ANACAP

The following is a summary of the main features and capabilities of this software for the analysis of concrete and steel structures. Many of these are unique to ANACAP and are not found in any other finite element computer program. ANACAP is designed to require only standard engineering material property data to be input by the client.


 

Overall Capabilities

  • Smeared-crack model for general 3D stress states
  • History-dependent cracking in three independent directions
  • Crack closure and re-opening for cyclic and non-proportional loading
  • Pre-cracking to model existing cracks
  • Preset crack direction for modeling material interfaces, construction joints, or known situations
  • Rough-crack modeling for aggregate interlock effects
  • Compressive plasticity utilizing the full stress-strain curve, including post-ultimate strain softening and crushing
  • Material damping applied at locally damaged regions of cracking
  • Hereditary creep relations with age and temperature dependence, valid for ages of 6 hours to 50 years and for temperatures up to 450°F
  • Age and temperature dependent strength and stiffness
  • Time and temperature degradation of modulus and strength at elevated temperatures
  • Shrinkage
  • Rock and soil modeling for foundation-structure interaction
  • Metal plasticity with strain hardening/softening, Bauschinger effect, and low-cycle fatigue/failure
  • Rebar plasticity with strain hardening/softening, bond-slip, and anchorage-loss

Cracking

Despite the advances made in modeling of concrete structures, analysts are often forced to use the simplified cracked-section modulus approach to calculate the load carrying capacity of reinforced concrete structures. This design-based approach, while acceptable for the design of simple beams and slabs, is not applicable in analysis, particularly under complex loading or thermal histories and for continuum structures. ANACAP employs the history-dependent smeared-crack model which predicts crack formation according to a principal stress/strain-based interaction criterion. Crack orientation, and hence anisotropy, is dictated by the principal strain directions at each material integration point.

Cracks are allowed to form in three directions, and once a crack forms, it may close and re-open, but it can never heal. This crack memory feature is essential for analyses involving load reversals such as earthquakes. The model includes residual tension stiffness algorithms for the gradual transfer of load to the reinforcement during crack formation and for shear retention to simulate the effect of crack roughness through aggregate interlock. The model also allows definition of pre-cracked directions for analyzing structures with existing cracks.

Crushing

Plastic flow under compressive stresses is implemented through a modified Drucker-Prager (combined Mohr-Coulomb and von Mises) yield condition. The compressive stress-strain curve is followed up the ultimate strength and into the strain softening regime where the material begins to unload due to internal damage and crushing. The model also includes hysteretic behavior due to loading and unloading in the strain softening regime. This capability is required when determining ultimate loads and ductility capacities of concrete structures and is unique to ANACAP.

Temperature Dependence and Degradation

At elevated temperatures often seen in nuclear applications, concrete exhibits a significant departure from ordinary elastic and creep behavior at lower temperatures. This occurs because of thermally activated damage that is evidenced by the degradation of the material properties, especially the elastic modulus, even during hold time at constant temperature. This material property degradation with time and temperature has been implemented in ANACAP for temperatures up to 450°F based on experimental data for the modulus, compressive strength, and ultimate tensile strength of concrete. This feature is required for evaluation of structural integrity involving long-term thermal creep and is unique to ANACAP.

Aging, Creep and Shrinkage

ANACAP's state-of-the-art concrete constitutive model also includes the effects of aging, creep and shrinkage as functions of time and temperature. The creep response of concrete is modeled through hereditary integral representation of the creep stress-strain history relations. Temperature affects concrete creep in a manner that is characteristic of an aging thermo-viscoelastic material. The creep compliance is defined from an extensive creep database that accounts for both temperature and age. Age-dependent stiffness and creep formulations have been implemented based on available experimental data. These include terms that handle aging for as early as 6-hour old concrete. This capability is needed in repair applications for mass concrete (particularly locks, dams and bridges) where loads may be applied before the concrete is fully mature, or for incremental and roller-compacted concrete construction to optimize construction parameters to mitigate cracking, Here, the strains depend not only on the duration and magnitude of the loading, but also on the time at which the load is applied.

Rebar-Concrete Interaction

In areas of large stiffness discontinuity, major cracks develop, and the interaction between the concrete and the reinforcement or other steel elements plays a major role in determining the structural response and failure state. Because of dislocation displacement and rebar de-bonding, slippage can develop between the steel and the concrete. ANACAP has capabilities for modeling rebar bond-slip based on confinement and anchorage-loss, utilizing bond strength data from rebar pull tests.

Existing Cracks and Weak Zones

In addition to the development of cracking as outlined above, ANACAP has the ability to model structures with existing cracks. This is done by pre-setting cracks in the material. A potential crack in a weak zone such as a material interface or a fabrication joint can also be modeled. This allows a crack to develop preferentially in a pre-defined direction, such as along a construction joint.

Damping

For dynamic applications, a concrete response model must include the effects of internal damping. General purpose computer code capabilities for treating concrete material damping in time-history analyses are limited because they allow only a constant damping ratio to be applied uniformly over the whole structure, which is the accepted practice in linear analysis. In nonlinear analysis, however, the dominant energy absorption mechanism is the time-dependent damage of the concrete in localized regions. ANACAP employs a crack-consistent damping model at the element integration point level. This damping is thus treated as a function of time and cracking status.

Viscous or Radiating Boundary

ANACAP has the ability to model infinite media by utilizing dashpot dampers at the model boundaries. The amount of stress/pressure wave reflection at the model boundaries can be defined by the user with force-velocity relationships.

Foundation-Structure Interaction

Because of the similarities in the behavior of rock/soil and concrete, ANACAP is adaptable to rock/soil-foundation modeling using yield and failure surfaces of the rock/soil material. This allows direct coupling of the foundation continuum with the structure for detailed foundation-structure interaction analysis. This capability is particularly well suited for determining the response of embedded structures since it can consider the variation of rock/soil characteristics and energy absorption capacities with depth.

 

 

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