Overview of combined analysis and member design

Tekla Structural Designer
2021
Tekla Structural Designer

Overview of combined analysis and member design

After creating and loading a model, you can proceed directly to a static design to rapidly design every steel and/or concrete member and wall for active gravity, static and ELF seismic load combinations.

For larger steel models you might potentially find it more efficient to adopt a two step process consisting of a gravity design followed by a static design.

If you have set up RSA load combinations via the Seismic Wizard these are catered for by running an RSA design.

Gravity Design

Gravity Design uses the results from a 3D analysis to design* every steel and concrete member and wall in the model for all active gravity combinations. (For steel and composite members, this includes both “gravity only” and “gravity and lateral” members).

The 3D analysis can be either first or second order, depending on the settings specified on the Analysis page in Design Settings.

Note: If working to the US head code and applying the The Direct Analysis Method refer to the suggested workflow for guidance on setting the analysis type appropriately.

Although your final design should consider all combinations, designing for gravity combinations only can be a useful way to rapidly pre-size the members in the model that are not subjected to lateral loads.

Because you may not have created lateral systems at this stage, the column nodes that are not in a rigid floor diaphragm are, by default, fixed horizontally. To change the default setting for this go to the General page in Design Settings.

Once members have been sized acceptably for gravity combinations, a Static Design can then be run to design the remaining combinations.

* Each member is either designed or checked according to its individual Autodesign setting.

Gravity Design Process

The processes performed when a Gravity design is run can be summarized as follows:

Process Description
Model validation Model validation checks are performed to detect specific issues before the analysis and design process begins.
3D pre-analysis processes In preparation for analysis a number of pre-analysis processes are undertaken as necessary, such as:
  • Decomposing slab and wall loads
  • Preparing loadcases and combinations
  • Meshing and diaphragms
  • Applying loads
  • Resolving vertical loads for application of global imperfections

The actual pre-analysis processes performed will vary depending on the specific model that has been defined.

3D analysis

A traditional frame analysis of the entire 3D model, with an option to mesh floors. This analysis generates a first set of results that can be used for the design of beams, columns, walls and slabs.

You can control whether a first, or a second order 3D Analysis is run by making the appropriate selection on the Analysis page in Design Settings. The actual options that are presented will vary depending on the design code being worked to.

The choice of linear or non-linear analysis is made automatically:

  • if the model has entirely linear properties a linear analysis is performed,
  • else if any non-linear properties are detected a non-linear analysis is performed.
Member Design Members are designed in accordance with the selected gravity design command:
  • Design Steel (Gravity) - designs or checks “gravity only” and “gravity and lateral” steel and composite members
  • Design Concrete (Gravity) - designs or checks concrete beams, columns and walls
  • Design All (Gravity) - designs or checks “gravity only” and “gravity and lateral” steel and composite members, plus concrete beams, columns and walls
Stability checks Sway/Drift checks and Wind Drift checks are performed for all columns and walls, (apart from any that have been manually excluded from the check

Static Design

Static Design uses the results from a 3D analysis to design* every steel and concrete member and wall in the model for all active gravity, lateral and ELF seismic combinations. The 3D analysis can be either first or second order, depending on the settings specified on the Analysis page in Design Settings.

Note: If working to the US head code and applying the The Direct Analysis Method refer to the suggested workflow for guidance on setting the analysis type appropriately.

If the model contains concrete members the results from Grillage and FE chasedown analyses are also used to establish a second and third set of design forces for those members. FE chasedown results are only generated if the model contains FE meshed slabs.

You can use Design Settings to turn off 'Design for FE Chasedown analysis results' for a specific concrete member type if required. This option is located under Concrete > Cast-in-place, then Beam, Column, or Wall > General Parameters as appropriate.

Members are designed for the appropriate design requirements as follows:

  • “gravity only” members for all active gravity combinations, (For more information, see: Design individual members for gravity only)
  • other members and walls for all active gravity, lateral and seismic combinations,
  • any members or walls defined as being in Seismic Load Resisting Frames, that are subject to seismic combinations (as opposed to seismic RSA combinations) and that meet the relevant criteria for the appropriate seismic provisions.

* Each member is either designed or checked according to its individual Autodesign setting.

Note: Additional considerations may be necessary when designing members and walls for seismic analysis. For further details, see Seismic design methods in the Seismic analysis and design handbook.

