Construction Stage Analysis Control

Enter the analysis conditions for a Construction Stage analysis. Define the final construction stage and select the analysis features among the features, which reflect the time dependent material properties and geometric nonlinearity. To achieve more accurate results for creep and shrinkage in a long duration construction stage, we may select the option of auto-generating steps.

• midas Gen considers the following items for a Construction Stage analysis:

  • Time dependent material properties

   

  1. Creep effects of concrete members having different maturities

  2. Shrinkage effects of concrete members having different maturities

  3. Time dependent compressive strength gain for concrete members

  4. Prestressing tendon relaxation

   

   

  • Expression of Construction Stages

   

  1. Creation (activation) and deletion (deactivation) of members with certain maturities.

  2. Loading (activation) and unloading (deactivation) of loads relative to time

  3. Change of boundary conditions relative to time

   

• midas Gen follows the procedure noted below for a construction stage analysis reflecting time dependent properties.

  1. Model the structure.

   

  2. Define the time dependent material properties such as creep and shrinkage. The time dependent material properties may be user defined or defined by the standards such as ACI and CEB-FIP. (Time Dependent Material(Creep/Shrinkage), Time Dependent Material(Comp. Strength))

   

  3. Link the defined time dependent material properties to the general material properties through which the change of the concrete member material properties with time are automatically calculated and reflected. (Time Dependent Material Link)

   

  4. Create the construction stages and time steps considering the true sequence of construction. (Define Construction Stage)

   

  5. Define the construction stages using the predefined Element groups, Boundary groups and Load groups. (Define Construction Stage)

   

  6. Specify the desired analysis conditions and carry out the analysis. (Construction Stage Analysis Control, Perform Analysis)

   

  7. Combine the results of the construction stage analysis and that of the completed structure analysis as necessary.

   

   

  Note
  Use Construction Stage Wizard for Building Structure to automatically handle the majority of the tasks pertaining to the steps 4 and 5 above.

The procedure for construction stage analysis is shown below.

From the Main Menu select Analysis > Analysis Control > Construction Stage Analysis Control.

Final Stage

Assign a stage to be considered as the Final Stage of the construction stage analysis.

Last Stage
Assign the last stage as the true last stage.

Other Stage
Assign a stage within the overall construction stages as the final stage.

Analysis Option

Include Non-Linear Analysis

Check to reflect the change of geometric shapes of the geometric nonlinear analysis in the construction stage analysis. Geometric Nonlinear analysis for Independent Stage Model cannot account for time dependent effects simultaneously.  However, with Accumulative Geometric Nonlinear Stage Model, the program not only accounts for time dependent effects but also has an option to select the Pretension Type of cables and calculates tangential displacements while taking Lack of Fit Forces into account.

Independent Stage

Geometric nonlinear analysis is carried out independently in models of each construction stage. This option is used for the backward analysis of a suspension bridge considering large displacement.

Accumulated Stage

Geometric nonlinear analysis is carried out with accumulated effects of the models of each construction stage. This option is used for the forward analysis of a cable stayed bridge considering large displacement.

Include Time Dependent Effect

Check to reflect the time dependent material properties such as the change of modulus of elasticity, creep and shrinkage in the construction stage analysis.

Include Material Nonlinear Analysis
Define the Plastic Material Data and then check on the Material nonlinear option in the Nonlinear Analysis Control dialog to use it on the construction stage analysis Control Data dialog.

Accumulated Stage

Geometric nonlinear analysis is carried out with accumulated effects of the models of each construction stage. This option is used for the forward analysis of a cable stayed bridge considering large displacement.

Non-linear Analysis

If 'Include Non-Linear Analysis' is checked on in the Analysis Option, enter the following information:

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
The selection of the convergence criteria for repeating or ending the analysis must be based on the condition to reflect the effects of various degrees of freedom. For example, in the case of Displacement Norm, if the displacement resulting from the corresponding analysis step is {D1}, and the total displacement accumulated from each step is {D2}, the Norm is expressed as . If this value is smaller than the specified value, a convergence is considered to have occurred and the program stops the iterative analysis.

Cable-Pretension Force Control

Define the method of applying the pretension forces of cable elements.

Internal force: Apply the pretension forces as internal forces.

External force: Apply the pretension forces as external forces.

Add: Add external pretension forces to the pre-existing tension forces of cable elements.

Replace: Replace the pre-existing tension forces of cable elements with applied external pretension forces.

Note
If the initial pretension forces are applied as internal forces, the forces in the cable elements become reduced due to the deformation of the support structure based on its stiffness. If the initial pretension forces are applied as external forces, the forces are treated as external loads to the support structure at the construction stage of pretensioning; hence the forces in the cable elements remain unchanged as the initial pretension forces at the corresponding construction stage.

Composite Section

If the 'Calculate Output of Each part' option is selected, the construction stage analysis results of composite sections by each Part will be produced. The analysis result for each Part can be checked in Result >> Result Tables >> Composite Section for C.S.

Load Cases to be distinguished from Dead Load for CS Output

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
Multiple erection load cases can be defined.

   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 a specific group.

Time Dependent Effect

If 'Include Time Dependent Effect' is checked on in the Analysis Option, enter the following information: Define the material properties related to creep and shrinkage in  Time Dependent Material (Creep/Shrinkage).

Creep & Shrinkage

Type

Select one of the options for considering creep and/or shrinkage.

Creep

Convergence for Creep Iteration

Specify the convergence requirement for ending the repetitive process in the analysis reflecting creep.

Number of Iterations: Maximum number of repetitions

Tolerance: for convergence

Only User's Creep Coefficient

Perform the construction stage analysis only using the creep coefficients entered by the user. Creep coefficients are entered by elements in Creep Coefficient for Construction Stage.

Internal Time Steps for Creep

Specify a number, which is used to divide a construction stage to create internal steps for considering creep.

Note

Internal Steps are applied in the process of analysis, and the corresponding analysis results are not produced.

Auto Time Step Generation for Large Time Gap

Specify a number, which is used to divide a construction stage to create internal steps when the duration of the construction stage is too long.

Note

Internal Steps are applied in the process of analysis, and the corresponding analysis results are not produced.

Tendon Tension Loss Effect (Creep & Shrinkage)

Check on to reflect the effect of prestressing tension loss of tendons due to creep and shrinkage. Define the tendon properties of the prestressing loss in "Tendon Property".

Variation of Comp. Strength

Check on to reflect the change of modulus of elasticity related to the change of compressive strength of concrete. The maturity variant property is defined in "Time Dependent Material (Comp. Strength)".

Tendon Tension Loss Effect (Elastic Shortening)

Check on to reflect the prestressing tension loss of tendons due to the elastic deformations of concrete.

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.

: 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.

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.

 Revision of Gen 2014 (v1.1)

Q1. There are construction stage load cases, namely, “creep primary”, “creep secondary”, “shrinkage primary” and “shrinkage secondary”. What are the meaning of “primary” and “secondary”, their differences and application?