Stress sign convention
Stress sign convention
(OP)
Hi all,
I was thinking when we draw tension/compression side of beam stress diagram? Attached below is a simple beam subjected to Uniform DL and Point load. This is longitudinal stress and tension is at top drawn, while compression side is drawn at bottom. Why tension side is drawn at top?Is this due to max moment is negative, hence top is tension and bottom compression (when sigma = Mc/I). Because the load is applied and the fibres of beam supposed to go undergo tension at bottom. Any clarifications is appreciated.
Cheers
I was thinking when we draw tension/compression side of beam stress diagram? Attached below is a simple beam subjected to Uniform DL and Point load. This is longitudinal stress and tension is at top drawn, while compression side is drawn at bottom. Why tension side is drawn at top?Is this due to max moment is negative, hence top is tension and bottom compression (when sigma = Mc/I). Because the load is applied and the fibres of beam supposed to go undergo tension at bottom. Any clarifications is appreciated.
Cheers





RE: Stress sign convention
Also, the enemy's gate is down.
RE: Stress sign convention
But what if we have continuous moment and a few max positive and negative moments. So the values tensile stress and compression side is varying. I mean isn't supposed to be tension bottom and compression top? (simple bending theory)
https://www.youtube.com/watch?v=IyRZ0WIhCdU
RE: Stress sign convention
In your sketch, as long as your applied loads are acting down then the bottoms of your beam is in tension.
RE: Stress sign convention
thanks for your input. Here in Australia we learnt it reversely, notably tension to be below axis.
So in the attached picture the tension side is drawn above axis, and in fact tension side is at bottom fibres (that's what I thought!)
Cheers
RE: Stress sign convention
If you are plotting the stress at a particular face you would normally follow whatever convention you use for stress; i.e. positive stresses above the axis and negative stresses below.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Stress sign convention
thanks for your comment. You are right it is all about sign convention, but as we know when the load is acting downward, the tension fibres are located at bottom portion. Which is graphically drawn above axis. I Believe it makes more sense to be below axis and follow normal stress block convention!
RE: Stress sign convention
Two examples:
The convention of drawing positive and negative moments on a diagram when related to shear diagrams "makes sense". When shear is "positive", the moment is increasing, and vice versa.
The convention of drawing moments on the tension side also makes sense, especially in things like load balancing of post-tensioned structures, as the shape mimics the tendon profile.