Seismic Retrofitting of a 28 Story Hotel
In this project, modern methods of retrofitting were introduced for the seismic retrofitting of the hotel building located in Tehran when I was working as a project manager in 2007. The 28 story Azadi Hotel was built during the 1970's by a German engineering company. The building is made of concrete and has continuous walls from the 2nd floor to the 28th floor. These walls partition the rooms and are made of reinforced concrete and they work as the preliminary resisting elements for the seismic. This is advantageous during an earthquake. The major deficiency was the existence of a soft story in the lobby, including poor quality concrete columns of up to 36 feet high.
The columns in the lobby, which were introduced to create a commercial space in the first story, are highly critical. These columns have very little stirrups and consist of poor quality concrete with average concrete compressive strength of 2500 psi. The shear walls, at upper stories, discontinue at first floor. In buildings built in the 1970s, columns usually only have standard vertical and transverse reinforcement.
However, the walls stop at ground level, introducing a discontinuity to the lateral load resisting system. This results in a structure with a soft story floor. Such designs are not allowed in seismic zones, because a high demand is placed on these columns during earthquake loading.
The preliminary design was to apply FRP to the columns and shear walls at the lobby level and the basement. However, it was clearly demonstrated that applying FRP alone could not be satisfactory. The FRP could solve the shear problem of the columns; however, the bending problems of the columns could not satisfactorily be solved. They drift too much, resulting into high bending moments. It was decided to introduce additional bracings with hydraulic dampers in the lobby level and in the first floor.
In order to prevent overturning of the structure, and reduce moments and shears at the base, 16 hydraulic dampers were utilized in the lobby and first floor. More than 215,000 square feet of carbon fiber reinforced polymer was also used to reinforce the columns and the shear walls in the lobby and the two basements floors. The dampers also, were installed with help of inverted V-bracings.
The owner has set the seismic performance level of the building to immediate occupancy for earth- quakes with a 475year return period. This means that the post-earthquake damage state should include very little structural damage. The basic vertical and lateral-load-resisting systems of the building should retain nearly all of the pre - earthquake strength and stiffness.
The retrofitted solution has following advantages:
- Limit the drift of the lobby that resolved the soft story issue
- Reduction of the shear and bending stresses
- Limit the loads in the bracing in acceptable levels
- The dampers allow a seismic health monitoring of the hotel in future