3-2 Building Description

The hospital structure has glass facades with a rectangular plan, as displayed in Figure 3-2. The building structure is made of reinforced concrete and includes five shear-resistant cores of reinforced concrete. The shear walls thickness is uniformly 40 centimeters, and the type of stories slabs is of reinforced concrete. Totally, the amount of concrete was around 40000 cubic meters, and the overall used reinforcement in the structure were about 4000 tons. The structure plan and the position of five shear-resistant cores can be observed in Figure 3-3.

b) The plan and main corridor of the building

a) A view of the building

Figure 3-2 View and schematic plan of Triemli hospital.

As the building consists of a concrete structure and due to the existence of large equipment, the weight of the building is extremely high. The thickness of most slabs is 25-45 centimeters and the slab thickness of the stories, which contain weighty facilities, would reach up to 60 centimeters. The building is located at the downward of a hill; the type of the soil is sedimentary with a relatively poor property. The major parts of the soil layer are composed of sediments and soft deposits including soft clay and poor sand, in which the shear wave velocity and loading capacity is very small. The building’s foundation was placed on reinforced concrete pile caps and 200 concrete piles with a diameter of 80-120 centimeters and a length of 30-35 meters. The piles transfer the dead and live loads of the building to the ground. The operation of all piles in load transfer is frictional. According to Figure 3-4, the piles are concentrated under the shear-resistant cores. The pile cap thickness is variable between 60 and 120 centimeters. Based on the Swiss code SIA 261 for seismic resistant design, the type of the soil containing the sediments is classified as type E.

Figure 3-3 The structural plan of the hospital and the positions of shear-resistant cores

Figure 3-3 The structural plan of the hospital and the positions of shear-resistant cores.

The hospital building considering the static dead loads and operation loads was designed by a consulting engineers group. However, for the structural analysis and design against earthquake loads and dynamic behavior investigations, I was invited to conduct the calculations. The significance of the structure seismic response comes from the fact that lateral displacement of the frame and the shear-resistant cores in earthquakes is a determinative point. Their values are very limited (about a few millimeters); so that in case of displacement increase up to the more than allowed values, the performance of the elevator and some other facilities would be disturbed. Due to the importance of the hospital, the operation of this building even in the critical situations and also after earthquake occurrence should continue without interruption.

Figure 3-4 The layout of reinforced concrete piles under the shear-resistant cores

3-3 Seismic Analysis and Design Challenges

Regarding the descriptions of the previous section, due to the existence of the thick shear-resistant cores, the behavior of the structure against the lateral loads is entirely rigid. Because of the importance of the lateral displacements, the modeling should be close to the real condition of the structure and the surrounding. About the support condition, it was decided that the set of the soil and piles would be simulated based on the CONE model. Figure 3-5 demonstrates the structure displacements and the contribution of pile, structure, and soil movements.

The foundation is placed on 200 buried piles, and these piles in some areas are not far from each other, so the pile group effect on the stiffness is a critical point and its negligence would

Figure 3-5 Structure displacements which should be separated in the design process

affect the accuracy of the results. It should be noted that as the structure supports are modeled with a lower stiffness (close to the real condition), the natural frequency (f) would decrease and as a result, the earthquake load obtained from the spectrum analysis also would reduce.

Moreover, the damping ratio of the concrete structure is small. If the foundation’s soft soil is modeled considering piles and dynamic effects of pile group, the friction and radiation damping (especially radiation damping) of the soft soil would raise which leads to decreasing earthquake forces on the structure.

Since in the seismic analysis the quake wave transfer through the soil thickness and toward the ground, the piles interact with each other; thus, the concept of the dynamic stiffness of the pile group is different fromthe static state. Figure 3-6 explains the concept of piles impact on each other during dynamic loads.

Based on investigations, the pile groups‘ reduction coefficient (group effect factor) was calculated as 0.11 percent in this project. It means that the pile group stiffness causes 90 percent decrease in dynamic loads, while the reduction factor in static loads (stiffness reduction) is around 40-60 percent. Although this trend may provide increasing the damping ratio and reducing the base shear, it would also increase the lateral displacements. For more clarification, the base shear amount of the building, when the fixed supports were used, was calculated as 50000 KN, and natural frequency was equal to 1 Hz. The estimated values of base shear and natural frequency were respectively decreased to 38000 KN and 0.4 Hz whenpile models were included.

Figure 3-6 concept of pile group effects.

Note that based on the analysis results, without considering the above options, six shear-resistant cores were required; however, utilizing five cores were adequate when the model was modified. Figure 3-7 shows the 3D view of finite element model of the building. In Figure 3-8 an example of dynamic stiffness of piles can be observed. Figure 3-9 also illustrates the design of armature installation inside the shear-resistant core.

Figure 3-7 The finite element model of the building.

Finally, I firmly suggest that engineers and designers try to consider the effect of soil-structure interaction in simulation and dynamic analysis of tall structures, particularly in the seismic zones, and also try to model the structure along with piles and foundation soil, simultaneously. If there is a pile group, its effect should be considered in the stiffness decrease of the whole model because stiffness reduction leads to decreasing the forces. It should be noted that by reducing foundation stiffness, the lateral displacements as a result of foundation rocking could enhance. Such rocking motions in tall buildings may cause significant lateral displacements at higher stories, especially at the roof. In a successful design, all the mentioned challenges are interpreted along with each other as a group.

Figure 3-8 An example of dynamic stiffness of piles

Figure 3-9 Design of reinforcements of the concrete core.

I would like to mention that Professor Oral Buyukozturk, Professor in the field of concrete structures at MIT Cambridge, evaluated the design of the building and expressed that quality of the analysis and design is highly satisfactory. This statement is extremely precious for me, and his positive viewpoint would definitely intensify professional credit.

Figure 3-6

Figure 3-7

Figure 3-8

Figure 3-9