beam to support weight of boat 1

[ajk1]

Given:
Need to store boat over the winter, in boathouse. The boat will be supported on 2 beams, that span 14 feet on centre. Owner tells me that boat weighs 3500 pounds (seems a lot to me) and that I should assume that two-thirds of the weight is on one beam only i.e. 2333 pounds point load at midspan.

Note: Currently the beams are timber, 5.5" wide x 6" deep, based on my on-site measurements. I would assume the load is applied as a point load at mid-span of the beam. When I check the flexural capacity based on #2 spruce, the beams are nowhere near to figuring. Even if I assume #1 Douglas Fir, the Mr is only about half of the required moment of resistance.

Required:
The owner wants to find beams that weigh less so that they are more easily manhandled into place. The beam would of course be laterally unsupported.


How to determine a lighter weight beam than the existing 9 pounds per foot wood beams. i.e. shape, material, etc. The only thing that I can think of would be aluminum tubular section.

Questions:
What do you suggest for beam material and beam shape? If you agree aluminum section, would it be 6061-T6 grade and what size? I can size it in steel, but not experienced in aluminum design.

 

[rb1957]

I would build a cradle, like a truss, to support the boat; rather than a brute force SS beam. I assume a Vee bottom to the boat, I assume power boat (no keel).

Very simple (light looking) trailers support boats very well.

another day in paradise, or is paradise one day closer ?

 

[azcats]

Can it be rigged so that the load is two point loads closer to either end?

 

[dhengr]

Ajk1:
It seems to me that we’ve seen this same problem/question on ET in the last few years. Maybe do a search on the problem. How do you get the boat high enough, out of the water, to get the beams under it? What do the beam bearing areas look like, and can the beams be slid around? Do the beams span 14’ or are they spaced at 14 o.c.? Put a 2’ x 4’ stack of foam blocks (or some such) on the water at one bearing point; put one end of the beam on it and push the beam across the slip to the other bearing point and lift that end into place. We all know and have seen that what works without failing is not the same beam as the one which meets the latest bldg. code. What’s the worst that can happen if the beam does fail? They all want zero weight, zero cost, with max. cap’y. And, after you waste your time (and our’s) on this, they will use the same beams next winter. Just make it a little easier to install them.

 

[ajk1]

to dhenger - boat is lifted with winch, so that is not a problem. As I said, there are just 2 beams, it is not a series of beams on centres. I think the answers to that and all your questions are in my original post. My job as engineer is to figure out a solution. Does anyone have a good solution? I wonder if ALUMA beams might work...I will look into that. If they work, I have answered my own question.

 

[rb1957]

I think the "center load on a SS beam" is too simple a solution for this problem.

Why not support outbd of the CL ?? 4 pads supported roughly normal to the hull, braced to the "heavy" beams on the ground ?

another day in paradise, or is paradise one day closer ?

 

[ajk1]

I can find only 10 foot span ALUMA beams so that seems not the answer, unless they have longer span. Maybe aluminum tubular section is best I can think of.

 

[CANPRO]

ajk1, I read this thread earlier today but didn't have a chance to respond - I was going to suggest aluma beams. I've seen them as long as 21'-0". Maybe a call to local formwork company or scaffold company would point you in the right direction.

 

[ajk1]

to CANPRO - that is very helpful. I will follow up on that suggestion. Thank you. Much appreciated.

 

[dauwerda]

You can check some smaller steel sections as well. An HSS6x4x1/8 or a HSS7x3x1/8 are both 8.16 lbs/ft and would probably have the capacity that you need (I did not run numbers). If you do go down this route, keep in mind that HSS is typically dual certified as Grade B and C now so you can take advantage of 50 ksi yield strength.

 

[ajk1]

I tried tubular section, but calculated it as aluminum, and found that if I wanted to limit the deflection to 1" (which works out to span / 168), then I need 6" x 4" x 5/16" tubular aluminum section (E of aluminum = 10,000,000 psi). Deflection, not strength governed. This tubular section weighs 6.62 pounds per foot. This is 28% ± reduction in weight from the existing 5.5" x 6" wood beam.

