Bending moment sign convention
Bending moment sign convention
(OP)
HI,
I am teaching structural analysis in a college. I have a question about the sign convention commonly used in industry. Most text book draw the positive bending moment above the beam (compression side), but some software (SAP2000) plot +M on the tension side. I personally prefer the tension side plot. Can you tell me which sign convention is more popular in industry of civil engineering?
It seems many other countries use the tension side convention too.
I am teaching structural analysis in a college. I have a question about the sign convention commonly used in industry. Most text book draw the positive bending moment above the beam (compression side), but some software (SAP2000) plot +M on the tension side. I personally prefer the tension side plot. Can you tell me which sign convention is more popular in industry of civil engineering?
It seems many other countries use the tension side convention too.





RE: Bending moment sign convention
RE: Bending moment sign convention
When drawing shear and moment diagrams (by hand), allowing the shear values to have the same sign convention as forces in the free-body diagram is the "logical" thing to do.
Then, drawing the moment diagram using the shear sign convention gives a graph with what you have described as the "positive bending moment above the beam".
The example in the book is easy to follow, put a single point load in the center of a simple beam and draw the diagrams by hand.
When these steps are performed one at a time, by hand, the above assumptions make sense. The software most likely "skips" the Free Body / Shear Diagram / Moment Diagram development reasoning, making the sign convention appear more arbitrary.
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
www.VacuumTubeEra.net
RE: Bending moment sign convention
Isn't there a way to reverse the sign convention in your software?
RE: Bending moment sign convention
RE: Bending moment sign convention
Start with a right hand coordinate system and a moment or torque has a direction using the right hand rule with the thumb pointing in the proper axis direction.
A positive moment is a moment pointing in the positive direction and located on the positive face. Using the BMD for the tensile stress is a simplification to help locate where the reinforcing steel should be.
Dik
RE: Bending moment sign convention
RE: Bending moment sign convention
Why the moment diagram is shown on tension side?
We cover the tension with rebar, so I think that's the reason for showing the moment on the tension side.
RE: Bending moment sign convention
RE: Bending moment sign convention
On the cantilever beam we put rebar on the top side and moment diagram we plot also on the top side.
RE: Bending moment sign convention
RE: Bending moment sign convention
If a students can understand the nature of problems there are not problems with reversing the signs of coordinate system.
RE: Bending moment sign convention
RE: Bending moment sign convention
RE: Bending moment sign convention
RE: Bending moment sign convention
RE: Bending moment sign convention
RE: Bending moment sign convention
I think terminology is confusing people, but I would think it most logical to have the moment diagram a similar shape to what you would drape a tendon rather than facing the other way.
h
RE: Bending moment sign convention
I don't think anyone mentioned "negative" bending for a simply supported beam. Drawing moment on the bottom side still means 'positive' moment.
I agree with many people that it is logical to have BMD a similar shape to the deflection. But choosing which convention depends on the engineering community you live in.
RE: Bending moment sign convention
RE: Bending moment sign convention
As for personal taste, I'm with Whyun and Ndtmj. I find drawing the BMD on the tension side of the member to be a bit more intuitive because of its (extremely rough) similarity to the deflected shape.
RE: Bending moment sign convention
RE: Bending moment sign convention