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Safe hoisting load on this truss beam

avivz

Aluminum
Joined
Jan 28, 2009
Location
Israel
What would be an undoubtedly safe load to hoist on this truss beam?
The horizontal pipe-section beams are about 3'' diameter, the columns are about 6'' diameter. The span between the two colomns is about 70 foot (~20m).

Assuming I hoist close to the column, will 2800lb (1.3 ton) is safe?
I marked the desired approximate hoisting point.

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Poor man's FEA. Hang a tape from the beam. Start lifting if the tape length decreases by more than your personal pucker factor tolerance stop. Everything about that beam construction screams don't lift using it. If you insist on proceeding propping it would be the way to go. Obviously none of the above is a recommendation. Hopefully we don't see you on the news or YouTube under heading of man collapses entire factory roof after seeking advice on infamous website Practical Machinist.
 
Message clear.
For my education, what makes this truss less capable than std beam used in simple 2 ton gantry? Is it the large span or something else?
 
You've been given good advice. Ignore it at your peril.

No engineering analysis can be made without knowing the exact diameters and wall thicknesses. Without those things we can only do a visual inspection. The visual inspection indicates this structure was built to the very lowest price point possible. There is no extra capacity in the system. ??? had a general good idea about checking sag, but that depends on the structure staying intact. Catastrophic failure on one of the weld joints can't be predicted, and could be expected. Places that build to low price points can't afford good welders.
 
Simple...proof test your lift ...its easily seen if the truss is deflecting down ,and the load is not lifting.........there is a picture of a light gauge steel sheet building with a tractor suspended off every frame all the way down the building...........the pic claims that the load is supported entirely by the corrugated sheet box structure .
 
Simple...proof test your lift ...its easily seen if the truss is deflecting down ,and the load is not lifting.........there is a picture of a light gauge steel sheet building with a tractor suspended off every frame all the way down the building...........the pic claims that the load is supported entirely by the corrugated sheet box structure .
If it's on the internet it must be true.
 
Consult the engineer that designed the structure. "Load Testing" without input from the engineer may result in imperceptible damage that could weaken the structure and cause it to fail later when exposed to extreme weather conditions or other disturbances.

RT
 
If was gonna do that...and I wouldn't...I'd shorten the 70' span as that is Killer #1. I'd shorten it by placing a vertical upright as close to the load as possible. Of course...there is no height specified but it looks like 10' or so I'd get a suitable length of something strong and wedge in it there.
 
Despite appearances it's not a truss. It's a stabilising structure intended to stop the upper ends of the steel pillars wobbling around in the wind. The bracing merely stops the long "beams" from significant bending under their own weight as the structure is assembled. When the building is complete the whole thing is a basic space frame. Quite stiff against twisting and diagonal forces but it cannot cope with significant vertical loading. That roof is very light.

Any serious vertical load will tear the small diameter diagonals and verticals out which may lead to total collapse or merely serious bending. There is huge difference between proper truss joints intended to cope with serious loadings and those needed to simply resist shearing style loads where its necessary to stop two parallel tubes a certain distance apart from moving lengthwise in relation to each other.

Clive
 
Message clear.
For my education, what makes this truss less capable than std beam used in simple 2 ton gantry? Is it the large span or something else?
It's not the material, it's the design intent. The simple I-beam used in the gantry crane has been sized for the intended use. There are cranes using trusses to support loads. The truss designed to support the roof has been optimized to the building design code for the locality with no extra capacity as the builder was likely bidding against other builders.

If you intend a truss to support additional loads you have to include this in the design requirements before you build it.

Additional temporary loads may be permitted under specified conditions, the engineer may be able to tell you off-hand or may have to perform calculations. In all cases the engineer should be consulted.

RT
 
1) purchase free-standing gantry hoist of the appropriate capacity.
2) install immediately below points indicated in your diagram.
3) if this seems expensive consider all the customers' cars below the indicated structural element.
 
One thing I don't think was mentioned by others (and I know you have said you are not going to try to use the beam)
Not knowing wall thickness, a chain (common to use for attachment point) wrapped around the pipe could dimple the wall (especially if the chain shift and rotates) and lead to a rapid failure. Looks like it might have been meant to hold mainly tension. Maybe.
 
This roof will have no extra capacity since the OP has no snow load or tornados to consider. In the Lake Tahoe area some roofs require a snow load of over 200 pounds per square foot. A roof like that, in summer, would be much safer to hoist from.
Bill D
 








 
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