New twin car lift sagging in middle!
Discussion
bigothunter said:
Kawasicki said:
All material deflects under load. When you take the load off does it return to it’s original shape? If it stays bent, it’s definitely overloaded.
Plastic deformation - how many fatigue cycles to failure? 
If it is staying in the elastic zone I wouldn’t worry too much.
Kawasicki said:
Not many.
If it is staying in the elastic zone I wouldn’t worry too much.
It's a civil engineering structure - no excuse for having a low factor of safety. Also those (naïve) holes in the vertical section magnify stress concentration by at least 3.If it is staying in the elastic zone I wouldn’t worry too much.
Those beams should be well inside the elastic region. And being steel, they should stay under the fatigue threshold (S/n curve).
Previous poster mentioned the old appearance adage. That beam deflection doesn't look right which strongly suggests it isn't.
Collectingbrass said:
If it looks right it probably is right, and that doesn't look right.
I would get an independent Structural Engineer to check it, pronto.
I would get an independent Structural Engineer to check it, pronto.
And tell the supplier they will be getting his report. Should ease negotiations towards refund or lift with higher load rating (5 tonne?)
So I’ve thought carefully and have taken on board all the help I have received here. Thankyou.
The first point is that it is a huge task to strip the ramp now that it is all panelled on top. I did not drill the crossmember at all to do that - but the 25mm plywood is secured to the ramp runs using 60 M12 bolts so it itself acts now to spread the load much more evenly than just 8 tyres as contact points. Because of the amount of work returning the ramp isn’t an option.
Secondly, I have thought about the liability side of it - imagine it collapses and someone is hurt or worse - no amount of money will put that right, so I think it is down to me personally to engineer my way to a point where I can rest easily.
The fact that the manufacturer is confident in the product and knows the exact maximum design deflection gives me good confidence that the rest of the system is up to the job, and looking at my other ramp it is identical except for the depth of those beams. The manufacturer admits it does look odd with the sag, but tells me that they are done like that so that ramp angles are easier and very low cars can come on and off with ease. They also tell me that some of you here are correct in saying that bolting it down will reduce sag. So I will do that right away.
I have designed some shear plates which we are making up out of 9mm steel. These will bolt on each end and at that point I think the performance will be much better. Since I won’t be drilling anything at all and it’s a bolt on change I figure this is my next port of call.
The first point is that it is a huge task to strip the ramp now that it is all panelled on top. I did not drill the crossmember at all to do that - but the 25mm plywood is secured to the ramp runs using 60 M12 bolts so it itself acts now to spread the load much more evenly than just 8 tyres as contact points. Because of the amount of work returning the ramp isn’t an option.
Secondly, I have thought about the liability side of it - imagine it collapses and someone is hurt or worse - no amount of money will put that right, so I think it is down to me personally to engineer my way to a point where I can rest easily.
The fact that the manufacturer is confident in the product and knows the exact maximum design deflection gives me good confidence that the rest of the system is up to the job, and looking at my other ramp it is identical except for the depth of those beams. The manufacturer admits it does look odd with the sag, but tells me that they are done like that so that ramp angles are easier and very low cars can come on and off with ease. They also tell me that some of you here are correct in saying that bolting it down will reduce sag. So I will do that right away.
I have designed some shear plates which we are making up out of 9mm steel. These will bolt on each end and at that point I think the performance will be much better. Since I won’t be drilling anything at all and it’s a bolt on change I figure this is my next port of call.
[quote=Julian Thompson
The insurance people come to inspect the garage periodically as all the cars are on one policy so I guess it would need to be deemed suitable by them. I’m sure they would pass it on the basis that the manufacturer of the ramp says it’s ok, so in the end I think this has to come down to me.
[/quote]
Slightly O/T, but you you mind sharing who you use for insurance? I’ve yet to find anyone able to cover all mine on one, and it looks like you also have some interesting machinery there (some overlap with mine).
The insurance people come to inspect the garage periodically as all the cars are on one policy so I guess it would need to be deemed suitable by them. I’m sure they would pass it on the basis that the manufacturer of the ramp says it’s ok, so in the end I think this has to come down to me.
