# Bending moment in a beam.

1. Mar 27, 2012

### sockless

So for school I'm designing a ramp. The problem is that I need to calculate various things in it, none of which I've ever been taught, since I'm pretty sure it's all uni level. I've asked multiple teachers, including my physics teacher and my technology teacher, and they haven't been any help.

Basically, my ramp is a 50.8 mm square aluminium tube with 3.25mm walls, which is 2m long. I want it to be able to hold at least 200kg, which I guess can be modeled as a point force in the middle (i.e. 1m). However, I don't have a clue how to calculate the bending moment, so that I can calculate the strength of the beam (via it's section modulus, which I have figured out). The ramp is supported at both ends and can be modeled as being horizontal. I've looked on the internet, but nothing makes any real sense.

I don't want the solution as much as I want an explanation on how to calculate the bending moment.

2. Mar 28, 2012

### huntoon

let's calculate beam deflection first. as if it were horizontal.

you have to look up the young's modulous for aluminum. Wikipedia says 69 gigapascals.

okay here's the most basic equation:

(length of beam * force) / (young's modulus * cross section area)

result is in meters.

length of beam = 2m
force= 200kgs*9.81= 1962N
young's mod=69Gpa (1pa =1 N/m2)
cross section= 50.82-(50.8-3.25)2= 319.6375mm2 =0.0003196375 m2

plug and chug:

2 meters * 1962 Newtons / 69x109Newtons/meter2 * 0.0003196375 m2

=0.00000017791894 meters

That probably didn't help. sorry.

3. Mar 28, 2012