Section Class 6

[dik]

Can this composite section ever be considered as a Class 2 section?

I thought it had to be symmetric in the plane of the loading to be Class 2. The effect of adding a channel for composite capacity is very limited and the original section is Class 1 and you can use the plastic section Zx. The resulting is not Class 2, I think, and the effect of using the composite section modulus Sx results in a lower moment capacity than the unreinforced section.

Interesting puzzle? Any comments?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TLHS]

I think definitionally it *may* be class 2, but unless it's super restrained I wouldn't count it as class 2. The channel portion needs to yield first, and the act of that yielding feels like it would significantly effect stability during the rest of the loading before the wide flange portion yields.

If you're really confident that your lateral bracing is good, then maybe.

I guess this is pretty similar to how it works with channels as it is. They can be class 2, but for laterally unsupported moment capacity they get grouped into the capacity formulas for class 3.

 

[TLHS]

Okay, more thoughts. I think it can be class 2, but the unbraced lateral torsional length depends on both the channel flange and wide flange being braced. There's likely enough stiffness in the web that bracing one braces the other, but I'd want to think about it.

It should definitely be in with channels and class three wide flanges as part of the 'All classes except Class 1 and Class 2 sections of I Shaped Members' category for LTB.

 

[KootK]

Can this composite section ever be considered as a Class 2 section?

Sure, so long as the plate elements of both members can be considered to be class 2 when the pieces are considered independently. The class business is just about plate bucking under various compression gradients.

I thought it had to be symmetric in the plane of the loading to be Class 2.

The requirements to use the plastic flexural capacity are:

a) that the beam be class 2 or stockier and;

b) that the load be applied through a principal axis. A principal axis is often, but not always, an axis of symmetry.

If you're really confident that your lateral bracing is good, then maybe.

Similarly, I think that what you want is not translational bracing but rather torsional bracing of the entire cross section. The impact of the torsional bracing would be to rectify the load such that the vector sum of it becomes principal axis loading per my .

This setup certainly seems like an inefficient way to reinforce but, then, I'm sure that dik has his reasons. From an LTB stability perspective, it's also a bit unnerving that one would have to take the compression flange of the w-beam well past full yield strain before yielding the compression flange of the channel.

 

[dik]

Thanks Koot... I've sent a note for clarification to the CISC... in the meantime, I'll look into calculating the plastic section modulus, and treat as a Class 2... I'll consider the Class 2 or 'better b/t valued for the channel flanges since with web will be fillet welded to the W section web...

The problem came about because one of the metal fabricators ran into an issue. The EOR wanted him to weld the channels to the W section for the full length of the W beam, with CJP welds at each flange. This reinforcing was not by me... this type of reinforcing is very inefficient even with normal fillet welds. This was overkill and I started looking into it. There are several hundred feet of CJP welds. I still haven't heard back about what the contract documents spec... usually shear loading equivalent to the UDL capacity of the fully supported beam... but dunno...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

should have included TLHS... sorry guy (binary term for PC)...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TLHS]

That's a ridiculous weld and I feel like you're likely to have deformation issues.

Also, what does CJP even mean on the backside of the channel? Is it the nominal flange thickness? The flange thickness at the widest point (have fun cutting a large part of the section out beveling that) ? The web thickness? The full depth of channel?

 

[dik]

I was thinking it was Class 2. It didn't make any sense that if you reinforced a Class 1 section that it would be less strong than the original section (even barring stability issues).

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

I didn't spec it... it's what the EOR asked for... For that type of composite construction, due to the limited capacity of the added channel, the length of composite member is often limited to a few feet, on either side of the beam centre. Full length CJP welds are unnecessary. I'm running with calculating the composite section Zx (Sx for Kiwis). I'll let you know how this 'shakes out'.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[canwesteng]

My understanding is that any load can be turned into two loads through the principal axis, so all loads would be allowed to use the plastic section modulus. I think maybe some confusion between plastic design in section 8 and plastic capacity in section 13. As far as I know, all rolled channels are class 3 sections, so my quick approach would be to use the Zx of the wide flange and ignore the channel. Sharpen the pencil approach would be to assume plastic stress distribution up to the column flanges and elastic beyond that. The last wrench to throw into the works is that the section cannot yield under service loads (since it now has two S.x values with the channel offset to the top), so you need to check S.min against SLS moment.

