can we camber and shore girder at the same time 1

[johndeng]

Just wondering for composite girders, I called cambers based on composite section, say 1" camber. But during concrete pour, the dead load on steel girder would be 2", should I call shore during concrete pour?

Thanks

 

[KootK]

You could. It's usually more economical to just design your steel for the extra weight. Google "concrete ponding". Most engineers will make some allowance for this. An extra 1/2" or 15% concrete weight. Stuff like that. Note that some screening methods produce a constant thickness slab that follows the curvature of your beams.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I was thinking of infill beams john. For girders, the economics of shoring might be more favourable depending on what you're shoring down too.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Come to think of it, why not just increase your camber?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jike]

Usually, we either camber out the DL or some percentage of DL OR shoring.

I see no reason you couldn't do both BUT WHY NOT JUST INCREASE YOUR CAMBER?

 

[johndeng]

To tell the truth, I used the modeling to size all the beams and cambers.
Then when I check, I need to make sure steel can take the dead load and without too much deflection. It looks like load capacity is ok, but deflection is big.

So basically all your cambers should design on non-composite steel beam properties, and we'll get larger cambers. Just wondering this should be the way for cambering.

Thanks KootK and jike!

 

[jike]

Yes, cambers should designed based upon non-composite steel beam properties

 

[RHTPE]

Keep in mind that if you do specify shoring, you should carefully review how the contractor proposes to shore the beams AND consider how the shoring will behave based upon what it bears on.

It's very simple to specify that shoring is required. It's not always a simple matter to determine if the shoring will actually prevent the deflection that you are concerned about. Further, the results will also depend on how the contractor will screed the concrete during its placement. If you expect the some camber to remain after concrete placement is complete, then the concrete screeding must be based on thickness of slab and NOT on elevation (usually not considered by some contractors).

From a constructability perspective, I believe it is wiser to camber for dead load deflection so that the slab can be placed to the specified elevation (i.e. level).

Ralph
Structures Consulting
Northeast USA

 

[wannabeSE]

One thing to consider if shoring. If the concrete is placed level on an unshored beam, the deck will be flat under it's own self weight. If it is shored, then the deck will deflect under it's own self weight when the shoring is removed.

 

[johndeng]

Hi wannabeSE: "If it is shored, then the deck will deflect under it's own self weight when the shoring is removed."
not really, shoring only removed after concrete being cured, the composite section will work so section properties is greatly increased, and wont deflect much after shoring being removed.

 

[JoshPlumSE]

My impression is that shoring is anathema to most engineers and contractors. Just too expensive compared to cambering and / or increasing size of beams.

I have to admit I have never been the EOR on this type of project though. I'm just basing this on the number of composite beam models I see (I work for a structural software firm). And, very, very few of them specify shoring. I don't even remember the last time I saw a model come in with shoring specified.... it's been awhile!

 

[canwesteng]

I'd probably just do a hand calc for shoring instead of building a model for it.

 

[RHTPE]

JoshPlum - I'm not so sure that it's expensive but rather can it been executed reliably? For a single level structure with good ground conditions the shoring will do what's expected. When shoring is used on a multi level structure its reliability depends on how the shoring is supported by the level below. Multiple levels of shoring (to insure a deflection-free support) then starts to get expensive. It also can begin to affect the trades that follow.

Ralph
Structures Consulting
Northeast USA

 

[johndeng]

also what's the min. length of beam that can camber?

 

[johndeng]

I think shoring is good for composite design. concrete dead load is 60psf, live load is 40psf.
non-composite steel will deflect under 60psf dead load and 20 psf construction load, that's almost the full load. then what's the purpose of composite?

 

[KootK]

I normally don't depend on composite action for my girders. It can be tough to align the deck flutes over the girder and I worry about stud effectiveness in a region of likely curvature discontinuity (and therefore cracking). Additionally, the benefit accrued never seems to be as substantial as with infill beams.

If you have access to the contractor for your project, I'd be sure to check with them about the "goodness" of shoring. You'll be tempting the cruel wrath of the rework gods otherwise.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Hokie93]

According to AISC, the minimum length of steel beam that can be cambered is 24'-0". There are other limitations as well (minimum beam depth is 14", minimum web thickness is 5/16", minimum camber is 3/4", etc.). All of these requirements can be found in the article "Specifying Camber" from the July 2006 edition of Modern Steel Construction (

http://www.modernsteel.com).

The primary reason for using composite beams and girders is strength and the ability to shallow-up beams and girders relative to non-composite framing. For the same size beam or same beam depth, you can typically space composite beams farther apart than non-composite beams, thus reducing the steel tonnage (though the savings is partially offset by the cost of the shear studs).

 

[wannabeSE]

johndeng,
I realize the post-composite moment of inertia is larger. The Steel Construction Manual table 3-120 has lower bound moment of inertia for composite beams. For instance, a W25x55 has a precomposite moment of inertia of 1350 and a post composite moment of inertia between 1970 and 4210. The original post mentions 2" deflection from concrete weight. If the deck is shored and the composite moment of inertia is 4x the bare beam, the floor surface may deflect the 1/2" from the weight of concrete when the shoring is removed. (This example is only meant to give a point of reference because there are other factors that affect composite beam deflection) In most cases, the floor flatness is much more important than the deflected shape of the beam below the concrete. Typically, I will look at the precomposite beam deflection to make sure the excess concrete I assumed is acceptable and the deflection is not ridiculously large.

 

[johndeng]

Thanks to all,

But I still feel shoring is a good way for composite beams. I will only need one mid point shoring for all steel beams, and the slab will be perfectly flat or slope.
The most important thing is that no matter how many times the new section I is bigger, if we put all dead load and construction load on bare steel, we don't utilize much of the composite advantages, and our beam sizes and depths will be no much difference.

 

[KootK]

Remember: for many applications, post-composite deflection is what governs beam design. It doesn't much matter how much your beam deflects while it's non-composite so long as it doesn't outright plastify. And you'll actually get more post-composite deflection if you shore the beams.

The benefits of composite construction are not negated when shores are omitted. Beam sizes are reduced. You will find, as evidenced by this thread, that unshored construction is overwhelmingly the choice of contractors and your colleagues in the structural engineering community.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.