## Moment Diagram Sign Convention Poll

## Moment Diagram Sign Convention Poll

(OP)

I'm curious about how many people use tension side vs compression side M diagrams.

If you have a simply supported beam subject only to a gravity load, which side of the beam do you draw the diagram, top or bottom?

If you have a simply supported beam subject only to a gravity load, which side of the beam do you draw the diagram, top or bottom?

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Obviously, I then draw the simple BM on the bottom. The old convention of the top being positive goes out of the window when you have to deal with frames, it gets in the way.

Michael.

Timing has a lot to do with the outcome of a rain dance.

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Textbooks go either way - frame analysis texts seem to always favor moment diagrams on the tension side, whereas basic statics and strenght of materials always focus on drawing the positive moment on the top of the beam.

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TOP!

Mike McCann

MMC Engineering

Motto: KISS

Motivation: Don't ask

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BA

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you should clarify your plot by also plotting the +ve axis (typically deflections +ve down, moments +ve up)

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But I have adjusted to RISA-3D, which shows moment on the tension side.

DaveAtkins

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STAAD, my good friend, however, draws it on the bottom and drove my old boss nuts.

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By international convention, using a RH coordinate system, a positive moment is a positive moment vector direction on a positive face (a face where the line normal to it faces in the direction of a positive axis) and also a negative moment vector direction on a negative face. A negative moment is a negative vector on a positive face and a positive vector on a negative face.

Dik

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BA

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The bottom line is that it is a good idea to indicate the sign if the situation isn't completely obvious. With the convention noted, it really doesn't matter.

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When I analyze a beam, I draw all my forces and moments in the positive direction, which is up, to my right, and counter-clockwise. I enter downward and leftward force and clockwise moments as negative values when I do calculations. Unless I forget, I show a sign convention on the drawing.

This makes entering data into spreadsheets and analysis programs, easier.

Nobody told me to do this. I just got in the habit.

JHG

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I prefer the compression side - I also prefer the toilet paper going over the top rather than under the bottom.

They are both very important to me.....NOT.

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The tension side vs compression side is an argument that I can live and let live, but not so for the toilet paper direction.

If the toilet paper goes over the top, then there is square footage of floor lost. Also, if a toddler or cat comes by and claws at it ferociously, it'll spool off a bunch of paper.

Come on man.

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Paper on top. An absolute necessity. More important than the moment diagram. If the cat spools it off, you're close enough to the toilet to have a solution for the cat.

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Later on I had a boss who had specialized in steel design. He always encouraged us to plot the diagrams on the compression side of the members. That way, it would be very easy to see where we were going to have to add flange bracing.

In the end, both methods are completely valid. It's just a matter of personal preference (or perhaps personal bias?) based on what type of work you're doing and what you learned in school.

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Mike McCann

MMC Engineering

Motto: KISS

Motivation: Don't ask

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must've been a good boss to work for ...

good thing you didn't do a spoonerism on "fits of tension" particularly with the adverb "huge" ...

me, i'll have two pickets to tittsburg, please

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He is retired and no longer has to draw or read moment diagrams.

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Mike McCann

MMC Engineering

Motto: KISS

Motivation: Don't ask

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I could be pedantic and say this is the correct way, but I don't want to offend all you "below the line" aficionados.

BA

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How can you possibly not agree with me? I have upon occasion drawn my bending moment diagram on both sides of the line, uniform load above and point loads below and occasionally the other way around. It makes it easier to draw to scale without having to add numbers together as you plot.

BA

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Doug Jenkins

Interactive Design Services

http://newtonexcelbach.wordpress.com/

## RE: Moment Diagram Sign Convention Poll

If you have performed as much Moment Distribution and Slope Deflection as I, you would see that consistency is the only thing that matters.

Michael.

Timing has a lot to do with the outcome of a rain dance.

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Likewise I can understand why steel designers would prefer showing the bending moment diagram on the compression side to understand which flange requires attention/restraint.