Finding the bending moment for maximum stress

[Jameseyboy]

Hi,

I am looking to use My/I to find the maximum bending stress for a beam

My problem is I don't know how to find the bending moment- the reason why:

The beam has more than one point of load, aside from the reaction points. (Pin joint cantilever beam)

The sum of all moments equal 0 so I don't know how or which bending moment to use.

 

[SteamKing]

Your problem is not clear. Perhaps there is a diagram showing the beam and loading which you can post?

In any event, if the beam is in equilibrium, the sum of all moments will equal zero. However, you are looking for the moments which are internal to the beam. If you have the reactions of the beam, construct the shear diagram, and from this, you can construct the bending moment diagram. Once you have done this, the max. BM should be clear.

 

[Jameseyboy]

Ahh ok, so I can physically get the answer from the BMD? Is this typically when it crossed the x axis? Then the actual value is THAT particular moment on the beam disregarding the other ones?

Sorry to be so vague

 

[SteamKing]

The maximum BM is just that: it's the value which lies the furthest above or below the zero axis. The points where the shear force is zero are also points where the BM is a maximum or minimum, since dM(x)/dx = V(x), according to Euler beam theory.

 

[Jameseyboy]

Right. So it's not the value I.e. Going from +2 to -2 meaning 4 ---> it's actually the absolute value from the x axis- so '2'?

 

[SteamKing]

Correct.

 

[Jameseyboy]

Fantastic. Cheers