It is of utmost importance that in seismic-resistant design the designer recognizes from the beginning that the dynamic forces in a structure can be controlled by proper selection of its structural system and by the amount and distribution of its reactive masses (masses that will react to the shaking of the building foundation). The smaller the reactive masses, the smaller the earthquake forces (inertia forces). The use of unnecessary masses should be avoided; any mass used in the building should have a seismic-resistant function. Damage due to the use of unnecessary masses are illustrated in Slides J59 and J60 for non-engineered buildings and in Slides J61-J64 for engineered buildings.
Damage
to a wooden house due to a heavy roof supported on a flexible frame.
1971 San Fernando Earthquake.
Damage
to the old portion of the Olive View Hospital in the 1971 San Fernando
Earthquake. This building had a very heavy tile roof supported on
unreinforced brick masonry and was neither designed nor detailed to resist
seismic effects.All of these old buildings suffered significant damage and were subsequently demolished. The lessons from such damage are clearly to avoid the use of unnecessarily heavy roofs and unreinforced masonry.
Elevation
of one of the wings of the Olive View Hospital Treatment and Care
Building, illustrating the presence of unnecessary masses, 1971 San
Fernando Earthquake.
Note the heavy earthfill (0.46 m) requiring a heavy slab in Slide
J61. This was located at a floor level where significant
discontinuity in stiffness and strength existed in the lateral structural
system. As a consequence of this unnecessary mass and of the
structural discontinuities, this wing of the building suffered the
significant damage illustrated in Slides J62-J64.
Building
of Slide J61, showing the collapse of the
ground roof supporting the heavy earthfill for the garden. The
collapse of the roof was due to shear failure in the columns supporting
the roof. Note also that the shear failure of the supporting columns
caused the precast concrete facade panels to fall from the spandrel girder
on the sidewalk because of poor connections of the non-structural panels.
Building
of Slide J62. Close-up of the failure
of the roof supporting the garden. Note the heavy earthfill and roof
slab with its watertight membrane that was needed to support the garden
soil. Note also the distorted and disrupted (distressed) corner
column, and how the pounding of the main building against the only
stairtower that remained standing caused this tower to tilt.
Building
of Slide J62. Close-up of the failure
of the roof slab supporting the garden shown in Slides
J61 and J62. Note the structurally unnecessary mass of the
soil required for the garden, as well as the waterproof membrane.
Note also at the top of the slide the distress in the beam-column joint at
the floor above the garden roof.
Copyright 1997, The
Regents of the University of California.
Structural Engineering Slide Library, W. G. Godden, Editor
Set J: Earthquake Engineering, V. V. Bertero