Field inspections of earthquake performance of buildings demonstrate that the simpler the building the better the behavior, all other parameters being similar. As pointed out by Dowrick, there are two main reasons for this: first, it is easier to understand the overall earthquake behavior of a simple building than of a complex one; second, it is easier to understand, formulate in drawings, and construct simple structural details than complicated ones. Symmetry and regularity in plan and elevation are desirable for much the same reasons. Symmetry is important in both directions of a plan. Lack of symmetry (in mass distribution and/or in stiffness, strength and ductility) leads to torsional effects which are difficult to assess properly and which can be very destructive. A plan layout with reentrant angles should be avoided (Slide J65). A regular rectangular plan building with asymmetrical stiffening, as illustrated in Slides J65-J68, should also be avoided because it will create significant torsional forces.
Torsional
effects created by irregular shape of building plan (L configuration) and
by a very stiff off-center core area in a rectangular (regular) plan
building.
Hotel
Terminal, Guatemala City. Overall view of this 6-story hotel,
illustrating the torsional failure of the second story during the 1976
Guatemala Earthquake.The reinforced concrete hotel in Slide J66 had a rectangular plan, but a very stiff service core area was placed anti-symmetrically near the far end of the building. The eccentricity induced by this service core was balanced in the upper stories (above the second story) by the solid masonry walls that were used as infills for the reinforced concrete frame located in the front end of this picture. However, these walls were interrupted in the second story (where the dining room was located) to have only glass so that there would be a view of the plants on the terrace and of the city. This interruption created a large eccentricity in this story which resulted in very large torsional forces that sheared the reinforced columns as illustrated in Slides J67 and J68.
View
inside the building of Slide J66 showing the collapse of the second story
due to shear failure of the second-floor columns. Note the
significant lateral displacement (interstory drift to the right) due to
the torsional rotation of the upper part of the building.
Close-up
of one of the collapsed columns of Slide J67. Note that the
upper floor has displaced to the right and dropped, and the top and bottom
sections of the column are now side-by-side. Although the columns
had lateral reinforcement (ties) these were not enough and at inadequate
spacing to resist the shear force developed due to the torsional moment
which originated in the second story. The failure described in Slide J68, emphasizes the importance of avoiding large torsional forces and the need for providing an adequate amount of transverse reinforcement with proper detailing.
Copyright
1997, The
Regents of the University of California.
Structural Engineering Slide Library, W. G. Godden, Editor
Set J: Earthquake Engineering, V. V. Bertero