Moment Diagram Sign Convention Poll 2

[271828]

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?

 

[rb1957]

top ... 'cause it's a +ve moment

 

[ishvaaag]

Bottom. I had a boss that said that drawings have not up and down, all is a matter of looking at it properly; so one may imagine a positive axis going downwards; and I would say this is the general convention I have seen in over 90% of the spanish, french and english literature that I may have examined over 3 decades of practice. I only started to see the contrary in the representation output of some analysis programs, and some times just as a device dependent on the choice of local axes.

 

[271828]

My old school taught tension-side moment diagrams, so in my case, I'd put the diagram on the bottom. Seems to give a better intuitive feel for which way the member is curving or displacing (kind of) and that's the side that gets the rebar.

 

[Splitrings]

I believe typical beam convention is positive moment is shown on the compression side. Frame convention is different. I believe positive moments are counterclockwise on both the left and right side for frame convention.

 

[paddingtongreen]

Tension side. Main reason is that that was the general convention where I learned and where I worked. Almost as strong a reason, it seems intuitive to me because it tends to match the shape of the deflected member.

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.

 

[LSPSCAT]

I draw moment diagrams on the tension side for many of the reasons listed above. This is particulary useful in frame analysis.

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.

 

[slickdeals]

Positive moment on tension side. Bottom.

 

[frv]

Tension side- same reason as others; that's the way I learned it.

 

[nutte]

I was taught in school (in the US) to draw positive moments on top of the beam. We were told that the rest of the world did it the other way, with the most obvious benefit of the moment diagram reflecting the deflected shape. Risa draws positive moments on the bottom. When I'm doing them by hand, I still put positive moments on top.

 

[msquared48]

Same as AISC, AITC, DWS, WWUB, etc., etc., etc.

TOP!

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

It can be either way. It really doen't matter.

BA

 

[rb1957]

i guess it only matters 1) if the boss tells you to do it one way or the other, and 2) if you're used to looking at it one way and come across an example plotted the other way.

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

 

[ash060]

I use compression side as that was what I was taught. I think that is the ACI does it, and I use the coefficients alot.

 

[DaveAtkins]

Top.

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

DaveAtkins

 

[ToadJones]

I draw on top...most textbooks i have ever read do the same.
STAAD, my good friend, however, draws it on the bottom and drove my old boss nuts.

 

[dik]

A positive moment is drawn upwards and negative downwards (opposite from where reinforcing is required in a concrete beam). I've often seen the convention where the positive and negative moments are drawn 'upside down' to reflect reinforcing placement. This latter convention is not standard.

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

 

[BAretired]

Those who are adamant about having it one way had better get used to the other way because both conventions are common.

BA

 

[271828]

Dik, int'l convention or not, a lot of people go the tension side route and they have decent reasons.

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.

 

[drawoh]

271828,

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