New analytical center of mass and rigidity

First released in Tekla Structural Designer 2025 Service Pack 3, a new analytical approach for calculating the Center of Mass (CoM) and Center of Rigidity (CoR).

Previously, Tekla Structural Designer, like most other software, provided an estimated CoR position, which could be inaccurate or non-existent in certain situations. The new analytical method overcomes these limitations, providing more precise calculations.

These new analytical calculations offer valuable insight into the eccentricity between mass and rigidity within a structure. This eccentricity is influenced by load positioning and the stiffness of lateral force-resisting systems. Minimizing this eccentricity is vital for effective seismic design in earthquake-prone regions.

While the CoM and CoR results are displayed, they are not currently used in any further calculations within Tekla Structural Designer.

Key aspects of the new calculations

Applicability: Calculations are performed for all "Modal Mass" and "Base Shear" type combinations defined in the model. The method is applicable in all situations, including models with rigid diaphragms, semi-rigid diaphragms, or no diaphragms at a given level.
CoM Determination: The CoM is determined based on the loads present in the relevant combination.
CoR Determination: The CoR is determined using analytical methods allowing for the mass distribution when there is no rigid diaphragm present.

Benefits of the new analytical CoM and CoR

These new calculations offer a comprehensive analytical approach for determining CoM and CoR for "Modal Mass" and "Base Shear" type combinations across roofs, slabs, diaphragms, floors, and the entire structure. They consider both the mass distribution and the structural stiffness of the entire building, including:

Interconnected core wall systems
Coupling of beam and frame systems
Independent diaphragms on floors
The true in-plane stiffness of semi-rigid diaphragms

Slab-based reporting remains supported and is no longer limited to defined rigid or semi-rigid diaphragm areas. FE meshed slab areas not designated as rigid or semi-rigid diaphragms are also included. By default, reporting is now level-based, meaning:

Plan bracing systems are considered
Interconnected rigid diaphragms, semi-rigid diaphragms, and plan bracing systems are reported as one

A new option is also available to report the CoR for the entire building.

Performing analytical CoM and CoR calculations

To perform analytical calculations, navigate to the Analyze ribbon. Then, click Calculate within the Center of Mass and Rigidity Group.

CoM and CoR calculations are performed for each defined modal mass combination (of either "Modal Mass" or "Base Shear" type). The results for the first modal mass combination are automatically displayed graphically in the active window.

For meaningful results, the modal mass combination should accurately represent the structural state for which CoM and CoR are needed. For example, use the seismic weight combination for seismic situations.

The Show Results button on the Analyze ribbon displays the analytical CoM and CoR results for the first modal mass combination in the active graphical window. Clicking this button repeatedly will cycle through the results for each additional modal mass combination.

The Scene Content window has been updated; under "Center of Mass & Rigidity," it now includes two entity categories: "Analytical" and "Approximate," as shown below.

The "Analytical" category contains the following sub-categories, allowing you to select where CoM and CoR information is displayed:

"Slabs/Roofs/Diaphragms" - Displays CoM and CoR for each slab, roof, or diaphragm.
"Floors" - Displays a single CoM and CoR for each floor.
"Model" - Displays a single CoM and CoR for the entire model.

Each of the above sub-categories then has controls which enable you to display:

"Geometry" - A red arrow for CoR and a blue arrow for CoM.

"Text" - The offset distance between CoR and CoM.

"Origin" - If required, a green arrow can be displayed to indicate the origin at each level from which the CoR and CoM coordinates are measured.

The "Approximate" entity category allows you to display the CoM and CoR calculated using the approximate approach. This was the only approach available in previous releases of Tekla Structural Designer. It has been retained to allow you to view approximate values for any other loadcase or combination.

Regardless of the approach used, hovering over a CoM or CoR in the graphical view displays a tooltip.

As shown above, the tooltip contains the following information:

CoM location (X and Y coordinates)
CoR location (X and Y coordinates)
The distance between the CoM and CoR pair

Tabular results for the new analytical approach are also available. Table Options are provided, allowing you to select "Model," "Floor," and/or "Slab/Roof/Diaphragm" results as needed.