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Moment Diagrams
5

Moment Diagrams

Moment Diagrams

(OP)
One thing that irritates me a little about RISA-3D is that it draws the moment diagram upside down -- at least based on my educational background.  For example, I was taught that for a simple span beam with uniform load, the parabolic moment diagram is drawn ABOVE the horizontal line.  However, I do know that some schools teach this differently.  I'm wondering -- do most of you draw moment diagrams as I do?  If so, I may call RISA to ask them to change their graphics.

DaveAtkins

RE: Moment Diagrams

Hi Dave,
I'm from Quebec Canada, and learned that the moment diagrams were drawn below the horizontal line. I guess the important thing to look at are the units and ± signs.
I don't know if this helps, but thats the way we work at our office.
Take care

RE: Moment Diagrams

Draw mine the same as you. I think Risa should change as well.

RE: Moment Diagrams

DAVE,

Ditto from me, compression on top equals positive moment. Thus, in your situation the parabola would be above the horizontal.

Also, it's not just RISA; I have used fast frame by Enercalc and it did the same thing. My boss thought it was odd too.

Rik

RE: Moment Diagrams

Dave,

I agree with you in my preference for postive moment drawn up.  However, a rationale once explained to me is that drawing them "upside-down"  (right side up to civeng and others in Canada!) is that the moment diagram approximates the deflected shape of the member.  But as civeng pointed out, it doesn't really matter as long as you keep track of the sign convetion.

Good luck with RISA, and let us know when they're going to change their output!

RE: Moment Diagrams

This really ought to be a user configurable option within the program.

I learned to do it both ways -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

RE: Moment Diagrams

I agree with Focht3...there should be an option. STAAD and SAP both allow you to draw the bending moment on the tension side (as positive) or compression side (as positive).

RE: Moment Diagrams

I learned that a positive moment produces a smiling face. Found little arguement with that. I heard the older guys learned it opposite.

RE: Moment Diagrams

2
I was taught, and prefer, moment curves showing up on the compression flange side.  I just wish that the "industry" would standardize one way or another - what a small issue, with potentially big disasters looming around it.

RE: Moment Diagrams

yes depends on the shear force diagram and whether you visualizing internal shear forces from the left or right of the imaginary section. Typically yes reactions up, loads down, at least Newton thought so, equal and opposite so to speak, so yes i prefer my bending moment envelope on top of the line in a so called simple beam illustration as an example, the bending moment being the area under the shear force diagram.  The problem seems to arise when we consider tension to be positive and compression negative, 'hogging' and 'sagging', etc... but yes still just signs going back to compatibility equations and Mohr's Circle, it is disconserting when you run a canned program and the world appears upside down,  anybody from 'down under' reading this thread, apparently when you pull the sink drain the water forms a clockwise vortex in your sink, is this true or is someone just having me on?

RE: Moment Diagrams

We learnt from our college days that though the BM diagram can be drawn either above or below the base line for a beam, it is preferable to draw them on the tension side.

The 'BMD on tension side' practice is helpful in drawing and identifying the tension face of any member This is more relevant in the case of concrete frames where you need to know the tension face to provide the reinforcement correctly.

I feel it is OK to draw the BMD on the compression side or tension side as long as the consistency is maintained.

Further, a single consistent convention helps in drawing the BMD correctly for columns in multibay multistorey frames.
 

RE: Moment Diagrams

When moment diagram drawn on the tension side the "belly" looks "similar" to the deflection diagram, that might help visualize the results. In Russia the choice was very strict: draw positive on tension side for civil engineers and on compression side for mechanical engineers. Does it really matter?

RE: Moment Diagrams

For most applications of moment curves on continuous beams, the form of the moment curve is easily visualized either way.  We use RISA software and they draw it on the tension side, while I'm used to compression side moment curves.  I simply tolerate it as its not that big of a deal with continous beams.  The shape of the curve is conventional enough to allow me to understand what's going on - even though I'm not used to it.

But when you get into more complex, or non-symmetrical, models, the moment curves are more unique and you have to struggle a bit to ensure that you know which way is tension or compression.

We always make it a policy to plot the analyzed model's deflected shape FIRST, before doing anything.  This allows us to check how the model is behaving (does it make sense?) prior to digging into the design and the numbers.

RE: Moment Diagrams

In university, I had a teacher who drew the moment diagrams (on the compression side) without plus or minus signs, just a curvature symbol in the appropriate direction inside the hatched region of the diagram.  I have kept using that approach as it also hints at the deflected shape.

tg

RE: Moment Diagrams

As far as I'm aware, the AISC beam charts always have drawn the moment diagram on the top (compression side).  That's also the configuration I've always used.  But, we also use RAMAdvanse in our office, and they show it on the tension side, so I've learned to adapt.

The other thing that has always driven me nuts is the lack of a consensus on sign configuration for positive/negative moments at a joint - some show clockwise as positive, and some show counterclockwise...I guess we just have to be on our toes!

RE: Moment Diagrams

Actually, I was just looking further at AISC, and (it's weird), when they get to the continuous beam diagrams, (cases 34 and on), they switch configuration to the positive on the bottom (tension) side...looks like even they're confused!

RE: Moment Diagrams

I like RISA the way it is. You can plot either the deflected shape or the moment diagram, or both.

It could cause confusion if they were opposite. Say the moment curves were shown on the compression side instead. You only plot the moment diagram on a multiple span beam.
 You go get coffee..

 Come back and for some reason think you are looking at the deflection curves and forgot about having plotted moment curves. Now you think the deflection is going in the opposite direction it actually is. Well, stupid things like this might happen.

RE: Moment Diagrams

haynewp - I thought that RISA had a checkbox in the OPTIONS window for:

[] Avoid coffee break brain farts

RE: Moment Diagrams

Yeah I wish...

RE: Moment Diagrams

Jae,
agree with the deflected shape first comment it facilitates an understanding of many things including the bending moment diagram, reaction/shear directions etc.,

RE: Moment Diagrams

lippie.

   What command in staad that will allow you to change the bending moment on the tension side (as positive) or compression side (as positive)?  Havent seen it...
 

RE: Moment Diagrams

nrguades

In SAP, I dont know - I have just heard. We use STAAD 2003 and the option is inside the INI file. They don't expose it through the graphical side because according to their people at the REI Conference, "Tech Support would be confused if they were looking at bending moment diagrams, not knowing the setting chosen by the user". So you have to open their staadpro20030.ini file in notepad and change the [MOMENT DIRECTION] = 1. I am not sure what thye have done in their new 2004.

RE: Moment Diagrams

DaveAtkins,

I used Risa-3D for quite a while, but not realized it cannot convert bending moment diagram, maybe because that's way I've got used to, and seems better as mentioned by Yalpol.

It is indeed very irritating when one got used to one way of looking but encountered a upside down picture. The converting capacity should be standard.

RE: Moment Diagrams

I always draw the moment diagram on the tension side so I can instantly identify the tension side, and see which way the members will bend toward.  It’s really helpful for identifying the deflected shape.

I feel this method intuitively goes hand-in-hand with concrete design, where you don’t want to make silly mistakes because you mixed up "positive" and "negative" moment.  

I never think of bending moment as positive or negative, because it doesn’t serve any good purpose.  I do, however, reference the moment to the side of the member it's on. Top moment, bottom moment. Top moment=top steel.  Bottom moment=bottom steel.

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