**1. Introduction**

The structural design of buildings is meant to resist the anticipated loads likely to act on a structure. The structural designer chooses the structural system to resist gravity and lateral loads. There is a variety of simple and complex structural systems used to resist combinations of vertical gravity loads and earthquake actions. The techniques available nowadays to alleviate seismic actions are numerous. The requirements of continuous load paths in the vertical and horizontal directions, stiffness and strength, and ductility are well-known requirements for the proper seismic design of structures [1].

Structural systems employing shear walls, bracing and moment-resisting frames, and combinations of them are now widely used by designers. Analysis and design

of structures are guided by design codes and cover the most encountered needs and requirements of typical buildings. However, sometimes, building owners wish to make changes to their properties after the completed buildings are finished and occupied due to some unforeseen needs arising later on. Structural engineers are then consulted to execute the desired changes in the building. Some of the desired changes could produce changes to the overall structural behavior of the building. The designers are required to investigate thoroughly the new configuration of the building based on criteria and requirements of design codes originally used during the initial analysis and design stages. However, in many cases, such investigation is not performed, and a decision is finally taken to execute the desired changes without examining the structural safety of the new configuration of the buildings. The changes take many forms according to the needs arising. In one example, a new multi-story building is built beside an old and similar one and the owner decided to connect the two buildings by a corridor bridge for easy and fast circulation. In another example, a multi-story building is connected to a nearby high retaining wall in order to brace the retaining wall and provide it with ample lateral support against rotation and sliding. Other examples of adding or removing walls, partitions, beams, or even, columns, in order to suit the spaces of buildings to some new business needs are also encountered in practice. Partial or complete fatal collapses of buildings were reported due to arbitrary, nonengineered changes.

Many previous studies investigated the effects of connecting buildings together by sky bridges, sky pools, and sky gardens built of reinforced concrete or steel [2–10].

On the other hand, numerous theoretical studies on twin buildings linked by damping devices striving for mitigating seismic effects on the linked buildings have been conducted [11–14].

In this chapter, the following cases of changes to the dynamic properties of buildings are investigated. This chapter is organized into four parts.

First: Two or more adjacent buildings connected together by bridges after the buildings are completed and occupied for the purpose of easy circulation and saving of time and effort.

Second: Buildings connected by reinforced concrete beams to nearby high retaining walls in order to provide strong lateral support to the retaining walls.

Third: Theoretical investigation and structural analysis of connecting two or more buildings together by rigidly linked sky bridges and the effects of this linking on the seismic performance of the connected buildings.

Fourth: Theoretical investigation and structural analysis of connecting two or more buildings with special devices that are designed to mitigate seismic response and reduce earthquake effects on the connected buildings.

These parts are discussed in the next paragraphs.
