Eartquake analysis of two different systems?

[drile007]

I have a reinforced concret building with 10 storeys. After Under 10 storey there are 2 storeys with steel frame construction. I don't know how to design this 2 storeys to the earthquake loads. I think that's wrong to just to design them on loads from specter. On the other way I think that I can. I just need to pay ettention to the modal mass participations ratios which should be under 90% for both systems (concrete and steel), since their basic period will be completely different.
Thank you for any comment.

Drile007

 

[msquared48]

Do I understand you to say that you have 10 stories of concrete construction over 2 stories of steel?

If so, is there a concrete foundation wall at the exterior wall line of those two steel stories, plus a composite concrete floor system?

Mike McCann
McCann Engineering

 

[drile007]

1) No, first 10 storeys are of concrete, 11 and 12 storeys are of steel

2) There is no concrete wall at the exterior wall line, and there is a composite concrete flor system

Thank you msquared48

Drile007

 

[prsconsultant]

This is quite common. What design code are you using? You may have different R (building response modification factors) for Steel and Concrete systems. You have to use the lowerer of two R values for the entire structure (unless the upper two floors can be classified as a pent house).

 

[drile007]

I'm using Eurocode 8...and we use q factors for reducing the elastic quake load (I think they are the same like yours R factors). We colled them ductility factors...
Why should I use the lower q(R) factor? I think I can took greater ductility factor for the steel as for the reinforce concrete. That means that I will have two load cases...
But there is onother question? What if I have these two systems (concrete and steel) in the same floors? It's completely different story? The height of overall ductility factor is dependent of the stiffnes, capacity and ductility factor of each system? Am I right?

Drile007