ACI 318 & footing - development of flexural reinforcement VS development of deformed bars in ten

[LearnerN]

In designing the flexural reinforcement in a footing, the development length calculated is of deformed bars in TENSION, and the development length distance is from the edge of the pier (as the critical section). This makes sense since the moment in the footing will be putting the steel in tension.

However, in ACI 318, there's also a specific section on the development of flexural reinforcement. But upon reading this section, is this moreso for beams instead of footings? Based on the wording in this section, it appears the flexural r/f section on development length is referring to flexural members/beams where there are points of max stress like at the face of supports or near the middle of the span of a beam. Does this section have any relevance for footing design?

 

[KootK]

For a square, concentrically loaded footing where 100% of the reinforcing will run the full length of the footing, development length is really the only concern.

That being said, I believe that the provisions for the development of flexural reinforcement do apply to all flexural members. There are more complex foundation elements out there that deserve the extra attention.

A raft foundation, for example, is very much like an upside down two way floor slab and many of the same concerns apply. Another common example is that of a load bearing wall at the edge of footing. Turned upside down, this is is the same as a slab bearing on a wall and, thus, the provisions for the anchorage of positive flexural reinforcement at supports becomes an issue.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

This brings up the issue of whether or not to cog (hook) footing bars. I do, some don't.

 

[KootK]

I've never understood the debate over the cogging, if that is indeed a word. To me, it seems like just a matter of calculation/detailing. The reinforcement has to be developed, somehow, beyond the critical section. Any bars that are hooked, could be made straight by either using smaller bars or extending the footing. It just comes down to the economic balance between rebar tonnage, concrete volume, handling, shipping, prefabrication, and excavation. Yeah, just those things.

I started out using straight bars exclusively, assuming that contractors would balk at the hooks due to complications in prefabrication and transport. I've hooked some footing bars now on some larger projects and have yet to hear a complaint.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

KootK...optimization is good and necessary, but there are many, many applications where hooking the bars makes a whole lot more sense than increasing the size of the footing to achieve a development length. I've generally considered hooking bars to be superior to additional length for development.

Cogging is a word, mostly used in Oz. It is similar to what we would do with anchor bolts by adding a washer and nut to the end rather than hooking the anchor bolt. Cogging rebar varies in process, but can be done as a "mushrooming" of the rebar end. I think cogging is used with 90 deg bends rather than 180 hooks.

hokie66....pls correct me if I'm off on the cogging process!

 

[KootK]

@Ron:

Thanks for the response. Much like the CRSI manual, I rarely encounter situations where development issues can't be solved simply through a more intelligent selection of rebar size. No footing extension required. All other things equal, is a hook better than no hook? Sure. But an engineer's job isn't to make the strongest possible structure. Rather, our job is to make the most economical structure that is strong enough.

I may in fact need an education on cogs. I thought that a cog was just a hook and was just wondering if the verb form "cogging" was commonly used by those in the as we might use "hooking".

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

KootK,
In Australia, 90 degree hooks are commonly, but not always, called cogs. 180 or 135, it is a hook. Yes, the verb forms "cogged" and "cogging" are used.