42 tall cmu firewall

huangyhg

42' tall cmu firewall
i'm looking at a 12" 4-story cmu firewall that the architect located at the major axis of symmetry of a building i am working on.  the buidling is u-shaped and the firewall splits the building into two l's.  this is a wood structure and there is an expantion joint at the same location.  my question has three parts: first, can i consider this wall to be an interior wall?  second, can i use the wood diaphragm to laterally support my wall? third, regarding the equivalent thickness defined by ibc 2006, table 721.3.2, does 12" block meet the minimum requirements, or does it need to fully grouted?  (i assume that the equiv. thickness is based on the wall thickness of the block.) to increase the amount of fun for this question, the structure is located in a high seismic region, cat c, i am very interested in any insight that  would be helpfull.  as always i greatly appreciate your comments.  
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interesting problem!  the firewall has to stay intact in the event either side burns away, or perhaps part of one side and a different part of the other.  i would be trying to extend the walls at the ends with perp walls forming an 'i' shape. and maybe pileasters between.
in a high seismic zone, depending on the type and jurisdiction, you may not be able to use masonry. let alone reinforced masonry.
dik

my understanding of a 'fire wall' appears to be the same as dik's. the wall must remain standing if the structure on either side burns away and collapses.
this generally means that you cannot use the structure on either side to brace the wall as the connections between the wal and the structure ould pull the wall over if the structure on either side collapsed.
this usually leads me to either:
1) free standing cantilever walls (tremedous pain-in-a@#).
or
2) a double row of 'fire walls's' eperated by a joint. then the cmu (or other material) wall on each side of the joint can be attached to the structure on that side of the joint. and if the wall and structure on one side collapse in a fire, the wall and structure on the otherside of the joint remains standing. architects hate this one as it chews up floor space, but sometimes it is the only practical solution.

depending on the aggregates available, there are several choices for the units used and the grouting(if necessary) to achieve a satisfactory fire rating.
generally, the fire resistance depends on the type of aggregate (lightweight is superior to normal weight) and equivalent thickness of the cmus (the volume of the solid material in the block divided by the face area). a given fire rating can be obtained by varying the aggregate types (usually the concrete density) and the percentage of voids in the block (thicker face shells give a higher equivalent thickness).
check with the local cmu producers to determine what is readily available. most have a combination of aggregate types and mold shapes to meet most requirements. in some areas, 14" and 16" thick cmus are commonly available.
depending on your local code and standards, core filling with grout, some insulation (or even sand filling in some jurisdictions) can be used to increase the equivalent thickness and the fire resistance or rating. properly applied materials (gypsum board, etc.) can also be used to increase the fire resistance of the wall assembly.
go to the national concrete masonry association (ncma) site at ncma.org and click in the upper right area to get access to over 120 tek notes on all subjects, including fire resistance and codes. when they ask for a location, just use any state and producers, since the technical information is national and the same for all.
one thing to be concerned with is the connections of the 4 story wood frame structure to the masonry firewalls - wood shrinks much more than the masonry, so there could possibly be some distortion on the upper floors through the years.
pilasters are not as effective as "l" returns on the fire wall if they can be incorporated into the building.
there are many options!
dick

thanks, i appreicate the help.
i would suggest a non-cmu, double firewall (tied to each structure) at that height and seismic zone.
what is the fire rating of the wall?
the wall rating is 2 hours...did i mention that it is sitting on hollow core planks?  it keeps getting beter.
a 2 hour rating is very minimal. even an ordinary 8" lightweight block will give you that with no filling of cores for structural purposes. a normal weight 8" unit would only require a face shell that is 3/8" thicker and is very commonly available.
12" walls are probably a better choice for the height and to provide adequate bearing for plank.
why not make the fire wall a structural element (8" or 12") and forget about the firewood framing?  - probably much cheaper even if 2 separated walls are needed for some strange reason.
the description of the structure is a litle hazy, so some suggestions may not ne applicable, except for the wall fire rating.
this topic has really been hammered in the past.  check these discussions out:
you can design a breakaway connection so the roof structure on either side can support the wall. we generally use a bent plate that wraps over the top of the bottom chord of the truss. the truss needs to be laterally braced at the points where the wall is braced.
it is best to locate a bond beam in the wall at this level.
if you want the wall to also act as a shear wall you could add a vertical slot to the clip that would allow a bolt to provide lateral restraint but allow the truss to fall downward should it collapse do to the fire.