# 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...

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8 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
seankoh (author)  kelseymh8 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
seankoh (author)  seankoh8 years ago
**please refer to my latest image for clearer view

Relevant equations
M = Force x distance
Volume of buoyancy is 1000m3
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?

frollard8 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...
seankoh (author)  frollard8 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.
8 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.
seankoh (author)  frollard8 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
8 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.