Construction Stage Analysis
Contrary to a conventional analysis method in which a structural model is analyzed as a whole while all loads applied at once, a construction stage analysis (i.e., construction sequence analysis) considers changes in geometry, material and loads during different phases of construction.
This kind of analysis method is selected for certain type of structures in which member forces and structural displacements accumulate as construction of structure progress. For instance, concrete structures may exhibit differential axial shortening during construction and this might induce additional moments into frames. For larger loads and/or for tall buildings, these moments can be substantial. In another example, a conventional analysis may produce maximum vertical displacements under dead loads at upper levels, but this might be also in error since such vertical displacements at upper levels may be negligible as construction of structure progress.
The program carries out a construction stage analysis by running each stage separately. For each stage, the program redefines geometry of analytical model, tracks loads on members added in stage, update material properties of concrete members (if time-dependent material effects are considered), and finally performs analysis. A final set of results after the last stage analyzed is produced at the end of analysis, which includes results from all stages analyzed. (note that the program does not provide results for individual stages).
To this end, it is essential that members/diaphragms should be assigned with stage numbers properly, so that a valid analytical model is ready for staged construction analysis. While the program runs a final check to confirm validity of model, it is still engineer's responsibility to make sure that a valid analytical model is created for stage construction analysis. The program provides two-tools to help engineers visually to check model before analysis: see Construction Stage Numbers and Show Stages commands, which are available on toolbar menu.
- Define stage numbers for diaphragms: Use command to assign stage numbers for diaphragms. This command opens the Stage and Duration dialog where diaphragm stage numbers and stage duration information can be set. Diaphragm stage numbers can be automatically generated by invoking Generate Stages command in the dialog (or they can be manually assigned).
- Define stage numbers for members: The program automatically assigns stage numbers to members upon closing the Stage and Duration dialog. This is referred to as the global setting for members. It is also possible to override the global setting and assign stage numbers manually to members. To do this, use command, which opens the Assign-Stage Number dialog. If gravity columns or walls are included in analysis ( see the option in the Story Diaphragm dialog for Gravity Members supporting two-way decks ), it is also possible to assign stage numbers to them.
- In case of considering
time-dependent material changes for concrete members, the followings should be
defined before analysis:
- Stage duration information: It is defined in days and it can be set in Stage and Duration dialog. Duration information is used to calculate creep and shrinkage deformations accumulated over the time period of stage duration. In addition, the program can consider changes in concrete elastic modulus over time.
- Concrete material models: opens the Concrete Material Models dialog, which shows available concrete material models with default settings. It is possible to create additional material models accompanied with a unique label.
- Assign concrete material models to members and to diaphragms: command opens the Assign-Concrete Properties dialog. This is used to assign concrete material models to concrete members such as columns, beams, walls, braces and diaphragms. For those not assigned, the program uses the default material models for them.
- Create Construction Stage Analysis load case: command opens the Load Cases dialog. Here, a load case for construction stage analysis can be created. Two options are provided for a construction stage load case: stage analysis with or without time-dependent effects.
For each stage, the program reconstructs analytical model with loads applied only in that stage. The first analysis considers instantaneous (elastic) deformations in members due to the applied loads. If time-dependent material effects for concrete members are included, the program performs a second analysis for the same stage, in which only such effects are accounted for. Then, instantaneous deformations are superposed with those from the second analysis, and these summed results constitute total deformations of the stage.
The second analysis in each stage is intended only to account for effects of time-dependent material changes observed in concrete members. It is assumed that structure does not change and loads on members stay constant and they do not produce any additional deformations while the second analysis is conducted.
- Changing in concrete elastic modulus over time: As concrete hardens, its material properties change over time. These changes impact concrete compressive strength and hence, its elastic modulus. In the first analysis of each stage, the program updates elastic modulus of concrete members based on age their ages if the option to consider stiffness changes is selected (i.e., changing concrete modulus of elasticity). Otherwise, the program uses nominal elastic modulus for concrete members.
- Creep-related deformations: Concrete members continue deforming over considerable length of time under sustained loads. Creep effects are considered in the second analysis of each stage. To this end, the program calculates creep-related deformations for each concrete member first, convert them to equivalent fixed-end-forces which produce axial and bending effects and finally, apply them to member during the second analysis. If the option to consider stiffness changes is selected (i.e., changing concrete modulus of elasticity), the program uses an age-adjusted elastic modulus (AAEM) for concrete members in the second analysis. Otherwise, the program uses nominal elastic modulus for concrete members.
- Shrinkage deformations: It is related to volumetric changes in concrete members due to loss of moisture content. Shrinkage effects are considered in the second analysis of each stage. The program calculates shrinkage related deformations for each concrete member first, convert them to equivalent fixed-end-forces which produce only axial effects and finally, apply them to member during the second analysis. If the option to consider stiffness changes is selected (i.e., changing concrete modulus of elasticity), the program uses an age-adjusted elastic modulus (AAEM) for concrete members in the second analysis. Otherwise, the program uses nominal elastic modulus for concrete members.
It is important to mention that the program does not explicitly consider the behavior of cracking in concrete members in creep and shrinkage calculations. However, they are implicitly included with crack factors as defined in RAM Modeler, which are applied to members during construction stage analysis.
It is also important to note that the program considers only dead loads (including member self-weights) during construction stage analysis. P-Delta effects are ignored and tension-only members are not included for staged construction analysis.