Abrupt change in stiffness

[Robbiee]

Hello,
Working on a new two-storey steel frame college building where the architect allows no vertical braces. I tried moment frames as a lateral resisting system, but story drifts exceeded the allowed limits, then I used some of the CMU infill walls as shear wall in combination with some moment connections in the framing of the roof where needed, which made drifts acceptable. The problem is that some of the walls I used in level 2 are not continued down to level 1 and level 1 is stiffer that level 2. Should I be worried about this abrupt change in stiffness?
Thanks for the inputs.

 

[JAE]

In a high seismic zone that would possibly create a vertical irregularity that should be considered in the design. What code are you designing with?

 

[Robbiee]

JAE,
It is the National Building Code of Canada. The area is not a high seismic zone. The seismic hazad index, Ie.Fa.Sa(0.2), is 0.52.

 

[kslee1000]

I could be wrong on this, if so, please correct me.

Because the infill-shear walls, shouldn't be the 2nd floor stiffer than the first foor, eventhough the steel elements are smaller? Since it is in low seismic zone, as long as all members in a joint can adequatelly carry their's share of loads, the walls are properly anchored to the beams, and all deflections are within allowable limits, I don't see much of problems. Do you have specific concern in mind?

 

[Robbiee]

My concern is that the code allows using static analysis for a building like this one with stiffness irregularity if its period is smaller than 0.5 sec. But, really, how you calculate the period correctly?

 

[kslee1000]

I evision a steel moment frame (1st story) supporting a rigid body (2nd story) above. Maybe you can find the period by computer modeling.