CMAA #70 Transverse Web Stiffener Sizing 1

[RockEngineer]

Here is a tough question for our crane design experts. The first few paragraphs are only background. The only question I have is at the end and it is specific to sizing the web stiffeners. I'd prefer not to make this into a complete lesson on design per CMAA #70.

I have a design for a 440 Ton Gantry crane with box girders 9'Hx3'W. It has fixed lifting points so there is no trolley. It's fixed lift points are at about the quarter points of the 100 ft span. It doesn't have full or half height diaphrams because there is no trolley and this works per all the CMAA #70 requirements. It does need transverse web stiffeners for web shear buckling according to 4.5.4.1 with a required spacing of 96". The designer was conservative and put the stiffeners at 60" oc. The problem comes in sizing the stiffeners per 3.5.4.3.

The stiffener sizing equation is
I>1.2*(h^3)*(t^3)/(a^2)
where h=Web height (108")
t= web thickness (3/4")
a= spacing of transverse stiffeners (actual?)
I= Minimum Moment of Inertia of the stiffener plate about the interface of the web plate. (i.e I=b*(h^3)/3 for rectangular bar)

All previous editions of CMAA #70 had this same equation but were very specific that the "a" to be used was the calculated maximum spacing not actual spacing if different. In the current 2010 edition it has been changed to read just "spacing of stiffeners". Many the equations for determining stiffener spacing were modified or added in the 2010 edition. I have looked and no errata has been issued for this edition.

So for this case if I space the stiffeners 96" as calculated
I=69 in^4 so I can use a 3/4"x7" flatbar

If I use the actual spacing as currently written in CMAA #70
I=177 in^4 so I would need a 3/4"x10" flatbar

Why would putting the web stiffeners closer together than required for web shear buckling require a much bigger stiffener?

I told you I had a hard question! I'm also going to ask CMAA but their process can be slow.

 

[nutte]

I’m not familiar with the CMAA equations, but maybe this will help. At a larger stiffener spacing, it takes less load to buckle the web, so the demand at the stiffener would be less. This would give you a smaller stiffener. At a smaller stiffener spacing, it takes more load to buckle the web, so there would be more demand at the stiffener. This would give you a larger stiffener.

That is, maybe the stiffener size equation is calculating the size necessary to prevent web buckling under any load, not the size necessary to prevent web buckling under the actual load. You’ll note that the applied load doesn’t appear in the equation for the stiffener moment of inertia.