Box section strength calculator.

[Martin Turner]

Hi, does anyone know the calculation to work out how much weight a length of box section could take in the middle?.
I am making a lifting beam to lift our boat up enough so the trailer can be removed.
The materials I have considered are 70mm and 80mm box section, both around 5mm wall.
The beam will need to be able to support 700-800kg.
Any advice will be greatly appreciated.

 

[Erie Fred]

You could try the "Beam Boy" online program, but in your case, I think FEA is required, as your design is not a constant cross section.

 

[Martin Turner]

You could try the "Beam Boy" online program, but in your case, I think FEA is required, as your design is not a constant cross section.

Thanks for your quick reply, I could just make it out of 80mm to be on the safe side.
I spoke to someone in the US that has this kit and the beam is 3” and 1/4” wall, roughly 75mm and 6mm wall.

 

[Agroshield]

Hi, does anyone know the calculation to work out how much weight a length of box section could take in the middle?

Structural design does not work like that.

Yes, you can calculate the ultimate load (i.e. failure load) of a particular section but that tells you virtually nothing. If you know the ultimate load, to what percentage below that load will you load the item? If it is a rope for life support, it would be 10%. Lifting chains have a 5:1 factor of safety (load to 20%).

99% of the time, the thing that governs section choice is allowable deflection. The joists in your house could deflect 50mm without failing but living in the house would be very unpleasant.

Your best bet is to find pictures of it and deduce the section size from there. See here, for example:

Brownell BL2 Boat Lift

I am thinking about purchasing a Brownell BL2 boat lift to remove my Pathfinder from the Ameritrail trailer. I want to be able to clean, polish and wax the hull and service the trailer more easily and I live about an hour from the nearest boat yard. So doing these projects at home is more practic...

www.mbgforum.com

For a given load, the midpoint deflection is proportional to the span, so the closer the two stands sit, the lesser material you will need. The factory version is good for 'up to' a certain span. If the span you want is less than the maximum (the most inboard points of the stands have to allow the trailer to pass though the gap), you can save a bit there or add to the factor of safety.

You also have to factor in that the original is made from "Heavy Duty American Steel" (the stuff that is all in uppercase is even stronger than normal heavy duty American steel). Clearly, our average duty UK steel could never match that...

 

[carbonarc]

Hi Martin Turner. A critical consideration is the span of your beam between supports. If the simply supported span is 3 metres then a 80x80x5 SHS [square hollow section] will be capable of supporting a 900Kg load at mid span with the bending stress not exceeding 200N/mm2. Good luck.

 

[nickk]

If I was in that situation,and it was only me at risk underneath and I didn’t want to employ a SE,then I’d carry out a test with my chosen steel bar ,then load it to 1.5 x projected load,measure the deflection,unload ,re measure deflection,if it returns to zero deflection I’d be happy to use that bar to support the boat.

 

[Martin Turner]

Hi Martin Turner. A critical consideration is the span of your beam between supports. If the simply supported span is 3 metres then a 80x80x5 SHS [square hollow section] will be capable of supporting a 900Kg load at mid span with the bending stress not exceeding 200N/mm2. Good luck.

It will be around 3 meters, our boat trailer from memory is 2.37meters at the widest part but then I need extra length to get to the centre of the stands.
Once the trailer is out additional support will be going under the boat before it’s worked on.
The US kit uses roughly a 3” beam with 1/4” wall so 75mm by 6mm.

 

[Martin Turner]

Structural design does not work like that.

Yes, you can calculate the ultimate load (i.e. failure load) of a particular section but that tells you virtually nothing. If you know the ultimate load, to what percentage below that load will you load the item? If it is a rope for life support, it would be 10%. Lifting chains have a 5:1 factor of safety (load to 20%).

99% of the time, the thing that governs section choice is allowable deflection. The joists in your house could deflect 50mm without failing but living in the house would be very unpleasant.

Your best bet is to find pictures of it and deduce the section size from there. See here, for example:

Brownell BL2 Boat Lift

I am thinking about purchasing a Brownell BL2 boat lift to remove my Pathfinder from the Ameritrail trailer. I want to be able to clean, polish and wax the hull and service the trailer more easily and I live about an hour from the nearest boat yard. So doing these projects at home is more practic...

www.mbgforum.com

For a given load, the midpoint deflection is proportional to the span, so the closer the two stands sit, the lesser material you will need. The factory version is good for 'up to' a certain span. If the span you want is less than the maximum (the most inboard points of the stands have to allow the trailer to pass though the gap), you can save a bit there or add to the factor of safety.

You also have to factor in that the original is made from "Heavy Duty American Steel" (the stuff that is all in uppercase is even stronger than normal heavy duty American steel). Clearly, our average duty UK steel could never match that...

Our boat is around 1500kg but it’s probably not going to be a 50/50 split, the front where this beam would be going is probably lighter than half the load.
I have some adjustable wind up stands as well do once the trailer is pulled out they will be going straight in before I work on the boat.
It’s just for cleaning the hull and as I will be using oxalic acid I don’t want to do any cleaning above our new trailer as it can eat away the galvaniseing.