Civil Engineering Reference
In-Depth Information
For prestressed concrete structures, prestress forces in the tendons can be
defined using INITIAL CONDITION option, which can add prestress
forces in the rebars. If prestress is defined in the rebars and unless the prestress
is held fixed, it will be allowed to change during an equilibrating static anal-
ysis step. This is a result of the straining of the structure as the self-
equilibrating stress state establishes itself. An example is the pretension type
of concrete prestressing in which reinforcing tendons are initially stretched
to a desired tension before being covered by concrete. After the concrete
cures and bonds to the rebar, the release of the initial rebar tension transfers
load to the concrete, introducing compressive stresses in the concrete. The
resulting deformation in the concrete reduces the stress in the rebar. Alter-
natively, modelers can keep the initial stress defined in some or all of the
rebars constant during the initial equilibrium solution. An example is the
posttension type of concrete prestressing; the rebars are allowed to slide
through the concrete (normally they are in conduits), and the prestress load-
ing is maintained by some external source (prestressing jacks). The magni-
tude of the prestress in the rebar is normally part of the design requirements
and must not be reduced as the concrete compresses under the loading of the
prestressing. Normally, the prestress is held constant only in the first step of
an analysis. This is generally the more common assumption for prestressing.
If the prestress is not held constant in analysis steps following the step in
which it is held constant, the stress in the rebar will change due to additional
deformation in the concrete. If there is no additional deformation, the stress
in the rebar will remain at the level set by the initial conditions. If the loading
history is such that no plastic deformation is induced in the concrete or rebar
in steps subsequent to the steps in which the prestress is held constant, the
stress in the rebar will return to the level set by the initial conditions upon the
removal of the loading applied in those steps.
Rebar force output detailed in ABAQUS [1.29] is available at the rebar
integration locations. The rebar force is equal to the rebar stress times the
current rebar cross-sectional area. The current cross-sectional area of the
rebar is calculated by assuming the rebar is made of an incompressible mate-
rial, regardless of the actual material definition. For rebars in membrane,
shell, or solid elements (see
Figures 5.6-5.8
), output variables identify the
current orientation of rebar within the element and the relative rotation
of the rebar as a result of finite deformation. These quantities are measured
with respect to the user-specified isoparametric direction in the element,
not the default local element system or the orientation-defined system.
The output quantities of rebar angles can be measured from either of the
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