Channel-braced flange?

[haynewp]

Say for instance you have 2 channels, running side by side 2 feet apart. The channels are connected at 3 feet on center with TS members welded across the top of each flange, only grating is sitting on top of the TS members, (like on a catwalk).

I would not consider the TS memebers as brace points for lateral torsional buckling since I think that both channels could buckle in the same mode.

However I have found disagreement among colleagues and am wondering what your opinion is.

 

[SlideRuleEra]

haynewp - That is a good question. I consider these applications to be braced for the following reason:

Assume that any meaningful load that the channels must carry is applied to the grating. Then friction (along with any minor anchorage) between the the grating and the TS members will be significant. The fact that the grating is a two dimensional panel itself will cause it (the grating) to act as a brace and keep any sidesway to a minimum.

Another factor that may help - often the channels are oriented in opposite directions. If this is true, they approximate ether a tubular section (with a 2 ft gap) or a wide flange section (with a similar 2 ft gap). Either case would have a certain amount of sidesway resistance itself.

Hope that I made my thoughts clear.

 

[AlohaBob]

I would consider the top flange braced at the weld points. Especially considering the rigidity of the grating. Is the grating welded down too? Or mechanically fastened? If the stresses are based on the channel support alone rather than on the composite section, the structure should perform better than predicted.

Is the catwalk simply supported, or do you have continuity over supports? You might have an issue with the bottom flange buckling.

 

[haynewp]

The channels are oriented in opposite directions, like this ) (

Grating is mechanically fastened, resting above the TS members not in contact with the channels. Only welds at the channel flanges are for the horizontal TS members. The catwalk is simply supported.

It looks like the twist due to the load being applied off of the shear center of the channels will offset since they face opposite directions.

thanks for your input

 

[JAE]

I guess I don't agree. Grating has very uncertain amounts of diaphragm stiffness. Also, the twin channels can bow/translate sideways with nothing to prevent this. So your unbraced length would be the full span length.

But you can use the combination of the two channel's properties to get the capacity. Not as a combined section property, but as the sum of the two parts.

So if you are in ASD, your allowable stress would be based on the summation of the rT's or whatever goes into the allowable stress formulae. So you combined rT would be based on

Sqrt((I1 + I2)/(A1 + A2))

Where I1 and I2 are each channel's moments of inertia and the A1 and A2 values represent each channel's area.

The grating is usually a square or rectangular arrangement and I would be hesitent to use it as a diaphram bracing these members. Especially if they are just sitting on the channels - using friction to brace structures is not a good idea. If they are welded, and if you get some kind of understanding of the in-plane shear rigidity of the grating, then maybe you could consider the grating as adding some level of lateral bracing. It doesn't take much stiffness and strength to brace, but if you don't have a clue what the grating even provides, how can you justify counting on it?

 

[VoyageofDiscovery]

JAE,

This is interesting as I now see there is disagreement here. When my peers and I design bridges for the construction stage with no composite deck or rebar, there is no way we could use the full span as you mention. We place lateral bracing against compression flange in the compression zones and thus use an unsupported length equal to the spacing of these braces.

Regards

VOD

 

[haynewp]

I have another case where the grating is screwed into to the top flange of the channels and there are no TS members or welds. The grating is segmented. I can see where this would kind of act like as a wood floor diaphragm, the 2-d panels resisting deformation along the length of the members.

I originally didn't have the grating sitting above the TS members in mind, maybe this justifies the TS points as being brace points.

However another case,

Looking at the horizontals that are placed for joist bottom chord bracing for wind uplift for instance, I think there are usually enough members tied together that the chance of them all buckling together in the same mode is very slim.

What if there were no grating and only horizontal TS members welded to the top flanges of the 2 channels, I would still say these are not brace points.

 

[JAE]

I would agree - braces need to have a load path just like everything else. In your case of two channels running parallel with TS braces at intervals between, all the TS's are doing is tying the two channels together. So my comments above still apply - that the unbraced length is the full span BUT you can use the sum of the properties of the two channels in determining the capacity, vs. using each channel by themselves.

 

[AlohaBob]

JAE I don't agree. Consider the behavior of a verendiel truss. Consider how much force must be developed to provide the restraint. You are considering that the connections are theoretically pinned. And the out of plane stiffness of the bracing elements is zero.

I've seen attibutable assumptions being peculiar to the client. IE, power plant work getting the most conservative assumptions. I which case, diagonal angle bracing was added to the traverse bracing below the level of grating within the depth of the main member. In which case now the "trussing effect" looks more traditional.

 

[SlideRuleEra]

Perhaps the differing views on the capability of grating comes from the many types of grating available. For example when grating is mentioned, I think of "industrial strength" such as 1.25" x 3/16" 19-W-4, good for a UDL of 716 psf for a 3 ft clear span. This size is quit rigid. Others engineers will certainly have their own frame of reference, just as valid.

