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271828 (Structural) (OP)
11 Feb 11 9:10
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?
Helpful Member!  rb1957 (Aerospace)
11 Feb 11 9:13
top ... 'cause it's a +ve moment
ishvaaag (Structural)
11 Feb 11 9:18
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 (Structural) (OP)
11 Feb 11 9:27
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 (Structural)
11 Feb 11 9:27
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 (Structural)
11 Feb 11 9:29
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 (Structural)
11 Feb 11 9:38
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 (Structural)
11 Feb 11 9:54
Positive moment on tension side. Bottom.

frv (Structural)
11 Feb 11 10:20
Tension side- same reason as others; that's the way I learned it.
nutte (Structural)
11 Feb 11 10:22
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 (Structural)
11 Feb 11 10:30
Same as AISC, AITC, DWS, WWUB, etc., etc., etc.

TOP!

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

Helpful Member!  BAretired (Structural)
11 Feb 11 10:32
It can be either way.  It really doen't matter.

BA

rb1957 (Aerospace)
11 Feb 11 10:41
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 (Structural)
11 Feb 11 10:59
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 (Structural)
11 Feb 11 11:36
Top.

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

DaveAtkins

ToadJones (Structural)
11 Feb 11 11:52
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 (Structural)
11 Feb 11 12:00
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 (Structural)
11 Feb 11 12:10
Those who are adamant about having it one way had better get used to the other way because both conventions are common.

BA

271828 (Structural) (OP)
11 Feb 11 13:16
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 (Mechanical)
11 Feb 11 13:20
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

steellion (Structural)
11 Feb 11 14:09
I draw them on the top, that's how we learned in school, though I acknowledge that bottom would be more logical and practical, especially for concrete.
JAE (Structural)
11 Feb 11 14:36
I think it is NOT intuitive to match the moment diagram with the deflection diagram.  By having them opposite, the eye avoids confusing the two.

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.

 
271828 (Structural) (OP)
11 Feb 11 14:40
"...I also prefer the toilet paper going over the top rather than under the bottom."

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.
Ron (Structural)
11 Feb 11 14:46
Top...it's the way I was taught and got stuck with it.  

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.
JAE (Structural)
11 Feb 11 14:56
My cat died last year.   catcatcatcatcatcat
 
JoshPlum (Structural)
11 Feb 11 15:02
I was taught structural analysis by a professor that specialized in concrete design.  Hence, he always forced us to plot moment diagrams on the tension side of the member.  That way, it would be very easy to know where we were going to have to place our flexural reinforcement.

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.   
msquared48 (Structural)
11 Feb 11 15:12
My early boss had huge fits of tension if we did not plot it to the top.  He had a patent on a bra design too.

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

rb1957 (Aerospace)
11 Feb 11 15:30
mike,

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  
hokie66 (Structural)
11 Feb 11 15:57
Tension side.  Like everybody else, just because that's the way I learned it.  Really, it makes no difference.  I always did wonder why we called the ones on the bottom positive and the ones on the top negative.
ToadJones (Structural)
11 Feb 11 16:06
I agree with everyone except BA Retired
He is retired and no longer has to draw or read moment diagrams.  
msquared48 (Structural)
11 Feb 11 16:12
No.  He is just of the age where everthing is done momentarily.  bigsmile

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

BAretired (Structural)
11 Feb 11 16:47
The CISC "Handbook of Steel Construction" under "Beam Diagrams and Formulae" show positive moments and shears above the line.  What does the AISC show?  Our Wood Design Manual shows it the same way.

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

BAretired (Structural)
11 Feb 11 16:55
Your Wartiness,

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

gobsmacked (Structural)
11 Feb 11 17:04
I have had many discussions about this. Of course it doesn't matter, really, but you should avoid referring to moments as positive or negative. A better description is 'hogging' or 'sagging'. (Though, in columns, you may have to turn your head to one side or the other - I draw a 'physics eye' when I want to identify the point of view) Analysts favour a mathematical approach so that in complex frames the interpretation is unambiguous. Designers prefer the intuitive approach. Whatever convention you adopt will eventually lead to a contradiction which requires you to think about the meaning rather than the sign (like the meaning of a wurd as distinct from its speeling). For example, the bending moment in an arch is best drawn to match the line of thrust, which the other sign would fail to reveal. However this nice point is lost in other circumstances. Just state your convention for both moments and shear in the introduction your calcs. Beware of dogmatic people. They are ignoring one principle in favour of another.
hokie66 (Structural)
11 Feb 11 17:34
The problem with that is that hogs sag.  I should have stayed out of this.  A better answer would have been "I forget".
IDS (Civil/Environmental)
11 Feb 11 18:05
Surely the first question we need to settle is, is a tensile stress positive or negative?

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
 

paddingtongreen (Structural)
11 Feb 11 19:47
putting the BM above the beam was fine, it was positive, below the line was negative. Then I moved on to fixed ended beams and the direction of rotation got a sign, clockwise positive and anti clockwise negative, or vice versa. So the fixed end moments now have different sign, even if they are both above or both below the beam, so tension side can be positive or negative.

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.

kikflip (Structural)
11 Feb 11 21:15
Each to their own really. I was raised with concrete design and always showed the bending moment on the tension side to show where reinforcement was required.

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.

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