4-2 The Structure Analysis

The geometry of the Azadi hotel and the materials used require necessity of seismic retrofitting. The foundation of the hotel is resting on a concrete MAT without piles. The thickness of the MAT reaches up to 200 centimeters in some areas. The initial sampling indicates that the quality of building’s concrete strength is unacceptable in some columns and shear walls. The poor quality of the concrete may refer to some parameters such as structure’s long lifetime, improper mixing, and low water-cement ratio. The compressive strength of concrete in columns and shear walls varies between 12 and 20 Mpa. The positive point in the concrete walls of this structure is the usage of appropriate ripped armatures. As shown in Figure 4-2, the main plan of the building is rectangular. Because of the lobby existence in the ground story, some of the columns are not continued to reach to the top floors, which actually causes the creation of a soft story. Soft story remain a challenge for most hotels due to architectural considerations.

Figure 4-2 Base floor plan of Azadi Grand Hotel

Figure 4-2 Base floor plan of Azadi Grand Hotel.

 

The structure is massive due to the usage of thick slabs, thick shear walls, and shear wall cores. The initial calculations and seismic controls showed that structure was able to tolerate only 20 percent of quake loads, especially in lower floors. As the structure is more slender on one side in comparison to the other side, it exists the probability of a building collapse or uplift. In this respect, exact analysis along with performing in-situ tests in order to provide an appropriate design of seismic retrofitting was an essential and inevitable issue.

It is of note that the primary aim was to improve and reconstruct the hotel. In addition to the principal objective, the seismic retrofitting was also performed because of the large mass, the building’s height, poor quality of concrete, and the existence of the soft story with 11 meters in height, which all represent the weaknesses of the structure.

Regarding the extreme weight and the high seismicity there will be a large base shear force. The columns were constructed about 40 years ago with inadequate reinforcements which do not meet the required capacity to withstand the shear forces. In addition, the lateral displacements and the axial stresses due to earthquakes are very high. As mentioned above, building uplift is a likely problem, and due to the extremely large weight, a local failure is possible in foundation.

In this situation, it should be noted that to increase the base shear resistance bracings were added. In addition, the weak concrete columns were strengthened with Fiber Reinforced Polymers (FRPs). In the current project, combinations of all mentioned methods were used.

The 3D FEM model with more than one million solid elements was utilized for the seismic analysis. In this simulation, the columns, beams, and the walls were modeled using shell elements. Figure 4-3 shows the finite element model of the structure.

Figure 4-3 The 3D finite element model

Figure 4-3 The 3D finite element model.

 

As mentioned, the technique used for retrofitting of this structure was adding the bracings and reinforcing with FRPs, which required a precise seismic analysis. Hence, the non-linear time history analysis was used, and input spectrum was determined by Dr. Zare and his colleagues from International Institute of Earthquake Engineering and Seismology (IIEES). The soil type was categorized as type II. Besides, due to the importance of considering the dynamic soil-structure interaction effect, the CONE model was used in order to simulate the soil effect on the structure.

Figure 4-4 depicts the design spectrum for a 475-years return period.

Figure 4-4 Design spectrum for the hotel with a 475-years return period

Figure 4-4 Design spectrum for the hotel with a 475-years return period.

Figure 4-5 shows the principles of the CONE model. The soil was considered as a homogeneous half-space layer. Although it is mentioned in the previous sections, I would like to emphasize again that considering the soil in the modeling results in decreasing the main frequency, enhance of the period; and consequently the structure behavior would be close to reality. It should be noted that the main frequency of the structure was calculated as 1 Hz, by means of Ambient Vibration method, which was used to calibrate the FEM model.

Figure 4-5 Modeling of soil, using CONE model

Figure 4-5 Modeling of soil, using CONE model.

 

4-3 Design and retrofit approaches

Seismic retrofitting of Azadi Hotel was performed based on various analyzes. As mentioned, additional lateral resistant elements like bracings were installed; the columns were reinforced with FRPs; also dampers were inserted at the intersection of bracings in order to reduce the forces on the columns. The dampers, which are schematically depicted in Figure 4-6, decreased the shear forces up to 30 percent. As a result, the problems of structure uplift were eliminated. The bracings also acted as distributors of lateral forces between resistant elements at the lobby. The hybrid has resolved the solution, i.e. dampers, bracing, FRP’s severe problems due to earthquake forces.

The steel plates were utilized to reinforce the critical regions of the columns, as is displayed in Figure 4-7a. Moreover, in order to improve the performance of reinforced beams with FRPs, some columns were modified from the rectangular shape to oval one and then were jacketed by FRPs because confinement performance of the FRP sheets is not appropriate in rectangular sections. The sample of this method is shown in Figure 4-7.

b) Implementation in practice

a) Schematic layout

Figure 4-7 Columns reinforcement of Azadi Hotel

 

Another major problem in the practical operation, was the installation of dampers and bracings. The dampers were prepared as packed units; but because of equipment limitations and particular operating conditions, it was not feasible to install the dampers as integrated units. Hence, the executive team divided the dampers into smaller pieces and implemented the welding to reconnect the pieces. All welding were tested for possible faults, and no problem was observed. However, this point was one of the unpredicted challenges.

 

4-4 Hints

A significant and instructive matter of the current project is that in the design of a large-scale project, the result of the design must be validated with the experimental results, and it is not acceptable to rely on only computational analyzes. For instance, in the present project, the response spectrum response of the site was determined, and the main frequency obtained using experimental methods. Another point is that the design engineer should always track the construction procedure, especially in the retrofitting step because of some unpredictable problems, like the installation of dampers of Azadi hotel. The other noteworthy point is the analysis inputs. The inputs in structural and geotechnical analyzes are of great importance. For clarification, in this project the input earthquake (spectrum and input accelerograms) effected the design process largely. This issue is a notable point particularly in the regions without microzonation.

Finally, I would like to remind that in the non-linear time history analysis of the present project, I benefitted a lot by consulting with outstanding researchers.

It is of note that nowadays with development and progress of computer softwares, conducting non-linear analyzes is widespread. The first mode in the push-over analysis is considered as the dominant mode and the design basis. However, in the tall building over than 100 meters, the second and even third modes are also determinative. Hence, it was not advisable to use the push-over analysis. According to research works of my previous advisor, Professor Helmut Krawinkler, from Stanford University, using the push-over analysis for multi-modal tall structures still have some uncertainties.

Application of FRP sheets in the lobby’s columns

Columns reinforcement using steel plates in critical areas