Story Drift Question 4

[Respawn]

I am running a ELF analysis on a model with rigid diaphragms and precast moment frames at the perimeter. There are moment end releases in those perimeter frame beams along with moment rotational springs. My period and mass participation look great. Even the story disp., increases as the height increases. However My story drift looks like this. Not sure why it is wacky like that. I haven't seen story drift distribute like this. Any thoughts?

 

[HTURKAK]

Notice that the drift is unitless.. That is , drift ratio.. calculated with dividing the relative displacement of a story with the story below to the storey height.

Apparently, max ratio ,0.0021 LT 0.01 better than OK..

Edit:

The following excerpt from ASCE 7 -16 showing the story drift ratio

The story drift ratios may also be used for stiffness irregularity check. ( Soft Story Irregularities Types 1a and 1b).

 

[271828]

I can't see the model very well, but the bases look pinned. If they are, and if the moment frame columns and beams are the same at the various levels, then I would expect the drift ratio to be a lot higher at the bottom story than the stories above.

 

[Deker]

As HTURKAK pointed out, your plot is of the drift ratio and not the displacement. Frame drift is typically dominated by shear, so for your pinned base structure the largest drifts will generally occur at the lower levels and decrease as you move up the building. See below for an example illustrating the difference between a plot of the displacement (red) and a plot of the drift (blue).

Since you have a tall first story, the drift at the lower level is exaggerated. Be sure to check for a soft story irregularity.