Construction Stage Analysis Control |
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A civil structure such as a suspension bridge, cable stayed bridge or PSC (prestressed or post-tensioned concrete) bridge requires separate and yet inter-related analyses for the completed structure and interim structures during the construction. Each temporary structure at a particular stage of construction affects the subsequent stages. Also, it is not uncommon to install and dismantle temporary supports and cables during construction. The structure constantly changes or evolves as the construction progresses with varying material properties such as modulus of elasticity and compressive strength due to different maturities among contiguous members. The structural behaviors such as deflections and stress re-distribution continue to change during and after the construction due to varying time dependent properties such as concrete creep, shrinkage, modulus of elasticity (aging) and tendon relaxation. Since the structural configuration continuously changes with different loading and support conditions, and each construction stage affects the subsequent stages, the design of certain structural components may be governed during the construction. Accordingly, the time dependent construction stage analysis is required to examine each stage of the construction, and without such analysis the analysis for the final stage alone will not be reliable. midas Civil considers the following items for a Construction Stage analysis The procedure for construction stage analysis is shown below.
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From the Main Menu select Analysis > Analysis Control > Construction Stage. |
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Assign a stage to be considered as the Final Stage of the construction stage analysis. Last Stage Other Stage
Restart the construction stage analysis from the stage specified by the user. Restarting the analysis from the modified stage will save time.
Analysis Type Linear Analysis Independent Stage Accumulated Stage Include P-Delta Effect Only Nonlinear Analysis Independent Stage Accumulated Stage Include Equilibrium Element Nodal Forces Include Time Dependent Effect Note 1
Note 2
If "Include Time Dependent Effect" is checked on in the Analysis Option,Click the Creep & Shrinkage Type Creep Convergence for Creep Iteration Number of Iterations: Maximum number of repetitions Tolerance: for convergence Only User's Creep Coefficient Internal Time Steps for Creep Note Auto Time Step Generation for Large Time Gap Note Tendon Tension Loss Effect (Creep & Shrinkage) Consider Re-Bar Confinement Effect Variation of Comp. Strength Tendon Tension Loss Effect (Elastic Shortening) Note Prestressing tension loss in tendons due to elastic deformations is caused by other loadings such as live loads, creep, shrinkage, prestressing other tendons, etc after the prestressing force is applied. Note that it is not the same as the elastic shortening loss, which is one of the instantaneous losses.
Click on the Number of Load Steps: Input the number of load steps for the non linear analysis. Maximum Number of Iterations/Load Step: Maximum number of iterations of analysis per Load Step. Convergence Criteria: Specify the basis on which to assess the convergence. Enter the norm values for Energy (Member force x displacement), displacement and member forces. Note
Click on Number of Iterations: Maximum number of iterations of analysis Convergence Tolerance: Tolerance for convergence
Dead Load is generally the most significant component of all the loads applied to construction stage analysis. The results of all the load cases except for Creep, Shrinkage and Relaxation of Tendons are lumped into CS: Dead Load. Here we can select certain load cases to be distinguished from the Dead Load and produce the results under CS: Erection Load. Add : Add an erection load case to be distinguished from Dead Load for C.S. Output. Modify : Modify an existing erection load case selected from the list. Delete: Delete the selected erection load cases from the list. Load Case Name: Select the Load cases to be distinguished from Dead Load Load Type for CS: Specify a load type classified into CS:Erection, which is differently categorized from CS:Dead. This is helpful in distinguishing the pre-composite dead load from the post-composite long term loads for the Steel Composite design as per AASHTO code. This is effective when the Auto Generation function is used for generating the load combinations. Assignment Load Cases: Select the load cases from the load case list to be classified as erection loads. Note
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Load Case |
Stiffness |
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Linear Static Load cases |
Linear analysis |
Equivalent Truss Element |
Nonlinear analysis |
Elastic Catenary Cable Element |
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Moving Load Case Settlement Load Case Response Spectrum Load Case |
Truss Element |
Apply Initial Member Force to C.S. :
Apply initial forces entered in the Initial Element Forces table to member forces for construction stages. The initial forces are applied to a specific construction stage at which the corresponding elements are first activated. This function is useful when we perform construction stage analysis from a specific construction stage by considering the member forces of the preceding construction stage as initial forces. Initial forces entered during construction stages will not be reflected in the geometric stiffness.
