Using the UBC 1997 seismic wizard

Tekla Structural Designer
2021
Tekla Structural Designer

Using the UBC 1997 seismic wizard

The Seismic Wizard guides you through the process of defining the information that is required in order to calculate the seismic loading on the structure in accordance with the Uniform Building Code.

This section runs through each page of the wizard and discusses the various options.

Starting the Wizard

To initiate the wizard:
  1. Click Home > Model Settings > Design Codes
    1. Set the Design Code for Seismic Loading to UBC
    2. Click OK
  2. Click Load > Seismic Wizard...

The Seismic Wizard will start, and you can use its pages to define the necessary information. The first page displayed is Site Specific Spectra.

Site Specific Spectra page

You can choose the analysis procedure to be used, either:

Property Description
Code Spectra Choose this option to use the UBC Horizontal Design Spectrum.
Site Specific Spectra This option is appropriate for locations which use another country's loading and design codes where the code spectrum is not relevant.

For this option you create a user defined spectrum by specifying the required curve parameters and slope options.

See: Code spectra and site specific spectra.

Next

Clicking Next takes you to the Basic information page described below.

Note:

No vertical spectrum is considered in the current release.

Basic information page

The Seismic Design Category (SDC) is determined on this page.

Note:

In ASCE 7-10 (not ASCE 7-05) there is an additional value of Cs calculated for a building < 5 stories and where T <0.5s. In this instance Ss is taken as 1.5 and the additional value of Cs should be used to give a final value of Cs. This clause has not been implemented in Tekla Structural Designer.

Property Description
Structure details
Height to the highest level This field defaults to the structure height (calculated from the base to the highest point on the structure).
Ignore seismic in floor (and below) Only floors above this level are considered when the seismic weight combination is determined.
Number of stories This field defaults to the number of ticked floors in the Construction levels dialog less the ones ignored in field above.
Soil Profile Occupancy
S - Soil Profile Type This field allows you to set the appropriate soil conditions, A-E - (A - Hard rock, B - Rock, C - Very dense soil/soft rock, D - Stiff soil, E - Soft soil. As defined in UBC97 Table 16-J.
Occupancy Category This field allows you to set the appropriate occupancy category, I-IV. As defined in UBC97 Table 16-K.
Importance Factor, I This is automatically derived from the occupancy category.
Override checkbox Checking this box allows you to override the importance factor, (some countries require different Importance Factors to those given in the US codes).
Seismic Zone
Seismic Zone This field allows you to set the appropriate zone, I-4. As defined in UBC97 Figure 16-2
Z - Seismic Zone Factor This is automatically derived for the seismic zone.
Near Source (for Seismic Zone 4 only)
Seismic Source Type This field allows you to set the appropriate source type, A-C.
Distance to Seismic Source The distance to the seismic source.
Next Clicking Next takes you to the Structure Irregularities page described below.

Structure Irregularities page

For SDC types B to F this page is used to indicate any irregularities in plan or vertically.

Property Description
Structure Plan Irregularities Check the appropriate boxes to define any plan irregularities.
Structure Vertical Irregularities Check the appropriate boxes to define any vertical irregularities.
Analysis procedure to be used Having specified any irregularities, you then choose the analysis procedure.
  • Use Static Force Procedure
  • Use Modal Response Spectrum Analysis
Note: Based on the irregularities you have defined, one or both of the above methods may be unavailable.

Any additional information, assumptions or warnings applicable to the selected a method are displayed for information.

Next Clicking Next takes you to the Fundamental Period page described below.

Fundamental Period page

For SDC types B to F this page is used to determine the fundamental period, TDir 1 and TDir 2.

Property Description
Fundamental Period Definition
Use approx fundamental period Ta This is automatically derived.

If RSA was selected on the previous page this option is dimmed.

User defined fundamental period If RSA was selected on the previous page this option is dimmed.
Use modal analysis Modal analysis is run to determine fundamental periods of the structure in Dir1 and Dir2.
Fundamental Period Dir 1 and Dir 2
Structure Type Select from Steel moment resisting frames, Concrete moment resisting frames, Eccentrically braced steel frames or All other structural systems
Approx fundamental period, Ta This is automatically derived.
Fundamental period, TDir 1, TDir 2 Depending on your choice of definition this will either be taken as Ta, or it can be a user defined value, or it will be calculated from the modal analysis.
Next Clicking Next takes you to the Seismic Force Resisting System page described below.

Seismic Force Resisting System page

This page is used to determine the response modification coefficient, R, and other factors in the directions 1 and 2.

