EngineerGeneral
Structural
How wide does the concrete slab need to be over the edge beam to have full composite action?
Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Composite Beam Question
- Thread starter EngineerGeneral
- Start date
- Status
LobstaEata
Structural
I recall 3 tests to determine the effective slab width (b) for computation of composite beam properties:
1. 1/4 x span
2. Beam spacing c to c
3. 12 x slab thickness
Use the lower value of the three.
Hope this helps.
1. 1/4 x span
2. Beam spacing c to c
3. 12 x slab thickness
Use the lower value of the three.
Hope this helps.
DaveAtkins
Structural
LobstaEata,
ThuSuperDore is asking about an edge beam.
My 1989 ASD says that shear connectors shall have at least 1 inch of lateral concrete cover.
DaveAtkins
ThuSuperDore is asking about an edge beam.
My 1989 ASD says that shear connectors shall have at least 1 inch of lateral concrete cover.
DaveAtkins
LobstaEata
Structural
Understood. My response considers that an edge been would have at least an inch of cover beyond the edge of flange, therefore the 1 inch lateral cover over connectors located along the centerline of beam would be greater that that. Call me crazy.
WOW now, hold on there. Your question is about like asking what color a circle needs to be for it to have an area of 4 in.^2!!!
A fully composite beam has enough shear connectors so that Sum(Qn) is at least as big as AsFy or 0.85f'cAc. For example, if Sum(Qn)/AsFy = 0.4, then it's 40% composite.
Like someone typed, there are limits on rib width, etc. for the situation to be valid at all. Check Section I3.2 of your 2005 AISC Spec. These do NOT tell you anything about whether a beam is fully composite.
A fully composite beam has enough shear connectors so that Sum(Qn) is at least as big as AsFy or 0.85f'cAc. For example, if Sum(Qn)/AsFy = 0.4, then it's 40% composite.
Like someone typed, there are limits on rib width, etc. for the situation to be valid at all. Check Section I3.2 of your 2005 AISC Spec. These do NOT tell you anything about whether a beam is fully composite.
LobstaEata
Structural
27...You bring up a fine point about the number of shear connectors indeed. I suppose I'm making too many assumptions with regard to the narrow request in the question.....and Dave....please accept my apology for such a curt response to your observation about what the questioner was asking. You are indeed correct.
OK...If it is considered that there is sufficient shear transfer between the steel and concrete, I beleive the code says that the limiting design value to be taken for full composite action must be the smaller of steel yielding or 85% of the concrete crushing limits. (However in a broader context, simply limiting the design value without regard to the overload failure mechanism might be considered unwise.)
I don't beleive either of these two strength limit states have anything to do with arriving at a "partial composite" percentage, as suggested in 27's post however. The AISC 9th edition Section I.4 states that partial composite action is determined by "q" times the number of connectors furnished between the point of maximum moment and the nearest point of zero moment, not the limiting strength of the two materials being joined.
My 4 sq.in. circle is blue.
OK...If it is considered that there is sufficient shear transfer between the steel and concrete, I beleive the code says that the limiting design value to be taken for full composite action must be the smaller of steel yielding or 85% of the concrete crushing limits. (However in a broader context, simply limiting the design value without regard to the overload failure mechanism might be considered unwise.)
I don't beleive either of these two strength limit states have anything to do with arriving at a "partial composite" percentage, as suggested in 27's post however. The AISC 9th edition Section I.4 states that partial composite action is determined by "q" times the number of connectors furnished between the point of maximum moment and the nearest point of zero moment, not the limiting strength of the two materials being joined.
My 4 sq.in. circle is blue.
LobstaEata (great name BTW), I never would've guessed you be an Obama voter!! My circle is red, LOL! Unfortunately, most circles were red. Oh well--I don't like McCain anyway.
I wasn't trying to say what was implied in your third paragraph. AsFy, 0.85f'cAc, and Sum(Qn) are just computed to set V' (talking 2005 AISC here, not 1989, which is very old when it comes to composite beam design). V' gives us what we need to figure out hte exact plastic stress distribution that's used to compute Mn.
I don't think of 0.85f'cAc and AsFy as "limit states," but just numbers that we use to help d the plastic stress distribution.
The real point was that the original question is invalid.
I wasn't trying to say what was implied in your third paragraph. AsFy, 0.85f'cAc, and Sum(Qn) are just computed to set V' (talking 2005 AISC here, not 1989, which is very old when it comes to composite beam design). V' gives us what we need to figure out hte exact plastic stress distribution that's used to compute Mn.
I don't think of 0.85f'cAc and AsFy as "limit states," but just numbers that we use to help d the plastic stress distribution.
The real point was that the original question is invalid.
- Thread starter
- #8
EngineerGeneral
Structural
Thank y'all for inputs. I believe I have seen something like: the concrete slab shoud have XX lateral cover over the studs at the edge of the slab.
So if I have W12x26 beams and 4" slab with 3/4" dia studs, how much slab I need over the center of an edge beam?
I don't think it's an invalid question.
So if I have W12x26 beams and 4" slab with 3/4" dia studs, how much slab I need over the center of an edge beam?
I don't think it's an invalid question.
LobstaEata
Structural
Ha...I didn't link the blue circle with Obama right away, so thanks for the chuckle. I did vote for the man by the way...since I just couldn't bring myself to cast a ballot that put moosehunt barbie a wheeze away from the presidency.
Keep in mind that I come from a generation that chooses to part with the green book only after rigor has set in. Not that there is anything wrong with LFRD. I consider myself to be enlightened in these times of change. In fact, some of my best friends are plastic.
You are quite correct about the original question though.
Thanks for the humor!
Keep in mind that I come from a generation that chooses to part with the green book only after rigor has set in. Not that there is anything wrong with LFRD. I consider myself to be enlightened in these times of change. In fact, some of my best friends are plastic.
You are quite correct about the original question though.
Thanks for the humor!
"I believe I have seen something like: the concrete slab shoud have XX lateral cover over the studs at the edge of the slab."
AISC Spec. Section I3.2d(6): "Shear connectors shall have at least 1 in. of lateral concrete cover, except..."
"So if I have W12x26 beams and 4" slab with 3/4" dia studs, how much slab I need over the center of an edge beam?"
The above section seems to answer this question, although I think I'd personally like to have more cover than that. This is usually not an issue because one would normally have the edge of slab at least to the edge of the W12x26 resulting in at least 3.25" lateral cover.
"I don't think it's an invalid question."
These questions are fine. The original question was invalid because it asked what cover results in fully-composite. Whether or not a beam is fully composite depends on the number of studs instead.
AISC Spec. Section I3.2d(6): "Shear connectors shall have at least 1 in. of lateral concrete cover, except..."
"So if I have W12x26 beams and 4" slab with 3/4" dia studs, how much slab I need over the center of an edge beam?"
The above section seems to answer this question, although I think I'd personally like to have more cover than that. This is usually not an issue because one would normally have the edge of slab at least to the edge of the W12x26 resulting in at least 3.25" lateral cover.
"I don't think it's an invalid question."
These questions are fine. The original question was invalid because it asked what cover results in fully-composite. Whether or not a beam is fully composite depends on the number of studs instead.
- Status
Similar threads
