aci 530 and unity equation

huangyhg

aci 530 and unity equation
when using aci 530, if the wall is reinforced you only have to check fa/fa + fb/fb <1.0 (or 1.33) for the masonry if the axial stress in the wall is greater than the allowable axial stress if the wall was unreinforced.
if it isn't, you can directly add the axial and bending compressive stresses together and compare to the masonry allowable compressive stress in bending (fa + fb < 0.33f'm).
can anyone explain the reasoning behind this?
i believe you may be mis-interpreting the aci 530 code.
isn't the 0.33f'm giving you the factor of safety of 3?
thanks for the replies,
i tried to figure out this one for a while today and it just seems strange that you can't do the same for unreinforced masonry. maybe it's something come from tests.
for a reinforced masonry wall {from aci 530-99):
subject to axial load - applied load must be less than pa (eq. 2-17 or 2-18).
subject to bending loads - compressive stress due to flexure must be less than f'm/3 & tension stress must be less than 0.4fy (sect. 2.3.2.1).
subject to bending and axial loads - applied load must be less than pa and compressive stress due to flexure and axial load must be less than f'm/3 & tension stress must be less than 0.4fy.
for an unreinforced masonry wall (or you may design a reinforced wall for the following):
subject to axial load - applied load must be less than pe/4 (eq. 2-15 & 2-11) and axial stress must be less than fa (eq. 2-12 or 2-13)
subject to bending loads - compressive stress due to flexure must be less than f'm/3 (eq. 2-14) and tensile stress must be less than that in table 2.2.3.2.
subject to bending and axial loads - applied load must be less than pe/4, and compressive stress due to axial and flexure must be less than f'm/3 and axial stress over allowable stress plus flexural stress over allowable flexural stress must be less than 1.
now, as a addendum, you can use the 4/3 stress increase, provided you are using load combinations from asce 7-95 or previous.  if you are using load combinations from after that you may not use the increase as the 0.75 factor is on the loads already.
sorry for the long post.  hope this helps.