 

[dik]

What I'm doing is checking the leg of the C section to see if the b/t is OK for Class 2 and if the W section is Class 2 or better, I'm calculating the Zx for the composite section, treating the composite section as Class 2. To ignore the effects of the channel is to eliminate the effects of reinforcing of the steel section.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

My understanding is that any load can be turned into two loads through the principal axis, so all loads would be allowed to use the plastic section modulus.

I disagree. The clause for plastic capacity limits one to uniaxial principal axis loading. The decomposition, by definition, creates a biaxial loading condition about the principal axes which would preclude the reliance upon plastic flexural capacity.

I feel that the spirit of the uniaxial/principal thing is precisely to preclude meaningful biaxial loading about the principal axes which complicates matters considerably. A member uniaxially loaded through one of its principal axes does not induce a commensurate lateral movement through the perpendicular axis such as that which we obnoxiously have to contend with when dealing with brick angles.

What I'm doing is checking the leg of the C section to see if the b/t is OK for Class 2 and if the W section is Class 2 or better...

That's what I would do as well. In the context of your particular problem, I'm not sure that it even matters what class the channel web is. The name of the game here is surely to add two, intermediately positioned flanges to the w-beam. The only meaningful roll that the channel web is likely playing is to, perhaps, nominally brace the channel flanges rotationally.

 

[dik]

The channel web is welded, albeit not a CJP... but sufficient to reduce any rotation.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

To be clear, the cross section that you're creating here will not wind up uniaxially loaded through a principal axis unless you brace it convincingly laterally and torsionally. As such, using the plastic flexural capacity is likely inappropriate unless you've got a bunch of clip angled infill beams hitting the back side or something along those lines.

 

[dik]

Thanks for the caution Koot... but the plastic section created will be close to the 'real' one...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

I'll buy that. If the Zx increase is slight then the non-principal-ness ought to be too.

 

[dik]

If the C and the W are at mid depth, Zx would be close to the summation of the two Zx properties. The Zx for the channel, being a lot less than that for the W...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TLHS]

I read a bunch of things over the last hour or so and I've changed my mind. I'm convinced that this is class three. The easiest thing is referencing Figure 2-8 in the handbook. Even an asymmetric cover plate is specifically called out as being a class three only check. Since the class of the section is the class of the worst element, a built up section with an asymmetric cover plate is enough to bump you to class 3. What we're looking at here is more asymmetric than that.

The argument that it shouldn't be less than the unreinforced section doesn't really hold. You can adjust effective dimensions to move from class 4 to class 3, but we are generally doing that at the extreme stress location of the beam, so there's not a huge stress gradient across the elements and there's no post yield behaviour in any of it.

In this case, the asymmetric elements will be at something like twice the yield strain when the section goes fully plastic and will be extremely yielded. Just ignoring the destabilizing effect doesn't really work. You'll have the yielded channel putting a torsion on the wide flange web at the time the wide flange web yields and has no reserve strength. The yielding web will presumably rotate and be more subject to buckling than tested class 1 and 2 sections were. You'll get some stiffening from the channel, but who knows what that actually does at that point?

I'm open to discussion on all of that. It's a little out of typical practice. I definitely can't find data on post yield behaviour of asymmetric beam reinforcement.

It still feels like an element the relative size of that channel would have a hard time destabilizing everything, but I don't know how you actually prove that.

 

[dik]

I think strictly speaking it is Class 3, also; that's why I raised the question. The problem is that Sx for the composite section is less than Zx for the W section... and the resulting moment is less. Therein was my problem. Adding the C section to the W section yielded a lower resisting moment. With the exception of certain stability issues, this doesn't make sense. I'm comfortable in treating the C section as part of the composite section and using the plastic modulus Zx for the composite. I'm still waiting to hear from the CISC (Canadian steel guys).


I concur... I'm like a magnet for this stuff... it seem to find me. I got involved by telling the fabricator to find out what the real attachment was; the EOR wanted a CJP weld, which I knew was wrong.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TLHS]

You can be class two with asymmetry, but you have to account for the asymmetry. It specifically says that in the definition. I can't find any reference to how one might do that, though.