Stress sign convention

[Shz713]

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

 

[allgoodnamestaken]

Someone else may correct me, but my understanding is that its just convention. Similarly why is positive shear up on the left side of a cut but down on the right? It could be the reverse, if you wanted. But we have a convention we use so that everyone is on the same page. Why is positive moment counterclockwise? etc etc.

Also, the enemy's gate is down.

 

[Shz713]

Thanks for your prompt answer.

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

 

[BridgeEI]

It's all nomenclature. In the US we are taught to draw positive moment above the axis. Other countries are taught the opposite. Most software has a toggle that will allow you to reverse the nomenclature to what you are used to seeing.

In your sketch, as long as your applied loads are acting down then the bottoms of your beam is in tension.

 

[Shz713]

Dear BridgeEI,

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

 

[IDS]

If you do a Google image search for bending moment diagram you will see that sometimes positive moments are plotted above the axis, and sometimes below (i.e. on the tension side).

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/

 

[Shz713]

Dear IDS,

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!

 

[hokie66]

Tomatoes, tomatoes. Potatoes, potatoes. There are reasons for doing it both ways, and neither is "better".

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.