cmu isolation

huangyhg

cmu isolation
lately we have been running into quite a few 5-6 story buildings with light-framed load bearing walls and sheathed shear walls. these buildings typically have masonry stair and elevator towers as well.  does anyone detail the interface between the diaphragm and cmu to allow for the code-required amplified story drift?  if so, how?
i hope someone responds to this thread, kusucks (is that a commentary on tonight's game?).
i am working on a project in sdc b, using braced frames without seismic detailing, and i must multiply my ultimate deflections by 3!!!  this seems excessive to me, but i will detail wide sealant joints to isolate interior cmu partitions if necessary.
daveatkins
our current project is a 6-story with steel moment frames at the first floor (no seismic detailing) and the calculated deflection at the top level with the amplification factor is +/- 6"!  any input on this subject would be greatly appreciated...
what it sounds like you have is a stiff cmu shaft set within a light framed, sheathed system or a moment framed system (which is not as stiff).
per the universal laws of physics ...i.e. hooke's law - force follows stiffness.  the lateral forces on the building will first attempt to move the cmu and not engage the full strength of the shearwalls...
your calculated deflection may be due to the shearwalls or moment frame and they may not be accurate as the cmu shafts will serve to stiffen the building and take more load, or most all the load.

there is no way that i would make the stair and elevator towers independent of the floor and walls.  not clear if the 6" drift is wind or seismic or much drift one would expect from wind but i doubt you would be able to make the joint detail work.  i would use both the wood walls and masonry stair walls as shear walls and connect the floor and walls to the masonry walls.  divide up the lateral load based upon relative stiffness of each shear wall and reinforce the corners of the masonry walls as required. in most cases that i run accross the wood exterior shearwalls are a lot longer than the masonry walls so they are stiffer and take most of the lateral load.  unless i'm missing something, the masonry walls have to be reinforced and designed to take earthquake loads from there own dead load anyway so why not take advantage of them and make the building work the way it wants to work.  
just wondering how you would keep a building roof from leaking with that much movement next to the cmu shaft.  would using an epdm highway pavement joint work or would you just build a wood curb around it and use sheet metal flashing?