Balanced Stiffness Strength and Ductility

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

Structures Should Be Provided With Balanced Stiffness, Strength & Ductility Between its Members, Connections & Supports


	“A chain breaks at its weakest link”. It is not worthwhile using strong, stiff and ductile structural members if they are not properly connected. Collapse and severe damage of buildings due to lack of good connections is common. Slides J95-J98 illustrate cases of such damage.


	J95. Mosque Building, El Asnam, Algeria, 1980 El Asnam Earthquake. View of the first story column and column-girder connection at the second floor level of the mosque.


	This building, located in downtown El Asnam, was part of a complex of buildings called “Cite Al Naser Market”. Except for part of the mosque building and the corner of another unit, all the other two- and three-story buildings collapsed completely, pancaking and resulting in a large number of deaths [4]. Note the shear failure of the column and of the column-girder joint due to the lack of adequate shear reinforcement. This lack of shear reinforcement was one of the main causes of collapse of the buildings in this market [4].


	J96. Galerie Algerienne Building, El Asnam, Algeria, 1980 El Asnam Earthquake. View of the unit of this 4-story reinforced concrete building which collapsed in the earthquake. Lack of adequate reinforcement at the column-girder connections was one of the reasons for the collapse of this unit.


	J97. Naiguata Beach Club Building, Naiguata, Venezuela, 1967 Caracas Earthquake. This was a single-story building (approximately 7 meters high) with a mezzanine at 3 meters above ground. This slide shows the damage at the connection between the column and the girder supporting the mezzanine [24].


	Note the 7-meter long column at the left edge of Slide J97 supporting the roof which suffered a permanent lateral displacement of 0.30 meters. The stiffening (shortening) of similar columns by the mezzanine girders (to the right of the slide) imposed large shear and rotation demands at the plastic hinges in these shorter columns. This example illustrates the need for avoiding sudden changes in stiffness and the need for balanced strength between members and their connections.


	J98. Four Season Apartment Building, Anchorage, Alaska, 1964 Alaska Earthquake. General view of the building after the earthquake.


	The 6-story building in Slide J98 had, as a structural system, post-tensioned concrete flat slabs supported on wide flange steel columns and two elevator and stairwell shafts of reinforced concrete walls. The lateral resistance to earthquake ground motions was essentially provided by the two slender vertical reinforced concrete shafts. These shafts failed at the ground floor level where all the vertical reinforcing bars in the shafts were spliced. The dramatic failure of this building due to improper splicing of the vertical reinforcement as well as poor selection of the lateral resistance structural system emphasizes the importance of following the main guidelines for seismic-resistant design given above.


	J99. Overall view of the 1200 L Street Apartment Building, Anchorage, Alaska, 1964 Alaska Earthquake. This 14-story reinforced concrete structure has as a basic lateral-resisting structural system a series of slender walls coupled by spandrel girders which worked as coupling girders. Unfortunately it appears that these spandrel girders were not designed (detailed) to work as coupling girders and therefore suffered significant damage in the earthquake. The damage consisted in the formation of X-shaped cracks as can be seen in this slide and in the close-up of Slide J100. The early failure of these coupling girders did not allow very much dissipation of energy in the girders and resulted in the walls working essentially as single cantilever walls with significant decrease in stiffness.


	Obviously the design and construction of the structural system of this building in Slide J99 did not follow the basic principle enumerated in the section entitled Selection of Proper System and Configuration for the Superstructure and discussed and illustrated in more detail in the section entitled Multiple Defense Lines. The coupling girders did not act as ductile and tough structural fuses.


	J100. Close-up of characteristic X-shaped cracks and failure of coupling girders or short spandrel girders in the building of Slide J99. These girders were not properly designed for the shear demands.


	J101. Close-up of the failure of one of the shear walls in the building showed incipient types of failure at the construction joints due to poor workmanship at the joints. The failure of this particular wall did not occur at the construction joint, and it was attributed to lack of adequate axial-flexural capacity [7]. Note the lack of any extra reinforcement at the edge of the shear walls.


	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