Development of Hooked bars in compression
Development of Hooked bars in compression
(OP)
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?
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?





RE: Development of Hooked bars in compression
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?
RE: Development of Hooked bars in compression
Dik
RE: Development of Hooked bars in compression
So if you are relying on the ultimate axial capacity of the column just above the footing the bars have to be developed there.
RE: Development of Hooked bars in compression
RE: Development of Hooked bars in compression
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.
RE: Development of Hooked bars in compression
RE: Development of Hooked bars in compression
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?
RE: Development of Hooked bars in compression
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.
RE: Development of Hooked bars in compression
RE: Development of Hooked bars in compression
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.
RE: Development of Hooked bars in compression
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.
RE: Development of Hooked bars in compression
RE: Development of Hooked bars in compression
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.
RE: Development of Hooked bars in compression