) is usually sized to carry compressive force by concrete section alone. For As (req'd) = 0, all bars are practically provided to avoid incidental loadings and seperation of concrete elements, which were, I think, the intents of the ACI minimum steel requirements, althouth not explicitly explained.