Bending moment sign convention 2

[ndtmj]

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.

 

[Lion06]

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.

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[chicopee]

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?

 

[Ussuri]

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

 

[dik]

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

 

[ndtmj]

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.

 

[lisica]

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.

 

[civilperson]

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.

 

[lisica]

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.

 

[civilperson]

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.

 

[lisica]

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.

 

[civilperson]

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?

 

[lisica]

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.

 

[DRC1]

Positve moments make beams smile.

 

[Lion06]

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

 

[whyun]

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.

 

[csd72]

I was taught to draw it on the tension side (Australian University).

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.

http://www.ecf.utoronto.ca/apsc/courses/civ214/Lectures/Moment diagrams 1.jpg

 

[ndtmj]

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.

 

[DWHA]

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.