PSI - Issue 81

Mykhailo Hud et al. / Procedia Structural Integrity 81 (2026) 372–376

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3b. Storeys 1 – 3: steel; storeys 4 – 9: timber. This structuring of the analysed variants made it possible to evaluate the influence of different material combinations on the spatial stiffness, dynamic characteristics, and potential load-bearing capacity of the frame system. 3. Result and discussion The study revealed numerical values for the natural frequencies and vibration modes for each material combination scenario. Furthermore, graphical visualisations of spatial displacements associated with the corresponding vibration modes were created.

Scenario 1

Scenario 1b

Scenario 1a

14

12

10

8

6

Frequencies, Hz

4

2

0

1

2

3

4

5

6

7

8

9

10

11

12

Modes

Fig. 2 Natural frequencies for scenarios 1, 1a, and 1b.

A comparison of the three frequency (Fig.2) sets indicates that configurations with lower stiffness, such as Scenarios 1a and 1b, exhibit significantly lower natural frequencies. Conversely, the model with higher stiffness – achieved through optimised material combinations (Scenario 1) – displays substantially higher modal frequencies.

Scenario 2

Scenario 2a

Scenario 2b

3

2.5

2

1.5

1

Frequencies, Hz

0.5

0

1

2

3

4

5

6

7

8

9

10

11

Modes

Fig. 3 Natural frequencies for scenarios 2, 2a, and 2b

Group 2 demonstrates the lowest natural frequencies among all configurations (Fig.3), indicating that the utilisation of timber throughout all storeys serves to reduce the overall stiffness of the structure. In contrast, variant 2a exhibits a pronounced increase in medium-range frequencies, indicative of a local stiffness enhancement attributable to steel in the upper storeys. Variant 2b is characterised by intermediate stiffness, positioning it between the stiffer models and those with the lowest stiffness.