HISTORICAL REINFORCED CONCRETE HIGH-RISE BUILDINGS

The seismic risk of a country is significantly affected by the presence of reinforced concrete structures built without reference to a seismic design code. This problem has a worldwide extension. In Italy approximately 60 percent of the building stock, with a large fraction of reinforced concrete buildings, is located at sites that were not considered seismic at the time of construction. Additionally, many reinforced concrete structures were built in the reconstruction period after WWII and are now at the end of their conventional life cycle, which could imply decay of the mechanical properties of their elements. Some of these buildings, at the same time, are now listed as part of the national cultural heritage. The case of the first reinforced concrete high-rise buildings, dating back to the 1950’s and reaching a height of 30 or more stories making use of normal strength concrete, is of particular interest. Design was based on an allowable stress approach without consideration of the ductility resources and, in the Italian case, without any requirement of seismic capacity. The design, however, is often based on very effective intuition in terms of structural concept. This point is well reflected in two high-rise buildings constructed in Milano, Italy, at that time: the Pirelli building and the Velasca tower. They present two different structural schemes, innovative for the time and very effective with respect to seismic action: the Pirelli with a shear walls scheme, the Velasca tower with a tube-in-tube one. The re-analysis of the two high-rise buildings, performed with today’s computational means and with reference to present code requirements, has shown high capacity resources and very effective behavior characteristics. As a conclusion, the seismic safety of existing reinforced concrete high-rise buildings necessarily implies their re-analysis and the consideration of many different aspects related to the period of construction and to the effect of time elapsed. Yet, from the structural view point, these buildings, designed with analysis tools not comparable to the present ones, were an engineering challenge and, as a result, are often found to reflect great wisdom and intuition in the structural concept.