Strut-Tie Model - Exterior Joint

[Cjpwdesign]

Dear All

Going through the book "Structural Concrete - Strut and Tie Models for Unified Design" by Wai-Fah Chen (of structural stability and plasticity fame)
I have come across a somewhat perplexing line in one of the examples - either that, or I am just missing the obvious. - see attached file
Under the section of Geometry of the STM and Forces, he states the column moment is half the nominal beam moment. No end conditions are given for either the beam or the column, so I am lead to believe that simple moment equilibrium about the intersecting centre would result in the column moment above and below the beam being equal and thus give the required result.
Is this the correct approach or am I missing something silly?!
Thanks, Chris

 

[KootK]

Is this the correct approach or am I missing something silly?!

Exactly right. Or, at the least, that 50/50 distribution between columns is what has been assumed. As you might imagine, that assumption will usually be an approximation and the attribution of moments between the columns will be based on the relative stiffness of the columns above and below the joint.

The 50/50 split comes up in seismic stuff a fair bit because, there, plasticity extends the validity of the assumption.

In an elastic model where the upper story is twice as tall as the lower, one certainly would not expect the split to remain 50/50 unless the upper column were massively larger than the lower.

 

[Cjpwdesign]

many thanks for the clarification. I can add a marginalia in now to that effect