If I have a 6' long steel beam with a load rating of 1000lbs, but I only need it to be 4' long, that should increase the load rating, correct? How do I calculate(or at least estimate) how much the load rating would change?
I'd like a load rating of 1200-1300lbs to have a safety margin, and since I only need 4' of total length I'm hoping that would put me in the ballpark.
What kind of beam? W-Flange? Simple supported at each end? Cantilever? What connection type? A structural engineer like me would want to know a lot more before certifying the change - because of liability issues.
You don't know if moment or shear controls the capacity. Or what connection type was assumed in the rating.
Max Moment for simply support at each end is (load x length)/4.
Max Moment for cantilever is (load x length)
Moment is used to calculate stress. Units are ft-lbs for moment and lbs for shear.
So 4/6 = 0.66. So 4 feet would have 66% the moment of 6 feet for the same load. To match moments you divide 1000/0.66 = 1500.
The problem is if beam shear controls it will be the same at the connections for either 4 or 6 feet for the same loading. Also the connection type could be the limiting case.
Jharr3 is spot on. If you are trying to do something a little better than guessing but don't have a lot of background Google can be helpful.
https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html
You have to find the right loading scenario for what you are trying to do and then read through and figure the proper variables but engineeringtoolbox.com has a lot of information. If someone is going to die if you get it wrong, pay someone with a PE license to get it right for you...
NYN
Agreed with ^. He is also assuming that it is horizontal which is just that - an assumption, but probably a good one. If it is loaded in compression, you'll probably do even better for improving safety factor if it was expected to fail in buckling at 6'. Tensile loading would be weird, not impossible, and not improved by shortening it.
What type of load is that rating for? If unspecified, it is probably assuming it is equally distributed. If you are loading it with a point load instead, you might actually not get the uprating you want.
In reply to jharry3 :
See, this is why I'm not an ME! :-)
Seriously this helps a lot, and your comments about shear makes perfect sense. It's not cantilevered, so at this point ~1500lbs is enough of a safety margin for me to do some more research on feasibility. Thank you!!!
New York Nick said:If someone is going to die if you get it wrong, pay someone with a PE license to get it right for you...
NYN
The failure of most supports "could" cause a death. If this is for picking up an engine it definitely could cause a death. This is why structural engineers won't just stamp drawings without knowing all the variables and conditions plus listing that limiting information on the drawing they stamp. Yes, I am a PE in structural engineering.
You can download BeamBoy software for free that would give you deflections, stresses, etc. Last I used it, the only beam shapes were rectangular and circular though (solid and tubular).
Beam is probaby fine, what holds it to things at the other end maybe not.
jharry3 said:What kind of beam? W-Flange? Simple supported at each end? Cantilever? What connection type? A structural engineer like me would want to know a lot more before certifying the change - because of liability issues.
You don't know if moment or shear controls the capacity. Or what connection type was assumed in the rating.
Max Moment for simply support at each end is (load x length)/4.
Max Moment for cantilever is (load x length)
Moment is used to calculate stress. Units are ft-lbs for moment and lbs for shear.
So 4/6 = 0.66. So 4 feet would have 66% the moment of 6 feet for the same load. To match moments you divide 1000/0.66 = 1500.
The problem is if beam shear controls it will be the same at the connections for either 4 or 6 feet for the same loading. Also the connection type could be the limiting case.
It has been 25 years since I had to do beam equations in engineering school, but this strikes me as an oversimplification? To get a real answer you would need to look at a cross-section of the beam and determine the max stress region. If it's a bending load, there would be a maximum tensile stress and and maximum compressive stress. The applied moment or shear load would be your input to this equation, not the output. I guess it is possible that because the cross-section isn't changing that everything reduces down to simple/linear math like you say. I can barely remember how to say "Laplace transform" much less do one.
I guess I should probably fill everyone in with the rest of the detail & let you know I'm not dead yet.
I was looking for trailer ramps that would support my Miata. On my old(heavier) Miata I used a pair of 700# rated ATV ramps with jack stands underneath them & they worked fine, but I wanted something a little more robust.
HF sells a pair of 6' 1000# ramps for $59 that I bought yesterday. I only need 3' or 4' long ramps, so I'll cut them down once I confirm the length I need. I've scaled the car, so I know the combined front weight is just under 1300# with me in it. I'll still put a pair of jack stands under the ramps for added security, but no one's life will be in peril if they fail.
I appreciate all the help(and concern)!
You will be fine cutting them, a 6ft "beam" with supports at each end will occur max deflection/stress in the middle. Assuming the same cross-section cutting the ramp shorter will decrease your max deflection and stress. I would keep the ramp as long as possible though because it makes pushing a car up way easier, especially if its lowered. Those 1000lb ramps probably have a sf=3 but i dont think i would trust them for a fullsize vehicle like a truck.