Property Description
Seismic force resisting system Dir 1 and Dir 2
System This field allows you to set the appropriate system, (eg. bearing wall, building frame, moment resisting frame etc.) from UBC97 Table 16-N.
Type This field allows you to set the appropriate type for the chosen system from UBC97 Table 16-N.
Coefficients & Factors Dir 1 and Dir 2
Response modification coefficient, R This is automatically derived, but you are given the facility to edit the calculated value.
System over-strength factor This is automatically derived, but you are given the facility to edit the calculated value.
Redundancy factor drift, ρdrift This factor has to be entered manually.
Redundancy factor, ρ This factor has to be entered manually.
Next Clicking Next takes you to the Effective Seismic Weight page described below.

Effective Seismic Weight page

This page is used to determine the Structure Seismic Dead Weight.

You should include those loadcases that you want to contribute to the Structure Seismic Dead Weight.

Note:

The effective seismic weight combination is used to develop the seismic design loading and is not used in any analysis of the structure.

Clicking Next takes you to the Localization page described below.

Localization page

On this page you can:

  • choose whether to perform seismic drift checks, and set the allowable story drift factor to be used
  • define appropriate localization parameters for using the UBC code outside of the US
Property Description
Skip seismic drift checks - Check this box if you do not require seismic drift checks to be performed.
Structure Type Select the structure type for determining the allowable story drift factor.
Allowable Story Drift Factor The factor to be used when calculating the allowable story drift is varies according to the Structure Type. Click 'override' if you want to enter your own value.
Include localization - Check this box in order to display the below 'ELF & RSA' and 'RSA only' localization settings.
SFP & RSA
Scale Factor between elastic and design spectra (Dir1 and Dir2) This field allows you to override the code value for the scale factor, (UBC code value = 1/R).
User Defined Static Base Shear (Dir1 and Dir2) This field allows you to override the code value for the static base shear, (UBC code value V = Cv x I x W / (R x T)).
% eccentricity for accidental torsion (Dir1 and Dir2) This field allows you to override the code eccentricity for accidental torsion, (UBC code value = 5%).
Note: The same value is applied to all floors in the structure.
Vertical seismic load effect combination factor This field allows you to override the combination factor for vertical seismic loading (usually a small additional factor for the dead loads in the combination), (UBC factor = 0.5 x Ca x I x Dead loads for strength design and 0.0 for allowable stress design).
RSA only (only available if RSA has been selected)
% Mass participation for RSA This field allows you to set your own required % mass participation, (UBC code value = 90%).
Note: This overrides the setting in Analysis Options/1st order seismic when running the Seismic Wizard.
Note: It is not usually possible to achieve 100% mass participation in a modal analysis so max value is 99%.
Note: This setting can benefit some users wanting nearer 100% to be conservative.
Scaling of forces (% of Base Shear) (Dir1 and Dir2) This field allows you to set your required scaling factors,

UBC code values:

Code ground motion = 90% x V / VDesign

Site specific ground motion = 80% x V / VDesign

Irregular structures = 100% x V / VDesign

Scaling of Deflections for Drift Checks (% of Base Shear) (Dir1 and Dir2) This field allows you to set your required scaling factors,

UBC code values:

Code ground motion = 90% x V / VDesign

Site specific ground motion = 80% x V / VDesign

Irregular structures = 100% x V / VDesign

Note: A zero factor means no scaling will be performed on deflections.
Finish Click Finish to automatically generate the Seismic loadcases and open the UBC Seismic Combination Generator described below.

UBC Seismic Combination Generator

The Combination Generator sets up seismic combinations from the following UBC 1997 "core" combinations that include seismic loads.

ASD - Seismic Combinations

5) 1.0D + 0.714E

6) 1.0D + 0.75L + 0.536E + 0.75S

8) 0.6D + 0.714E

Note:

In ASCE, in the above 0.714 equates to 0.7 and 0.536 equates to 0.525. Tekla Structural Designer utilizes the ASCE factors - you can however change them if required.

LRFD - Seismic Combinations

5) 1.2D + 1.0E + 1.0L + 0.2S

7) 0.9D + 1.0E

(Where E is the seismic component of the loading in the combination.)

Property Description
Initial Parameters page
Delete / Replace combinations Select whether to:
  • Delete all previously generated combinations
  • Replace only combinations generated by this run
Scenario If you need to generate combinations for a range of different scenarios, ensure you type in (or choose from the droplist if appropriate) a scenario name. This scenario name is then added as a prefix to all the combinations generated in the this run of the generator. You can then rerun the generator to create additional scenarios.
Clicking Next takes you to the Combinations page described below.
Combinations page
Generate Select the seismic combinations to be generated with factors as specified.
Keep existing Seismic Combinations If you want to retain previously set up Seismic Combinations you can select this checkbox to do so.
Note: Warning - some factors may no longer be correct for these combinations after rerunning the Wizard.
Next Click Next to specify the service factors.
Finish Click Finish to generate the Seismic combinations.
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