huangyhg

deflection of reinf. masonry wall backing up brick
i am currently designing a 32'-0" tall, 12" block wall supporting long span roof joists. i have determined for my loading conditions that #5 at 24" o.c. is adequate for strentgh. when i presented this to the boss he thought deflection might be a problem since the wall is backing up brick. i usually do not perform a deflection check for out-of-plane loading on a masonry wall (ibc 2003/asd). i have determined the theoretical defl. to be about 1.5" and l/600 equals 0.64". any thoughts? to meet the l/600 i would have to grout the wall solid.
add'l info:
dl 1650 plf
ll 1100 plf
wind 16 psf
sds 0.32
6' parapet
thanks in advance for your comments!
with that sort of axial load, you may be able to justify the bottom as fixed (for deflection) which will help bring the deflection down.
csd
csd72,
thanks for the input. i guess my real question is simply do you think a reinforced masonry wall backing up brick veneer has to meet the l/600 criteria or just the 0.007 h limit stated in section 3.2.5.6 of the aci 530-02?
what are you using for the eccentricity of your axial load?
if you are using a 6" deep ( parrellel to direction of joist span) bearing plate, with edge set back 1/2" from edge of cmu, (per section 104.4 of sji standard spe) your eccentricity would be 2.31 inches.
depends on which building code you are using.
also,are you multiplying the wind loads by 0.7 for serviceability?
lkjh345,
based on the 1/2" from the edge as you stated and the diagram (on page 43 of the vulcraft manual) of the top chord bearing of lh / dlh joist which shows the resultant reaction 2" from the end of the joist i calculated the eccentricity to be 1-5/16". i used 1.5" for my design. would you agree or am i missing something? on a side note, do you put a minimum eccentricity on a wall (0.1xwall thickness). i only see that requirement for a column.
thanks,
j
jechols:
you are correct. using the diagram on p43 of vulcrafts manual (with center of loading 2" from end of joist) you do get about 1.5 inches of eccentricity.
personally, i usually use 1/6 of the wall thickness as minimum eccentricity. i don't re
lkjh345,
that makes sense. i am going to try to treat the wall fixed at the base as suggested by csd72, multiply wind load by 0.7 (thanks csd72), and increase bar size to reduce mcr. maybe by adjusting those 3 parameters i can at least get close to l/600.
since technically i need a relief angle anyway, another option would be to install the angle at mid-height of the wall which would counteract worst case condition while also elminating l/600 requirement.
thanks again to all who responded.
csd72 - you might provide a reference for that 0.7 factor. i'm not sure where that comes from.
jechols -
the concept of a relief angle near mid height is outstanding!
without the relief angle you may end up with an uncontrolled crack near that location if you did have excessive deflection.
as usual, just make sure you flash it properly to shed moisture from the cmu wall.
this also solves the conflict between the dissimilar long term expansion of the brick in comparison to creep/shrinkage of the concrete masonry or any steel or concrete in the structure. a brick veneer wall 32' high without relief is stretching it as you recognize.
dick
jae-
i believe the 0.7 factor is a result of the strength wind loads using 50-year wind speeds, but serviceability only needs to be designed for 10-year wind speeds. this is discussed very well in the aisc design guide #3 (i think) - the one about serviceability of steel buildings.
for non-hurricane zones, the factor for 50-year to 10-year is around 0.84. 0.84^2 is about 0.71, rounded down to 0.7.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]