Static Design Process

The main steps in the Static Design process are summarized below:

Process Description
Model validation Model validation checks are performed to detect specific issues before the analysis and design process begins.
3D pre-analysis processes In preparation for analysis a number of pre-analysis processes are undertaken as necessary, such as:
  • Decomposing slab and wall loads
  • Preparing loadcases and combinations
  • Meshing and diaphragms
  • Applying loads
  • Resolving vertical loads for application of global imperfections

The actual pre-analysis processes performed will vary depending on the specific model that has been defined.

3D analysis

A traditional frame analysis of the entire 3D model, with an option to mesh floors. This analysis generates a first set of results that can be used for the design of beams, columns, walls and slabs.

You can control whether a first, or a second order 3D Analysis is run by making the appropriate selection on the Analysis page in Design Settings. The actual options that are presented will vary depending on the design code being worked to.

The choice of linear or non-linear analysis is made automatically:

  • if the model has entirely linear properties a linear analysis is performed,
  • else if any non-linear properties are detected a non-linear analysis is performed.
FE chasedown An FE chasedown analysis is only performed if required, see: FE chasedown
Grillage chasedown A Grillage chasedown analysis is only performed if required, see: Grillage chasedown
Member Design Members are designed in accordance with the selected static design command:
  • Design Steel (Static) - designs or checks “gravity only” and “gravity and lateral” steel and composite members
  • Design Concrete (Static) - designs or checks concrete beams, columns and walls
  • Design All (Static) - designs or checks “gravity only” and “gravity and lateral” steel and composite members, plus concrete beams, columns and walls
Stability checks Sway/Drift checks and Wind Drift checks are performed for all columns and walls, (apart from any that have been manually excluded from the check

RSA Design

RSA Design is used to design* any members defined as being in Seismic Load Resisting Frames, that are subject to seismic RSA combinations.

RSA load combinations must be set up in advance of RSA Design by running the Seismic Wizard (choosing the option to use Modal Response Spectrum Analysis).

Note: Additional considerations may be necessary when designing members and walls for seismic analysis, see Seismic design methods in the Seismic analysis and design handbook.

* Each member is either designed or checked according to its individual Autodesign setting.

Choice of Design Steel, Design Concrete, or Design All

The appropriate design command depends on the materials you have used in the model.

In simple terms:

  • If your model consists of steel members only, you can run the design by using the Design Steel commands.

  • If your model consists of concrete members only, you can run the design by using the Design Concrete commands.

  • If your model consists of a mix of both concrete and steel members, you can run the design by using the Design All commands.

Tip: For structures that are mostly steel but have a few concrete members: instead of running Design All, you could run Design Steel (in order to focus on the steel design) before switching to Design Concrete for the remaining members.

In this way during the steel design phase you are not running grillage and FE chasedown analyses when they are not required .

For more details on how each command affects the analysis process, see the following:

Design steel

  • Performs a 3D analysis.

  • Does not perform grillage chasedown or FE chasedown analysis.

  • Designs or checks all steel elements.

  • Does not design or check concrete beams, columns or shear walls.

Design concrete

  • Performs a 3D analysis and a grillage chasedown analysis.

  • May also be required to perform an FE chasedown analysis.

  • Designs or checks all concrete beams, columns and shear walls.

  • Does not design or check steel elements.

Design all

  • Performs a 3D analysis and a grillage chasedown analysis.

  • May also be required to perform an FE chasedown analysis.

  • Designs or checks all concrete beams, columns and shear walls.

  • Designs or checks all steel elements.

Design force sets used for steel and concrete member design

Steel Member Design Forces

The 3D analysis results are the only results set used in steel member design.

Concrete Member Design Forces

Up to three sets of analysis results will exist for concrete member design as follows:

  • 3D analysis results will always be used to design all beams, columns and walls.
  • Grillage chasedown results will exist for gravity loadcases if the model contains any concrete beams, in which case they will also be used to design all beams, columns and walls.
  • FE chasedown results for gravity loadcases will also exist if the model contains 2-way spanning slabs.

    By default these results will also used for concrete beam, column and wall design.

    Note: You can use Design Settings to turn off 'Design for FE Chasedown analysis results' for concrete beams, columns or walls if required. This option is located under Concrete > Cast-in-place, then Beam/Column/Wall > General Parameters (as appropriate).
Oliko tästä apua?
Edellinen
Seuraava