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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.

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: 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.

Formulation

What equation is used to apply curvature and wobble coefficients?

Answer: Frictional losses due to curvature and wobble effects are calculated in terms of the tendon force P_x at distance x from the jacking end, expressed as follows:

where:

• P₀ = prestressing force at the jacking end
• P_x = 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.

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.

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:
  
  
  1. Specify the desired tendon jacking forces through Assign > Tendon Loads > Tendon Force/Stress.
  2. Run analysis, check tendon forces, and adjust tendon jacking forces to compensate for elastic shortening.
  3. Repeat this last step until the desired tendon forces are obtained.
• Target force. Elastic-shortening losses may also be disregarded using tendon target force.

P-Delta effect

How are P-Delta effects handled when suspension-bridge prestressing forces are modeled using tendons?

Answer: When tendons are modeled as 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.