Effective width of concrete flange with radial steel beams 1

[Chappie]

I have a project where there are radial steel beams that have fixed ends to columns at the perimeter of a circular floor(2 beams in an X). The concrete slab on top is to be composite with these diagonal steel beams. The max sagging moment is at the centre of the circular floor, where the radial beams meet. What should be assumed for the effective width of concrete flange when calculating the composite beam moment capacity at midspan? Compression is occurring in two orthogonal directions at once, and the closer to the centre of the circle you get, the less distance there is to next adjacent beam - and the larger the sagging moment becomes...

Has anyone come across this situation before?? Would be very much appreciated any info or ideas on this!

 

[hokie66]

So are there only two beams which cross? If so, the effective flange width for each beam would be just the same as if it were just one beam, that is based on the appropriate multiple of slab thickness. You would design it as a two way system, so each direction would take half the load.

 

[Chappie]

Thanks for your reply. Yes there are only two diagonal beams that cross the centre. There are some other secondary beams as well, but they do not cross the centre point.

If you consider the effective flange width for each diagonal beam to be the same as if it were just one beam then isn't the concrete at midspan (centre of cirle)being used twice in compression? although the compression is in orthogonal directions...I was thinking of using only half the effective flange width that you would use for each beam if there was only one beam.

Also, the text books suggest that the eff flange width should not be more than the distance to the adjacent beam (which is simple for a grillage of beams), but with the radial beams this reduces to nothing at the point where they cross.

 

[hokie66]

Where the beams cross, they are not adjacent, they are intersecting. So the flange goes straight through in both directions. Being compressed in both directions is good, not bad. Incidentally, the same thing applies to the steel beam flanges.

This is similar to a slab supported on 4 sides by walls. You design the slab in each direction to take its share of the load depending on the aspect ratio. As your floor is circular, each of the main beams will take the same load because they will deflect the same, assuming that they are the same size, are loaded by the secondary beams in the same manner, and have full moment continuity where they cross.

 

[Chappie]

Yes your explanation makes good sense, thank you very much!

 

[hokie66]

You are welcome, and as you are a new member, welcome to the site.