|Mitigation of Disasters in Health Facilities: Volume 3: Architectural Issues (PAHO)|
|Chapter 3: mitigation measures in hospital design|
|Problems of form and volume|
Concentrations of mass
The problem in question is caused by high concentrations of the total mass of the building on a given floor because heavy elements, such as equipment, tanks, storerooms, files, etc., have been placed there. The problem worsens the higher the heavy floor is, since the seismic response accelerations also increase upward and there is consequently a greater seismic response force at that point and a greater possibility of collapse.
The architectural design of these buildings should place whatever areas involve unusual weights in basements or in separate structures near the main body of the building. In cases in which for topographical reasons large amounts of water must be stored at high elevations, independent towers should be selected for this purpose instead of towers attached to the main building.
The seismic design of frames aims at ensuring that the damage produced by strong earthquakes occurs to beams, not columns, since there is a greater risk of building collapse from the latter type of damage. However, many buildings designed according to seismic resistance codes have failed for this reason. These failures can be grouped into two categories:
· Columns with less resistance than beams
· Short columns
There are several reasons why the value of free length is reduced drastically and the result can be considered a short column:
· Partial lateral confinement of the column by dividing
walls, facade walls, retaining walls, etc.
· Placement of slabs at intermediate levels
· Location of the building on a slope
Short columns are the cause of serious failures in buildings subjected to seismic excitation, since their failure mechanism is fragile. The most appropriate solutions in the case of all kinds of wall that impede the free movement of the column consist basically of placing the wall in a different plane from that of the column, or in separating the wall from the column by means of joints. In the case of buildings with intermediate levels, the architectural design should consider locating the columns outside the transition line between the levels. Finally, on sloping land, the foundations of the columns ought to be sunk at greater depths.
Several types of architectural and structural plans lead to the formation of so-called "soft" stories, that is, stories that are more vulnerable to seismic damage than others, since they have less stiffness, less resistance, or both. The usual plans are:
· A story that is significantly taller than
· Interruption of vertical structural elements on the floor
· Construction on a slope
The first case frequently arises because of a desire to place greater masses at certain levels of the structure, usually for technical reasons (equipment requirements, etc.) or aesthetic reasons (the building's appearance at the access levels, etc.). As a result, stiffness on the floors in question weakens due to the higher elevation of the vertical elements and of the resistance.
The interruption of vertical elements of the structure has proven to be the cause of multiple partial or total collapses in buildings subjected to earthquakes. The reason is that the floor on which the elements are interrupted has greater flexibility than the others, thus aggravating the problem of stability; in addition and mainly, however, a sudden change in stiffness takes place, causing a greater accumulation of energy on the weaker story. The most common cases of such interruption, which usually occurs by virtue of size, form, or aesthetic reasons, are the following:
· Interruption of the columns.
· Interruption of structural walls (shear walls).
· Interruption of partition walls, conceived erroneously as nonstructural, aligned with frames.
Setbacks in the mass of a building are generally used because of city planning requirements relating to illumination, proportion, etc. However, from the standpoint of earthquakes, they cause sudden changes in stiffness and mass and accordingly give rise to the problems mentioned above of concentrating the destructive energy in the floors near the area of the sudden change. In general, the aim should be to make the transitions as smoothly as possible in order to avoid such concentration.
Inverted setbacks should be avoided in seismic areas, since they also involve a serious risk of overturning, as mentioned with the distribution of mass (see Figure 25).
Graphic interpretation of "irregular structures or framing systems," from the Commentary to the SEAOC Recommended Lateral Force Requirements and Commentary. Reproduced in Arnold, Christopher and Reitherman, Robert, Building Configuration and Seismic Design (New York: John Wiley & Sons, 1982, p. 8). Reprinted with permission of John Wiley & Sons.