Seismic loadcases

On completion of the Seismic Wizard process, seismic loadcases are automatically created and applied in the two building directions, Dir1 and Dir2. Seismic torsion loadcases are also created.

Equivalent Lateral Force (ELF) Loadcases

For structures not requiring application of accidental seismic torsion (eg ASCE7 SDC A)

The following lateral seismic loadcases are created:

Seismic Dir1 - loadcase type: Seismic
Seismic Dir2 - loadcase type: Seismic

For all other structures:

The following lateral seismic loadcases are created:

Seismic Dir1 - loadcase type: Seismic
Seismic Dir2 - loadcase type: Seismic

The following seismic torsion loadcase is also created:

Seismic Torsion - loadcase type: Seismic

Note:

+ and - factors are applied in the combinations to achieve the possible options - +Dir, -Dir, +torsion and -torsion.

Viewing the Seismic Forces

After analysis, the magnitudes and distributions of the seismic forces can be viewed directly in the Results View via the “Seismic” button on the Results ribbon after selecting the required a seismic loadcase.

Note that, for seismic torsion loadcases created by the ELF method, a system of lateral, radial “push-pull” forces are applied to the structure to produce the overall torsion.

The Seismic Design report includes a table giving the total level loads F_i and eccentricity values Ecc used to calculate these forces.

Response Spectrum Analysis (RSA) Loadcases

The following lateral seismic loadcases are created:

Seismic Dir1 - loadcase type: Seismic RSA
Seismic Dir2 - loadcase type: Seismic RSA

The following seismic seismic torsion loadcase is also created:

Seismic Torsion - loadcase type: Seismic

Note:

In Tekla Structural Designer the seismic torsion loading and analysis is static, even for the RSA method; only the lateral RSA analysis is 'dynamic' i.e. a modal analysis + RSA. This is a common method of seismic analysis i.e. dynamic lateral analysis + static torsion.

Note:

Enveloping is used in the combinations to achieve the possible options - +Dir, -Dir, +torsion and -torsion.

Viewing the Seismic Forces

Note that, for seismic torsion loadcases created by the RSA method, the application of these forces depends on the diaphragm options that have been applied.

For a rigid diaphragm single point torsion loads are applied.
For semi-rigid diaphragms or for levels with no diaphragms, radial “push-pull” forces are applied to the structure to produce the overall torsion.

The Seismic Design report includes a table giving the total level loads F_i and eccentricity values Ecc used to calculate these forces.