COMPOSITE STEEL-CONCRETE BEAMS

[ACE58]

I am currently designing a steel frame medical office building & the architect has requested I use unshored composite steel-concrete floor beams to minimize depths & cost. The only reference I have for this type construction is the 9th Edition of the ASD Manual. I have followed the procedure detailed there for std. simple span beams & have designed a number of them already. My question pertains to the main beams which support these simple span beams on the column lines. If one of these main beams (28' long)supports simple span beams @ 9'-4"c/c from each side (bay sizes 21' & 9') what value is used for the effective width of concrete slab, "b" (step "C" in design example 15) for these main beams? Using either L/4 or (21+9)/2 both result in what seems to be an excessive number of shear studs compared to a past job I am referencing (designed by another firm). Any advice would be appreciated.

 

[Lion06]

The effective width "b" is as the AISC manual states. Bear in mind that you don't need to account for 100% of 0.85f'c or Asfy when you are calc'ing the shear studs. It is often more cost effective to use a lower percentage composite than 100%.
Also, note that the studs can be limited by the concrete or the steel. It sounds like yours may be controlled by the steel area rather than the slab with what sounds like (in your opinion, anyway) such a large slab area.

 

[csd72]

These beams are often designed as continuous steel beams (not composite) with the continuity providing the economy.

They can be designed as composite also and the effective width is calculated the same way as for the secondary beams (the compression force is perpendicular to the other beams and therefore not effected). You need to take into account that the ribs are paralell to the beam in this direction.

Being a hospital, you should also use AISC design guide 11 to keep vibrations down to an accepatable level. Operation room equipment can be highly sensitive to vibration and you should get information from the client on this).


csd

 

[Galambos]

i'm not sure ive ever seen a steel beam or composite beam designed as continuous in an office building other than in cantilevered construction.

 

[csd72]

Galambos,

You got me there, I assumed it was the same as Australia where this is fairly standard practice.

csd

 

[lkjh345]

Csd72:

Interesting. Continous composite beams are not common practice in the states. For my own curosity, how is it done typically? Is the column interupted and 'stacked' above and below the beam with the beam continous through the joint? Do you neglect the concrete slab at the negative moment region?

ACE58:

JMHO, but if your going to be doing quite a few composite beams, I would invest in a computer program. We use RAM, but there are many others out there. The savings in time ($$'s) will pay for the program in very short order.

 

[csd72]

lkjh345,

No the beam is just designed as a continuous beam with no studs. It works out about the same size as a simply supported composite beam.

Composite beams are often designed as unshored to keep site costs down and therefore the continuous beam also helps to reduce deflection in the girder during pouring.

csd