Weak last floor (seismic)

[vaquers]

At the time I´m designing a 7-story control tower building. The principal characteristics are:

1) Lateral force-resisting system: special concrete shear walls.
2) Rigid diaphragm condition: at each floor, because I have concrete slabs without mayor discontinuties.
3) Vertical force-resisting system: two and one-way slabs spaning between concrete beams, which are resisting on columns/shear walls.
4) Seismic Design Category D.
5) At the last floor the architects must accomplish a 360º view, so they want to build a steel frame structure with a curtain wall.

Actually only the last item is the one of interest. I didn't find any restriction in ASCE 7-05 regarding the last story abrupt change of stiffness (I only found that I have to increment the design forces by 25%).

As the steel frame structure is more flexible than the concrete structure I'm expecting to have larger drifts but less inertial forces (of course I have to check the drifts). In fact I wasn't worried about these until today when another engineer told me that he didn't like the steel frame structure on top (instinct he said).

In my opinion, without having any construction consideration into account (the money is not the issue here), these type of structure is achievable. What do you think? Regards,

Sebastian

 

[hokie66]

A soft level is a problem down low, not at the top. Control towers must have unlimited visibility.

 

[vaquers]

Dear hokie66,
I'm totally agree with you, I'm aware of the problem of having a soft level down low. It's my opinion that only the drifts are going to be magnified because of having the weak floor on the top. If they are below the allowable drifts, I don't see why these configuration couldn't be possible.