Day 6: Short Stories & Poor Building Configurations
Structural Investigations & General Takeaways
november 2, 2017
Today started like all of the other days except for one difference: we took a thirty-minute Uber ride south towards the mountains and then a 5-mile walk in the blistering heat to arrive at Av. Canal de Miramontes. In our database, there appeared to be many damaged buildings clustered in the vicinity and we wanted to investigate why that was the case. There we met up with Dr. Mario Rodriguez (who I met on Day 2), so he would show us the significant damage along certain streets in this area.
Seismic Recording Station: Reviewing the Records
Along Miramontes there are primarily residential buildings with an average height of 5-stories. The map below shows the nearest seismic recording station in Parque Jardines de Coyoacán (Station # JC 54) indicating a peak ground acceleration of 220 gals (1 gal = 1 cm/s2), equivalent to about 0.22g. To the right of the map is the acceleration record from this station for N-S, E-W, and vertical motion. Note the significant spike in magnitude in the N-S direction nearing a time of 70 seconds. Also located below is a response spectra which plots of building period versus spectral acceleration Sa for the 1985 and 2017 earthquakes, as well as the design spectra for the site (orange, blue, and grey lines; respectively). It is important to note that how both earthquake spectra exceed the design spectrum.
ABOVE: Parque Jardines de coyocan amax: 220 gal (left), ACCELERATION RECORD (CENTER), rESPONSE SPECTRA (RIGHT)
Structural Damage in Miramontes
There appeared to be a few factors that resulted in the concentration of building damage around Miramontes:
- the motion was predominantly in the N-S direction, same as the orientation of the street;
- the region is located on transition zone soil characterized by highly-compressive clayey layers alternating with sandy alluvial deposits; and
- many buildings exhibited designs with either soft stories or captive columns.
Our investigations of apartment buildings along Miramontes revealed wide shear cracks-to-partial collapse of masonry infill walls and significant structural damage to first-floor columns. The team concluded that the severe damage to masonry infill likely led to loss of stiffness in the building requiring the columns to carry considerably more demand. Dr. Rodriguez pointed out that despite the susceptibility of unreinforced masonry infill walls to severe damage, they are quite prevalent in Mexico due to their low cost, ease of constructability, and construction tradition of over a hundred years.
Further down the street we saw families moving out of a six story apartment building where columns had severe cracks and partial/complete collapse of masonry infill wall panels.
On the adjacent street, we found two nearly identical apartment complexes, differing only by the first floor design. The building shown on the right had continuous masonry walls, while that shown on the left had an open garage making it susceptible to soft-story behavior.
The apartment building, shown above on the left, exhibited shear and flexural cracks on columns and exposed rebar on its deep spandrel beams. The most important thing I learned from this case was that open first-floor garages have a tendency to reduce the building stiffness at this level where the shear force is the greatest.
Lessons From Today's Investigations
- Be careful of first-floor soft stories with low stiffness particularly where shear demands are expected to be high. Use columns with larger cross-sections or more reinforcement so these members (and the story as a whole) can achieve a greater stiffness.
- Unreinforced masonry infill is brittle and can experience significant non-structural damage. It should be noted that intact infill walls do add stiffness/strength to buildings with reinforced concrete frame systems and when these infills fail, stress will be redistributed to the frame and may lead to column damage.
- Avoid designing short/captive columns that tend to fail in a brittle manner via shear as shown in the image below. If short/captive columns are unavoidable, they need to be designed to resist high shear demands using larger cross-sections or more closely spaced shear reinforcement.
