am i missing something

huangyhg

am i missing something?
i am trying to verify some masonry spreadsheets and i am having a hard time with some things.  
if you have a section with axial load and moment, is it correct to assume that you can analyze the section for each individually and superimpose the results to get max stresses?  this is reinforced masonry.
i got some results for the combined loading that surprised me a little, but i took the results blindly and verified them through mechanics, not design.  needless to say, it did work out.  i then analyzed the section for axial load only, and for moment only.  i thought i could superimpose the masonry stresses and the steel stresses to get the same stresses that i got when using the combined loading, but it isn't working out that way.  does anyone have an idea why?  
am i just completely missing something.
this is asd, and i thought that for the elastic range of stresses (i know it's not really elastic if it cracks, but we assume so for design), that the principle of superposition applies.
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without knowing more details about what you did and how, my first thought is: is it really elastic/linear?
for axial load, it probably behaves elastically, but for bending, i would imagine that your masonry behaves as a non-tension material, i.e. non-linear behaviour.
did you re  
n was taken into account for each case individually.
is your axial compression greater than or equal to mc/i?  if so, it should behave elastically, no? if not, i think you need an interaction diagram, like in reinforced concrete columns.
you should be able to superimpose the stresses, p/a +- mc/i, where a and i have been appropriately "transformed" using n.
what method are you comparing this to?
pmr-
i was using p/a+bending stress due to moment only.  since this already took n into account, i thought that should work.  am i thinking about it wrong?
i don't believe you can superimpose axial and bending analyses.  you need to analyze the section for combined bending and axial load.  an axial force puts the masonry in compression, which allows the masonry which would otherwise be in tension to contribute to the bending resistance.  an example of the analysis for beams is given in the old aci sp-3 publication.   
miecz is right on.  the axial load will actually increase the moment capacity of the masonry in most cases.  this is because masonry capacity is typically limited by flexural tension.  since the axial compression reduces the net tension the flexural capacity increases.  as a result superimposing the axial and flexural stresses will not work out.
if you are assuming a cracked section for either case (same properties if superimposing or combined axial + bending), i don't know why the results are coming up differently. same section modulus, same effective area, linear elastic. the spreadsheet may not be calculating based on a cracked section if the axial is high enough under the combined case.  
the more i really think about, i think others are correct in that you may not be able to directly add p/a and mc/i if the section is cracked.  the neutral axis location is not a function of applied axial or flexural loads, therefore it is not as straight forward as i originally thought.
hmmm... something to think about next time i'm "sitting on the throne".