Initial Tangent Displacement for Erected Structures
This function calculates real displacements of the elements, which will be created in the next stage, considering the rotational angles of nodes resulting from each current construction stage. This functionality is used for fabrication cambers for structural steel and precast concrete members.
All: Calculate real displacements for all members.
Group: Calculate real displacements for specific groups.
Lack-of-Fit Force Control
This function relates to forward construction stage analysis for a cable stayed bridge. When a certain cable is about to become activated in a particular construction stage, the end nodes of the cable are subjected to the displacements of the immediately preceding construction stage. In order to install the cable in place, the cable needs to be tensioned to the nodes in the deflected structure. Such imaginary tension force necessary to fit the cable into place is referred to as Lack-of-Fit Force. This value is calculated based on the difference in lengths obtained by projecting the end nodes of the immediately preceding construction stage onto the x-axis of the cable element.
In forward construction stage analysis of a cable stay bridge, the Lack-of-Fit Force plus the pretension obtained from the initial state analysis is applied as pretension in construction stages. This results in the tension forces of the initial state analysis at the final stage without having to analyze backward analysis.
A typical cable stay bridge consists of 3 spans. Such cable stay bridge includes Key Segment closure. Just prior to placing the Key Seg closure, the elements at each end of the Key Seg closure exhibit deflections. Placing the Key Seg in the deflected structure will result in discontinuous deflection and deflected angles, which in turn results in incorrect analysis after the connection, compared to the results of the initial state analysis. For correct analysis. forced displacements at each end of the Key Seg are calculated and then converted into equivalent member forces, which are then applied to the Key Seg at the time of placing the Key Seg. This then leads to the same results at the last stage after connection as that of the initial state analysis. The forced displacements (equivalent forces) applied to the Key Seg are also referred to as Lack-of-Fit Force. This process enables us to perform forward analysis without resorting to finding pretension forces in cables through backward analysis
In order to calculate Lack-of-Fit Force, we need to define a Structure Group for the truss and beam elements for which Lack-of-Fit force will be calculated. We then check on the Lack-of-Fit Force Control and select the Structure Group defined earlier from the combo box.
The results such as member forces or nodal displacements used in calculating Lack-of-Fit force can be checked from Result>Result Tables>Construction Stage>Lack of Fit Force>Beam or Truss.
When we apply Lack-of-Fit Force to all the cable and Key Seg elements in general cable stayed bridge analysis, we can omit the process of calculating the cable tension forces during construction, and forward analysis alone enables us to design a cable stayed bridge.
Note
1. The results of real displacements can be checked from:
Results > Deformations > Deformed Shape > Stage/Step real Displ.:Real displacements by construction stages
Results > General Camber: Camber Graphs are produced.
2. Lack of Fit Force option must be used with Internal Force type under Cable Pretension Force Control function. Cable-Pretension Force Control function is used to control Cable-Pretension for construction stage analysis of a cable stayed bridge. In general, due to force redistribution the resultant cable forces differ from the Pretension Loads entered. External Force type is selected when the user directly enters cable pretension. In that case the program does not change the cable force values and use the user defined values. Hence in order to use pretension obtained from the completed state, select Internal Force type and Lack-of-Fit Force Control to automatically calculate pretension for each stage.
Consider Stress Decrease at Lead Zone by Post-tension
Select a method of computing stresses over a transfer length in a post-tension model. This feature is applicable only when a Transfer Length is entered in the Tendon Profile dialog.
Linear Interpolation: Linearly interpolate stresses from end anchorage zone to stress-free zone
Constant: Stress* : Stresses over a stress-free zone are computed by multiplying stresses, without considering a Transfer Length, by a constant ratio. For example, enter 0.5 to use 50% of stress for the stress over a stress-free zone.
