"Jim Wilkins" wrote in message news
"Carl Ijames" wrote in message
news
"Jim Wilkins" wrote in message news ...

I think I misunderstood your layout so I better stop "helping" :-). The
link you posted to a splice diagram showed an I beam, so I assumed you
were using two C channels back to back to simulate an I beam with a thick
vertical web. If that's the case I still think my three splice instead of
one layout would be almost as easy to assemble and would be stronger.
Just for confirmation, the program I use (Engineering Power Tools, free or
shareware for more features at www.pwr-tools.com) gives peak stress 24.9
ksi and 1.32" deflection for 1000 lbs at center and 16' span for one C
channel.

--
Regards,
Carl Ijames

Your assumption is correct, two channels back to back with sections of
3/8" plate between them at the ends as hangers and as splices between
8' lengths. The 3" x 16' track I've been using to move logs in and out
of a shelter hangs from a roof beam at the center splice and this 4" x
16' one will too, in another shed that has become storage for squared
timbers. At this point creating a 16' free span gantry from the 4"
channels is just a design exercise, to see what I could make from my
materials.

I do like your idea of staggering the joints but it prevents
assembling the four channels as two separate 8' sections with higher
load capacity than the 3" channel I'm using now, one to load the
trailer and the other to unload and stack, and I don't have another
piece of 4" channel to cut in half. My 3" and 4" channels came from a
stack of used pallet rack parts at a surplus store that closed. The
nearest metal dealer recently quoted me $120 for a 4" x 20' channel,
cut in half so I could bring it home.

I have extra 3" channel and 3.5' of W6x9 beam that I picked up to
reinforce the splice somehow, perhaps bolted on top to go with a plate
under the tension flange, but that challenges my limited ability to
calculate and I have no idea how to figure the assembly's resistance
to buckling, I proof test my homebrew hoists on a stump with a 1000
Kg crane scale.

Thanks for confirming my numbers on the stress and deflection of the
4" channel. I'm never sure I didn't miss something with those
calculators. Is it true that the peak stress on two channels would be
half as much, and acceptable for the working load?

-jsw
================================================== =

First, the usual disclaimer, I'm not an engineer and this isn't professional
advice :-). But I've read a lot and done a lot of calculations on
mechanical prototypes I was designing that worked as designed. Yes, if you
put two channels in parallel with the same total load the peak stress is cut
in half. For 1000 lb concentrated at the center of an 8' span a single 4"
channel will deflect 0.165" and peak stress will be 12.5 ksi, and a 3"
channel will deflect 0.383" and have peak stress 21.7 ksi. Adding a second
4" channel cuts the deflection to 0.083" and the peak stress to 6.3 ksi, and
the doubled 3" deflection and stress become 0.191" and 11 ksi. That says
that the 4" channel is almost twice as strong so one 4" channel is worth
almost two 3" channels. How about if you bolt a 4' piece and an 8' piece of
4" channel to an 8' piece, back to back with the ends flush at one end.
That gives you your desired doubled 4" x 8' channel with 4' extra sticking
out one end, which doesn't hurt so long as you have room in your shed for it
to still fit. Then take the other 4' x 4" channel and bolt it with ends
flush to the other 8' channel, and then bolt on a 4' piece of 3" channel
with the bottom flanges aligned so the crane trolley can roll the entire
length. If your doubled 3" channel is sufficient now, this should be more
than enough, and you can unbolt the 3" channel and bolt together the 4"
assemblies to make the full 16' span with staggered joints.

Whenever you have a live load, like people walking on a deck or lifting a
load that is moving and bouncing, the usual rule of thumb is to triple the
load to calculate the peak stress. Then apply whatever safety factor you
feel appropriate. For something used once with no one anywhere near that
can get hurt I might go down to 2x but for this I'd go at least 4x -
remember, I'm not a professional. So if your beam steel is rated at 25 ksi
yield I'd keep the peak stress below 25 ksi/12 = 2.1 ksi, maybe less.
Others might say that is way too conservative but I never, ever want
anything I'm walking under to fall on me :-). Running those numbers in
reverse for the 4" channels, 2.1 ksi / 6.3 ksi * 1000 lbs = 333 lbs for a
rated load for your 8' span 4" assembly. I said I was conservative.

--
Regards,
Carl Ijames