PSI - Issue 64

Maximilian Fehr et al. / Procedia Structural Integrity 64 (2024) 885–892 Maximilian FEHR, Michael BAUR, Giovacchino GENESIO / Structural Integrity Procedia 00 (2019) 000 – 000

887

3

Key:

Existing concrete walls Retrofitted steel braces

Y

C C’

M

M C

Center of mass

Center of stiffness (before) Center of stiffness (after)

ca. 22.0 m

C’

X

Frame A

Frame B

ca. 44.0 m

Fig. 2, Floor plan (3 rd - 6 th floor) and highlighted the lateral structural elements

The external steel braces were chosen, as they caused minimal to no disruption to the hospital's operations. In addition, the reinforcement measures in the courtyard and the steel structure resulted in a short construction time. The steel braces were connected to the existing RC slabs and in the substories with post-installed anchors. The influence of the retrofitting measures in form of steel braces can be seen by the change in the modal analyses for the existing and retrofitted building in tables 1 and 2. Only relevant eigenmodes with high effective modal mass are shown in tables 1 & 2.

Table 1. Effective modal mass in percentage for the first 6 eigenmodes (existing building HOF) Mode Period ( s ) Effective modal mass in x direction (-) Effective modal mass in y direction (-) 1 2.264 17.08% 21.70% 2 1.751 39.15% 13.13% 5 0.847 0.29% 15.63% 6 0.585 6.81% 5.97%

In table 1 the first two eigenmodes have a high effective modal mass percentage in both the X and Y directions. This distribution is the result of torsional modes. In addition, the influence of the very poor structural integrity of the structure in plane and in height can be seen. To reduce the torsional effects due to the eccentricity, two external steel braces were placed on the facade. This allowed the center of stiffness to be transferred towards the center of mass, significantly reducing torsional vibrations and their effective modal mass fraction. Table 2 shows the results of the eigenvalue analysis of the retrofitted model. The eigenmodes have now a dominant direction and only small parts perpendicular to it, i.e. only a very small torsional component. The addition of the bracing elements greatly improved the highly eccentric floor plan and the overall structural integrity of the building.

Table 2. Effective modal mass in percentage for the first 6 eigenmodes (retrofitted building HOF) Mode Period ( s ) Effective modal mass in x direction (-) Effective modal mass in y direction (-) 1 1.332 44.68% 1.93% 2 0.969 0,63% 52.05% 3 0.762 9.40% 1.21% 6 0.446 17.87% 2.44% 7 0.367 4.25% 9.00%