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
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
RE: Moment Diagrams
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
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
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
RE: Moment Diagrams
RE: Moment Diagrams
RE: Moment Diagrams
RE: Moment Diagrams
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
RE: Moment Diagrams
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
tg
RE: Moment Diagrams
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
RE: Moment Diagrams
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
[] Avoid coffee break brain farts
RE: Moment Diagrams
RE: Moment Diagrams
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
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
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
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 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.