Composite vs. Non-composite Steel Beams

[LPPE]

What would be the cheaper way to do it?

If steel beam depth is not an issue, would it be cheaper to design the floor beams as non-composite?

Obviously, with composite design, the weight of the steel beam is less, but is the savings in steel weight then offset by the labor to install shear connectors?

 

[a2mfk]

I think the answer would depend on availability and cost of local labor, but moreso on your actual beam design (load, span, etc.). I think AISC may have a guideline for how much an installed stud may cost vs steel tonnage, the rules of thumb change with steel and labor costs, but I would just run a typical beam design and then check the economics.

I think for the most part composite beams in buildings such as office buildings, especially when there is repetition, will win out over non-composite, and you get the benefits of reduced deflection and an all around stiffer floor since the slab and beam are mechanically tied together.

 

[a2mfk]

This is from a 10 year old MSC article (quick search on AISC.org), I think I may call a local steel fab you have worked with to ballpark you a couple of numbers for steel stud installation and for steel tonnage costs:

Favor the use of partially composite
action in beam design. Although shear
stud installation costs vary widely by
region, on average, one installed shear
stud equates to 10 lb of steel. Fully composite
designs are not usually the most
economical because the average weight
savings per stud is less than 10 lb.
Modern Steel Construction / April 2000
Figure 2. Web penetration reinforcement of an I-shaped beam.
Sometimes, the average weight savings
per stud for 50 to 75 percent composite
beams can exceed the point of equivalency.
In some cases, non-composite construction
can be most economical. A
caveat: make sure that the beam in a
composite design is adequate to carry the
weight of the wet concrete.
• When composite construction is
specified, the size, spacing, quantity
and pattern of placement of shear stud
connectors should be specified. It
should also be compatible with the type
and orientation of the steel deck used.
When evaluating the relative economy of
composite construction, keep in mind
that most shear stud connector installers
charge a minimum daily fee. So, unless
there are enough shear stud connectors
on a job to warrant at least a day’s work,
it may be more economical to specify a
heavier non-composite beam.

 

[steellion]

That's a good summary, a2mfk. I always use partially-composite beams unless it is a really small footprint or if it is something like a mezzanine floor. My justification is, if all that concrete is there, why not use it to your advantage? It allows smaller beams and helps dampen deflection and vibration. Plus, contractors expect it in this region; therefore, it's no big deal.

Don't mix and match though. If you're going to use composite, put at least a few studs on every beam to make sure the steel frame is "locked" into the composite slab everywhere.

 

[msquared48]

I think that it is hands down for composite for highrise construction, but the other way around for one or two story structures. Asa to where the break point might be stoiory wise, I don't know.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[LPPE]

My case is a bit of an odd-ball. I have a steel framed second floor (concrete on metal deck on steel framing) with steel columns. Top of steel elevation is roughly 20 feet above finished ground floor. Thats why steel beam depth is not an issue here.

The client does want to support brick veneer, so the spandrel beams will be L/600 for deflection.

Therefore, I guess I'm leaning towards composite construction.

 

[a2mfk]

I've used it on two and three story buildings extensively, from moderate to large office and school buildings...

With your last comment about brick, mind your Ps and Qs on camber and deflection, and compatibility of parallel beams.