New - Staged construction analysis and design

Tekla Structural Designer
2024
Tekla Structural Designer

New - Staged construction analysis and design

Newly introduced in Tekla Structural Designer 2024 - building upon Tekla Structural Designer's existing automated analysis and design features, staged construction analysis (SCA) and design is now supported.

This exciting development allows the analysis to recognize the changing state of a structure over time, which can potentially result in a more economic design.

While staged construction might appear to be a complex process, you should find that the way it has been implemented in Tekla Structural Designer makes it simple to use.

The information below is intended only to give a brief overview of this new analysis, the general workflow and main new settings associated with it. For comprehensive details and guidance, please refer to the new Help Topic Staged construction handbook

Background

We start as always with an appreciation that all analysis is approximate. Next, that this is especially so for reinforced concrete structures, due to a number of factors, principal among them being:

  1. The complex nature of the material behavior which involves cracking and creep and is dependent on the time of loading - both in terms of when it is applied and its duration.

  2. How the building is constructed; typically in stages of some discrete duration based on levels.

The first factor is generally addressed with the long established features of stiffness modification factors and associated cracked/ uncracked/ partially cracked status.

For the second factor, the “Building Analysis'” (BA) of the entire final structure for all load cases (self-weight, dead, imposed/ live etc) occurring simultaneously in various combinations can be inaccurate for buildings, especially tall/ massive ones, constructed in stages. In this case some amount of deflection, such as that resulting from axial shortening (of columns and walls) and displacement of transfer structures due to the self weight and construction loads, tends to be ‘constructed out’ to a large degree as successive new stories are built, with floors at the prescribed level throughout, on top of older stories. This behavior can lead to a quite different set of results, the degree of difference depending on the proportion of the self-weight+construction load to the entire load, together with the manner, duration and timing of loading and construction.

Such construction related deflection effects are discussed in this long standing Help Topic Why chasedown analyses are included in Analyze All (Static). Tekla Structural Designer has always included chasedown analysis for this reason - that together the BA and Chasedown analyses produce an upper and lower bound range of results that have traditionally been used for concrete building design and which inherently account for this typical staged manner of construction while being efficient and conservative. More rigorous staged construction analysis results would generally sit within this range, rendering this additional, more complex and potentially time consuming process unnecessary. In addition, there would typically be a significant degree of uncertainty involved in such an analysis, such as in the timing and duration of loading and stages. This means it can be difficult to rely on a single such analysis - a range of analysis results generated by multiple runs might need to be considered, further extending the complexity and duration of the analysis and design process.

Nevertheless, there has been a desire among our users for some cases to have the option of a more rigorous and direct assessment of staged construction effects, potentially giving greater accuracy and economy of materials. This is answered in this release by the new Staged Construction Analysis feature, which takes account of the effects discussed above in a rigorous and comprehensive analytical manner. Furthermore, we have striven to make the new process transparent, simple to use and efficient.

Scope

  • Analysis - While the above discusses concrete buildings, SCA can be used for the analysis of buildings of any material/ mix of materials.

    • The SCA (static) Analysis can be any of; 1st & 2nd Order linear, 1st & 2nd Order Nonlinear. The static part of Seismic RSA analysis can also come from an SCA analysis.

  • Design:

    • Concrete - concrete design (of members, walls, slabs and foundations) will consider SCA design forces for all Regional Codes

    • Steel - in the first release steel design will consider SCA only for the Eurocode (all countries) Regional code.

    • Design Using Tekla Tedds - design using Tekla Tedds will consider SCA results for all of; precast concrete members (Eurocode only), timber/ wood members (Eurocode and United States regional codes only) and masonry walls (United States regional code only).

  • (For further scope details see the Limitations and Assumptions section of the Staged construction handbook)

Benefits of SCA

  • Firstly, from the above discussion, there need be no concerns that previous designs need be revisited on the basis that they might be unsafe without consideration of results from SCA.

  • However there might be a concern that designs could be more economic by considering a less extreme “envelope” of design forces. With SCA the envelope can now be tightened up by replacing the BA results with those from SCA giving potential for more economic design and reduced material use.

  • Using the recommended approach, where both SCA and chasedown results are considered, concern about the need to consider multiple SCA scenarios can be avoided, saving design time.

  • SCA considers different concrete stiffness modifiers for different loading conditions - analysis of wind and seismic combinations can use relevant short term stiffness adjustments. This means reduced 2nd order effects under these conditions offering potential for more economic design and reduced material use.

  • Engineers have sometimes maintained separate models for the consideration of wind and/or seismic drift checks - this can now be achieved in a single model again saving design time (see below for more on this).

  • 2nd order effects for gravity combinations may also be reduced offering further benefits for economic design and reduced material use.

  • There is also the option to consider SCA in isolation or SCA along with BA (i.e. excluding chasedown results) which, we believe, would more closely emulate design capabilities in some other software (note however that we do not recommend this route without a clear understanding of the chasedown procedures in Tekla Structural Designer).

Basic Workflow and New Settings

New Setting - the key new setting that controls the running of staged construction analysis and its use in design is in the Design Setting dialog headed “Staged Construction Analysis” as shown in the picture below.

