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View Full Version : Truss vs. I-beam for hoist?



jpump5
03-21-2011, 07:44 PM
I want a way to lift objects both in my garage and in front of the garage,with a beam starting at the back of the garage and sticking out 3 - 4ft. through the front over the door.
While looking at weights and prices of I-beams, I started wondering if I could use less
steel by building a truss like I've seen supporting roofs in factories.These are typically
built of angle,top and bottom with rebar diagonals between them. I'm just wondering if anyone has tips, ideas, examples.(BTW I'm thinking I'd be lifting 700# cantilevered 4 ft.
from support.)

Pro-Fab
03-21-2011, 09:54 PM
Either will work, but it is quite likely that you don't have enough clearance for a truss. Also if you are running the hoist outside on a trolley, you will need a larger flange than what you will find on a standard truss. If you are going to run it outside over the door, you will have to make modifications to the header.

jpump5
03-21-2011, 10:32 PM
Trolley is out due to overhead door.I just want one lifting point outside and several inside,
I was planning on adding steel header on columns ( on sides of the door) for support
As for clearance,there is about 2ft. between top of header and inside peak of roof.

raferguson
03-21-2011, 10:52 PM
Trusses require lateral support to work. Roof trusses are attached to the plywood or other roofing material, and often braced on the bottom as well.

Designing a truss is not a trivial matter. Designing for an I-beam or H-beam is easier.

I agree that a truss would be lighter, but an I-beam would be simpler. Have you considered what size beam you would use? Generally, you can save weight with a larger beam that is not as thick. One time I had to design a beam with limited headroom, and ended up specifying the heaviest 6 inch beam that was available. Sounds like you have plenty of headroom.

I took a quick look at the stress numbers for 750 pounds at the end of a four foot cantilever, and came up with a 4 inch I-beam, which weighs 8 pounds per foot, so not crazy heavy. I did not try to look at deflection, however, which might call for something bigger. Another question is how closely it would be supported inside, and how it will be supported. You don't want to cave the roof in. 20 feet long times 8 pounds per foot is 160 pounds, maybe $150, so not prohibitive.

I would still add a trolley, even if it would not go out the door, convenient inside.

Richard

rlitman
03-22-2011, 12:08 AM
Could you install the beam with the trolley inside, and use an engine hoist for your outdoor lifting?
The cantilevering idea just doesn't sound good.

Trusses absolutely require side support, and your roof truss would not work. For it to work on a roof, you need multiple trusses, with a deck (and lower supports) preventing lateral and twisting movement.
You could build a box shaped truss. Think about what is used to support stage lighting.
But that would only save weight off an I beam, might not be any stronger, would require some serious engineering, and probably rules out using a trolley.
An I beam with a cantilevered end would also be a risky setup, because while the beam may be strong enough to hold your load "by the numbers", I beams left unsupported fail by twisting, and can drop the load at a much lower stress than you may expect.

DSW
03-22-2011, 12:09 AM
Trusses require lateral support to work. Roof trusses are attached to the plywood or other roofing material, and often braced on the bottom as well.

Designing a truss is not a trivial matter. Designing for an I-beam or H-beam is easier.

I agree that a truss would be lighter, but an I-beam would be simpler. Have you considered what size beam you would use? Generally, you can save weight with a larger beam that is not as thick. One time I had to design a beam with limited headroom, and ended up specifying the heaviest 6 inch beam that was available. Sounds like you have plenty of headroom.



Very true. Trusses, especially those that will be used for overhead lifting, are not a design on the fly sort of thing. There are any number of basic references for simple wide flange or I beam spans. Someone here posted up a few good links in the past from a hoist and trolley manufacturer for example.

Bgbkwndo
03-22-2011, 02:20 AM
[QUOTE=jpump5;489045]I want a way to lift objects both in my garage and in front of the garage,with a beam starting at the back of the garage and sticking out 3 - 4ft. through the front over the door.
built of angle,top and bottom with rebar diagonals between them. I'm just wondering if



First thing you should do here is forget the idea of using rebar in the fabrication of a truss!!!

Bgbkwndo.

oxygen454
03-22-2011, 02:34 AM
[QUOTE=jpump5;489045]I want a way to lift objects both in my garage and in front of the garage,with a beam starting at the back of the garage and sticking out 3 - 4ft. through the front over the door.
built of angle,top and bottom with rebar diagonals between them. I'm just wondering if



First thing you should do here is forget the idea of using rebar in the fabrication of a truss!!!

Bgbkwndo.

I agree, rebar is not ment for that type of application. Even if rebar was used, it has to be weldable (W400 if I remember right)

jpump5
03-22-2011, 08:09 AM
Thanks for replies-
I was thinking i would need a larger,more expensive i-beam.I guess a truss is not the way to go.
I still need to look at means of supporting beam.

I realize truss design is not a simple matter.Was looking at the possibility of copying existing design from similar application.

The reason I mentioned rebar for diagonals is that I've seen it used in trusses of steel buildings.
Never thought about the weldability of rebar.

Kelvin
03-22-2011, 08:49 AM
I'm no engineer but I think another issue you need to consider is whether the truss or I-beam or whatever can twist in the middle under load, because many things are a lot stronger in one direction than in other directions, if that makes sense. In other words, if the supporting member can twist, in many cases it can fail a lot easier than if it was prevented from twisting.

