# What size I Beams to carry a 600lb load across 5ft span

Discussion in 'Boat Design' started by Michael Hyder, Jan 4, 2021.

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### Michael HyderJunior Member

Happy 2021 to all!

First, a big thank you to all the people here, I truly hope to return the favor some day when I learn a bit more.

Question: (See diagrams and Table One)
A) What type and B) How many - 6061 T6 I Beam(s) ???
-to carry a 600 lb load across a 5'4" span on a pontoon boat?

Specifications:
26ft Aluminum Pontoon Boat, 11 ft Beam, 34in Diameter logs, span b/w anchor points 5ft 4in. Load consists of a 600 lb Diesel engine/pump combo and is distributed more or less evenly across center 4 ft of the span. Assume boat will not be subjected to huge axial or longtitudinal loads (waves , etc).

I'm trying to maintain a balance b/w cost and safety. Hopefully a 3 in or 4 in beam(s) is sufficient (maybe that is overkill??) I tried to 'YouTube'ing' my way through the engineering calculations, but thought It'd be safer checking with the forum rather than trusting my math. Probably a simple problem for most experienced boat builders, but I'm just learning. If anybody can simply 'guestimate' off the top of your head. " Better to use a 4inch beam, all from Table One are sufficient", "Something like, any 3 or 4 inch beam is sufficient", etc. I' be very grateful.

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### TANSLSenior Member

In the worst case, assuming a beam of length "l" with its ends simply resting on the tubes and assuming that the weight, P, of the motor is uniformly distributed along the length of the beam, the maximum bending moment that occurs has a value of:
Bm = P * l ^ 2/8
This bending moment, divided by the minimum resistant modulus of the beam section (which you will find in the profiles catalog) will give you the value of the maximum stress that your beam is supporting.
If you manage to embed the ends of the beam, with the same load "P", you would have: Bm = P * l ^ 2/12 (33% less than in the previous case), with which the supported stress will be 33 % lower.
The supported stress must be less than the maximum allowed by the applicable regulations.

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### Doug HalseySenior Member

Wouldn't the worst case have the load concentrated at the center, instead of uniformly distributed?

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### gonzoSenior Member

You don't have enough information on the structure. The attachment of the beams to the pontoons will have a huge effect on the forces. The diagram shows a simply supported beam with an off center unevenly distributed load. The beam will be rigidly attached to the pontoons, which will rotate when beam deflects due to the load creating a moment. The diagram shows only a dead load without considering live loads which can be very large. Further, being a boat, the dynamic forces are often larger than the static ones. You need to approach the problem from a naval architectural perspective where everything is in motion. For example, the assumption that there will not be large axial or longitudinal forces is flawed. Catamarans have forces in all 6 axes. Rather than work this out from first principles, it would be easier to look at examples of successful boats that have similar loads in similar work conditions.

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### TANSLSenior Member

Yes, you are right but I was writing from memory and did not remember the formula for the bending moment in that case. In any case, what I have proposed will always be worse than what actually happens.

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### fallguySenior Member

Another thing to consider is the mountings of the load suggest the load is supported by two beams; not one. But I do not see how this is considered a uniform load.

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### fallguySenior Member

A point load would be the worst. So, I, must disagree. Then, as Gonzo suggests, a man or two men working on the engine, or other dynamics, increase the load. So does the decking, and the house.

At some point, you have to consider whether the load needs to be more uniformly distributed..

I would start by determining what beam supports a 600 pound point load as a back of the napkin approach. But even that opens a question. Any beam will deflect, so you must determine allowable deflection that will not cause other things to fail.

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### TANSLSenior Member

Youre right, sorry , I have clouded, you do very well to disagree.
In effect, it is not a uniformly distributed load and neither is it a point load. To be exact, it would be necessary to see what supports the motor has on each beam (which, by the way, could be 2 or some more), where each of them is, and the load that each support exerts on the beam. But the precision achieved in this way does not compensate for the results obtained by the "simplified method". More important than all that will be figuring out the maximum allowable stress.

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### gonzoSenior Member

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### Michael HyderJunior Member

The beams will be bolted to on each end to the pontoon, engine is mounted on the I beam with a simple Angle bracket, bracket will extend across one or both beams (depending if there are single or double beams). The engine is bolted with 6 bolts each side extending about 16 inches. Not exactly a point load, the weight is distributed over 16 or so inches.

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### gonzoSenior Member

I think that for simplicity and ease of maintenance, the drip pan and mounts could be a single unit. The bottom of the pan may be a thinner gauge than the sides. It should be installed on soft mounts instead of bolted rigidly to the beams.

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### TANSLSenior Member

I think it will be enough to use the formula: Bm = Pl ^ 2/8
You don't have to complicate your life more than necessary. A good technician must also know what is sufficient in each case.

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### fallguySenior Member

So, you will never be standing next to it to service? Start off with a point load, determine an allowable deflection that will not damage other things. All I am wiling to give you is a starting point. I am no credentialed engineer. But smart enough to know you and a friend might be working near or on it, even perhaps the same side and suddenly you are at 600 pound concentrated near center of beam load.

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### Michael HyderJunior Member

Gonzo, I used the calculator, added a distributed load of 37.5 lbs/ inch (over 16 inches) along the center. Added 2 fixed points at each end. Result was 8,391 lb/in. So what do I make of that ??? )) Maybe better to simply mount the engine directly on the deck, which is 3/4 " Marine grade plywood. 4 X 8 panels across cross members.??? Yeah, the engine sits 6 inches higher on deck, but safer???? But back to the main question, are 3 or 4" I beams enough, every 2ft o.c. across the pontoons??

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### fallguySenior Member

You need to determine max allowable deflection and then use the loads vs various beams to determine how much each beam option will deflect.

use a spreadsheet; makes life easier and you can see errors easily

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