huangyhg

moment diagrams
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

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
draw mine the same as you. i think risa should change as well.
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
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
this really ought to be a user configurable option within the program.
i learned to do it both ways -
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).
i learned that a positive moment produces a smiling face. found little arguement with that. i heard the older guys learned it opposite.
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.
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?
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

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