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volumetric change induced stresses
i have come across a situation where an engineer has undertaken a detailed analysis of the bending moments induced in the columns of a tall building caused by shrinkage. i have been able to find a small reference on this kind of effect in one text but not much.
my question is this: in a tall reinforced concrete building, the floors would be expected to shrink at roughly the same rate and time. would any forces theoretically induced in one subframe by the restraint (fixity) between it and the floors above and below be of serious consideration or would they cancel?
the building in question comprises a post tensioned floor scheme; however, would this make a difference on the quantum frame forces. understandably the shrinkage affects the pt losses but i think this is a separate issue.
nb
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concrete shrinkage is a volumetric phenomena that manifests differently in different shapes. for large masses of concrete such as large square or round columns, the shrinkage is more uniform. in thin section members, the manifestation is perpendicular to the thin section, thus greater appearance of cracking in flatwork. often this is simply a concentration of strain in two dimensions that occurs orthogonally, but results in tighter cracks (sometimes not visible to unaided eye)in thicker members.
keep in mind that shrinkage is a function of the mix design. you might have several different mixes being used in the same building. you will not have uniform shrinkage among the different mixes. many issues in a mix design affect the shrinkage...aggregate type and size, cementitious material content, water-cement ratio, air content....all of these will affect the amount and type of shrinkage that will occur. balancing shrinkage strains in a large building could drive you nuts!

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