masonry stair tower cc loads

huangyhg

masonry stair tower c&c loads
when designing an 8"cmu 21ftx15ft exposed stair tower, five stories tall, how would i go about reinforcing the wall for c&c loads on the 21ft face in  a 90pmh wind zone?  it would seem that i would have to place bond beams every other course to span horizontally.  
i'd rather not reinvent the wheel here, so i'd appreciate any suggestions.
hate these abbreviations.  what are "c&c loads"?  but if you are just talking about wind loading, why can't you brace the walls with the stair landings to cut down your span?
try to place bond beams at some appropriate spacing - perhaps 4 ft. to 8 ft. on center and design the walls to span between bond beams, and design the bond beams to take those tributary loads and span to intersecting corners.
this is probably a total rookie question, but if you have a 5-story high, rectangular box wall, won't the wall want to span horizontally between the endwalls instead of vertically?
if you do tie the wall to the stair landings, i would think that you would get a lot more two-way action than spanning simply from landing to landing.
21 ft seems like a pretty long span for a bond beam what kind of wind loads are you getting? does the building code in your area allow for the 1/3 increase? you could also try spaning the masonry vertically to a concrete beam at mid height, and seeing if that will work.  you might need a wide beam though.
yikes...ash060 - you are right - i failed to see the 21' dimension.  that is pretty long for an 8" cmu bond beam....even for a 12" cmu.
tricky problem.
can you talk the architect into allowing a steel girt at each floor?  i mean, on the inside of the cmu.
daveatkins
i suggest using the landings and stairs to brace the walls. i suggest placing bond beams just beneath each landing all around the perimeter.
you may have to look at several locations to determine the maximum wall height.  you may still have to reinforce the walls vertically depending upon the floor to floor height.
good luck!
what i have done in the past, not necessarily for stair towers (and i agree that it is not the best solution - masonry contractors hate it and it tends to slow construction down) is to provide horizontal concrete bond beams at whatever spacing is required. you can play with the concrete strength as required to get your spans and strength required. just recently, i used it on a high school fine arts theatre where the cmu walls were 50 feet tall. i spanned horizontally between pre-cast concrete pilaster/columns spaced about 22' apart using concrete bond beams at 48"o.c. vertically. in my case, the exterior auditorium walls were to be painted and the architect used the bond beams as accent stripes at various levels.   
galambos:
i have encountered this situation several times in the past.
my method is to design the section of the wall between landings to span horizontally. by assuming a span from edge of landing to edge of land (in plan view) i get a span somewhere around 14 feet or so. this is doable with 8" bond beams. don't forget to include the are of steel in your joint reinforcing in your calculations. it is small, but every little bit helps.
i then design vertical 'strips' the width of the landing to carry the load vertically between the landings.
this assumes that the landings can either transmit this horizontal force into the larger floor slab diaphragm (at the landing at floor elevation) or can carry the horizontal load into the masonry wall that is on the short side of the stair tower at the intermediate landing by some means of attachement.
this method appears rational to me.
regards.  
lkjh:
your description didn't mention it but i assume that you then put a large vertical element at the end of landing (where the stairs connect) to carry the reaction component from the 14 foot horizontal span. the vertical element needs to transfer those forces to the landings above and below.
i prefer to have the wall continuously connected to the landings and stairs with intermittent clips. i then don't have to deal with trying to span the wall horizontally.