This model is intended for unreinforced concrete and similar materials, and is the default model for concrete and masonry materials in the program. Tension and compression behavior are independent and behave differently. The force-deformation (stress-strain) curve is used to determine the sign of compression, which can be positive or negative. The point having the largest absolute value of stress or force is considered to be in compression, so that the sign of compression can be either positive or negative. Likewise, the concrete model can also be used to represent a tension-only material whose behavior is similar to concrete in compression.
This model is primarily intended for axial behavior, but can be applied to any degree of freedom. Reinforced concrete is better modeled using the pivot, degrading, or Takeda models.
A non-zero force-deformation curve should always be defined for compression. The force-deformation curve for tension may be all zero, or it may be non-zero provided that the maximum force value is of smaller magnitude than that for the compression side.
A single parameter, the energy degradation factor f, is specified for this model. This value should satisfy 0.0 < f < 1.0. A value of f = 0.0 is equivalent to a clean gap when unloading from compression and dissipates the least amount of energy. A value of f = 1.0 dissipates the most energy and could be caused by rubble filling the gap when unloading from compression.
Compression behavior is modeled as follows:
Initial loading is along the backbone curve
Unloading to zero occurs along a line nearly parallel to the compression elastic line. The line is actually directed to a pivot point on the extension of the compressive elastic line, located so that the unloading slope at maximum compressive force has half the stiffness of the elastic loading line.
At zero force, reverse loading toward tension occurs
Subsequent loading in compression occurs along the previous unloading line if the energy factor f = 0.0, and along the secant from the origin to the point of maximum previous compressive deformation if the energy factor is 1.0. An intermediate secant from the horizontal axis is used for other values of f.
Tension behavior, if non-zero, is modeled as follows:
Initial loading is along the backbone curve
Unloading occurs along a secant line to the origin
Subsequent loading occurs along the unloading secant from the origin to the point of maximum previous tensile deformation.
See the figure for an example of this behavior for an energy degradation factor of f = 0.7.
See Also: