T -Beam section

[JStructsteel]

I have a beam that is a L shaped (precast bleacher seats), the stem is 18, flange is 78". I have designed the flange outside of a 2:1 depth to width as a one way flat slab, but was going to consider the whole section for deflection calc.

Does this seem logical? I have some intermediate plates too along the flange, connected to another stem so its not going to deflect even as much as calculated. Unfortunately the review is being critical about this.

 

[BAretired]

Using the whole section for deflection is unrealistic. Would it be adequate if you consider a quarter of the width of flange as effective?

BA

 

[Agent666]

For serviceability, the effective flange widths for determining the effective section are usually taken less than those for determination of the moment strength. Rather than spanning as a one way slab the long way won't loads on the flange try span the short way back to the stem, placing the system into torsion (though maybe your connection to the next bleacher prevents this).

Not clear from your question, but which way up is the stem of the L? In compression or tension?

 

[JStructsteel]

Stem of the L is in compression. Delfection from just using the flange is L/160 to L/900 considering the whole section. I think its conservative to say I am somewhere in the middle, and not do a extensive analysis to check. Plus, I have the plates securing the flange side to the next bleacher.

 

[Agent666]

Well if the flange is in tension and you want to use it you would first need to justify that its not cracked under an appropriate loading scenario (if cracked its obviously ignored completely and you are back to square one). Only compression regions are generally used to contribute to the section second moment of inertia, concrete in tension is ignored (except for the tension stiffening effect in working out your effective moment of inertia). Guessing somewhere 'in between' is not really a correct approach. Work out the cracked moment of inertia/effective moment of inertia correctly in accordance with whatever code you are working to accounting for the loading, reinforcement distribution, etc and see what the deflection and crack widths are like. Depending on your plate detailing you may or may not (more likely) get composite action between the bleachers. Worst case at the extremities you don't have the other bleacher there maybe to rely on, so I'd be making sure it works on its own, and if there some unquantifiable benefit in joining them its icing on the cake..

 

[BAretired]

From the thread title, T-beam section suggests the web is below the flange which, for a simple span, means the stem is in tension and the flange in compression. That was what I assumed. If the flange is in tension then, unless the section is pre-stressed, the flange should probably be ignored for deflection calculations.

BA

 

[JStructsteel]

Sorry, my bad. Stem in tension. Sunday and working too much!

 

[Guest090822]

Perhaps posting a sketch of beam and its supports would be helpful. The descriptions in the previous posts make no sense.

 

[IDS]

That seems like a huge difference in deflection from just increasing the width of the compression flange with the tension reinforcement area remaining the same.

You are considering cracked section properties, creep and shrinkage effects, etc?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/