Story Drift Results do not Match Joint Deflections

[JoshH726]

Could there be any reason that the story drift results would not match joint deflections? I have a case where I define a node as a story, and compare the story drift results to the same node joint deflections. The story drift does not contain a Y-drift (Y is vertical), but the joint deflections do contain Y deflections. Any ideas? I attached a screen shot for reference.

 

[JoshPlumSE]

In the current RISA, you define story joint for each direction. In your case, you have defined an X direction story joint, but not a Y direction story joint. That's why you're only getting the story drift reported for the one direction.

 

[JoshH726]

JoshP, I'm not sure if that is the reason. See attached. The story drift I defined was for all three directions. I'm certainly open to suggestions. Thanks

 

[JoshPlumSE]

Sorry, I didn't look closely enough at your first image.

There is no Y direction drift because Y is your vertical axis. But, you are getting X and Z direction drift, right? And, the X and Z direction drift does match your X and Z joint deflections.

 

[JoshH726]

JoshP, correct, Y is the vertical direction. I guess it makes sense that you don't have "story drift" in the vertical direction. However, on a separate model (see attached) (again Y is vertical), I only have Y drift? Is it something internal to the program?

 

[JoshPlumSE]

I don't have an explanation for that. It doesn't seem correct to me, but I'd have to see the model to know for sure.

Also, what version of the program are you running? There may be an old bug that I don't remember. If I know the version I could look at the release notes for subsequent releases to see if there was anything corrected that sounds similar to what you're seeing.

 

[JoshH726]

Version is 13.0.1 (64-bit) (standalone license, if that helps). Attached is the model. I did scale up the Wx service load to increase the lateral drift just to see it gets picked up.

 

[JoshPlumSE]

Ahhhh, that makes more sense now. Though I had to scratch my head for a while before it started making sense....

Story 1 is at a 0 ft elevation. Since there is no Story 0 defined, story 0 will be assumed to also be at a 0 ft elevation! And, story drift calculations blow up (divide by zero errors). Hence the program refuses to do the calculation.

Try instead to define joint N12 (which is at -5 ft elevation) as your story 0. Then you should start to get drift results.

Apparently I was wrong about the Y drift not being reported when the Y axis is vertical. It will still be reported based on the "height" difference in the Z direction. Though I would not personally use this for anything as it is not a true drift.

 

[JoshH726]

Yes, that does make sense now. I guess to close out the thread, I would want to make sure that my story 1 node never has a y = 0 coordinate. Thanks for you help JoshP