During earthquakes, adjacent buildings with insufficient separationdistance often collide with each other. This collision called seismicpounding and each time it occurs, buildings are subjected to short lateralimpact force in which buildings are not accounted for in the design. Theseimpact forces produce high-amplitude, short-duration local accelerationswhich could induce severe structural and non-structural damage in thebuildings. Some evidences of such damages were observed in manyearthquakes as shown in Figure (1.1). This figure presents one of thecommon pounding damage, which occurred due to the large difference inthe dynamic properties, and in the heights of closely adjacent buildings.Another type of pounding damage is shown in Figure (1.2) which occurredat mid-height of columns when floors of closely adjacent buildings havedifferent levelsAlthough, some modern codes such as Uniform Building Code(UDC-88), National Building Code of Canada (NBCC-1990) and theEgyptian Code for Loads (ECL-93) may include seismic separationAs a high effective thermal conductors and isothermal devices thermosyphons have been widely used in energy and different industrial applications due to their simple construction, small thermal resistance ,broad operating limits and low fabrication costs.Thermal analysis of thermosyphon performance is theoretically and experimentally presented in the steady state operation of the closed two-phase water/copper thermosyphon. The main objectives of the current study are to develop a simplified, rapid and understandable engineering theoretical model for the thermosyphon operation. Anumerical, one-dimensional, steady tate flow model is substantially presented for the vertical, gravity assisted, closed two-phase thermosyphon. The model accounts for the cooling flow regimes, the heat transfer processes of individual sections, the thermosyphonthermal losses, the local change of the working fluid thermophysical roperties, the local heat transfer coefficient and correction for the expanded liquid pool height in evaporator. In addition, to validate thepredicted mathematical model results, an experimental investigation isultimately conducted. Also, The study investigates the effects of changethe operating parameters: heat load, cooling rate and liquid fill ratio onthe thermosyphon performance are considered. Furthermore, the axialvapor and outer wall temperature distribution, as well as the heat