Shored or Unshored Composite Construction? 1

[abusementpark]

Do you generally design or specify shored or unshored composite steel construction? Obviously, each has its drawbacks.

Do you always do one opposed to the other? I'm curious what everyone's thoughts are.

 

[271828]

At the firms I worked for, all composite constructon was unshored. I'm sure others do differently at times, but I think unshored is more common.

LOL, there was only one shored job in the offices I worked for. Some un-named engineer accidentally turned on the "Shored" option in RamSteel and the entire job got designed as shored, but constructed as unshored. The error got by the designer, his supervisor who stamped it, the detailer, the shop drawing checker, the fabricator, and the erector. Didn't get by gravity, though. Those 34' long W12x14 didn't stand a chance when they dumped the concrete on there! Pretty large building and it was all in teh air before they discovered the error.

 

[Lion06]

We always design for unshored construction. My understanding is that it is much more expensive to shore beams than to add some steel weight.

 

[SteelPE]

We always us unshored construction for as well.

 

[lkjh345]

Have always done unshored. Seems to be the local norm.

I feel pretty sure if there was a cost savings in shored construction, several of our local contractors would not have hesitated to push for it. They are usually not too shy about those type of things.

 

[dik]

Generally unshored, but sometimes for thicker topping slabs, it is necessary to provide shoring and sometimes for beams, they are designed for a simple support post at mid span.

Dik

 

[haynewp]

Unshored always. We have had to specify to shore the just the deck itself during construction when it was a very large single span.

 

[RHTPE]

Just be sure to specify the design basis (shored or unshored) in your Structural Notes. Having been around long enough to have experienced what happens when you don't shore floors designed as being shored, you really don't want to see what can happen. Also be sure that the beams' compression flanges are adequately laterally braced, particularly if the metal doesn't provide adequate lateral bracing or there is no metal deck.

Designed a suspended shoring system once for a formed concrete slab + composite steel beam floor - EoR declared the beams did not require shoring. First concrete placement proved otherwise.


Ralph
Structures Consulting

 

[jike]

We also give the contractor a conservative estimate of dead load deflection so that he has no basis to ask for more money for additional concrete.

 

[kslee1000]

Ralph:

I can see the beam would twist somewhat under the unbalance load from concrete pour. Was that the case?

 

[RHTPE]

kslee:

That job goes way back. The problem was one of excessive deflection, both vertically and horizontally. The beams were very light sections and the slab was an early version of post-tensioned concrete. We ultimately shored the beams.


Ralph
Structures Consulting

 

[abusementpark]

Thanks for all of the responses.

For those that design as unshored on a regular basis:

Does the construction load case usually end up governing?

Also, how do you handle the issue of wet concrete ponding during the pour? Do you camber the beam to negate the effect or do you design the noncomposite beam with anticipation of extra concrete?

 

[271828]

At the last two offices I worked at, we added an extra inch of concrete for ponding. We stretched the beam spacing as far as the deck tables allowed, so that made me sleep a bit better at night. I think it was probably pretty accurate in most cases. We cambered 85% of the precomposite DL for beams over 29-30', assuming a min. 3/4" camber.

Does the construction case govern? Depends on your approach. It rarely did with mine. Vibrations often set the member sizes. Camber gets rid of construction DL deflection, so that case is not controlling anything. Increasing member sizes is generally the thing that helps with vibe.

I'm sure some offices see this differently, but that worked well for us and we actually got some compliments from contractors on the designs.

 

[abusementpark]

Are you required to analyze a composite floor system for vibrations? My office seems to stick to using deflections as the only serviceability criteria.

 

[kslee1000]

Construction stage is critical, though not necessarily governs the design. Camber is normal practice. I don't add add'l concrete in the estimate for span deflection, but ask contractors (in the bid document) to reflect it in their unit price, especially when floor has small slopes. Vibration? Never done that for industrial floors. But maybe necessary, if you choose ligh weight long span floor framing system with intense human activities involved.

 

[RHTPE]

Camber in concrete construction is always an interesting challenge, though I'm not sure how it's handled when placing slabs on metal deck. Many contractors simply use a laser to establish top of concrete being placed. Which will not correlate with cambered members or with beams that do not deflect to level when loaded with concrete. Most contractors are reluctant to use screed rails that control concrete thickness (but not elevation).


Ralph
Structures Consulting

 

[271828]

"Are you required to analyze a composite floor system for vibrations?"

As a serviceability limit state, it is never required by a spec. or code. It is highly advisable to check on a routine basis for steel-framed floors. If you don't, then sooner or later, you're going to have a floor with complaints. Besides, if you use the FloorVibe software (

http://www.floorvibe.com),

it takes probably less than a minute to check for a typical bay.

Floors that are very likely to have lively responses are pretty much any with open-web steel joists and regular composite systems with lightweight concrete, 5.25" slab or thinner. When you get into the normal weight 5.5" or more slabs, vibe is seldom an issue.

Any floor subject to rhythmic activity like dancing or aerobics should be checked. Also, any floor supporting sensitive equipment (microscopes, other imaging equipment, electronics manufacturing, etc.) should be checked. These also take almost no time with FloorVibe.

 

[kslee1000]

Yes, it is. Theoritically the beam should be at level after the placement of concrete (it is cambered for DL effect), however, I don't truely believe it would occur the way as had in our wish. And I think that's the seed for later argument.

 

[271828]

Ralph, we've cambered literally thousands of beams in screeded-level, unshored floors, using steel deck. Never had a problem. We measured camber in the fab shop, on the ground at the jobsite, and in the air, and it was amazing to us how accurate the cambering was. If I remember correctly, most of the members had 3/4" or 1" camber and none were off by more than 1/16". In the air iwth the concrete on there, they were almost exactly level. I'm inclined to think camber problems are mostly something people are concerned about for little reason.

 

[RHTPE]

271828: For concrete on steel beams, I believe the results that you found, assuming the support constraints are reliable when you calculate DL deflection.

My experience has been primarily with all-concrete structures - in that arena the situation is much different, and if you can get a contractor to construct the formwork to the desired concrete element's required camber, your create a problem for finishing the top of the floor. Very often we found that the as-constructed camber did not deflect to level.


Ralph
Structures Consulting