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

888

4

3. Influence of the connection stiffness To gain a better understanding of the influence of connection stiffness between steel braces and the existing HOF hospital building, a sensitivity analysis was carried out. The analysis involved gradually increasing the shear and axial stiffness, while performing eigenvalue analysis and evaluation at each step. The stiffness was logarithmically increased from 10 0 to 10 8 kN/m. The range reflects the axial and shear stiffness of a wide variety of anchors and their arrangements. The upper limit was set at 10 8 kN/m. This was found to be stiff enough in all calculations to represent a perfectly rigid connection. Figure 3 illustrates the change in eigenperiod for the first mode in the x-direction as the connection stiffness varies. The graph reveals three distinct regions, which represent the different rates of change of period and effective modal mass with respect to the connection stiffness. It is important to note that already a big change in frequency came due to the in-plane bracing of the steel structure at the top floor (6 th story). Along the region A, which occurred between a stiffness of 10 0 and approximately 10 2 kN/m, there was little to no rate of change and therefore no significant impact on the natural frequency or the distribution of effective modal mass. Region A still exhibits the same behavior as the unstrengthened building, which suggests that the strengthening is ineffective. Region B, spanning a stiffness of 10 2 to approximately 10 5 kN/m, displayed a significant rate of change, indicating the transition from the ineffective retrofit to the full effective one. The transition can be seen on both the natural frequency and the distribution of effective modal mass. Beyond a stiffness of approximately 10 5 kN/m (region C), the eigenvalue analysis and effective modal mass shown again little to no rate of change. This indicates that the connection stiffness is large enough to ensure the full effect of the strengthening. The same distinct regions are observed for the y-direction.

C

C

Fig. 3, first eigenperiod (X-direction) of the building as a function on the connection stiffness

Through this sensitivity analysis, after an initial determination of the effective stiffness of the connections, it is possible to estimate its influence on the structural performance. In addition, depending on the region (A-C) in which the effective stiffness is to be found, more precise calculation methods may be unnecessary as they may not affect the final result due to the little rate of change regarding the frequency and effective modal mass. The hospital building consists of two distinct connections. Firstly, the stiffness of the steel-to-steel connection is determined. In this configuration, the existing steel section is extended and connected to the truss using four pre stressed M20-10.9 bolts that pass through the end plates. Since the steel sections are incorporated in the structural finite element model, only the stiffness in tension of the four bolts is considered. According to the methodology outlined in EN 1993-1-8:2005, both shear and axial stiffness were calculated. In the case of prestressed bolts, the shear stiffness can be assumed to be infinitely stiff if the bolts have been designed to prevent slippage under the given load level. The axial stiffness is only computed for tension, as compression can be assumed to have an infinite stiffness as depicted in table 3.