SCOPE This report is concerned with the prediction of strains in columns frnm the mnment the columns become part nf a structure, It ...
SCOPE
This report is concerned with the prediction of strains in
columns frnm the mnment the columns become part nf a
structure, It also attempts to predict differential column
movements in reinforced concrete or composite struc-
tures nf significant height.
In reinforced concrete structures (Fig. la), differential
movements between shearwalls and columns and between
neighboring columns due to elastic, shrinkage, and creep
shortening are of concern. In compnsite structures (Fig.
1b, c) that combine either an interior core or peripheral
beam-column frames made of concrete with the rest of the
structure framed in structural steel, the shortening of the
steel columns relative to the reinforced concrete core or
the reinforced concrete peripheral system is of concern.
EFFECTS OF COLUMN SHORTENING
The shortening of columns within a single story affects the
partitions, cladding, finishes, piping, and so on, since
these nonstructural elements are not intended to carry
vertical loads and are therefore not subject to shortening,
On the contrary, partitions and cladding may elongate
from moisture absorption, pipes from high temperature
of liquid contents, cladding from solar radiation, and so
on, Details for attaching these elements to the structure
must be planned so that their movement relative to the
structure will not cause distress.
The cumulative differential shortening of columns
causes the slabs to tilt with resulting rotation of parti-
tions, as shown in Fig. 2. Modern dry-wall partitions can
be detailed with sufficient flexibility along their peripher-
ies and at the vertical butt joints to permit their distortion
without visible distress (Fig. 3). Plaster and masonry
partitions, which were quite common in the past, are
characteristically rigid and brittle and have limited ability
tn undergo distortions without cracking. When a slab
carrying such partitions is subject to differential support
displacements, the partitions must be detailed around
their peripheries to allow movement relative to the frame.
Value of Specific Creep
Specific creep values can be obtained by extrapolation of
results from a number of laboratory tests performed on
samples prepared in advance from the actual mix to be
used in a structure, It is obvious that sufficient time for
such tests must be allowed prior to the start of construc-
tion, since reliability of the prediction improves with the
length of time over which creep is actually measured.
A way of predicting basic specific creep (excluding
drying creep), without testing, from the modulus of elas-
ticity of concrete at the time of loading was proposed by
Hickey[ ]31on the basis of long-term creep studies at tbe
Bureau of Reclamation in Denver. Hickey’s proposal was
adopted by Fintel and Khan, “4,”) A simpler suggestion is
made in this study.
Let 6<=denote the specific creep (basic plus drying) of
6-in. -diameter (150-mm) standard cylinders (V:S= 1,5 in,
or 38 mm) exposed to 40!% relative humidity following
about 7 days of moist-curing and loaded at the age of 28
days. In the absence of specific creep data for concretes to
be used in a particular structure, the following likely
values of e,= may be used:
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