Channel Toe up or Toe Down? 1

[LowLax]

I have an existing runway beam with a cap channel. The runway needs to be reinforced to carry a new higher capacity crane, so I am adding a channel to the bottom flange to get the extra capacity.

When I did the calcs I had the bottom channel toe up so I had a doubly symmetric section (top and bottom channels are the same size). Now I'm thinking it's going to be much easier to weld with the bottom channel toed down.

Maybe I haven't had enough coffee this morning, but is there a problem with doing this? The I value would increase slightly, but there is such a small area at the extreme bottom fiber that I'm worried I am going to reach yield sooner.

 

[Stillerz]

I would like to say that in referencing continuous welds, I was referring to mill-duty crane runways only.

 

[miecz]

Ignoring fatigue considerations, I calculate that the AISC360 allowable strength of the section with a W14x34 and 2 C10x15.3s is 2% higher if the bottom channel legs point down, rather than up. A strange result, since the elastic stresses are 19% higher that way. Having been brought up to minimize stresses, it's difficult to think of the legs down section as the better one. But it is, according to the current code.

 

[racookpe1978]

It would appear that, with the lower channel being built "legs down" on the bottom of some beam, the effectiveness of the "fabricated" channel is greater with the legs down.

Legs down on the bottom beam (or legs up on the top channel), the tip of each of the legs is further from the centerline of the whole beam.

Legs up on the bottom (or legs on the top channel), the two legs are closer to the middle of the entire beam, and so the strength (ending resitance) of the entire beam assembly is less. (Welding access and probabilities of corrosion and the like skipped for a few minutes.)

But, looking at a single channel by itself, legs up or legs down would become more a matter of convenience and connections and welding, not leg position.

But I must stress that I am only going to go with the intermittent flow on this one .... 8<)

 

[cntw1953]

In the LRFD world, how you take care of unstiffened channel legs? With some strength reduction factor?

 

[miecz]

cntw1953

I don't think there is any reduction unless the flange width to thickness ratio exceeds 0.38[&radic;]E/F[sub]y[/sub].

 

[trainguy]

Guys,

I typically avoid plug welds, but I love ring welds.

tg

 

[Stillerz]

cant tell if you are kidding trainguy....
ring weld? weld ring?

 

[trainguy]

No joke intended Stillerz.

Drill a large hole (e.g. 1 inch dia.) and make a fillet weld on the edge to connect to the adjacent piece. Could be a terminology issue, I suspect. Similar to a slot weld.

tg

 

[JAE]

ringwraiths chasing the ring?