Moment Diagrams 6

[DaveAtkins]

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

 

[civeng]

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

 

[ChipB]

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

 

[rday]

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

 

[crossframe]

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!

 

[Focht3]

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?"

 

[lippie]

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).

 

[AlohaBob]

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

 

[JAE]

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.

 

[connect2]

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?

 

[trilinga]

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.

 

[yakpol]

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?

 

[JAE]

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.

 

[trainguy]

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

 

[denoid]

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!

 

[denoid]

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!

 

[haynewp]

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.

 

[JAE]

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

[] Avoid coffee break brain farts

 

[haynewp]

Yeah I wish...

 

[connect2]

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