Maximum Height of Rectangular Beam?

[MDDSI]

I am curious is there is a real limit to the height to thickness ratio of a rectangular cross section on a beam? We are considering a vessel with a wall consisting of a large, rectangular flat plate. The whole vessel will need to withstand pressures from fans, which results in around 1 psi of uniform pressure on the wall. Based on flat plate calculations, the wall is not strong enough on its own to withstand the pressure, so I am looking to add reinforcing stiffeners (rectangular flat bar) to reduce the unsupported plate span.

However, the stiffeners I am looking to use are 1/2 flat plate, and I am getting a required height of something around 7 inches tall. This begs the question though, how thin could I make the stiffener? A 1/8" thick stiffener at 20" tall just does not seem right, but I cant quite quantify why. The stiffener is welded to the wall on one edge, so I am thinking that the Roark gives in chapter 15.1 are not quite applicable. I have seen some sources that limit stiffening rings under external pressure to 8*thickness, but again, I have not seen much to quantify the reasoning.

So, for a beam of a rectangle cross section, what is the limitation on height to thickness?

Please let me know if I need to explain my situation further. Thank you!

 

[JStephen]

Is the stiffener loaded in compression or tension at the free edge?
In the latest AISC specification, there is an equation for allowable stress/strength of a flat-bar turned on edge. In your case, using it would be overly conservative since one edge of your bar is supported laterally by the plate.
Also in the AISC specification, you should find limits on width/thickness ratios of flanges or other compression elements on a built up beam.
I don't remember how it works in the newest AISC, but you can treat part of your plate and the flat bar as a composite beam. In the older AISC, they gave equations that gave you a reduced effective width to use in that design.
In the current API tank codes, they allow 16t of the flat plate to be considered effective as part of the composite beam, and limit the outstanding bar to 16t effective width, if I remember correctly.

 

[racookpe1978]

Your pressure (1 psig ???) is very, very low, but may be correct for ventilation work.

Stiffness in wind loads (outdoors ??), earthquake, and static loads between the supports may control.

A very thin very high "rib" fastened only by welds along one edge can fail under buckling if the main plate ever comes under compression loads against the rib. Tension loads would act to tear the very thin rib in two, or tear out the fillet weld holding the rib on.

 

[MDDSI]

Thanks for the replys,

Yes, its a pretty low pressure, we mostly deal with spray dryers/ducting. So while the pressures are low, the steel is typically 12 gauge, so not all that strong. It may be that my method of solving the stiffening rib sizes is overly conservative. Again, I break it down and assume the 12 gauge skin takes up no load, and all load is transferred uniformly to the stiffening rib - thus behaving like a simply or fixed beam.

JStephen, the free edge would be unloaded always. Only the edge that is welded to the 12 gauge skin would need to withstand 1 psi of pressure (positive and/or negative pressure depending on where it is placed i the system). I will see if I can track down the AISC sections you mention. Thank you!

 

[SAIL3]

I would suggest getting a structural engr involved in the design......