I have a similar ramp for my dirtbike from there, the part i would be most concerned about is the plate flange that lays on the trailer failing. I could see that plate yielding and peeling back, dropping the ramp.
mitch5 said:Those 1000lb ramps probably have a sf=3 but i dont think i would trust them for a fullsize vehicle like a truck.
ShinnyGroove (Forum Supporter) said:jharry3 said:What kind of beam? W-Flange? Simple supported at each end? Cantilever? What connection type? A structural engineer like me would want to know a lot more before certifying the change - because of liability issues.
You don't know if moment or shear controls the capacity. Or what connection type was assumed in the rating.
Max Moment for simply support at each end is (load x length)/4.
Max Moment for cantilever is (load x length)
Moment is used to calculate stress. Units are ft-lbs for moment and lbs for shear.
So 4/6 = 0.66. So 4 feet would have 66% the moment of 6 feet for the same load. To match moments you divide 1000/0.66 = 1500.
The problem is if beam shear controls it will be the same at the connections for either 4 or 6 feet for the same loading. Also the connection type could be the limiting case.
It has been 25 years since I had to do beam equations in engineering school, but this strikes me as an oversimplification? To get a real answer you would need to look at a cross-section of the beam and determine the max stress region. If it's a bending load, there would be a maximum tensile stress and and maximum compressive stress. The applied moment or shear load would be your input to this equation, not the output. I guess it is possible that because the cross-section isn't changing that everything reduces down to simple/linear math like you say. I can barely remember how to say "Laplace transform" much less do one.
yes, its an over simplification on purpose. I don't have enough information to give an answer I could certify so don't want to give any indication that doing what is proposed is ok. Just covering my Professional Engineer rear end.
If the failure mode is at the top of the ramp where the connection plates are bolted to the ramp structure, a shorter ramp wouldn't really help much. The potential failure of the bolts is a combination of shear and normal stress. To really know you need some measurements to draw a Free Body Diagram and a reasonable guess at the material properties.
But I agree with mitch5 that the actual factor of safety is likely more than the 1.5x you're planning to apply. If the ramps are designed against fatigue failure, then occasional use is a less severe loading case.
I finally unpacked & assembled the ramps today - the plates are separate pieces that bolt on & are much thicker, about 1/8". I wanted to bend them to give more of an angle to hook into the slots on the back of the trailer, but when I tried to use the trailer to bend them, it just bent the trailer deck instead...
Ouch, that sounds frustrating.
If you had been able to bend the plates by muscle, I would be worried about them holding up under the weight of a vehicle. Unless you rigged up some giant leverage mechanism to increase your applied moment.
In reply to Pete Gossett (Forum Supporter) :
It's funny (or not...) how the weakest point in the system will rear it's head. I needed to lift parts into the back of my truck years ago. So I bought a H-F bolt-in truck crane. The crane performed fine. The sheet metal floor of the truck bed however - notsomuch...
vsquaredbyrho said:Ouch, that sounds frustrating.
If you had been able to bend the plates by muscle, I would be worried about them holding up under the weight of a vehicle. Unless you rigged up some giant leverage mechanism to increase your applied moment.
I was using the 6' ramp as leverage against the trailer deck.
Pete Gossett (Forum Supporter) said:I guess I should probably fill everyone in with the rest of the detail & let you know I'm not dead yet.
I was looking for trailer ramps that would support my Miata. On my old(heavier) Miata I used a pair of 700# rated ATV ramps with jack stands underneath them & they worked fine, but I wanted something a little more robust.
HF sells a pair of 6' 1000# ramps for $59 that I bought yesterday. I only need 3' or 4' long ramps, so I'll cut them down once I confirm the length I need. I've scaled the car, so I know the combined front weight is just under 1300# with me in it. I'll still put a pair of jack stands under the ramps for added security, but no one's life will be in peril if they fail.
I appreciate all the help(and concern)!
I bought a set of these ATV ramps to load my Miata on the flatbed trailer, and they work really well. Very light and seem very strong. Rated to 3000#.
https://www.tractorsupply.com/tsc/product/reese-towpower-ramp-arch-3000-lb-pair
I also use them to load a golf cart into my truck bed when needed.
I picked up my old mower this morning.
I'm gonna need to get some longer boards before I haul it again, it actually slipped off while I was loading it, but fortunately they had a forklift & were able to get it centered on the deck.
Also, about those HF 1000# capacity ramps...
:-/ Looks like I'll be ordering some different ramps today.
Yeah, whoever assumes HF uses an actual (tested, not design) SF of 3 in their products is far more optimistic than I.
I ended up ordering these 5' aluminum ramps for $254 with free shipping(and they're supposed to arrive next week!).
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