Mild Steel Limits?
- Thread starter Spence
- Start date
Mild Steel Limits
When you say 3", do you mean a 3" by 3" square tube with a 3/16" thick wall? You would also have to provide the unsupported span length and whether it was loaded at a single point, multiple points or uniformly loaded. Dimension of the point loads from the supporting points would also be necessary.
When you say 3", do you mean a 3" by 3" square tube with a 3/16" thick wall? You would also have to provide the unsupported span length and whether it was loaded at a single point, multiple points or uniformly loaded. Dimension of the point loads from the supporting points would also be necessary.
Spence, we'll need a sketch of exactly what and how.. Are ya talking about supporting a boat from 4 simple overhead beams? or are ya talking about jack stands where the tube is acting as a vertical supporting column ?? not being picky, just that I can't "see" what exactly you are doing..
OK. Here is my plan. A trailer is going to bring my boat this weekend, and it has a 100 inch W deck. The boat will be resting on 4 X 4 on the deck so I can slide in the 3 X 3 beams in question. The trailer is 100inches wide, so I need something strong enough to take the load when we screw up the jack stands. I already have 140in beams ready, but don't know yet where I will position them on the stands since the trailer needs wheel space between the jacks.
Attachments
Very close, Spence.. Because of the position of the center of gravity and levelness and loading issues, I would want each beam to be rated for half of the load minimum.. 140 inches is right at the max for a bending collapse with half load.. I wouldn't do it with two beams. If you can weld the other beam onto the original with a continuous full fillet top and bottom that should work fine..
Another engineer type should check this for a good second opinion .. Ted??
Another engineer type should check this for a good second opinion .. Ted??
I get the same thing, using the lowest grade of HSS (hot formed A501) with Fy=36 ksi.
Two such beams side by side and sharing the load have twice the capacity. Same with one on top of the other.
BUT, stacking one on top of the other and welding them together (stitch welding would be adequate to carry the lateral shear) would make it stronger still. Roughly the same as a 6x3 tube. It will also be considerably stiffer, like around 3 times as stiff.
I got the same concern as yourself with the single beam option. On welding the sister beam I'm tempted to weld spaced joints since it saves sticks(Lincoln buzz box), but it's better to have a solid beam. I'll double them up for 3 X 6. It just occured to me, the joined wall should now provide a webbing of sorts to better what the straight 3 X 6 tubing would normally provide in characteristics.
Thanks!
I used to have an old static load table manual that I've lost years ago. You just ran down your length and type of tubing and it gave you max load. (I took 60% of that for our designs).
I'll do the double beams with continuous weld and hope the transformer holds out. It's going to be hairy seeing the boat hanging 3ft in the air. I'm going to weld cable loops on the jacks to tie them together, lay them on plywood over crushed stone, and I'll "U" bolt a 2X6 to tie the end beams together where they lie over the jacks for added safety. From then it will be covered for winter lay up.
Thanks.
Spence ... is this application to slightly lift your boat from a trailer, have the trailer drive away and then lower to the ground or another trailer?
If so this isnt a simple case of material strength, but a 'lifting body' that is not prone to deflection under load ... severe bowing when your 3500 lb. boat starts straining and bowing those beams.
If so, Id tie those two beams together with 2 or more cross beams so that this 'lifting body' has supreme stiffness so that if something 'slips', another part of the lifting body can take over and handle the 'surprise load'. Id also at least 'double' the capacity of such beams ... for just in case.
If so, and for those who can calculate some beam stresses (Im not anywhere near any data books, IE3 values, etc.) ... If you agree, please do some flexural calcs for simple point loaded beams to be sure that this will handle the shear loads AND he doesnt wind up with something like a 400lb. fat lady sitting on top of a flimsy pole-vault pole due to BIG deflections. Id still recommend a FS of at least 2 to cover .... uh-oh's. Id be pretty suspicious of an A-18 3x3 being able to support such a load across such a span distance without a LOT of beam deflection ... but I dont have any data handy, just an engineers 'intuition'.
If so this isnt a simple case of material strength, but a 'lifting body' that is not prone to deflection under load ... severe bowing when your 3500 lb. boat starts straining and bowing those beams.
If so, Id tie those two beams together with 2 or more cross beams so that this 'lifting body' has supreme stiffness so that if something 'slips', another part of the lifting body can take over and handle the 'surprise load'. Id also at least 'double' the capacity of such beams ... for just in case.
