back to back channels

huangyhg

back to back channels
i have a long beam made from back-to-back channels. they are bolted together at the web. the beam is loaded vertically along its length and is supported at one end (cantilever). how do you determine the bolt spacing and bolt size which hold the channles together?
thank you,
genesj
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what is the orientation of the channels with respect to the load?  assuming the load is a gravity load, are the channel webs vertical or are they parallel to the ground?
yes. the beam is horizontaly arranged with the webs vertical to the ground.
thank you.
if the loading is distributed evenly on the channels, then theoretically, there is equal movement and loading on each channel, and the force in the bolts would be effectively nothing.  in reality, you have to ensure that one channel isn't loaded more than the other, etc.
a conservative way to approach it would be to say that only one channel is loaded, and the bolts are required to transfer all of the load to the other channel.
assuming a uniform and equal loading on each channel, i would take a percentage of the total load, say 40%, and say the bolts have to transfer that amount of load to the other channel.  if the loading is eccentric, i would redo my design accordingly.
genesj:
if i understand the problem correctly...simple beam theory would indicate that the shear load is taken equally by both channels thus there is no need to bolt them together...you should look to possible secondary effects (bending in the other plane) or check the requirements of the code that applies to your problem.....i'm not familiar with current code requirements so not any help there.....
ed.r.
why don't you use a wide flange?  is this a retrofit where you have to use the channel that's already there?  
it depends on the assumptions you made when designing the back-to-back channels whether you design a bolted connection to hold them together.  if they are sized with the strength of two channels added together then you don't really need a connection between them for bending strength.  if they are sized as a doubly symmetric i-shape then you do need a connection between them.  the aisi spec gives design equations for the spacing of such connectors but i couldn't find one in the aisc.  the easiest thing to do might be to provide enough bolts to develop the full shear flow across the plane between the webs.  that's pretty conservative, maybe overly so, but will get the job done without days of research.  the bolts should be placed as close as practical to each flange.
genesj,
look at pg 5-50 section f6 of the green aisc (1989 asd) steel manual. this section addresses the minimum back/back channel connection requirements.   from your description the requirements in this section is all you need.
even though you have a cantilever these requirments should still apply. you still have a flexural member (a beam)and it is a built-up (not rolled)   
i am trying to qualify an exisitng structure. thank you all for all the input. section f6 in the aisc helps.