S obsessed
03-22-2011, 10:28 AM
I know you are already going to use an I or H beam but I will throw this out there on the subject. A truss is not going to handle a single point load like an I or H beam would. The span between the webs on the truss where there is no support will possible sag asyou roll by them. The H or I beam is better in that the top and bottom flanges are tied together the whole way. Inbetween webbing on the truss, it would have to act like a small I beam and that is not what they are designed for, their load is supposed to be spread out more.

MoonRise
03-22-2011, 11:29 AM
Standard off-the-shelf roof truss, used as a cantilevered lift point?

Nope.

Could an engineered truss-like structure be used as a cantilevered lift point? Sure. Go pay the engineer to design such a thing for you. Not all that difficult.

But remember that a lifting point up in the air has to have the correct structure underneath it to transfer the loads properly.

btw, rough rule-of-thumb for building cantilevers is to make the structure 'behind' (into the building) 3x as long as the cantilever extends out. So for a 4 ft cantilever, you want 3x4=12 ft of beam/joist/structure properly supported going back 'into' the main structure.

Did you consider just using an engine hoist to lift stuff up?

tenpins
03-22-2011, 08:52 PM
Thanks for replies-
I was thinking i would need a larger,more expensive i-beam.I guess a truss is not the way to go.
I still need to look at means of supporting beam.

I realize truss design is not a simple matter.Was looking at the possibility of copying existing design from similar application.

The reason I mentioned rebar for diagonals is that I've seen it used in trusses of steel buildings.
Never thought about the weldability of rebar.

I think Truss is the wrong word for what you've described. Its a bar joist. Joists need to be bridged and diaphragm-ed. Use a beam, or some tube steel. Check for the difference in an "I beam" and an "wide flange beam".

I seen guys fork up a load of beefy bent plate onto unbridged joists, the weight of it caused several of the joists (25 foot by 2 feet deep) to twist and deform. OSHA regs say that joists must be bridged before bundles of deck are landed on them.

jpump5
03-23-2011, 08:06 AM
Yeah,The bar joist for lifting beam idea wasn't great. (torsion,single point loading etc.)
I assumed it was called a "truss" because of it's appearance.
Engine hoist isn't my favorite option,but not ruled out.

-Thanks

A/C Guy
03-23-2011, 12:43 PM
There is a major difference between a truss and I-beam. I-beams are used for trolleys because you can load the bottom of the I-beam. You can not run a trolley along the bottom of a truss. The bottom of a truss is never to be used for a load. If you use a truss, you must run the trolley along the top of the truss.

As others mentioned, an I-beam would be a better choice due to the twisting that will occur in a truss.

rlitman
03-23-2011, 01:59 PM
There is a major difference between a truss and I-beam. I-beams are used for trolleys because you can load the bottom of the I-beam. You can not run a trolley along the bottom of a truss. The bottom of a truss is never to be used for a load. If you use a truss, you must run the trolley along the top of the truss.

As others mentioned, an I-beam would be a better choice due to the twisting that will occur in a truss.

I beams are prone to twisting too.
I don't get why it matters whether or not a trolley is run on the top or bottom of a truss.
Just look at a truss bridge. The roadway is often on the bottom.

A/C Guy
03-26-2011, 12:16 AM
I beams are prone to twisting too.
I don't get why it matters whether or not a trolley is run on the top or bottom of a truss.
Just look at a truss bridge. The roadway is often on the bottom.

The construction of the truss makes a huge difference. Most trusses have the webbing attached to the top of the bottom piece and attached to the bottom of the top piece, so when loading from the top, you compress the joints. If you load the bottom, you are attempting to tear apart the joints. Does that makes sense?

Boilermaker237
03-26-2011, 10:16 PM
to get inside the garage roll up mount another i beam under the beam on a set of roller trolleys ...roll the beam out the door then use the trolley on the beam to roll it into the garage.... like was said above the 3 to 1 rule sounds right for the length of the rolling beam.

jpump5
03-27-2011, 10:27 AM
to get inside the garage roll up mount another i beam under the beam on a set of roller trolleys ...roll the beam out the door then use the trolley on the beam to roll it into the garage.... like was said above the 3 to 1 rule sounds right for the length of the rolling beam.


I was thinking of something like that but there is not enough height under open door.
Lifting objects out of p/u bed is the main purpose of outside lifting point and that point
needs to above door .(height of bed + object + sling + chainfall + beam etc. is more
than height of door opening).Also, when load is lifted,won't rolling beam pivot on first trolley,pushing trolley at far end upward on stationary beam,requiring a different type of trolley?

Boilermaker237
03-27-2011, 07:26 PM
we had fabricated a roller to place between the beams to allow for the lift on the tail end.You want to lift plate out of the bed right?how much overhead room will there Be?

jpump5
03-27-2011, 07:55 PM
we had fabricated a roller to place between the beams to allow for the lift on the tail end.You want to lift plate out of the bed right?how much overhead room will there Be?

I have 7' 5" under garage door ,p/u bed is 2' 6".It would work for lifting plate out of truck but
would reduce headroom in garage and might not have height to lift taller objects out of
truck.(chainfall , sling, beam and trolley must take up at about 3ft.,That would only leave about 2ft. for height of object).

Donald Branscom
03-27-2011, 08:34 PM
[QUOTE=jpump5;489045]I want a way to lift objects both in my garage and in front of the garage,with a beam starting at the back of the garage and sticking out 3 - 4ft. through the front over the door.
built of angle,top and bottom with rebar diagonals between them. I'm just wondering if



First thing you should do here is forget the idea of using rebar in the fabrication of a truss!!!

Bgbkwndo.

Absolutely right. No rebar.