**2. Buildings rigidly connected together long after completion without being designed as such**

A need to connect two adjacent buildings together by a bridge or corridor arises sometimes long after the buildings are constructed and occupied. Fast and easy circulation of occupants and goods is a clear benefit of linking the buildings. There are uncountable examples of buildings linked together by a bridge for similar purposes. **Figure 1** shows two hospital buildings connected by a steel sky bridge over a street with a span of around 22 m. The steel bridge is partly supported by the stone walls,

*Some Risky Practices in Earthquake Engineering That Need More Research and Evaluation DOI: http://dx.doi.org/10.5772/intechopen.108445*

#### **Figure 1.**

*Steel sky-bridge connecting two parts of a multistory hospital building in Amman, Jordan.*

and partly fixed to the walls using steel plates and bolts, **Figure 2**. The sky bridge was erected long after the two buildings were constructed when the need to use the two buildings as one hospital came into existence. Often, in such cases, a thorough seismic structural investigation of the linked buildings is not carried out.

The connecting bridges are usually built of reinforced concrete or steel. The link may be supported directly on the beams' or slabs' tops of the two buildings. The concrete of the beam or slab tops may be removed and the steel bars exposed in order to splice or weld the steel bars of the linking bridge to the steel of the beams or slabs of the buildings. Steel sections (for example, Section I) may protrude into the beam or slab concrete, or be connected by plates and bolts. In all such cases of connections, the linking bridge becomes rigidly connected to the two buildings. A new complex building is, therefore, created, which comprises the two buildings and the linking bridge. The stiffness, mass, and damping matrices of this complex replace the individual matrices of the two buildings during analysis. Seismic analysis of this complex proves that its structural behavior is completely different from that of the isolated individual buildings.

In order to obtain some insight into this case, a seismic analysis of two buildings connected by sky bridges is carried out by the author. The two buildings are 18 and 13 floors in height, with different plan configurations. Sky bridges are modeled as beam columns, and hence it is a rigid connection. The buildings are connected together by

**Figure 2.** *The connecting steel sky bridge from below.*

one, two or three sky bridges, and seismic investigations are carried out for different cases. El Centro earthquake time- acceleration recording is utilized [15]. SAP2000 analysis results revealed that the modes of vibration of the connected buildings are quite different from those of the individual buildings. The following **Figures 3**–**5** are the results of SAP200 analysis of two linked buildings. One of the outputs of the analysis shows that the modes of vibration are out of phase, **Figure 3**, and they depart from each other [15]. For example, in the structure with one sky-bridge, horizontal displacements in the right building frame U1 in **Figure 3** for the two

#### **Figure 3.**

*Out-of-phase mode of vibration and deformation of two connected buildings, a case of 1 sky bridge, with permission of the author [15].*
