interstory drift braced frame vs moment frame

[Groszni]

hello eng-tips users,

im comparing intestory drift between the braced frame and moment frame. You can see the attachment for the graph. On the lower level the braced frame have lower value than the moment frame, but when it comes to the top of the building the interstory drift of moment frame is lower than the braced frame? how could it be? what is the cause of it? because im pretty sure the brace frame is more stiffer than the moment frame. what are some parameter that affect the interstory drift? thank you.

 

[CELinOttawa]

I think what you are seeing is an amplified effect of the moment frame version being most rotated/having most drift only once there is a good amount of load above it (wind or seismic). You don't get all that much drift from one or two storeys above a frame. For the braced frame, the amount of drift is restrained but also far more about the incremental floor to floor amount. So: Where with a moment frame the loads from all the floors above keep bringing the rotations further and further out, the nature of the braced frame means that these loads are very efficiently transfered through the braces and it is only marginal elongation that you observe.

At least that's my interpretation of your results. Saw a similar effect on the design of a braced frame tower in Christchurch, although I was just the vibration engineer on the job. The Architect made a last minute change and my colleague who designed the tower had to add a "kink" in the first floor brace frame. What a drift problem! The kink would tend to "straighten" under high level service load. Solved, but huge frame...

Will keep an eye on this one for other people's thoughts! Good luck...

 

[KootK]

My guess is identical to CEL's, I think. You've fundamentally got three sources of inter-story drift:

1) Shear deformation over the height of the story.

2) Bending deformation over the height of the story. Basically axial strain in the frame columns.

3) Rigid body rotation of the story as a result of the curvature occurring below the story. Basically the tilt angle at the bottom of the story projected over the story height.

For #1, we would expect a braced frame to be much stiffer than a moment frame and to produce less inter-story drift.

For #2, we would expect a braced frame to be less stiff than a moment frame because it is very likely that smaller column cross sections will be used for the braced frame.

For #3, we would expect a braced frame to exhibit quite a bit more rigid body rotation than a moment frame because #3 is a result of the cumulative effect of #2.

So, given enough stories, the effect of #2 + #3 starts to dominate over the effect of #1. And that's mostly the result of #3.

As an interesting experiment, you could model your braced frame and moment frame using identical column sections. In that case, I suspect that you'll see moment frame inter-story drift being more severe at all levels, regardless of the number of levels. The inter-story drifts would gradually converge, however, with #3 asserting its dominance as height increases.



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.