Masonry Magazine April 1986 Page. 34
the columns. Other floor systems used an earth fill between an upper and lower reinforced concrete slab. In some instances, the earth fill was replaced with a pyramidal light gage steel framing system. Many examples were observed of multiple-floor "pancakes," in which several levels of floors fell straight down, leaving the columns upright. In such cases, it appeared that that the slab-column connections had not been properly designed and detailed for shear and moment transfer.
As a result, beam-column connections were observed to have come completely apart, resulting in structural collapse, even though the individual columns and beams had suffered relatively little damage.
Structural Steel Frames
Steel is a relatively expensive building material in Mexico and therefore only a limited number of steel frames have been built. There are, however, some steel frame buildings in Mexico City, and their high lateral flexibility tended to place them in a period range which made them susceptible to high inertial forces.
One of the most serious failures (Pino Suarez) involved a steel frame building. Most structural steel shapes must be imported, while plates are produced in Mexico. Therefore, steel buildings are generally made with tubular columns built up using welded or riveted plates, and with open-web joists welded from plates and bars. A framing system which was used for several buildings consisted of built up box columns connected by a floor truss which appeared to be similar to a heavy steel floor joist. This results in a lightweight framing system which is probably quite economical.
The first is an 11-story steel frame located in the Roma district. The structure appeared to have three bays in the longitudinal direction (80 feet) and one bay in the transverse direction (30 feet). Lateral resistance in the transverse direction was provided by concrete shear walls which extended the full height of the structure. In the longitudinal direction lateral resistance was provided by the box column-floor truss system and possibly an interior elevator shaft. The building survived the earthquakes and was still standing, and in this sense had performed very well.
However, there was considerable glass breakage in the longitudinal direction indicating the frame had experienced large deformations. In addition, the structure had a substantial permanent drift in the longitudinal direction. It is questionable whether the frame can be used again due to the excessive flexibility.
A very dramatic failure of a steel high-rise building having the box column-floor truss framing system occurred at the government office complex at Pino Suarez. The total complex consists of two 14-story towers and three 22-story towers which rise out of a common one-story base. The smaller towers were located at each end with the three large towers in the middle.
During the earthquake, the 22-story tower at the south end of the complex collapsed in the transverse (south) direction and fell over the 14-story tower, bringing them both down in a heap of twisted steel and concrete which extended into the adjoining street. Fortunately the offices were not occupied since work does not begin until 8:00 a.m. However, a cafeteria in the first floor of the 14-story tower was open and occupied by an unknown number of people.
The middle, 22-story tower was still standing but had a very noticeable drift at the top of almost 36 inches to the south. The exterior facade consists of unreinforced concrete masonry panels connected to the steel frame by a clip angle at each story level. These have deformed relative to the frame but have only come off in one or two cases. This indicates that they have provided almost no lateral resistance.
At the third story level of the middle tower, the corner columns on the south face are both buckled in the longitudinal direction of the frame which is most probably the weak axis of the column. In buckling, the column has pushed off the metal cladding on the exterior of the column. The 22-story south tower which failed appears to have come apart at the top of the third-story level.