Composite Beam Design - 9th vs 13th

[rharting]

I've recently noticed that in the 9th edition, Vh is defined as the maximum of 50% of the concrete beam width [Vh=(0.5)*0.85*F'c*Ac] and 50% of the steel beam section (Vh =0.5 As*Fy). The thirteenth edition calculates V' as 100% of these same areas (eq. I3-1a V'=0.85*f'c*Ac or I3-1b V'=AsFy).

The 9th edition states in the code to use a minimum or 25% of Vh which then equates to a minimum of:
0.25*0.5*0.85f'cAc
0.25*0.5*AsFy

The Thirteenth commentary (page 16.1-317) states "Thus Designers should be careful when evaluating the strength of existing composite beams with 50% composite action" with % composite action being defined as sumQn/AsFy. In essense, AISC is saying that we should be careful when looking at composite beams when sumQn<0.5AsFy.

So 2 questions immediately come to mind:
1)Existing beams originally designed with composite action below 50% are now below 25% composite action. Should engineers be concerned with this change?
2)Existing beams originally designed with 100% composite action (sum Qn=0.5AsFy) are now considered to have 50% composite action. Thus any existing beam designed with the ninth edition as partially composite will fall below this 50% threshold. What precautions should engineers take?

 

[a2mfk]

I think we are find if we keep this a secret from existing beams. Shhhh.... They'll never know, and just keep on thinking they are composite.

 

[Lion06]

I think you're trying to compare apples to oranges. There are two somewhat significant differences between the 9th ed. and the 13th ed. when it comes to composite design.

1) This goes directly to your questions on % composite action - The 9th ed. uses an allowable shear stud value, while the 13th ed. uses a nominal (unreduced) value. The end result is that you end up with a comparable number of studs regardless of which method you use (if you take (FyAs/2)/(Qn/2) you'll get the same number as FyAs/Qn. Granted the factor for Qn might not be exactly 2, but it's not 1.

2) The 9th ed. had a limit for unshored construction on the tension stress in the steel. The 13th ed. has no such limit, only an ultimate moment. The moment capacities for unshored, composite beams are higher using the approach in the 13th ed.

 

[rharting]

PA (Congrats on getting your PE btw)...

I'm not sure I totally agree that it's apples to oranges. The change definitely poses a problem. Consider this a A992 gr. 50 W18x40 As=11.8 in^2(assume slab is large enough and steel controls) w/ 3/4" dia studs with the deck running perpendicular and f'c = 3.5 ksi

9th edition (Qnall=12.5 K/stud)
25% composite action = 6 studs per side
100% composite action = 24 studs per side

13th edition (Qn = 15.91 K/stud nominal)
25% action = 10K per side
100% composite action = 38 studs per side
50% threshold where 'special consideration needs to be applied' = 19 studs per side (which would equate to 80% composite action under the 9th edition).

So if this is indicative of other designs, anything designed as less than 80% composite action under the green book requires caution? And anything designed under 41% under the green book(10Studs/24studs) now falls below 25% composite action under the black book (currently programs like enercalc will not design a W18x40 with anything less than 10 studs).

 

[Lion06]

rharting-
Thanks for the congrats.

I see the 12.5 k/stud in the green book. Where is the 15.91 k/stud from the black book coming from? I see a range of 17.2 to 21.5 (assuming 1 stud per row and depending on weak or strong).

I expected a slight variation in the number of studs for a given % of composite action (as noted in my first post with the comment that the factor on Qn isn't exactly 2). The point I was trying to make is that just because the 9th ed. uses FyAs/2 or 0.85f'cbh/2 doesn't mean that it is off by a factor of 2 from the 13th ed. and the reason is that the stud shear values aren't the same between the two versions.

 

[rharting]

I will always assume weak location in existing conditions. I can't dictate where the studs were put (and trying to locate their location would be impossible).

I was using 60ksi for the stud strength, but i see i should be using 65. Which changes it slightly to...
25% - 9 studs
100% -35 studs
50% - 18 Studs (which is still 76% composite under the green book).

So essentially if it was designed under that 76%, AISC believes we should be 'careful'... that gives me some unrest.