# Bending moment calcs

Hi ,

I need to find out the forces and bending moments on, put simply a table. 4 stands are welded to a flat plate to resemble a table. An 8 tonne mass is placed on the table, which is distributed equally on the 4 corner stands. The stands are 2m high and the flat top is 2m by 0.4m. Each stand is a box section 25mmx25mmx2.5mm thick

If a stand fails(something bumps into it) how do i work out the distribution of the force on the three corners/stands? and the bending moments also?

thanks

Rich

## Answers and Replies

nvn
Homework Helper
radiarich: Are the legs bolted to the floor? If not, the table could (would) become unstable. I am assuming the legs are bolted to the floor. Assuming your mass is approximately uniformly distributed over the plate, the axial load on the leg nearest the failed leg would be P = 39.23 kN. And the y-direction bending moment on the top end of that good leg would be My = 13.08 kN*m.

The leg cross section you suggested in post 1 would be severely overstressed, even with all four legs intact. If all four legs are installed and not damaged, then your square tubes must have a minimum cross section size of 70 x 70 x 5.0 mm.

But if one leg fails or is removed, then your square tubes must have a minimum cross section size of 120 x 120 x 5.0 mm.

Post the square tube cross section sizes available to you, and the material specification and tensile yield strength of the tubes, so I can check some numbers.

Thanks for your message nvn,

yes the stands/legs will be bolted to the floor, onto rails i think. The 8 tonnes should be uniformly distributed over the table, and therefore the 4 stands in each corner.

Can i ask the formulae you used to work out the min cross sectional box size? the critical load i believe needs to be above 20,000 N or 2 tonnes (plus i will have to include a safety factor) to prevent failure.

also from using the yield stress of plain steel( i think is about 360 MPa and E=210000MPa) and the buckling formula I found that, if all 4 legs intact, the minimum box section dimensions were around the minimum 25x25x2.5 or 3mm, to prevent buckling under a critical load of 2 tonnes on each leg (8 ton/4 legs).

There are several box section dimensions i could use 30x30x2.5 all the way up to 160x160x... and the stands, the 8tonne object and the table are all to be made out of plain carbon steel.

Last edited:
nvn