[/quote]
Slightly O/T, but you you mind sharing who you use for insurance? I’ve yet to find anyone able to cover all mine on one, and it looks like you also have some interesting machinery there (some overlap with mine).
Julian Thompson said:
I have designed some shear plates which we are making up out of 9mm steel. These will bolt on each end and at that point I think the performance will be much better. Since I won’t be drilling anything at all and it’s a bolt on change I figure this is my next port of call.
Seems your beams are 120mm deep (4.75") against a design recommendation of 200mm (8").Might be worth considering that variance
classicaholic said:
Can you see a gap under one side of the feet when its raised, if you can then bolting it down will make a big difference, very easy to do with through bolts as long as your concrete base is thick enough.
Good point.Plus - have a think if you can bolt the tops of the columns to the walls or maybe even the ceiling. This may interfere with the travel of the lift itself though, as tolerances might become critical with too much constraints.
classicaholic said:
Can you see a gap under one side of the feet when its raised, if you can then bolting it down will make a big difference, very easy to do with through bolts as long as your concrete base is thick enough.
Why would there be a gap under one side of the feet?As i tried to explain earlier, there is no side force on the posts, the lift platform is literally hanging on the cables within the open sided posts, the only force is downwards.
If the lift is correctly set up, there will be some side to side and front to rear movement of the platform between the posts, when the effective length of the crossbeam becomes slightly shorter due to the deflection, the cable will just be very slightly off vertical but there will be no side force (or very little) on the posts.
Edited by oakdale on Thursday 20th July 15:14
Kawasicki said:
Plus - have a think if you can bolt the tops of the columns to the walls or maybe even the ceiling. This may interfere with the travel of the lift itself though, as tolerances might become critical with too much constraints.
Really?Reacting unintended loads into the garage walls or ceiling seems dubious - it could result in cracking of the building structure especially if loads are tensile.
As this thread continues, I'm starting to feel that cowboy solutions are gaining preference. My engineering background makes me cautious especially in workshops. But hey what's wrong with risk - certainly makes life more exiting
Edit: I meant to write 'exciting' but perhaps 'exiting' was right
Edited by bigothunter on Thursday 20th July 15:36
bigothunter said:
Julian Thompson said:
I have designed some shear plates which we are making up out of 9mm steel. These will bolt on each end and at that point I think the performance will be much better. Since I won’t be drilling anything at all and it’s a bolt on change I figure this is my next port of call.
Seems your beams are 120mm deep (4.75") against a design recommendation of 200mm (8").Might be worth considering that variance
I might have missed them but I’ve not seen anyone here run any proper calculations so far on this to actually create a solid textbook solution for me to follow, and it would be difficult I guess unless the person doing the calculations knew the exact spec of the beam internally? (I wonder what “internally reinforced” actually means?)
I’m not sure the shear plates are a bodge - and also they’re removable/modifiable if they don’t please?
Julian Thompson said:
I might have missed them but I’ve not seen anyone here run any proper calculations so far on this to actually create a solid textbook solution for me to follow, and it would be difficult I guess unless the person doing the calculations knew the exact spec of the beam internally?
You'd also have to define how much (or rather little) deflection you actually want to achieve. For example if you wanted to halve the current deflection without increasing the height of the cross-member, you'd need to (very) roughly double the thickness of the box-section walls. That's assuming the box section is hollow of course; if it has internal bracing then the walls aren't the only thing providing the current rigidity.
ETA: I bet the current box-section is nowhere near 9mm think!
Edited by kambites on Thursday 20th July 15:42
Julian Thompson said:
The other ramp that doesn’t bend isn’t 8 inches, it’s probably 6.5 and we don’t have 8 inches to play with (fnarrr) without extending below the existing structure.
The other ramp does bend - just not enough for you to notice. The level of visual deflection of this weaker beam looks alarming which does not instill confidence.I would not expect any significant deflection from a properly engineered structure. Certainly less than you would notice visually.
DickyC said:
Depth of beam. Not thickness of section.
You can check this by seeing how thin the webs are on steel beams.
Gassing Station | Homes, Gardens and DIY | Top of Page | What's New | My Stuff