Composite structure in AISC

[drasticxx]

Hi guys

We all know the concept of development length for steel bars in concrete(ACI CODE),now of composite beams with I steel beam (AISC)for example if the steel and concrete are acting together using steel concrete bond or using shear studs ,in this case there is no need for development length principle(as ACI saying) and I can cut the (I) steel section at face of support,is that what AISC is saying

To summarize I steel section can be cut at face of support(when shear studs and concrete-steel bond ensuring there is no slip between the two materials),,is that true, or is AISC handling the problem in different way?

Thank you all

 

[KootK]

In my opinion, there's always some version of development length at play in composite members. A flexural cross section can't utilize composite elements unless you can push/yank on them. And you can't yank on something unless it's developed in some way.

In order of preference, I would either:

1) Extend your steel section over the support some distance or:

2) Evaluate the section's ability to resist load without considering the steel section until the steel section is developed. Partial development can be utilized.

We can provide more help if you supply more detail about your problem. Span, loads, member sizes, reinforcing, support detail....


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.

 

[wannabeSE]

Typically, shear studs are required on both sides of the maximum moment and equally spaced between the point of maximum moment and zero moment. Also, the shear studs must be at least 8" from the edge of concrete in the direction of shear. I don't think the concept of development length apply to composite beams with shear connectors. The shear connectors just need to be strong enough for the shear and not break out of the concrete.

I don't know anything about designing encased composite beams that rely on bond strength rather than shear connectors. Does AISC have provisions for encased composite beams without shear connectors?

 

[KootK]

AISC gives you three options for encased beam design. At least one of them allows for shear transfer via bond. Testing actually indicates pretty good performance.

In a normal composite beam (deck slab & studs), the tensile capacity of the steel beam isn't available until close to midspan, where enough studs are in place beyond the inflection point. I consider that the "development" of the steel section. Mostly just semantics.



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.

 

[drasticxx]

KOOTK,

I have to stay, I am so proud of your knowledge and way of thinking..so happy to be here with us in the forum.

Regarding our point,I don't have specific example,I am just try to generate way of thinking, as you said many research has utilize the ability of studs or bond to prevent slip and achieve section capacity before slipping(In middle span), but for near the support, I didn't see anything, so I raise my question, if there is a way of solution using AISC, and accordingly what you said regarding development length make sense ,but is it proven by the code ,and what equation shall we follow?or this area may need more research as far as i can see?

 

[KootK]

Thanks for the kind words drasticxx.

but for near the support, I didn't see anything, so I raise my question, if there is a way of solution using AISC, and accordingly what you said regarding development length make sense ,but is it proven by the code ,and what equation shall we follow?

I don't know of anything in AISC that directly addresses this situation so we might be stuck with first principle, make it up as you go, design. The usual case with encased beams is that of a steel beam already supported by a column that subsequently has concrete cast around it. As such, reaction shear transfer into the steel beam happens instantaneously and stresses can migrate into the concrete gradually, as shear transfer allows.

In the scenario that you've proposed, it is the reverse. Reaction shear starts off being 100% in the concrete and has to quickly make its way into the steel. The detail below shows one way of accomplishing that. In my opinion, the biggest "development" issue is anchoring that first concrete strut that emanates up from the support. The situation is quite similar to the ACI provisions for anchoring positive flexural steel at supports and, effectively, extends the steel beam to the support.

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.

 

[drasticxx]

Thank you again