This page contains frequently asked questions related to tendons.
General FAQ
How does SAP2000 internally handles tendons modeled as elements?
Extended Question: What is the internal procedure SAP2000 uses to develop and analyzed models that contain tendons modeled as elements? Could you please provide some sketches illustrating this?
- How are the tendons attached to the other elements in the model? Using constraints?
- How is the tendon load applied? Does the program calculate strain equivalent to the specified prestress force and then apply this to the model?
Answer: See Tendon implementation in SAP2000.
How does SAP2000 internally handles tendons modeled as loads?
Extended Question: What is the internal procedure SAP2000 uses to develop and analyzed models that contain tendons modeled as loads? Could you please provide some sketches illustrating this?
- How are the tendon loads calculated?
- How are the tendon loads applied to the elements?
- What could go possibly wrong? Are there any potential pitfalls that I should be aware of?
Answer: See Tendon implementation in SAP2000.
What equations are used to apply curvature and wobble coefficients?
The friction losses due to curvature and wobble effects are calculated using standard equations available in the literature. See for example friction losses article at structural wiki.
How does the SAP2000 treat P-delta effects of prestressing forces modeled by tendons for a suspension bridge?
If the tendons are modeled as loads, the tension stiffening effect is not included. However, the compression on the deck will cause P-delta effects.
If the tendons are modeled as elements, the tension stiffening of the tendons and the compression softening of the deck will cancel, as should be the case for bonded tendons.
What is the difference between prestressed and post-tensioned tendons? Are both types of tendons available in the program?
Only post-tensioned tendons can be currently (as of V14.0.0) directly modeled in the program. The "Prestress Type" field on the "Tendon Section Data" form is currently inactive and not being utilized by the program as it is intended for future use only.
It is still possible to model pre-tensioned tendons with some additional considerations. Please see page Modeling different types of tendons for additional information.
How can I model prestressed (pre-tensioned) concrete members in SAP2000?
Please see page "Modeling different types of tendons", section "Pre-tensioned members".
The program seems to be considering elastic shortening stresses when a tendon modeled as elements is first stressed. How can I disregard these elastic shortening losses?
In the current version of the program (V14.0.0 as of June 2009), the elastic shortening losses are always considered when tendon load is applied to tendons modeled as elements. This approach is directly to applicable to pre-tensioned construction. However, for post-tensioned construction, the elastic shortening losses are compensated for by further stressing the tendons to the desired tension. There are two approaches to handle this in SAP2000:
(1) Use the following iterative approach to disregard shortening losses when the tendon is first stressed:
- Use "Assign > Tendon Loads > Tendon Force/Stress" menu command to specify the desired tendon jacking forces.
- Run the analysis and check the tendon forces.
- Adjust the tendon jacking forces for compensate for the elastic shortening; run analysis and check the resulting tendon forces.
- Iterate, until the desired tendon forces are obtained.
(2) Use tendon target force to disregard shortening losses when the tendon is first stressed. However, please note that the tendon target force does not currently enable to include friction and anchorage losses. This capability will be added in a future version of the program.
Why I am getting different results for tendons modeled as loads and tendons modeled as elements?
Extended Question: For my model of prestressed continuous girder, the prestressing force after the losses due to anchorage slip, cable curvature and friction only is different when define tendon as load and as element. Could you please clarify why the results are different?
Answer: When tendons are modeled as elements, forces due to elastic shortening losses are automatically accounted for and this may be causing the difference. Tendon loads for tendons modeled as elements are applied as equivalent strains. This causes compression of the concrete and consequently relaxes the force in the tendons, which are modeled as compatible with the concrete section.
Therefore, modeling the tendons as elements will produce smaller tendon forces than when the tendons are modeled as loads, assuming that all other tendon parameters (such as losses) are the same. You may also find it useful to review Watch & Learn video "Tendons" available from http://csiberkeley.com/Support_WL_SAP.html , which further illustrates this.
Do I need to model tendons as loads or as elements when evaluating creep and shrinkage?
Tendons can be modeled both as loads or as elements, but modeling the tendons as elements is more accurate, since various tendons losses (due to elastic shortening, creep, shrinkage and relaxation) are calculated automatically from the tendon material properties. For tendons modeled as loads, these losses would need to be estimated and entered on the "Tendon Loads" form.