If so, and for those who can calculate some beam stresses (Im not anywhere near any data books, IE3 values, etc.) ... If you agree, please do some flexural calcs for simple point loaded beams to be sure that this will handle the shear loads AND he doesnt wind up with something like a 400lb. fat lady sitting on top of a flimsy pole-vault pole due to BIG deflections. Id still recommend a FS of at least 2 to cover .... uh-oh's. Id be pretty suspicious of an A-18 3x3 being able to support such a load across such a span distance without a LOT of beam deflection ... but I dont have any data handy, just an engineers 'intuition'.
Thanks Rich. After the trailer drives off I'm going to center support the load with vertical wood beams and building a pile of old car tires under it just in case. I'm renting the trailer and paying people, so this is why this is a quick solution for now. I'm not crazy about it either and I'm going to be on pins and needles all the way. It'll need to hang there for the winter until i come up with a plan to lower it.
But 2, 3X6 fabricated beams seems like overkill for this span and weight even though it is wide. Consider. It is common and industry standard that boat trailers, with 2 X 3 X 1/8 frame and a wheel base of 96" is rated for greater than 3500 lbs, and that is with load peaking shock(bumps,road potholes,etc) safety margin designed in.
Also, I mentioned I researched how warehouses handled heavy loads for pallet racks which are simple 14 and 16 gauge formed channels. In one place you can purchase these up to 144 inches and greater than 5000lbs.
I'm just trying to get an idea of what is safe, but won't make me feel better either. :cry:
I missed the part where you need to slide the 3x3 tubes under the boat that's supported by 4x4 timber. Side-by-side is what you can accommodate.
HSS 3x3x3/16 Ix=2.6 in^4, Sx=1.73 in^3
Deflection of a single beam with a point load at center span, and the ends simply supported, is Pl^3/48EIx, so with the full 3500 lb on a single tube, you get (3500*140^3)/(48*30E6*2.6)=2.56 inches of deflection. Doubling the number of beams halves the deflection, and the bending stress, assuming they're sharing the load equally.
But, two beams together with the whole weight of the boat on it (3500 lb) is right up against the yield strength of the material. You'd need 4 or more such beams to carry the load with a reasonable factor of safety. Deflection would be 1/4 of that above, or about 5/8". Bending stress for the same configuration is Pl/4Sx, or (3500*140)/(4*1.73)=71 ksi, nearly double the minimum yield strength of 36 ksi of those tubes. 4 tubes would be a minimum.
Since you're laying them side-by-side, there is no value in continuous welds down both sides for the whole length. You just need to be sure they're both carrying the whole load together, especially at/near the center of the span, and at the ends, though supporting them on common timber at the ends should ensure they're sharing the load well at each end.
Use the above mental exercise at your own risk. Without seeing the project, boat, supports, knowing the steel grade for fact, etc., this is just messing about with numbers (and boats). Be careful, and if, whatever you end up doing, doesn't feel right, Don't Do It!.
Edit: Looking yet again, I see you have two beams at each end of the keel, presumably sharing the 3500 lb total displacement of the boat. That sounds reasonable, assuming the load is shared equally between each pair (or more) of tubes. Stitching them together will increase the lateral stiffness, but a bit of bracing is still probably a good idea, as RichH alluded to above.
HSS 3x3x3/16 Ix=2.6 in^4, Sx=1.73 in^3
Deflection of a single beam with a point load at center span, and the ends simply supported, is Pl^3/48EIx, so with the full 3500 lb on a single tube, you get (3500*140^3)/(48*30E6*2.6)=2.56 inches of deflection. Doubling the number of beams halves the deflection, and the bending stress, assuming they're sharing the load equally.
But, two beams together with the whole weight of the boat on it (3500 lb) is right up against the yield strength of the material. You'd need 4 or more such beams to carry the load with a reasonable factor of safety. Deflection would be 1/4 of that above, or about 5/8". Bending stress for the same configuration is Pl/4Sx, or (3500*140)/(4*1.73)=71 ksi, nearly double the minimum yield strength of 36 ksi of those tubes. 4 tubes would be a minimum.
Since you're laying them side-by-side, there is no value in continuous welds down both sides for the whole length. You just need to be sure they're both carrying the whole load together, especially at/near the center of the span, and at the ends, though supporting them on common timber at the ends should ensure they're sharing the load well at each end.
