nisee

National Information Service for Earthquake Engineering
University of California, Berkeley

      The seismic forces that develop during the vibratory response of a structure to earthquake ground shaking at its foundation are inertia forces whose intensity depends on the product of the mass and acceleration.  Hence it is of the utmost importance to reduce the mass of the structure to a minimum.  Thus when the designer is confronted with the problem of selecting the structural material, it is necessary to analyze the mechanical characteristics of available materials, normalized to their unit weight.  In doing so, it becomes evident that among the traditional structural materials - timber, masonry, concrete and metals (steel and aluminum) - the most efficient earthquake-resistant material for low-rise buildings is timber.  However, these buildings should be carefully designed and constructed, provided with proper lateral bracing and all of their components tied together from the roof down to the foundation.  The importance of adequate lateral bracing and of tying together the components is illustrated in Slides J22 and J23.

      During the M = 6.5, February 9, 1971 San Fernando, California, Earthquake, numerous modern split level and two-story wood frame houses were severely damaged because at their first story level the garage walls were inadequately braced (as illustrated in Slide J22).  The large garage openings greatly reduce the area of walls available to resist the lateral and gravity forces and introduce large eccentricities which lead to significant torsional loading.

      The effects of insufficient lateral bracing and of inadequately tying together all the components of a building were clearly demonstrated in the M = 6.7 May 2, 1983, Coalinga, California,  Earthquake.  Numerous porches collapsed (as illustrated in Slide J23) because they were not properly tied (anchored) to the main structure of the building and/or did not have their own proper lateral bracing.

      Unreinforced masonry is very susceptible to damage during earthquake ground shaking. Solid brick masonry is very heavy and its tensile strength, and therefore its flexural strength per unit weight for in-plane and out-of-place seismic forces, is very small.  Catastrophic failures of unreinforced masonry, as illustrated in Slides J24-J30, have occurred in many earthquakes.

      Old unreinforced masonry buildings, whose walls are not properly connected to the floors, roof, and transverse walls (interior and exterior), constitute a threat to the occupants as well as to people that may be walking in the neighborhood because the walls start to fall as soon as the building vibrates when subjected to even moderate ground shaking.  The devastating effect of even moderate earthquake shaking is illustrated in Slide J24 which shows what remained of the downtown area of the city of Managua after the M = 6.2, December 23, 1972 Managua, Nicaragua, Earthquake.  A large number of the 5000 people killed in this earthquake were killed by falling masonry walls and roofs supported by these walls.  This danger is also illustrated by Slides J27-J30.

      When large panels of unreinforced masonry are used as veneer or infilling in wood, reinforced concrete or steel framed buildings, they also constitute a severe hazard because large portions of these panels can easily be dislodged from the frame of the building as illustrated in Slides J25, J28 and J29.

      Properly reinforced masonry can be used effectively in seismic-resistant construction, but unreinforced masonry can only be used in small panels that are properly framed (confined) by structural elements.

      Concrete is a relatively heavy material which, like masonry, has a very small (practically negligible) tensile, and thus flexural, strength.  Therefore, it is usually reinforced with steel when used in structures.  When the concrete is properly reinforced with steel it can be used effectively in seismic-resistant construction, but it still has relatively low strength per unit weight when normal weight aggregates are used.  The use of lightweight aggregate concrete offers a significant advantage in seismic regions.

      For regions of moderate to high seismic risk it is necessary to reinforce the concrete structural members carefully: the proper amount and correct detailing of the reinforcing steel plays an important role in the seismic response of a reinforced concrete structure as illustrated in Slides J31-J35.

      Steel is a manufactured material, with usually excellent quality control, that is fabricated in structural shapes.  While its stiffness per unit weight is practically the same as any other traditional constructional material, its strength and particularly its ductility and toughness per unit weight are significantly higher than concrete and masonry materials.

      Because of its high strength per unit weight, the slenderness of steel structural members usually exceeds significantly the slenderness of similar structural members made of other traditional materials.  Thus buckling becomes a serious problem, and the higher the yielding strength of the steel the greater the danger of buckling.  Most structural shapes are formed by plate elements which can undergo local buckling, particularly when strained in the inelastic range.  Therefore, in earthquake-resistant design, the compactness requirements for the cross section of the critical regions of structural members are more stringent than for design against normal (standard) loading condition.  Another problem in attaining efficient seismic-resistant construction of steel structures is in the field-connection of the structural members.

      Slides J36-J41 illustrate some of the above problems in attaining efficient earthquake-resistant design of steel structure.

      As illustrated in Slide J38, during the 1971 San Fernando Earthquake the diagonal braces were stretched beyond their elastic limit.  The excessive deformations (buckling and yielding) of these braces resulted not only in the failure (rupture) of similar braces in the other north wall, but also in significant damage to the rest of the building. 

      The Cordova Building, shown in Slides J39-J41, is a six-story office building with a penthouse.  The earthquake-resisting structural system was designed to be a moment-resisting space frame (full moment resisting in the east-west direction and partial moment-resisting connections in the 20-cm reinforced concrete wall service core (near the north facade) where the penthouse was standing, and a 10-cm southeast corner, which contributed significantly in the seismic response of the building.  The steel connections were shop welded and field bolted with high-strength bolts.  The main earthquake damage occurred in the first story and at the penthouse whose walls collapsed [7].  The first and second story steel columns, located at the south end of the building, suffered severe local buckling as illustrated in Slides J40 and J41.  As illustrated in Slide J40, the local buckling of the w14x30 southeast corner column, which occurred just below the second floor level, was so severe that the flanges tore away from the web and the web crimped, resulting in shortening of the column by about 3.8 centimeters.  The mid-story stair landing was connected to this corner column, making it shorter and therefore stiffer than the other columns [7].

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

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