8th Sep 2013, 11:04 PM #1
How much can I safely lift on this structure?
I'm splitting this off from Christian's http://www.woodworkforums.com/f65/hoist-176037/ thread, because it's a bit different. I would like to ask people who have practical experience lifting heavy stuff from existing structures: How did you figure out that it was safe? Did you try to calculate forces? Do you have engineering knowledge? If not, what rules of thumb did you employ?
Now I know the safety aspect of lifting can get people worked up. Could we just try to keep it constructive and objective please?
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9th Sep 2013, 05:17 AM #2
If I was making a gantry and wasn't sure I would have a look at the size and length of the steel in one that is already design rated and go from there.
9th Sep 2013, 08:56 AM #3
Are you on Kiwi time Phil? Any earlier and you'd be up before you went to bed!
Yes what you say makes perfect sense, and I have some dimensions from commercial units saved for that reason. Which I can post if people want. But I was more interested in assessing the option of using my shed frame.
If I went to the maker of my shed and asked how much can I lift I suspect the answer would be 'Zero. It's not designed for lifting, it's designed to keep the rain off.' If we take that at face value, the question then becomes, 'How can I modify my existing structure to make it more lift-worthy, and how do I quantify (estimate) the Safe Working Limit?'
9th Sep 2013, 09:58 AM #4
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I work in a slightly different manner..
My theory is along the lines of
" If when you lift it the building breaks, It was too heavy "
Sorry I just couldn't resist.Warning Disclaimer
9th Sep 2013, 10:01 AM #5
I've done some research to make my own gantry but I haven't had time to put it all together to come up with a design.
Australian Standard AS 1418-1 (2002) [you can view it on the internet] details cranes, hoist and winches and discusses loading and design.
Based on the design parameters I intend to play around with some finite element analysis software: FRAME3DD - Static and Dynamic Structural Analysis of 2D and 3D Frames
to see how a design would work. You can get the section properties online from the manufacturers, e.g. OneSteel Publications/Software: Tubeline® Rectangular Hollow Sections
If you want to calculate what size beam you need for a given load I found a good tool to calculate this: Universal Beams
However it doesn't give you any information regarding a support structure.
9th Sep 2013, 08:36 PM #6
Thanks for the links Christian. But I don't understand any of em. I was hoping for some short cuts, but maybe there aren't any.
Back to "Applied Mechanics Made Simple". It's a book. Remember those?
9th Sep 2013, 08:56 PM #7
The major variables of course are how heavy, what sort of structure are you lifting from, and what sort of lifting, ie static lift or lift and travel.
Can you explain a little further, maybe post some photos?
9th Sep 2013, 09:22 PM #8
Time to make some comments, firstly a couple of months ago there was an article in MEW where someone made a gantry from wood. definitely NOT the way I would go but probably worth a read. Secondly, my shed trusses are strong enough that if I were making a gantry I would embody some of the design. They consist of two horizontal members of steel tube about 60mm outside diameter & about 6mm wall thickness. These are about 350mm apart, one above the other & diagonally braced with solid bar about 15mm in diameter. Depending on the span you want, you could do something similar. I would not put bolts in shear to hold it together, but would make it so that the horizontal truss sat on two lugs welded to your uprights & just use large bolts to hold it together. (16mm or thereabouts.) The legs do not have to be so strong as they are mainly in compression. If you want the max lift to be 1 ton, then the legs each side only need to support 1/2 a ton & if 2 legs each side, then each leg only supports 250 Kg. or thereabouts. (Maybe 50mm x 50mm x 1.6mm square tubing?) I would make the bottoms of the legs each side at least 1200mm (if you have the room ) to give it some stability. I would also triangulate the top of the legs to the truss to stop it skewing sideways. The only thing to be careful with is the method of supporting the truss on the legs, if you just weld something on the legs to take all the weight, the welds could just tear out. I would weld some angle iron (60mm x 60mm x 8mm?) onto the tops of the legs & mount the truss on top of that. I'm definitely NOT a design engineer, but have built a few things with no failures yet. NEVER put any body parts under a suspended load. Hopefully others with more design Knowledge than me, will now chip in & refine a design for you.
