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.
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This page is devoted to frequently asked questions (FAQ) related to tendons. |
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Tendon types
What is the difference between prestressed and post-tensioned tendons, and are both available to SAP2000?
Answer: Prestressing is when tendons are stressed within formwork before concrete is placed and allowed to harden. Once sufficient strength is achieved, prestressed tendons are released, causing a compressive axial force and a bending moment which is typically designed to be opposite that of the anticipated load application.
Post-tensioning, on the other hand, is when precast reinforced-concrete segments are cast with ducts. Once segments are in position, tendons are passed through these ducts, then tensioned to adjoin the segments and subject them to a compressive force.
Post-tensioned tendons may be directly modeled within SAP2000, while prestressed tendons may be modeled using techniques described in the Modeling different types of tendons article.
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The option for Prestress Type Tendon on the Section Data form is currently inactive, but intended for application in future versions. |
Modeling
How does SAP2000 handle tendons which are modeled as objects?
Extended Question: How do tendons attach to other structural objects and how are tendon loads applied?
Answer: For response, please see the Tendon implementation in SAP2000 article.
How does SAP2000 handle tendons which are 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.
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How are tendon loads calculated and applied to structural objects, and what are the differences between modeling tendons as either objects or loads?
Answer: For response, please see the Tendon implementation in SAP2000 article.
How are prestressed concrete objects modeled in SAP2000?
Answer: For response, please see the Prestressed-tendons section of the Modeling different types of tendons article.
Can tendons span through gap elements?
Answer: Yes, tendons may span gaps when the tendons are modeled as elements, and not when modeled as loads. The Modeling segmental-bridge joint openings tutorial presents an example bridge application.
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Formulation
What equation is used to apply curvature and wobble coefficients?
The friction Answer: Frictional 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:
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terms of the tendon force Px at distance x from the jacking end, expressed as follows:
where:
- P0 = prestressing force at the jacking end
- Px = effective prestressing force, at location x, after frictional loss
- µ = curvature friction coefficient
- α = cumulative angle (in radians) of change in the tendon-profile tangent between x and the jacking end
- K = wobble friction coefficient
- x = position along tendon from jacking end
Why are results different when tendons are modeled as either loads or objects?
Extended Question: When I analyze a prestressed continuous girder, why is the prestressing force after anchorage-slip, cable-curvature, and frictional losses different when tendons are modeled as either loads or objects?
Answer: Prestressing forces should be smaller after short-term losses are applied to tendons which are modeled as objects because the software automatically accounts for these forces by applying tendon loads as equivalent strains which compress the concrete and, through compatibility, relax forces within the tendons. When tendons are modeled as loads, on the other hand, these effects must be estimated or calculated by hand, then manually specified within the software.
It may be useful to review the Tendon Watch & Learn video, which discusses this topic in greater depth.
Do tendons recover stiffness after exceeding tension / compression limits?
Answer: Yes, tendons which exceed limits do recover stiffness after loading is reversed. For example, tendons with a compression limit of zero will shorten without stiffness when compressed. When load reverses, this shortened length will recover without stiffness, then full stiffness will engage elastically once original length is reached.
Additional information is available in Context Help.
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Response
How do I find the elongation of post-tensioned tendons?
Answer: Tendon elongations may be derived through post-processing of axial force, tendon area, and Modulus of Elasticity.
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How may elastic-shortening losses be disregarded when tendons, modeled as objects, are first stressed?
Answer: Elastic-shortening losses are always internal to computation when load is applied to tendons which are modeled as objects. This formulation realistically simulates the effects of prestressing construction techniques. During post-tensioned construction, however, tendons may be stressed further, after the onset of elastic shortening, to compensate for elastic-shortening losses. This may be accounted for through either of the following two methods:
- Iterative approach. Disregard the elastic-shortening losses which occur when tendons are first stressed through the following process:
- Specify the desired tendon jacking forces through Assign > Tendon Loads > Tendon Force/Stress
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- Run
- analysis
- , check
- tendon forces
- , and adjust tendon jacking forces
- to compensate for
- elastic shortening
- .
- Repeat this last step until the desired tendon forces are obtained.
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- Target force. Elastic-shortening losses may also be disregarded using tendon target force.
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However, please note that |
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this application does not |
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enable |
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the inclusion of friction and anchorage |
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Why I am getting different results for tendons modeled as loads and tendons modeled as elements?
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loss. These sources of loss should be accounted for in the specification of target-force value. This answer is specific to SAP2000 V14.0.0 (June 2009). |
P-Delta effect
How are P-Delta effects handled when suspension-bridge prestressing forces are modeled using tendons?
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" loads, the tension-stiffening effect is not included in formulation. However, the compressive forces which develop within the bridge deck do account for P-Delta effects.
When tendons are modeled as objects, tendon tension-stiffening cancels with deck-section compressive softening, as should be the case for bonded tendons.
Long-term effects
Should tendons be modeled as loads or as objects when creep and shrinkage are evaluated?
Answer: Tendons may be modeled as either loads or as objects when evaluating creep and shrinkage, though modeling as objects should be more accurate since this formulation uses tendon material properties to automatically consider tendon losses due to elastic shortening and long-term effects. When tendons are modeled as loads, these losses must be estimated and then specified on the Tendon Loads form.