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# Calculate bending moment of a beam and the size of the beam to be used?

Please refer to the uploaded images. (there are two images) When the right of the weight is lifted up, I need to know the formula to calculate the bending moment of the beam according to the rest of parts I have and how to calculate the suitable size of the beam to be used. I appreciate your help very much...

## Discussions

Best Answer 10 years ago

I guess you don't have your C.E. textbook handy. Try the following Web site, since you aren't able to Google "bending moment of beam":

http://www.efunda.com/designstandards/design_home.cfm

Answer 10 years ago

Thanks kelseymh and frollard for your helps. To frollard, As the bending moment is at its maximum when one of the weight is lifted out of the sea, the rest of the bending moment measurements are not neccessary for me to find out. I could only know how to calculate the easy question on bending moment. This one is too tough for me.. Let's assume the buoyancy force is 40N.. Volume of buoyancy is 1000m3

Answer 10 years ago

**please refer to my latest image for clearer view

Relevant equations

M = Force x distance

Volume of buoyancy is 1000m

^{3}buoyancy force is 40N

Mass = 40kg

The attempt at a solution

Moment of Mass = 40kg *9.81 m/s

^{-2* 30m = 11.72 KNm}Moment of the structure (horizontal, above water)= 10 * 9.81 * 45 = 4.41 KNm

Moment of the structure (vertical, above water) = 10 * 9.81 * 40 = 3.92 KNm

Moment of the structure (horizontal, under water)= 10 * 9.81 * 22.5 = 2.21 KNm (how to deduct the buoyancy force??)

Moment of the structure (vertical, under water)= 10 * 9.81 * 22.5 = 2.21 KNm (how to deduct the buoyancy force??)

Total M = 12.75KNm

In this case, how do I relate to the size of the stainless steel to be use ? what is the suitable diameter?

10 years ago

To solve for the moment, dont you also need the lengths of the other parts, volumes for buoyancy, delta-height of movable weight...

Answer 10 years ago

Hi frollard, Please refer to the added image, the volumes of buoyancy is unlimited as it is considered as under the sea. There lengths and delta height of movable weight are added.

Answer 10 years ago

Engineer friend says: the bending moment varies along the beam. you also need the cross section of the beam or the moment of inertia to know what "size" to make it.

Answer 10 years ago

Thanks kelseymh and frollard for your helps.

To frollard,

As the bending moment is at its maximum when one of the weight is lifted out of the sea, the rest of the bending moment measurements are not neccessary for me to find out.

I could only know how to calculate the easy question on bending moment. This one is too tough for me.. Let's assume the buoyancy force is 40N.. Volume of buoyancy is 1000m

^{3}Answer 10 years ago

That helps - what I mean is that if one of the weights moves out of the water, its buoyancy will change. You stated it's mass, but how much does it weigh in and out of water? I'm probably not gonna be of much help on this one - linking my meccy engineer friend.