Staged construction FAQ

What is the difference between Load Objects and Load Objects if Added?

Extended Question: On the Load Case Data for Construction Stage Analysis form, what is the difference between Load Objects and Load Objects if Added?

Answer: Load Objects will apply the specified loading to all objects in the specified group, regardless of whether or not they were added in the current stage. Load Objects if Added will only load objects which are in the specified group and were added in the current stage.

Why do shear and bending stresses disappear after changing staged-construction section properties?

Answer: For response, please see the Staged-construction operations article.

How are creep, shrinkage, and secondary prestress effects extracted from staged-construction results?

Answer: For statically indeterminate structures, the contribution from individual sources of tendon loss may not be obtained because of the interrelationship among loss components. For example, shrinkage strains may cause stresses which affect creep behavior, therefore their individual contributions may not be separated unless the shrinkage effect on creep is neglected.

For statically determinate structures, creep, shrinkage, and secondary prestress effects may be estimated through post-processing. For example, response may be evaluated both with and without creep consideration, then creep behavior may be estimated from the difference between these results.

A procedure available to both statically determinate and indeterminate structures is as follows:

1. Replace the original staged-construction load case with a sequence of chained staged-construction load cases, as described in the Obtain results for individual stages of a staged-construction load case tutorial.
2. For each effect to be extracted, create a separate load case. Examples include load-application, post-tensioning, creep, and shrinkage effects.
3. Create a load combination to extract the desired forces which result from a single type of loading.

To demonstrate this procedure, assume there is a single staged-construction load case with the following stages:

• Stage 1 – Erect the first set of segments and apply self-weight (duration = 0 days)
• Stage 2 – Apply post-tensioning (duration = 0 days)
• Stage 3 – Create an empty stage in which creep and shrinkage take effect (duration = 7 days)
• Stage 4 – Erect the second set of segments and apply self-weight (duration = 0 days)
• Stage 5 – Apply post-tensioning (duration = 0 days)
• Stage 6 – Create an empty stage in which creep and shrinkage take effect (duration = 7 days)

Replace this set of stages with a chained sequence of six load cases, listed as follows:

• LC1
• LC2
• LC3
• LC4
• LC5
• LC6

To obtain only post-tensioning forces, create the load combination which follows:

• PT-Only = LC2 - LC1 + LC5 - LC4

Similarly, to obtain only creep and shrinkage forces:

• Creep-And-Shrinkage = LC3 - LC2 + LC6 - LC5

How is design done for the individual stages of a staged-construction load case?

Extended Question: I need to do a code check for the steel members within a certain step of a staged-construction load case. The code checker uses a design load combination for checking. Is it possible to define a load combination which includes the results of a certain staged-construction step?

Answer: Each stage or sequence of stages for which design must be performed may be replaced by a standalone staged-construction load case which uses stiffness at the end of the load case for the previous stage. The design load combination may then be created for these new load cases which represent the individual stages of staged-construction analysis.

How are results obtained only for a certain stage of a staged-construction load case?

Extended Question: For each stage, staged-construction results are cumulative, meaning they incorporate response from all previous stages. Without manually post-processing output, how are results obtained for any given stage?

Answer: The staged-construction load case may be replaced by a chained sequence of stage-construction load cases, each using stiffness at the end of the previous load case. Results for a single stage could then be obtained by creating a load combination which includes the current load case with a scale factor of 1, and the previous load case with a scale factor of -1.

For additional details and a sample model, please see the Obtain results for individual stages of a staged-construction load case tutorial.