Cable Stressing Loads [FEA]

A Cable Stressing Load applies the jacking force of one or more cables to the structure. It converts each cable's Pretension into an equivalent initial-tension load, so that stressing a stay, a hanger, or an external tendon produces the correct axial force in the cable and the corresponding reactions in the anchorages and the rest of the structure.

It is a load-generating parameter set. It adds nothing to the stiffness matrix and no mass; it builds a load vector from the geometry and pretension of the cables it selects.

The magnitude comes from the Pretension parameter on the FE Line itself, not from this object. The Cable Stressing Load decides which cables are stressed, in which case, and what fraction of their pretension is applied. A cable whose Pretension is zero contributes nothing no matter what is set here.

General

Load Case: The analysis case the resulting load belongs to. This is a selection parameter — the generated forces appear only in this case and in any combination that includes it. A stressing load with no case assigned produces no effect anywhere. In staged construction, this is normally the case attached to the stage in which the cable is actually stressed; assigning it to the wrong stage applies the jacking force before or after the structure has the geometry and stiffness to receive it, which changes the force distribution substantially.

Element: The cable to stress — an FE Line, or a group of them. This is a connectivity parameter: it selects the members whose Pretension is converted to load. The referenced line should be a cable-type element with a non-zero Pretension. Pointing at an ordinary beam is not an error but is rarely meaningful.

Define together with Use Active Elements and Equation to Filter Elements, which offer two alternative ways of choosing the same thing; use whichever expresses your intent most directly rather than combining them unnecessarily.

Use Active Elements: When YES, every active cable in the stage is stressed, rather than only the element named above. This is the convenient choice for a stage in which a whole family of stays is tensioned at once, and it keeps working as cables are added to later stages without editing the load. When NO (the default), only the named Element (or group) is used.

Engineering influence. Switching this to YES broadens the load to every currently active cable — including any you did not intend, such as cables that were stressed in an earlier stage and remain active. Re-stressing an already-stressed cable in a later stage double-counts its jacking force. Use the filter below, or explicit elements, when the active set is broader than the set being jacked.

Equation to Filter Elements: An expression evaluated per element to narrow the selection. Use it to stress a subset — one side of a bridge, a numbered range of stays — without building a group for every combination. Leave it empty to apply no filter. An expression that matches nothing produces a load object that silently does nothing, so verify the count of elements actually stressed rather than assuming the filter matched.

Stressing: The proportion of each cable's Pretension applied by this object, entered in this sheet as a percentage.

Enter 100 (or 100%) for full stressing, not 1. The cell is a percentage: the sheet divides what you type by 100 and displays the value back with a % suffix. Entering 1 applies 1% of the Pretension, and entering 0.4 for "40%" applies 0.4% — understating cable force, camber and reactions by a factor of 100, with no error. (The underlying stored parameter is a fraction where 1 = 100%, which is what the agent API and ParamML see; the percentage convention applies to this spreadsheet cell.)

Engineering influence. The applied force is Stressing × Pretension, so the response of a linear model scales linearly with it. Values below 100% represent partial stressing — the natural way to model a multi-stage jacking sequence, where a stay is taken to 40% in one stage and to full force in a later one. Take care that the values across stages express the total intended at each point, not increments, unless that is how you have set the sequence up; getting this wrong is the usual cause of final cable forces that are a multiple or a fraction of the target.

A value of 0 applies no force. Values above 100% over-stress the cable and are legitimate for checking an over-tensioning condition. Negative values reverse the jacking direction, which is not physically meaningful for a cable and will usually put it into compression, where a cable-type element carries no load in a nonlinear analysis.

Results sensitive to this parameter: cable forces, deck profile and camber, anchorage reactions, and in a staged model the locked-in forces carried into every subsequent stage.

Verification

• Check the axial force reported in the stressed cables and compare it with Stressing × Pretension. This is the most direct confirmation the load was generated.
• Confirm the number of elements actually stressed matches your intent, particularly when using Use Active Elements or a filter.
• Check the deflected shape: stressing a stay should lift the deck at the anchorage. No movement means the load was not generated — usually a missing Load Case, a zero Pretension, or a filter matching nothing.
• Check anchorage reactions and confirm the structure is in equilibrium under the jacking force.
• In a staged model, review cable forces stage by stage and confirm they follow the intended jacking sequence rather than jumping to full force early.
• Hand-check a single stay: the vertical component of the jacking force should equal the change in the supported reaction.

Common mistakes

• Zero Pretension on the cable. The stressing load then applies exactly nothing. This has a documented history in this codebase — the parameter was once undeclared, which produced a jacking force of zero with no warning — so confirm the value is actually set on the FE Line.
• Assigning the load to the wrong stage, applying jacking force to a structure that is not yet erected or is already complete.
• Leaving Use Active Elements YES across several stages, re-stressing cables that were already tensioned and multiplying their force.
• Entering 1 in the Stressing cell to mean "full", which applies 1% of the pretension. The cell is a percentage — enter 100.
• Treating Stressing as an increment when the sequence expects a cumulative total, or the reverse.
• Stressing a cable modelled as an ordinary beam, which carries compression and hides the error.
• Forgetting that the load lives in a case — reviewing results in a combination that does not include that case and concluding the stressing did nothing.