Gantry hoist on rafters

[grego]

I have a wood truss garage(20'x24') which I had them install enginered trusses for extra load and or storage in the middle. I was told as a whole, I could store 2000 lbs across the whole area. It is 4' wide and 24' long. I have installed 3/4" plywood to span across the area to make a floor.
I would like to put unistrut across 75% of the span and install a 800 lb electric hoist. I would like to install wheels so I can move it side to side. I would be using unistrut fasteners and hardware unless someone knows a better way.

Am I on the right track or is this an invitation to something bad happening?
At the most, I would probably only use 80% of the hoist capacity. I am using the ASYLUM thread and Shopnut's front loft hoist as insperation.
http://www.garagejournal.com/forum/showthread.php?p=1843980#post1843980

Thanks in advance for any help.

 

[snorky18]

Honestly it's hard to say from the information that you've provided without knowing your specific truss design and desired loading characteristics (a bunch of light loads spread out vs single heavy load in one place), so this might be a question best left for the engineer who engineered your engineered trusses.

4' wide x 24' long = 96 square feet. 2000 lbs "across the whole area" would be 2000lbs / 96 SF = 20.8 pound per square foot, which is a very, very light (low) floor rating for even thinking about storing most garage accessories (that are generally quite heavy and dense).

 

[grego]

For extra support, I could increase the span to go from wall to wall so the channel rests on the top of the walls and then just have the additional support along the trusses.
Not sure????

 

[EOC_Jason]

What is the strength of the unistrut?

In reality this is best left to someone with an engineering degree to be safe, or at least someone with expert knowledge on structural designs.

 

[Steevo]

I thought about doing pretty much what you are suggesting. I planned to run a dual-track Unistrut trolley system on the surface of the ceiling, supported by all-thread rods up through the sheet rock and through another set of unistrut sitting on top of the truss chords. I figured the combined lengths of unistrut would distribute the load over at least the three or four nearest trusses at all times.
I ran the idea by a contractor/builder friend, who told me he thought it was a bad idea to purposely hang high concentrations of weight from the trusses, by any method.

Burst my bubble . . .

Then again, in my younger (dumber?) days, I used an old car seat belt, hung over a floor joist under a friends deck, with a come-along hanging from that, to pull engines . . .

But if you sandwiched your track between two sufficiently strong boards like in the linked example, that then sat on top of the end walls, or on posts buried in the end walls, you wouldn't be suspending it from the trusses, and would only need to calculate the load capability of your "beam".

 

[snorky18]

What is the strength of the unistrut?

In reality this is best left to someone with an engineering degree to be safe, or at least someone with expert knowledge on structural designs.

It depends. Unistrut/Superstrut comes in a million flavors, he didn't tell us which one he was considering using. Strenth depends on the dimensions of the unistrut as well as the thickness of the metal used.

If you're thinking about hanging the unistrut from the inside of your trusses, which are probably spaced at 16" or 24", the strength of the unistrut is the least of my concerns about the setup. I'd verify your floor loading capapbilities first.

If that floor won't hold your load, then there's not much reason to spend time or money making a crane that will lift something up there that has no where to go.

 

[snorky18]

I thought about doing pretty much what you are suggesting. I planned to run a dual-track Unistrut trolley system on the surface of the ceiling, supported by all-thread rods up through the sheet rock and through another set of unistrut sitting on top of the truss chords. I figured the combined lengths of unistrut would distribute the load over at least the three or four nearest trusses at all times.
I ran the idea by a contractor/builder friend, who told me he thought it was a bad idea to purposely hang high concentrations of weight from the trusses, by any method.

I'm very much in agreement with your contractor friend. If you want to see how much weight wood will hold, go hang weights off a tree branch, and keep anything with a pulse away from the load. The roof truss of a structure that you live in is not a good place to experiment with concentrated loads that it is not designed to handle.

Sorry to burst your bubble

But if you sandwiched your track between two sufficiently strong boards like in the linked example, that then sat on top of the end walls, or on posts buried in the end walls, you wouldn't be suspending it from the trusses, and would only need to calculate the load capability of your "beam".

The walls are 24' apart, that would be a heck of a wooden beam to span that far and take any appreciable load.

 

[Outlawmws]

You already used a healthy chunk of your 2000 lb design capacity with the 3/4" ply you put up there. Then you want to add whatever your planning to store, AND add in 1-200 lbs of uni-strut and electric hoist to that...

I think you are done...

You need to have a separate support structure for the hoist and uni-strut channel.

 

[GN4WHLN]

The trusses may take 2000 lb over the entire area but you would be placing a point load on a small area that may cause a failure - one that could lead to a series of failures. You might consider using a small I beam that runs wall to wall and welding it to structural channel or square tube verticles that bolt to the wall and floor. This would let the load be transferred into the slab. Better yet, A frame the ends. so the wall isn't loaded by any movement of your load. I would dump the unistrut all together. I think the origional design is asking for something bad to happen. Picking up a 4-500 lb load is no joke. You really should use structural steel and welded construction.

 

[koditten]

I have a 4" I beam lagged to my cieling. The one side is supported buy a post bolted to the wall. The free end gets a temporary post when I do any lifting. No probs yet.

I weighed the beam and it weighs the same as I do. I figured if I can stand up there, it can hold a beam.

KO

 

[Vegaman_Dan]

Trusses are meant to support the weight compressing upon it from the top surface/roof. The geometry is completely different in supporting a load pulling down on the truss from below. You'll very easily pull the roof down with you if you attach to the bottom of the trusses.

If you really want to do this, then you'll need to attach to the top of the trusses where they meet the roof panels and hang supports from that to your tracks. Awkward, expensive, and complicated.

It may be cheaper and easier to see if you can get some steel trusses fabricated that can go in next to the wooden ones for this purpose instead. Certainly safer.

 

[olytdi]

Trusses are meant to support the weight compressing upon it from the top surface/roof. The geometry is completely different in supporting a load pulling down on the truss from below. You'll very easily pull the roof down with you if you attach to the bottom of the trusses.

If you really want to do this, then you'll need to attach to the top of the trusses where they meet the roof panels and hang supports from that to your tracks. Awkward, expensive, and complicated.

It may be cheaper and easier to see if you can get some steel trusses fabricated that can go in next to the wooden ones for this purpose instead. Certainly safer.

^^^
What he said...consider a gantry.

 

[Steroblan]

Leave the structure alone and ditto what GN4 said. Unistrut really isn't designed for your use. Neither are your trusses.

 

[cyamaha2007]

^^^ i see unistrut used in these apps allover in commercial settings. It has accurate load charts and a built in safety factor. When we built the cabins garage we built it with the need to hang 5 deer inside. Thread all rod attaches to the top cord of the truss. The other end pokes through the ceiling and attaches to a I beam, A electric hoist is hung off that we have lifted a 3000lb boat off its trailer. NO issues