Considering the possibility of friction providing lateral restraint, how does this sound?
The coefficient of friction could likely be anywhere between say 0.05 to 0.35 depending on the following:
1. Grating (say plain, painted, or galvanized)
2. Supporting members (say plain or painted)
3. Field conditons (say dry, wet or "greasy&quot

It seems to me that the friction force, 5% to 35% of applied load, will safely exceed the nominal 2% bracing force needed to provide lateral restraint.

In any event, the National Association of Architectural Metal Manufacturers (NAAMM) has two informative grating manuals for free download (.pdf) at

http://www.naamm.org/mbg

 

[AlohaBob]

My preference is never to rely on friction to provide structural ties. I practiced in California, so I was accustomed to justification for this. Grating is associated to me with industrial applications and vibrations which may tend to undermine a friction connection.

Even nominal tack welding of grating seems unreliable with temperature affects and vibrations naturally working to undermine that connection.

So I specifically engineer it and detail the welds or connections.

I could understand where someone might doubt the rigidity of swaged grating. But only in comparison to the welded types. Then the camparison becomes a matter of relative rigidity. So in all fairness, consider the rigidity of metal decking, or plywood sheathing, even gypsum wall board is considered effective to stabilize the flanges of steel members under some applications.

I would consider the force necessary provide that lateral stability where there's doubt. Say it were 10% of that force in plane that produced the instability to counteract that buckling effect out of plane. Then model that load path and its stiffness.

 

[VoyageofDiscovery]

Hi JAE,

I must have had a brainfart in my last post. I should have said cross frames or diaphragms instead of the lateral bracing. I agree that the TS connected to the top flange alone would not provide lateral stability. However, if the TS were deep enough to consider as a diaphragm, then the compression flange in the channel could be considered supported between TS spacings.

Regards

VOD

 

[jetmaker]

heynewp,

If I understand the problem correctly, and assuming that the catwalk is laterally restrained so that shaking loads do not cause the basic sections to rollover, you could check to make sure that the C-channels will always try and roll in opposite directions regardless of where the load is applied chordwise. If that is the case, then the TS members would act as constrains as long as they are welded and not pin joined.

If there is inadequate lateral restraint, or the loading causes a same direction rollover, then treat the TS as pinned joints with an elastic stiffness, or none to be conservative.

jetmaker

 

[flamby]

Hi Haynewp,
I would agree with most of the above replies. My approach in case of doubts is very elementary - go back to basics. Six degrees of freedom -translational fixity in all 3 dimensions are needed to make it pinned. If not, ignore anything and everything. It might be a Load, not Restraint.
Regards.

 

[GoodnPlenty]

Hello
All responses can be correct. I have studied this type of condition and the TS member can provide sufficient bracing as a rotational support. If the restraint moment capacity is sufficient the channels can be considered braced. If the TS moment capacity is insufficient then the channels are ?partially? braced.

Lateral Torsional buckling can be stopped with rotational restraint. This can be calculated. Joseph Yura has written many papers on this problem and the solutions/evaluation is straight forward.

With the information you've presented, I would say that it is likely that the channels will behave as braced.

Good luck

 

[AlohaBob]

I modeled this with RISA. I find the stiffness out of plane for TS 1x2 at 2 ft oc provided a stiffness out of plane roughly equal to the stiffness in plane for a pair of C12x20.7 spanning 30 ft. Using a 1 K point load in two directions for comparison, nothing was overstressed. I check stresses at the fixed TS connections. The welds would appear to be minimal to accomplish the purpose. Now add the grating!

The load capacity of the channels varies by ten times adjusting the unbraced length for these stringers.

Actual conditions are surely different, and I didn't look at local stress in the channel flange, but it would seem to me that with proper detail, you can justify added composite benefit from the proposed framing.

 

[SmithJ]

If the channels are simple supported, I would argue that the TS members provide bracing for the compression flange since the compression flange would always be the top flange. I am considering the effects of wind uplift to be negligible since we are talking about grating here and not a solid deck.

Although I agree with JAE that the channels could bow sideways (since there are only two if them), in this particular application, loads are being applied vertically and not horizontally so lateral torsional buckling should control instead.

I also do not use friction to justify bracing. I know it exists and always leave that as an added sfety factyor that could come in handy somewhere down the line.

Haynewp, Just a thought. Why are you welding the TS members above the top flanges? Couldnt you drop them into the plane of your channels and have the top flush with the top flanges, as so ... )-(. They woould be able to act as structs for your horizontal, diagonal bracing in the plane of the deck. I think this would provide more stability than merely welding them above the channels.


Thanks,
JS.

 

[3dboy]

just put stiffeners between flanges of the channel at every Ts and call it good. I would definitely NOT use the friction factor with grating, unless you are going to compute the actual contact area which is going to be very small. and also only rely on the weight of the grate. I can't believe that the combination of light weight and reduced area will yield any significant load at all.

 

[haynewp]

SmithJ, it's an existing project so the TS's are already welded to the top flanges.