Diaphragm Design - Troubles and Woes
Diaphragm Design - Troubles and Woes
(OP)
I am trying to gain some experience for diaphragm design and actually run some numbers.
I am getting intimate with Terry Malone's book and have a good feel for what to do in simple cases (simple span stuff). Ran some numbers on his simple example problems was able to match his answers.
I feel confident that now I can tackle my projects complicated rigid diaphragm with a multitude of lateral resisting elements per level (/sarcasm).
I layout my drag struts and chords. (CMU walls as lateral elements)

I go to my sturcutral model (using RAM strutral systems) and look at wind load in the y direction. Cool, I find applied loadings and shear reactions in my shear walls. Total wind load of ~40 kips with sum of reaction be =40 kips. Model utilizes the rigid diaphragm assumption

I begin to dig in and draw my shear diagram for my diaphragm... nice... goes to 0 on both ends (with the assumption of a linear plf applied to edge of the diaphragm ~140plf).

Next I draw my moment diagram... NOT NICE does not go to 0 on both end.

I am in horror as to what I see... how can this be...
I then call over an experienced engineer to ask what is going on and he mentions that he expected the diagram to not go to 0. I was ignoring the torsion effects due to the center of rigidity of the diaphragm not = the center of the applied load.
I check my model. They do not align.

I go to look at my forces in the x direction and sure enough they are large enough that I would not want to ignore them when looking at loading page up and down (sum to be 0 though which is good).

Gut feel... this feels odd. Why do these forces effect my diaphragm moment diagram? Shouldn't my moment at the end of my diaphragm go to 0?
Do you guys have good way to explain what is going on?
A lost engineer.
Thanks in advance for advice.
S&T
I am getting intimate with Terry Malone's book and have a good feel for what to do in simple cases (simple span stuff). Ran some numbers on his simple example problems was able to match his answers.
I feel confident that now I can tackle my projects complicated rigid diaphragm with a multitude of lateral resisting elements per level (/sarcasm).
I layout my drag struts and chords. (CMU walls as lateral elements)

I go to my sturcutral model (using RAM strutral systems) and look at wind load in the y direction. Cool, I find applied loadings and shear reactions in my shear walls. Total wind load of ~40 kips with sum of reaction be =40 kips. Model utilizes the rigid diaphragm assumption

I begin to dig in and draw my shear diagram for my diaphragm... nice... goes to 0 on both ends (with the assumption of a linear plf applied to edge of the diaphragm ~140plf).

Next I draw my moment diagram... NOT NICE does not go to 0 on both end.

I am in horror as to what I see... how can this be...
I then call over an experienced engineer to ask what is going on and he mentions that he expected the diagram to not go to 0. I was ignoring the torsion effects due to the center of rigidity of the diaphragm not = the center of the applied load.
I check my model. They do not align.

I go to look at my forces in the x direction and sure enough they are large enough that I would not want to ignore them when looking at loading page up and down (sum to be 0 though which is good).

Gut feel... this feels odd. Why do these forces effect my diaphragm moment diagram? Shouldn't my moment at the end of my diaphragm go to 0?
Do you guys have good way to explain what is going on?
A lost engineer.
Thanks in advance for advice.
S&T






RE: Diaphragm Design - Troubles and Woes
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Diaphragm Design - Troubles and Woes
RE: Diaphragm Design - Troubles and Woes
To prove this to yourself, try running your model with all orthogonal walls removed. Then the torsional moment will be resisted entirely by the walls parallel to the load, and the moment diagram should close.
Your shaft walls appear to be resisting torsion as closed sections, hence the opposing forces. If you were to disconnect the shaft walls (model as 4 independent walls) you would get a force distribution more like bhiggins described.
RE: Diaphragm Design - Troubles and Woes
Kootk, thanks for the post. I believe I will do some form of moment correction as outlined in the link you posted.
I also felt a little skeptical about the magnitude of the torsional forces that RAM was spitting out in the bottom left of the building. I will dig into that.
RE: Diaphragm Design - Troubles and Woes
Now onto the more difficult stuff haha.
RE: Diaphragm Design - Troubles and Woes
Also, shouldn't your shear diaphragm's first reaction from the left then add to the magnitude of the shear instead of subtracting from it? I may be mistakenly reading your reaction diagram...
RE: Diaphragm Design - Troubles and Woes
RE: Diaphragm Design - Troubles and Woes
RE: Diaphragm Design - Troubles and Woes
RE: Diaphragm Design - Troubles and Woes
RE: Diaphragm Design - Troubles and Woes
Basically, diaphragm force distribution to composite wall assemblies is incredibly complex, especially when you add wall openings. The best reference I've found for doing this type of analysis is J.R. Benjamin's Statically Indeterminate Structures. But if your goal is just to practice doing rigid diaphragm design and perhaps have a shot at verifying the force distribution determined by the software, I would make it easy on yourself and disconnect the wall segments so that they don't behave as composite assemblies.