When to NOT Use Composite Steel

[KootK]

Whenever I go through a costing exercise for a concrete deck over steel framing system (joists / beams), it seems as though composite steel beams always come out on top. The system is shallower, the tonnage is reduced, vibration characteristics are improved... Fire protection issues seem to push the pendulum even further in the direction of composite steel beams.

So my question is this: should ALL steel beams be composite beams when there's a suitable topping slab above? Is there any simple span deck slab / beam framing situation in which it would make more sense to use non-composite steel framing?

Thanks.

 

[abusementpark]

I have heard that there needs to be enough studs on the job to make the setup of the stud welding machine worth it. We have found non-composite systems with joists to be cheaper for a lot of two story steel structures with small footprints.

 

[KootK]

Hmmm... got any idea what that "critical mass" number of studs would be? That argument might be especially valid here as we are just using the system for a moderate sized, one story mezzanine.

We've been contemplating joists for this application too. I'm concerned about the vibration characterisics however.

 

[JAE]

Try calling AISC - they have a department that helps engineers with preliminary framing concepts. They might have some idea of number of studs, spans, etc. where composite kicks in as more economical.

 

[Lion06]

Is it true that vibrations are improved? Doesn't DG # 11 recommend using composite properties even if the beams aren't composite because the friction is enough to get composite action for the load under consideration when doing vibration checks?

I am interested to hear the results of this "poll".

 

[KootK]

Yes, that is exactly what DG #11 says. In addition to friction, they cite the roughness of screws / puddle welds etc. Numerically -- by DG #11 -- there is no improvement. I guess I was expecting that there would be some improvement in the finished product however.

 

[abusementpark]

I saw someone on here say that 1000 studs was a good number to keep as a rule of thumb (i.e. if going composite yields less than 1000 studs on the job, then is better to stick with non-composite).

 

[lkjh345]

abusementpark:

Yes, 1,000 studs is a good round number to consider when deciding on whether or use composite steel or not.

I have had converstations with one of the owners of a local steel erection company (who also used to be a practicing structural engineer) and its his opinion, through looking at the costs of raw material, fabrication, and erection, that somewhere between 750 and 1,000 studs is where the material saving of using composite beams begins to overcome the cost of setting up the stud welding machine and calibrating it at each job site. I forget what he says he figures into a bid for setting up and calibrating the welder, but I do remember it was a surprisingly large number. More than a nominal amount.

 

[abusementpark]

Do they all use the stud welding machines nowadays?

 

[archeng59]

Based on personal experience on two projects, not my own, the composite system is more flexible with respect to vibration issues. The reason we use a composite system is to reduce the amount of steel used for the beams. That reduces the stiffness of the system, which is required to reduce vibration issues. For offices where we had full height partitions to dampen the vibrations, the reduced stiffness was not a problem. For offices where we have modular furniture systems that do not provide the damping, the reduced stiffness is a problem. I've worked with two owners who have buildings constructed using composite floor systems and neither met DG 11 criteria for an office/walking criteria even though the composite floor framing was met stress requirements and were within acceptable deflection criteria.

 

[csd72]

The type of structures that I design rarely have sufficient repetition for it to make much difference in material.

What I look for is simplicity, whatever system has the simplest erection procedure is generally the one I choose as up to two thirds of the cost is in the non material costs.

There are a few areas where I would definately NOT use composite beams - those areas where propping is difficult and you would need to use unpropped beams and those buildings where it is very likely that additional openings will be required.

 

[ishvaaag]

When making a Mathcad worksheet for composite deep beams I found another case where ensuring the composite action was no worth mechanically speaking: one was trying to rigidize a very stiff deep girder with say ... butter spread overhead?

That the case appears in practice may depend upon the case, but is a feasible one.

 

[jike]

I would not consider using composite for buildings with numerous floor openings since they interupt the composite action of the slab. This would include those buildings that anticipate future openings.

We have typically used composite beam construction for fire rated commercial buildings. For unrated commercial buildings, we have gone with open web joists.

 

[KootK]

Here's a question that's likely to be foolish:

Why aren'e composite studs welded to their respective beams in the shop. Tripping hazzard for workmen during the transportation / erection process?

 

[jike]

How would you erect metal deck with studs already on the beams?

For projects without metal deck, how can an ironworker walk along the top of the beam without tripping? "Cooning" would also be a problem!

 

[KootK]

Right -- installing the deck. I'll see if I can find some way to delete that post...

 

[ishvaaag]

Expanding the subject of the concrete head not being at some cases a positive contributor to moment stregth, the worksheet demonstrated that following the code specifications and for the case (a very stiff steel girder by itself) the moment strength of the composite girder never amounted to more than the mere steel contribution.

 

[HgTX]

KootenayKid: It wasn't an entirely unreasonable question. In bridge construction, there is mixed opinion about shooting studs in the shop vs. the field (and OSHA decided that shooting studs in the shop would be a "de minimus" violation as long as proper fall protection is used). But bridge construction is different from building construction.

Hg

Eng-Tips policies: faq731-376

 

[hokie66]

And KootenayKid, it once was done that way in buildings before the common use of metal deck for formwork, and before OSHA decided the studs were dangerous. Still is in some countries without OSHA.

 

[SteveGregory]

How about using/omitting studs on girders in moment-resisting frames?