Max.allowable width of flange in box girders 1

[CHARLESCOSTA]

What is the maximum allowable width of flange between the two webs of a box girder forordinary mild steel plates and manual welding such that the entire width can be used to calculate section properties?

 

[miecz]

What design code governs your design?

 

[kelowna]

I am not sure the answer is straightforward. It will certainly depend on the code used, but it should be related to flange and web thicknesses and flange stiffeners, both transverse and longitudinal.
More than the maximum width of flange, the important issue should be the effective width of flange, and shear lag calculations will give you that.

 

[CHARLESCOSTA]

Jmiec,
The code to be used in this case is indian code IS:800-86.
It doesn't clearly mention the requirements for box girder in detail. For single web the maximum flange outstand is given as 16t.Doubling it for box girder for flange between two webs , it comes out to be 32t where t is flange thickness. However in crane girders having box section i routinely find flange widths close to 50t.Thats the reason i want to find if in some reference this requirement is clearly mentioned?

 

[miecz]

Our old steel code limited the b/t ratio for unstiffened I shapes to 95/F[sub]y[/sub][sup]1/2[/sup], where F[sub]y[/sub] is in ksi. For carbon steels, this corresponds to your width limit of 16t.

For box sections, the limit was 238/F[sub]y[/sub][sup]1/2[/sup]. As you can see, the limit for box sections is more than double the limit for I shapes. The thinking is that a plate stiffened along two edges is less susceptilbe to buckling than a plate stiffened along one edge and free at the other.

A b/t ratio of 50 seems to be will into the range where the effective width should be reduced.

 

[CHARLESCOSTA]

Jmiec
can you please give the details of your code

 

[miecz]

The old Code I'm referencing is the 9th Edition of the Steel Construction Manual, published by the American Institute of Steel Construction. It has been replaced by later codes, with similar, more sophisticated provisions for b/t ratios.