Channels bolted back-to-back to improve load capacity 1

[patelam]

Channel C8, 14ft long, simply supported is supporting the grating and loads on the grating. The channel is failing in bending. To improve the strength of channel, another channel of same size and 8ft length is added back-to-back to existing channel. The channel is connected with one row of snug-tightened bolts at mid-depth, the bolt spaced at every 2'-6".

The intentions to add another channel is to improve the strength of original channel beam that was failing in bending.

Would the new channel added help the original channel in resisting the load? Or the original will still act as in independent channel receiving no benefit from new channel in resisting the load

 

[JoshPlumSE]

1) At the very least, doubling up the channel will double the capacity. That's even if they act as totally independent members.

2) If they are connected often enough to act as a single member, then you can calculate the the moment of inertia and torsional properties and such of the equivalent wide flange and calculate the bending capacity that way. It should be significantly stronger than twice the capacity of the original channel.

 

[jayrod12]

In the end, the new channel delivers the load to the existing channel at the ends of it. Assuming you're centering the 8 ft piece on the 14 foot piece, you're essentially just applying two point loads at 3 feet from each end of the 14foot piece. Why not just put in a full length piece?

Potentially your idea may reduce the maximum moment in the 14 foot channel, but it also may not. Let's just do a quick thought experiment:

Original single channel only
Load from grating applied to beam line = 1000 lbs/ft assumed
Maximum moment on single channel = 24.5 kip-ft

With 8 foot segment added assuming equal load share between channels for the 8feet.
load at ends of 8 foot segment = 2000 lbs.

load on 14 foot channel would be 1000 plf for 3 feet, 500 plf for 8 feet, and 1000 plf for final 3 feet. Plus a point load of 2000 lbs at 3 feet from each end.
Maximum moment = 20.5 kip-ft.

Maybe that's enough of a reduction for you, maybe not. Again, that's just a thought experiment. You'd also need to ensure that the last bolt between the channels can adequately transfer the load at the end of the reinforcing channel. I'd also not be fastening it along the length of the reinforcing channel, rather I'd be providing two bolts at each end. Otherwise in reality the slip in the connections will likely result in the original single channel picking up the majority of the load anyway.

 

[patelam]

@joshPlumSE and @joyrod12, great thoughts sharing.

 

[KootK]

joyrod12... I could see that sticking, optimistic as it may be.

 

[jayrod12]

joyrod12... I could see that sticking, optimistic as it may be.

I fear that would give people a false sense of my personality. But ragerod just doesn't sound great either even though it's more truthful.

 

[phamENG]

Hey - either one beats "pharm"...

 

[rb1957]

but the two channels would be enforced to deflect the same, no?

another day in paradise, or is paradise one day closer ?

 

[jayrod12]

they would yes, except for the whole bolt slop potential issue. And that the reinforcement is only for 8 feet of a 14 foot member. Obviously there would be some form of composite action, however I'd be defaulting to the most conservative analysis in this case. I also assume grating on channels is in some form of industrial application and I've always been a bit shy about pushing the envelope. I've seen how those companies treat their infrastructure. Having a bit of extra capacity around never hurts for a structure in an abusive environment.

 

[rb1957]

yes, the "truth" is very complicated, and not nearly as good as it appears (at first blush of dawn).

welding the two channels would be better.

full span would be better.

stacking the channels (if there is a free surface) would be much better.

another day in paradise, or is paradise one day closer ?

 

[WinelandV]

And we haven't even jumped into the "are the loads present when this is getting added" question yet.

 

[KootK]

We discussed some interesting, related things in this thread recently. It get a bit abstruse but there are some implications associated with shifting the shear center of the combined section along the length of the beam.

Were I to do this -- and I would be willing to -- I'd do this much like joyrod has suggested: as a sistering problem, paying attention to the shear transfer at the ends of the sister. Without welding or slip critical bolting, I'm pretty pessimistic about any true composite action materializing.

 

[KootK]

I feel that this model is about the right level of sophistication for the problem. And it suggests the appropriate attention to shear transfer.