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# A better understanding of bending moments in beams

MusicFreakArchiGuy

I'm brushing up my concepts of elementary structural design. I can solve numerical problems of shear force and bending moment easily. I can draw their shear force and bending moment diagrams perfectly. I don't understand one part of the concept in its real life application.

Okay, lets say a simply supported beam having point load at the centre encounters 40kN-m bending moment in the centre. Then, what exactly is 40kN-m ? How is this 40kN-m acting on one metre length of the beam?

I need an explanation like this. - For example, the speed of a moving car is 45 kmph. So, it will cover 45 kilometres in one hour. I need such kind of explanation for bending moment.

senjohnblutarsky

The load you describe isn't acting over a metre of the beam.  It's acting at a single point because it is a torque loading.  Torque factors in distance from the fulcrum as well as the force applied.

This is why you have a unit containing both a unit of weight: kN, and a unit of distance, m.  If you were told that the same mass occurred as a point load at another point on the beam, you'd have a totally different number.

MusicFreakArchiGuy

I'm still not getting what you're saying in first paragraph.  Please elaborate.

"If you were told that the same mass occurred as a point load at another point on the beam, you'd have a totally different number. "

Yeah, I agree, as we solve numerical problems, the resulst differ. I know this.

Non Sequitur

The description is rather simple. Hard to see what else can be added to water it down further.

tintt

Hold your arm out the side horizontal, parallel with the floor. Put a weight in your hand. That's torque - the tendency of your arm wanting to rotate back down. If that weight was very near your shoulder, it wouldn't take you as much to resist it because your torque arm is shorter.

Taking a shot in the dark here, but are you trying to understand what the bending moment means with regard to stress in the beam, either in compression or tension?

You're asking about what 40kN-m is, and that concept is fairly simple (as has been described by others), but it doesn't really tell you a lot about whether your beam is going to fail or not. You'll need to know the size and shape of the beam and the material properties to be able to figure out if the material can withstand the stress that 40kN-m will impart in the beam.

My next shot in the dark is regarding deflection so ...?

If neither of those is what you're looking fo

JawkneeMusic

I'll give you an explanation of a different ? that should answer it.  Take an inclined plane calibrated friction out.You're pushing down now instead of a / of a meter 1. it's a multiple of the force.  now put it horizontal.

JawkneeMusic

These r what u r actually lOOkin 4

tintt

such clarity

Volunteer

You are overthinking the concept. It is a unit of torque. 40,000 Newton-meters is the torque 40,000 Newtons would have over one meter. If the load was 20,000 Newtons and the beam was two meters long the torque would still be 40,000 Newton-meters (20,000 times 2). 80,000 Newtons over 1/2 meter would be - you guessed it - 40,000 Newton-meters.

Medusa

I think this might help.

thatsthat

++ lifesaver.

curtkram

here's a picture.  the unit is foot-pound, and the equation is foot * pound. I'm still on the american units instead of metric.

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