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.

 

[Denial]

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.

 

[hokie66]

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.

 

[alex8]

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.