Sign Convention !

[Hudhaifa]

Hi all
As a matter of interest to learn how to design a reinforced concrete buildings, I have started reading a book named "Reinforced Concrete Design (7th edition) to EuroCode 2, by Mosley ... "

My questions may seem to be obvious and easy, but it really confused me. While I am reading Analysis Section, the book considers the following sign convention;

the question is how the equation shown in the photo below has been found ? I mean the signs of all actions (loads) and Moments.

Should not the book show the direction of M(AB) and M(BA) in the free body diagram to expect the sign of them ?

Hope find anyone who could explain to me.

Thank you

 

[Agent666]

Maybe I'm not getting it, but the shear I think you refer to as being shown is in fact a reaction in the top picture with the loading? I don't really get why you think the shear is shown and moment isn't. Neither shear force or moment diagram show arrows to assist in the interpretation. The top diagram is a load/external reaction diagram?

If shear is above the member segment on a free body diagram its going up working left to right, moment acts in a direction on any free body diagram consistent with tension on that face of the member. Take your hands and twist at each end of an imaginary member in the air to put tension due to bending on the same side as the diagram, the twist direction your hands are making is the direction/sign convention of the moment (EDIT - at any point on the beam).

 

[271828]

"Moment" is being used in slightly different senses here.

End moments coming out of analysis methods like moment distribution are like support reactions. They're usually considered positive when counterclockwise in the global coordinate system.

That's different from a "bending moment" which is with respect to the member local coordinate system. Typically, a positive bending moment puts the bottom in tension.

 

[Settingsun]

It's because I'm having a terrible time remembering what i saw in the diagram by the time I scroll down to type the reply! I thought the arrows were labelled as V.

 

[Hudhaifa]

Hello again,
Hope u did not forget the subject .

I have contacted one of the authors through the publisher.

Please see attachment No 1 addressing the sign convention point.

Attachment No. 1

https://files.engineering.com/getfi...3d-a996-5e4b84a1a80a&file=Attachment_No_1.pdf

Attachment No. 2, he solved one exampled for more clarification. However, to understand the example u need to have the table below.

Attachment No. 2

https://files.engineering.com/getfi...e-bea3-3a1d2e553966&file=Attachment_No._2.pdf

Regards.