Structural analysis of stiffened cold form sections

[FLR84]

Hi everyone,

I'm wondering if anyone has any tips on how to determine the member capacity of cold formed cee sections with stiffeners attached? For example, a back to back C35030 section with C30030's bolted to each side of the web? Or a back to back C30030 section with two C35030's bolted horizontally across the top and bottom flanges, effectively doubling the area of the beam's flanges?

With the first example, I assume that the principle of superposition would apply and I can first model the back to back C350 section in a structural analysis program and find the load factor. I can then change the member to the stiffener (back to back C300) and find the load factor for that. If when added together, the load factor is greater than 1, then the 'composite' section should be strong enough. Is my logic correct?

With the second example, I have no idea how to find out the member capacity without doing FEA as most structural analysis programs do not allow for this shape of section to be entered. At the moment I'm estimating it by doubling the member capacity in bending (more than double the flange area so bending capacity should be more than double?) and making sure that the shear force doesn't exceed the shear capacity of the web of a regular back to back C35030 section. Does anyone have any ideas to be more precise?

Thank you.

 

[FLR84]

Here are some illustrations of the sections I'm talking about:

 

[JoshPlumSE]

I might run something like this through CFS.... to find out all of it's properties. That is IF the program can handle something like this.

 

[FLR84]

Josh, thanks for the reply.

I can get the Ixx value but no more it seems.

 

[human909]

I'm curious on why you are using these members. It looks like an interesting solution to whatever structural challenge you are trying to address.

 

[FLR84]

human909:

Using them to build larger span cold formed buildings without having to resort to hot rolled sections.

 

[dhengr]

FLR84:
Some stiffening elements will have limited allowable stresses as a function of their buckling stability, width/thickness ratios, their unsupported edge conditions and their location on the built-up member. You have to be careful how you fasten the various elements together, and what you actually account for in that fastener design. This always involves screws, rivets, bolts, bearing on a hole in fairly thin material. Otherwise, I would calc. section properties just as we usually do. The calc. of members section properties is kinda bitchy because of bend radii, various small stiffener shapes, etc. You should be able to pick section properties for the various individual elements out of various industry tabulations. Then combine them using our std. moment of inertia of areas, parallel-axis theorem for moments of inertia, and the like.

In your right sketch you have a shear flow problem btwn. the added flg. elements (C30030’s) and the two main elements (C35030’s) as this relates to the combined section. A load on the top flg. will load all of the elements as a combined section, through the shear flow determined fastener connections at the flgs. By the way the elements are combined/nested, it just looks like it loads all the elements. Then I would add some staggered screws in the webs of the C35030’s.

In your left sketch, you have quite a different critter. A load on the top flg. will load the C350’s directly (but not the C300’s), and the C350’s will transfer a portion of the loading to the C300’s as a function of their relative stiffnesses (stiffnesses of the C350’s vs. C300’s). This load transfer would take place through staggered and spaced bolts (whatever) through the webs. This is not a shear flow related problem like the right sketch is. Of course, you also want the web fasteners, just to tie the members together so they act as a combined section, and to improve web buckling conditions.