If "Composite Section for Construction Stage" is used, this function cannot be applied.
Beam Section Property Changes
Select whether to consider the presence of tendons for calculating section properties.
Constant: Do not consider the effect of tendon for calculating section properties.
Change with Tendon: Calculate section properties considering the effect of tendons. In case of post-tension, use a net section with duct areas excluded before grouting, and use a transformed section with duct areas included after grouting.
Frame Output
Calculate Concurrent Forces of Frame: corresponding force components of elements during construction stages. When a max or min value of a particular force component (say strong axis moment) is found, the corresponding other force components (say shear and axial forces) are also calculated.
Calculate Output of Each Part of Composite Section: Check on to calculate stresses and forces pertaining to each of the concrete deck part and the girder part. Otherwise, output will be produced for the total composite section.
Self-Constrained Forces & Stresses: Check on to calculate self-restraint forces and stresses due to creep, shrinkage and temperature gradient pertaining to each of the concrete deck part and the girder part. In steel-concrete composite beams, due to compatibility conditions, creep and shrinkage of concrete part of the cross-section (concrete slab) results in a redistribution of stresses. When deformation of shortening happens in the concrete part, the steel part of the cross-section prevents free deformation of concrete. As a result of restrained deformation, tensile stresses appear in concrete slab, Due to equilibrium, compressive stresses in the steel part of cross-section appear as well. Also, these self-restraint stresses of composite sections can occur due to nonlinear temperature gradient in the cross-section. These self-restraint stresses of composite sections can be checked in the Results > Result Tables > Composite Section for C.S. > Self-Constraint Force & Stress.
Remove the conditions for a construction stage analysis. The construction stage analysis is not performed in this case.
Note 1
The following Load Cases are automatically generated when construction stage analysis is completed.
Load Case |
Results |
Description |
1. CS: Dead Load |
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Results due to all loadings excluding Erection Load and the effects of Creep, Shrinkage and Tendon Prestress |
2. CS: Erection Load |
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Results due to dead loads, which are separated from CS: Dead Load, defined in Construction Stage Analysis Control Data dialog |
3. CS : Tendon Primary |
Reaction |
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Deformation |
Deformation resulting from tendon prestress |
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Force |
Member forces resulting from tendon prestress |
4. CS: Tendon Secondary |
Reaction |
Reactions caused by Tendon Prestress in an indeterminate structure. |
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Force |
Member forces caused by Tendon Prestress in an indeterminate structure. |
5. CS: Creep Primary |
Reaction |
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Deformation |
Deformation due to imaginary forces required to cause creep stain |
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Force |
Imaginary forces required to cause creep stain |
6. CS: Creep Secondary |
Reaction |
Reactions caused by creep in an indeterminate structure |
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Force |
Member forces caused by creep in an indeterminate structure |
7. CS: Shrinkage Primary |
Reaction |
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Deformation |
Deformation due to imaginary forces required to cause shrinkage stain |
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Force |
Imaginary forces required to cause shrinkage stain |
8. CS: Shrinkage Secondary |
Reaction |
Reactions caused by shrinkage in an indeterminate structure |
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Force |
Member forces caused by shrinkage in an indeterminate structure |
CS: Summation |
Reaction |
1+2+4+6+8 |
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Deformation |
1+2+3+5+7 |
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Force |
1+2+3+4+6+8 |
Note 2
Tendon Primary (CS) vs. Secondary (CS)
Tendon Primary represents member forces caused by Tendon Prestress forces. Tendon Secondary represents member forces resulting from Tendon Prestress forces acting in an indeterminate structure. To check analysis results, Primary and Secondary can be regarded as internal forces and external forces respectively. For design, however, the program internally recalculates member forces due to Primary considering the translation of neutral axis so as to use them as internal forces, and member forces due to Secondary are used as external forces.
Save Output of Current Stage
From V671, results for the current stage can be saved and printed. In the previous version, output was available only for accumulated forces and stresses.
Q2. If I look to member forces to design an element, should I look to primary or secondary?