9th Sep 2013, 09:41 PM #9
Generally the main issue is bending of the top beam. You can work out the bending stress as follows:
Bending stress (MPa) = M / Z
M is the bending moment in Nmm. Work this out using the length of the beam in millimeters x the load in kg x 9.81 x 0.25
Z is the section modulus in mm3. You can find this from steel tables like these http://www.onesteel.com/images/db_im...e5_Feb2010.pdf
The bending stress should be less than 66% of the yield strength of the steel. Unless you know otherwise you are generally safe to assume 250MPa.
9th Sep 2013, 10:08 PM #10
The way I read it the load applied at the apex will place the sloping beams in compression, which will try to spread the columns apart. Two obvious solutions would be (a) place another column under the apex, or (b) place a horizontal tension strut between the columns, forming a closed triangle. The extra column would get in the way more, so the horizontal would be preferable.
Let's say I want to lift 1000Kg. I need to know how much force will be compressing the beams, and how much will be tensioning the strut. I know the angle of the beams is needed but I don't have that info in front of me right now. I think the compression in the columns is simply half the load - plus whatever they already carry.
Ian, how much have you lifted with the setup pictured, and how much would be be willing to lift? That actually has some similarity to the point I'd like to lift from, in that it's mid-span on a beam supported by another beam. Is that a skillion roof?
9th Sep 2013, 10:40 PM #11The way I read it the load applied at the apex will place the sloping beams in compression, which will try to spread the columns apart. Two obvious solutions would be (a) place another column under the apex, or (b) place a horizontal tension strut between the columns, forming a closed triangle. The extra column would get in the way more, so the horizontal would be preferable.
I don't know anything about the engineering measurement side of things. I just use Matthews method only I try to stop before the building falls down. I also tend to over engineer in order to compensate.
As I have said in the other thread I have a lifting beam made from 100mm x 6mm SHS and reo triangulated above. Given the right supports it could probably lift a truck. This was just the material (recycled) that I had available. It is made from about three bits welded together.
9th Sep 2013, 11:22 PM #12
Lifting loads impose bending in the lifting beam (or truss), and bending loads in the frame uprights, and possible pull out forces where the frame is bolted to the shed slab.
Perhaps the "easiest" option is to make any hoist free standing -- brick piers supporting a steel beam.
the alternative of modifying the existing structure to support a hoist can get tricky very quicklyregards from Sydney
10th Sep 2013, 12:23 AM #13
Modern steel sheds (I have one myself) are designed to withstand distributed wind loads. That's not to say it won't withstand heavy point loads but that's not something that can be sorted out in a couple of forum posts.
The quality of shed engineering design is highly variable, well it is in SE Queensland anyway, I look at the structure of some sheds and shake my head. You wonder how an engineer could have have signed off on it.
Rolled C section beams are a lot more fragile than old school RSJs or triangulated trusses. You can't just sling a chain around a C section, if you bend the edge that could make the C section fail right there and then, or it might wait until the next big storm.
If I needed to lift 1 tonne from one of my roof beams and there was absolutely no other way to do it, I would be propping the beam as close to each side of the load as I could AND I would have a tight fitting 75 or 100 square hardwood or RHS spacer between the top and bottom flanges at each of those points. For the chain point, I would plate the top of the C section with 1" hardwood and fit a couple more spacers.
Good luck, let us know how you get on.
10th Sep 2013, 12:37 AM #14
Get an engine crane or build a roller gantry... As mentioned newer sheds really aren't suited to additional loads like this.
This is the gantry i built for my shed out of left over 100UC i had from a job. It breaks into 3 parts and i can change the length of the top beam if need be. From memory i think its good for about 2t. Only problem is i cheaped out on the wheels which only have bushes not bearings so it's very hard to move with weight on. I built it specifically to move my lathe around.
10th Sep 2013, 12:45 AM #15
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