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06/06/2019 4:34 pm
The 8% of the base shear applied to the roof as I understand is to account for higher mode effects which may increase the storey shear on upper floors. These questions mainly apply to residential design.
1) For a 2 storey building (ground, first and roof), is it necessary to apply the 8%Vb? As the 2nd mode shape is where the roof does not move and therefore wouldn’t increase the storey shear on the upper floor.
2) For a 3 storey residential building with a concrete first floor supported on stiff concrete/masonry walls on ground floor ,then lightweight timber framing above. Is there a correct approach to calculating the seismic loads? My approach is: Upper two levels in GIB/NZS3604 designed for ductility 3.5. Calculate total seismic mass and base shear for u=3.5 and apply 8%Vb(3.5) to roof, design accordingly. Ground level, designed for the total base shear but for ductility 1.25.
I know another approach suggested in 1170.5 commentary is to design the upper floor (penthouse) using parts (ch8 of 1170.5), however when 2/3 of the house is timber this approach seems wrong. Or should the ground floor be designed for the over-strength shear of the first floor? Or could the upper two timber floors be designed as if they are effectively on the ground as the lower floor is so stiff?
Thoughts?
Fazil Patel liked
19/06/2019 6:04 pm
Yes, the 8% base shear applied to the roof level is to account for higher mode effects but this also assumes a linear first mode shape. The fact that you have identified that this may not apply to structures with vertical stiffness irregularity is good.
1) I would say that you should still apply the 8% in this case(assuming the building is roughly vertically regular). Just because the modeshape of the second mode at the roof is low doesnt mean that there will not be additional demand at the first floor. There are photographs of tall concrete shear wall buildings with cracking and mid-height where higher mode effects have occured.
2) For designing such a vertically irregular building, the Equivalent static method may not apply. check the conditions in 1170.5 that you have to reach to be able to use it. Otherwise you may need to go to response spectrum or maybe pushover. It is tricky when designing two systems with different ductility. Don’t forget to consider the overstrength of the GIB shear walls.
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