Multiple Defense Lines

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	*National Information Service for Earthquake Engineering* *University of California, Berkeley*

Structures Should Have the Largest Possible Number of Structural Defense Lines


	A structure should have the largest possible number of internal and external redundancies. While a high degree of static indeterminacy is desirable, this is not sufficient. In order that a building be efficient in resisting severe earthquake shaking, it should have sufficient ductility, toughness and stable hysteric behavior under repeated cycles of deformation reversals. To achieve this it is necessary to proportion (size) and detail its members, connections, and supports so that all the inelastic deformations are constrained (controlled) to develop in desired regions and according to a desirable hierarchy (guideline No. 6), and are dispersed in a sufficiently large number of regions over the plan and height of the whole structure (which can be accomplished by following the requirement stated in the main guideline No. 8). A good example of the advantage of following these, and some of the other main guidelines, is illustrated by comparing the behavior of two buildings, Banco de America and Banco Central, during the 1972 Managua Earthquake. The main structural features and damage of these two buildings are illustrated in Slide J89 through J94.


	J89. Overall view of the Banco Central (left) and the Banco de America in downtown Managua, Nicaragua.


	The Banco Central was a 15-story reinforced concrete frame building with an eccentrically placed penthouse at the top of the eccentrically placed service core and two-level basement. While this building suffered severe structural and non-structural damage during the 1972 Managua Earthquake (which led to the demolition of the upper 12 stories), the other building - the Banco de America - which is somewhat taller (the tallest building in Managua in 1972) suffered very little damage and was repaired. The Banco de America is a 17-story coupled shear wall core, concentrically located, reinforced concrete building with two basements.


	J90. Plan view of the Banco de America, Managua, Nicaragua. This building generally performed very well during the 1972 Managua Earthquake. Its excellent performance can be attributed to the symmetry and uniformity of distribution of the masses and structural stiffnesses throughout the building.


	The structural system, which can be considered as a combination of the ductile walls with a framed tube, is an excellent system for seismic-resistant design, providing several lines of defense whereby the behavior of the whole system can accommodate the demands of a severe earthquake [12, 26]. The shear walls had only minor cracking. The only structural damage was the shattering of several of the deep coupling girders of the shear wall core because a rectangular hole was introduced in the center for passing through an air conditioning duct. In the lower stories the only visible damage was the spalling of the marble covering the shear wall core as illustrated in Slide J91. The damage was easily repaired. Most of the floors and walls had no significant damage and the stairs were in excellent condition, which was the complete opposite in the case of the Banco Central Building as illustrated in Slide J94.


	J91. View of the core service walls and floor area at the second story of the Banco de America, Managua, Nicaragua. Note that few of the marble tiles that cover the reinforced concrete shear walls have spalled off. This was the only visible damage in this story after the 1972 Managua Earthquake. Compare this with the damage illustrated in Slide J94.


	J92. Typical floor plan above the fourth floor of the Banco Central Building, Managua, Nicaragua (see Slide J89). This building had a reinforced concrete frame as the basic structural system.


	Note that the overall configuration of the reinforced concrete system of the tower (whose plan is shown in Slide J92) was not symmetric. This was due to the presence of a different structural system at the two short ends and due to the presence of the eccentrically placed reinforced concrete walls surrounding the service core which caused significant eccentricities between the center of rigidity and the center of mass. Note that in the transverse direction the framing beams had a clear span of about 13 meters, and were only 2.0 x 7.0 meters in cross section, Due to the large flexibility of these floor beams the contents of the building were not protected during the 1972 Managua Earthquake. The non-structural damage to ceiling, partitions and external tile walls was so heavy that it was difficult to walk on the floors. The stairs were covered with debris (Slide J94). There was also significant structural damage that resulted in the need to demolish the tower of this building.


	J93. Overall view of the Banco de America (left) and the Banco Central Buildings, Managua, Nicaragua. 1972 Managua Earthquake. This slide illustrates the damage to the infill hollow tile wall that was used at the end of the Banco Central Building. Because this end was very close to the center of rigidity, it did not suffer large lateral torsional deformations.


	J94. Banco Central Building, Managua, Nicaragua. View of the stairway after the 1972 Managua Earthquake. Most of the stairs were covered with debris that resulted from the failure of the hollow tile partitions surrounding the stairs. The damage (structural and non-structural) and the protection of the contents of this very flexible moment resisting frame building were in sharp contrast with those observed in the taller but symmetric combined coupled shear wall-tubular frame structural system of the Banco de America Building. These two buildings were located on diagonally opposite corners of a street intersection.


	The University of California, Berkeley Copyright 1997, The Regents of the University of California. Structural Engineering Slide Library, W. G. Godden, Editor Set J: Earthquake Engineering, V. V. Bertero


	Site Design: Vivian Isaradharm, Oct. 97. Mail to: eerclibrary@berkeley.edu