Use the above mental exercise at your own risk. Without seeing the project, boat, supports, knowing the steel grade for fact, etc., this is just messing about with numbers (and boats). Be careful, and if, whatever you end up doing, doesn't feel right, Don't Do It!.
Edit: Looking yet again, I see you have two beams at each end of the keel, presumably sharing the 3500 lb total displacement of the boat. That sounds reasonable, assuming the load is shared equally between each pair (or more) of tubes. Stitching them together will increase the lateral stiffness, but a bit of bracing is still probably a good idea, as RichH alluded to above.
Thanks, tkanzler. My suspicions of the low 'moment of inertia' (IE3) of the two 3x3s being quite low and would yield a large deflection/bow, thus functional instability during the 'lift'. When it come to bendy stuff, tall & thin is always better; putting 'stuff' inside those 3x3s wont help much as stiffness is more a function of geometry than material strength.
A very long long time ago I did structural/stress analysis for mega-lift stuff and 2 3x3s simply didnt seem adequate - thanks for the verification.
A very long long time ago I did structural/stress analysis for mega-lift stuff and 2 3x3s simply didnt seem adequate - thanks for the verification.
I agree, that they don't need full length welds.. just need to be stabilized.. seems like we all agree that it is best to put the other two beams under for four total and because they need to be stabilized, some stitching top and bottom is in order. Now who has the doughnuts and coffee for the engineers meeting?
My preparation for Friday is moving right along. One pic is the second doubled beam. The other is my 4 jackstands I made today. They are missing the T post diagonal braces.
I was trying to find the "base" to "upright" ratio for stability(the Cheops, or is that Kufu factor). One MFG jackstand I found on the web had a 17inch base for a 33inch upright. This looked a bit narrow for me and may work on level pavement. But my back lawn is less certain, so made my stands base 24 and upright 30. They'll rest on gravel pads well stamped.
Tomorrow weather willing I'll weld in the diagonals. I placed one of the four dock screws I will use in one of the pipes. These are cheap and they're 1 3/8 "' thick solid steel.
My beam is seen here sistered. This thing is %^$%^ heavy man. Nothing can bend this, trust me.
I'll send pics of the results of the move either way it goes.
I was trying to find the "base" to "upright" ratio for stability(the Cheops, or is that Kufu factor). One MFG jackstand I found on the web had a 17inch base for a 33inch upright. This looked a bit narrow for me and may work on level pavement. But my back lawn is less certain, so made my stands base 24 and upright 30. They'll rest on gravel pads well stamped.
Tomorrow weather willing I'll weld in the diagonals. I placed one of the four dock screws I will use in one of the pipes. These are cheap and they're 1 3/8 "' thick solid steel.
My beam is seen here sistered. This thing is %^$%^ heavy man. Nothing can bend this, trust me.
I'll send pics of the results of the move either way it goes.
Attachments
The irony is if I wasn't limited to the 3.5 inch clearance of the 4 X 4, I could employ a little trick
I do with swinging boom hoists I have made. I use rebar as cantilever to help support the weight. The use of the tensile strength of 1/2 rebar is very strong. 5/8 better still.
Here is a drawing of what I could do that would definitely solve the problem.
I do with swinging boom hoists I have made. I use rebar as cantilever to help support the weight. The use of the tensile strength of 1/2 rebar is very strong. 5/8 better still.
Here is a drawing of what I could do that would definitely solve the problem.
Attachments
yup.. if that re-bar were really stoutly welded at the ends, that would raise the "moment of inertia" of the beam to the point that only one reinforced tube on each end would be required..would need to be spaced about a couple of inches out from the square tube.. would work just like the spreaders and the shrouds of the mast .. Your double beam looks good and should do the job just fine .. Good stuff, Spence
Mild Steel Limits
This may be a bit late but could you ask the person transporting your boat to place it on 8"x8" wood timbers rather than 4"x4"? That would make a big difference as you would be able to slide a deeper structural steel member under the boat. If it's not too late, you could use your already fabricated tubes in the vertically stacked position rather than side by side.
This may be a bit late but could you ask the person transporting your boat to place it on 8"x8" wood timbers rather than 4"x4"? That would make a big difference as you would be able to slide a deeper structural steel member under the boat. If it's not too late, you could use your already fabricated tubes in the vertically stacked position rather than side by side.