  • Simply enable the “Include staged construction analysis” setting to include SCA as part of the design process

    • Then indicate with the subsequent check boxes which analysis types the results of SCA should be used in place of. By default the “3D building analysis” option is set off. Optionally you can also disable use of either or both of the chasedown analyses - with all other analysis types disabled, only the SCA results are used for design*.

      *Analysis types required for other design processes are still run (as part of Design and Analyze All (Static)), such as for the sway/ drift checks (Building analysis) and slab design (FE chasedown).

    • Note the highlighted text in the above dialog regarding steel design.

Basic Workflow Steps

  1. Create and load the model as usual.

  2. Review the default (automatically created) construction stages and edit if necessary.

  3. Create a staged construction load combination.

  4. Review concrete modification factors.

  5. Enable and configure SCA in Design Settings > Analysis (as discussed above).

  6. Run overall design via the Design Ribbon options, review results and output as usual.

Brief Detail

  • Review construction stages - stages are automatically created for both existing and new models based on levels in the model set as floors. For more explanation and guidance see the Staged construction handbook.

    Stages are reviewed/ edited via the new Review View > Show Alter/State > Staged Construction attribute view, as shown below. You set the view [M]ode to enable graphical adding, removal or editing of entities in a stage.

    The Review view has a handy animate feature making it very easy to review stages, especially for larger models, as shown below:

  • Review/ edit construction stage durations - a tabular dialog of the stage data can be opened for review and edit as shown below - note particularly the editable Stage Duration and Stage Time calculated from these.

    • While we expect the automatic default assignment of entities to stages shown above will be appropriate in many cases/ buildings, it is likely that the actual stage durations may be markedly different to the default of 4 weeks for every stage. We therefore recommend that serious consideration is always given to the stage durations for your particular project and that you edit the default durations to appropriate values.

  • Create a staged construction load combination - this is a specific new Load Combination Class “Staged Construction” as shown in the picture below. This combination describes the load that will be applied at each construction stage and must be defined for SCA to be run and used in design.

    • Once added, you then configure the staged construction combination for; Included load cases, Number of stages in Recent stages, factors for Recent and Older stages and optional allowances for construction loading for both Recent and Older stages - for more explanation and guidance see the Staged construction handbook.

  • Review concrete modification factors - there is a comprehensive set of new modification factors specific to SCA as shown in the picture below.

    • All the factors have sensible default values which you are recommended to review and edit as you require.

    • Note in particular that there are new modification factor sets specific to Design for both Persistent and Transient loading at both Strength and Service levels, as well as for Seismic Loading. These modification factors are automatically used as appropriate in the staged construction analysis of combinations, which themselves are automatically classified as Persistent or Transient based on their constituent Load case types. For more explanation and guidance see the Staged construction handbook.

    • You may also wish to review/ edit the default Building Analysis modification factors especially for Wind/ Seismic (ELF) drift checks - see below for more on this.

  • Enable and configure SCA in Design Settings - see above.

  • Design - For the running of design, results review and output etc there is no change to the process of previous releases. With design for SCA enabled, the design process automatically considers the SCA results - when these govern design it is noted in the design details in the usual manner, as illustrated in the picture below:

Other Aspects of SCA

  • Direct SCA - although not required as part of the workflow, once the process discussed above has been carried out, SCA can be run directly via the buttons in the new “Staged Construction Analysis” group on the Analyze tab of the ribbon, as shown below.

    Following this analysis (and also following the design process outlined above), the SCA results can be viewed in the same manner as for the other analysis types, as shown in the picture below. This also applies to all the usual reporting options for analysis results.

  • Drift Checks for Concrete Buildings for Transient Loading

    As part of the overall design process, in all previous releases the Building Analysis (BA) results were used both for Sway and Drift checks AND for member Design for all combinations at both strength and service levels. The default BA modification factors are set for determination of second-order effects and so reflect long term properties, as described in this Help Topic Use of modification factors. This meant that, by default, the displacements used for the drift checks for transient loading - Wind and Seismic (ELF) - would be overestimated. To obtain more realistic deflections and hence drift checks, it was necessary to run a second analysis process after adjusting the Building Analysis modification factor values to reflect short term properties (similar to the default values for Modal Analysis (and RSA)). The new SCA now enables assessment of drift using analysis with short term properties and member design from analysis using long term properties in a single design process as follows:

    1. Enable SCA for design as described above:

      • Note that any loading can be excluded from SCA (and it can even be set to have a single stage) if you are not concerned about the construction stage effects on results (this is not a requirement or a recommendation).

      • This means that the analysis for Gravity design uses the relevant “persistent loading - strength” modification factors and hence analysis results at the relevant level of 2nd order effects.

      • An added benefit is that analysis for ULS design of (transient classed) wind combinations uses the new transient modification factors and hence potentially reduced 2nd order effects.

    2. Edit the Building Analysis modification factors to values relevant for wind (or seismic) assessments* then run a single design process.
      • *The recommendation is that these are reset to be the same as those for SCA “transient loading-service” or “seismic loading”
        • Note that by default these factors are the same (based on ACI guidance).

      • Both Wind and Seismic (ELF) drift checks are based on the analysis of wind loads in isolation. SCA does not (cannot logically) solve individual loadcases, so the loadcase results used for wind drift continue to derive from the Building Analysis. With the adjusted factors these checks will be less onerous (as desired).

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