As of version 6.16.0001, the new tension stiffening approach for the Quast and modified Quast methods replaces the previous, globally effective approach for anisotropic damaged material. It acts locally, at each concrete integration point, and separately in both principal strain directions. The differences from program versions prior to 6.16.0001 are intentional. A comparison with the previous behavior is only possible using older program versions. Further information is available in the corresponding theory chapter of the online manual for concrete design in RFEM 6:
What Has Changed
| Topic | Previous Approach | New Approach |
|---|---|---|
| Effect | Global change in tensile behavior for an entire concrete layer or cross-section | Local at each concrete integration point and separately in both principal strain directions |
| Activation | The maximum reinforcement strain across all reinforcement layers activated the tension stiffening | Each reinforcement layer contributes with an activation factor based on its remaining capacity, its angle to the principal tensile direction, and its location |
| Direction and Location | No dependence on the reinforcement direction; limitation in consideration of the location across the thickness | The direction and location of the reinforcement are taken into account |
| Consequence | Activating the reinforcement is also possible where the reinforcement cannot be effective, for example, on the compression side of a section subjected to bending | Tension stiffening is effective only where the reinforcement can interact with the concrete |
When Results Are Similar
Under simple bending perpendicular to a single reinforcement direction, with one layer of reinforcement on the tension side, without significant shrinkage or initial strains, and with a base material that behaves nearly linearly elastic under tension up to the point of cracking, the results are qualitatively similar to those of the older versions.
When to Expect Deviations
| Situation | Expected Deviation |
|---|---|
| Reinforcement in Multiple Directions | Reinforcement layers in unrelated directions no longer influence each other. Tension stiffening is only effective where the direction of a reinforcement layer aligns with the principal tensile direction of the concrete. |
| Reinforcement on Both Sides | Only reinforcement located in the tension zone and acting in the correct direction contributes. On the opposite side of a section subjected to bending, tension stiffening is no longer activated without cause. |
| Shrinkage-Dominated States | The concrete strain, including initial strains (such as those due to shrinkage), is projected onto the direction of the reinforcement. Tension stiffening can therefore remain active when the concrete is subjected to tension due to restrained shrinkage, even if the strain in the reinforcement alone would suggest compressive stress. |
| Nonlinear Base Material | Tension stiffening supplements the stress in the base material. For damaged material, the behavior may therefore differ from the previous approach. |
| Areas Without Contributing Reinforcement | Where there is no reinforcement, where it acts in the wrong direction, where it is too far away, or where it is subjected to compression, no tension stiffening is added. These areas show the brittle tensile behavior of the underlying concrete material. |
Convergence
The simplifications in the previous approach may have masked nonlinearities and, in some cases, appeared to improve convergence. The new approach prioritizes directional consistency and locality.
Isotropic Damage Material
For isotropic damage material, the angle factor is replaced. The two current principal tensile strains are projected onto the reinforcement direction and normalized to the larger principal tensile strain. This yields a single activation value for both principal directions, because an isotropic damage model does not provide information about which direction has failed.