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Height of Building/Shear Wall design

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Hello 
I am looking for a some advice on table 16-N of the 1997 UBC  - as a New Zealand Engineer I am not really sure the background for some of the height limitations.  
I have a "theoretical" building in a zone 4 region with a height of 260 feet from basement slab to roof.  UBC97 table 16-N will allow a maximum height of the building  to be 240 feet for a Concrete Shear-Wall only system.   Accordingly I would require a combined SMRF and Shear Wall system for over 240 feet - unless this height was not measured from the lower basement level.  
Question 1 - If I have a rigid basement structure and shear walls only as the lateral resisting system above "ground" level the height from ground level to the roof is only 230 feet, so does this justify a shear wall only system given it is under 230 feet measured from ground level?

Another question I have relates to  Shear Design of Concrete shear walls - the UBC does not seem to consider in depth that shear walls will have Potential Plastic Hinge Zones at the wall base just like a "beam-column joint" might have in a ductile moment resisting frame at the column face  - I cannot find any attempt to try and suppress undesirable "shear yielding" through capacity design in the instance of flexural yielding at the base of the shear wall.  When I am designing the shear strength for a ductile wall in New Zealand I would always design shear capacity of ductile walls to overstrength according to capacity design principles with a much reduced shear strength contribution from the concrete - is this not required in the UBC, am I missing something?  Or would I just follow the normal simple shear design rules in Section 1921 of the UBC97.
Any replies would be much appreciated.
Thanks
Tim.



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