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Bending moment sign convention
2

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.

RE: Bending moment sign convention

I've always learned to draw a positive moment on the compression side of the beam.  Most engineers I know show it (and prefer to see it) that way.  Any time I encounter software that shows positive moments on the tension side of the beam it seems odd.  Most engineers automatically think of "negative" bending in a continuous beam over supports or at a cantilever support.  I've never heard someone refer to negative bending for a simple span or positive bending at a cantilever support.

RE: Bending moment sign convention

2
Agree with StructuralEIT. To see why, pulled out the old 1960's Beer/Johnston Statics book to refresh my memory:

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

www.VacuumTubeEra.net r2d2

RE: Bending moment sign convention

Not only with structural problems as mentioned above but also with problems dealing with machine design, AISC manuals, ME,CE  handbooks.
Isn't there a way to reverse the sign convention in your software?

RE: Bending moment sign convention

I went to a UK university where we were taught to draw the positive moment on the tension side.
 

RE: Bending moment sign convention

Positive moment on the tension side is not in keeping with the international conventions.

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

(OP)
Thank all for your reply. I like to clarify my question.  The two convention systems are consistent in terms of "positive" or 'Negative", i.e. upward bending is positive and downward is negative. The only difference is on which side you plot the the positive moment. SAP2000 plots the positive moment on the tension side, which clearly indicates the direction of curvature. The moment on the bottom side of beam (for a simply supported beam) is regarded as positive.   

RE: Bending moment sign convention

I plot the moment digram on tension side.
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

The compression side and the tension side are not fixed. Consider a cantilever beam or a continuous span. Positive moment, (defined as compresion on the top), is drawn above the center line of the beam.

RE: Bending moment sign convention

Of course its not fixed. But the moment side is not fixed also.
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

Positive moment is normally drawn on the positive side of the x-axis, (above the zero line for mathematics). Reversing the signs of Cartesian Coordinates is confusing and does a disservice to your students.

RE: Bending moment sign convention

Well we are learnt to be more engineers than matematicians.
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

The sign convention of Fixed End Moments have a different formulation whigh is also agreed upon amoung the engineering community.  As a confusing rebel, what do you use?
 

RE: Bending moment sign convention

In my country and in most of Europe, the bending moment is plot on tension side. That's convention and we are using it. Neither of us can't change that. So, depending on country where you work, use one or another convention.

RE: Bending moment sign convention

Positve moments make beams smile.

RE: Bending moment sign convention

If someone ever talked to me about "negative" bending for a simply supported beam, I would look at them like they have two heads.

RE: Bending moment sign convention

DRC1 explained it best.  My college professor used to draw the moment on the positive side, then drew a smile inside representing the deflected shape.

RE: Bending moment sign convention

(OP)
To StructuralEIT

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

I have seen both ways. Just make sure they lable it correctly. I like positive moment above the axis, but my boss does is the opposite. As long as it is labled correctly it shouldn't matter.

RE: Bending moment sign convention

Whatever convention is adopted, it has to apply to 2D (and even 3D) frames as well as to one-dimensional structures like continuous beams.  Arguments based on "x axis" or "positive side" or "upward" or "above" cannot readily be extended to a general beam member in a three dimensional structure, and so are of limited relevance.

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

I think SlideRule made the case best.  When you have a upward reaction at a left side support, the shear is drawn above the line.  So the area of the shear diagram is positive, and it is logical to draw the moment diagram increasing from the left end.  Not sure, but I think they are called positive and negative moments because of this convention.  As others have said, it really doesn't matter as long as the understanding is there.

RE: Bending moment sign convention

Put a log over a stream and walk over it. You have a positive moment with compression on the upper surface. This is known as a smiling moment.

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