I took the yield of aluminum to be 35 ksi and prorated the resisting moment of a steel HSS of that size by the ratio of yield of aluminum / yield strength of steel. Would that procedure for moment calculation be about right?

How much deflection would be safe to allow? I feel 1" might be the maximum.

 

[ajk1]

Also, does any reduction in capacity need to be made for the 14 foot laterally unsupported length? Is there a ratio of length-to-width less than which no reduction in strength is required? 14 feet span / 4" width = 42

 

[BridgeSmith]

If the lightest weight is the objective, and the owner is willing to pay for that, then a fabricated aluminum shape that is larger (deeper) in the middle and tapered down at the ends will give you the best capacity-to-weight ratio.

 

[ajk1]

Yes, something to be considered. Thanks.

 

[wannabeSE]

If the built-up aluminum section is welded, the temper will be lost and the yield strength will be greatly reduced. Is this beam permanently supported so it is only lifted into place one time (beam support hoist). If yes, how does the expense of aluminum compare to the cost of lifting the two heavier beams 1 time?

 

[amlinerrichard]

The boat has to be driven into the boathouse. Put straps in the water under the boat. Bring each end of the strap up and run them through rollers back down to the walkway in the boat house. Crank the straps to raise the boat. You look at the loads on the boathouse walls . You will have to put a few beams from wall to wall to keep the boathouse walls vertical.

 

[BridgeSmith]

Why would you set any limit on the deflection? I can't see how it would matter, other than a slight increase in stability.

Speaking of stability, you could go with a tall, narrow beam to maximize the bending capacity, and then add a wide end plate (maybe with handholes for easy handling?) or bearing plate to keep the ends from rolling over. That assumes the beam ends rest on a floor, or something else wide enough to support the end plate or bearing plate, of course.

 

[BridgeSmith]

One other consideration I just thought of is there could be a minimum bearing width where the for the keel rests on the support beam. Aluminum or steel may provide a bearing surface that is too hard for the keel of the boat. One of the reasons timber is the go-to material for these types of supports is that it is relatively soft and conforms to the surface of the keel, spreading the pressure evenly over the contact area. If you decide on a metal beam, I would give some thought to the contact area/pressure and consider adding a bearing pad to the top of the beam.

 

[3DDave]

You could probably knock about 1/4 of the weight off those beams by milling out the middle to make it an I-Beam shape without changing the strength in a noticeable way. If you taper the milling, you could probably get to 1/3 the weight; the bending resistance at the supports is the minimum so it doesn't need much web there. You may wish to do this as pockets to leave the ends full section to prevent damage from dropping the beams or dropping things on them.

 

[PEinc]

How about making each of the 2 beams out of 4 each, pressure treated, 2x10's. Each single beam could be made out of a pair of 2x10's so that the lifting weight of each half beam is about 16% lighter than your 5.5x6 wood beam [(2x1.5x9.25)/(5.5x6)] = 84% area = 84% weight. Then, after setting each pair of 2x10's, bolt them together to make a 6x10 beam (= 4 each 2x10's standing vertically, side by side, perhaps even with some blocking between the 2x10's to make them wider and more stable). If you really want to reduce lifting weight, make the quadruple 2x10 beam out of 4 each 2x10's, setting them one at a time before bolting them together. PT 2x10's and galvanized all-thread rods are readily available and cheap.
S = 1/6 x 5.5 x 6.0 x 6.0 = 33.0 in.[sup]3[/sup] for the 5.5 x 6 beam. I = 99.0 in.[sup]4[/sup].
S = 4(1/6 x 1.5 x 9.25 x 9.25) = 85.5 in.[sup]3[/sup] for the quadruple 2x10 beam. I = 396 in. [sup]4[/sup].

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