Existing Composite Steel Beams < 25% Full Composite

[Boiler106]

Using the governing AISC 360-10, i have determined that an existing building has beams that are less than 25% fully composite, however, they are still adequate by the numbers.

This is less than the *recommended* limit of 25% by AISC and it does not conform to the latest AISC ductility requirements given in the commentary section I3.2d. Neither of which are codified, as i understand.

I have my own opinion, but I'm looking for others on how might you address this in a peer review. Thoughts?

 

[dik]

With the Canadian steel code, if memory serves, you require 25% for calculation of deflections but not for strength. I think you require 40% for flexure... bit fuzzy; it's been decades.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JLNJ]

I do not know the rationale for removing the 25% requirement from the Specification and burying it in the Commentary.

The Commentary does say that the partially composite "effective" section modulus formula given in C-I3-4 "should" not be used for ratios less than 25%. If I were the peer reviewer, I'd say, "OK, the code doesn't prevent you from using less than the 25%, but how did you determine the partially composite section properties if you are advised not to use C-I3-4?"

I find C-I3-4 to be pretty generous since you are using the root of the stud ratio. Say you have just 1/4 of the studs you need for full composite action, S_s=50, and S_tr=200. You still end up with an S_eff of 125, which is 62.5% of S_tr. The most bang for your buck is right at the 25% minimum. There must be some testing to back this up. Below 25% maybe there is a risk of sudden slippage and non-ductile failure.

 

[SethGuthrie]

In Ram Structural System we used to enforce this lower bound % composite action, but in more recent versions we have made it optional (through the stud criteria dialog), so that users can evaluate beams that are < 25% composite. I know from the various users I have helped that this is pretty commonly done. The language of the code has also changed over the years, so it is more of a recommendation than a requirement now.

 

[JLNJ]

Seth

So what does RAM use to determine the partially effective properties if AISC is saying NOT to use C-I3-4?

 

[SethGuthrie]

Here is a page from the program manual that gets into that. Basically we don't change our methods if the option is set to design it compositely.

 

[JoshPlumSE]

i have determined that an existing building has beams that are less than 25% fully composite, however, they are still adequate by the numbers.

What do you mean by "they are still adequate by the numbers"?

My tendency is to design members like these as non-composite. I allow for minimum studs for deck connectivity and such. But, I wouldn't want to rely on composite action for deflection or strength.

If I had to do it, then I'd use an I_Lower_Bound moment of inertia for deflection (C-I3-1). If that allows the member to pass deflection checks, then I'd be good with those checks. However, I'd have an issue with strength calculations if it doesn't pass as a naked steel beam.

 

[dik]

When I was looking at composite action 30 years back... papers indicated that with even button welds connecting deck to the beams, although not accounted for they provided composite action to loading in excess of 50% of the design load... I was looking at composite loading for OWSJs at the time.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Boiler106]

JLNJ - i think we're on the same page as to how to approach this, by putting it back on the designer rather than making a binary judgment.

Seth - thanks for weighing in. It's interesting that engineers are going below the limit despite long standing recommendations. Also, I'm using Ram for my analysis and we've spoken many times on the phone over the length of my career.

josh - thanks for your response but im looking for how to approach this from a peer review perspective

 

[dik]

I've often used 25% composite for deflection (reason for using nominal headed connectors, else I don't use them... and 50% min for composite action. The second floor of the Cornwall Centre in Regina was designed for 50% composite. I sent 3 or 4 different framing systems out to a local contractor (VK Mason) for costing... 3" SLW concrete on T30V deck... provided a 2hr fire rating at the same time.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik