Welcome from Mexico City!
The Exploration Begins
January 26, 2018
I arrived in Mexico City around 6 A.M. local time, but by the time I made it through customs the city was just waking up. Almost immediately after making it to the hotel, I was back in a car to make it to a meeting with collaborating researchers who were already “on the ground” in Mexico to discuss and find answers to some questions and concerns for the following days of investigation. What followed was a full day of meetings with engineering professors from Universidad Nacional Autónoma de México (UNAM) as well as meetings with local structural engineers who were open to discussing which of their building designs had and had not been damaged in either of the 1985 earthquake and more recent September 2017 earthquakes.
As we drove from place to place, I began to discover that Mexico City has an extremely diverse array of architecture, but I had little chance to explore it beyond what I could see outside on my car window. Yet it is extremely interesting how the different styles coexist, sometimes with quite contradictory architecture spanning across a single façade.
And also the food is amazing here! I tried three different type of quesadillas for dinner my first night, one involving what I believe was pumpkin flowers, another with spiced potatoes, and all with very innocent looking sauce that would have the back of my throat burning after the first few bites. I believe the group of professor’s and researchers really bonded that night over how not used to spicy food most of us were.
But back to the technical structural engineering part of the trip!
Our goal for this research trip is primarily to record data for future use in assessing and understanding the factors that led to the damage in these buildings, as well as starting to explain why some buildings collapsed while the building right next door went nearly completely undamaged. One of our main interests is the study of the dynamic responses of these buildings to try to glean some information on how resonant frequencies could react to the excitation frequencies of the earthquakes. The team we have gathered this trip is focused on using accelerometers and the natural low level vibrations or movements of buildings to evaluate the dynamic characteristics (mode shapes and frequencies) of these buildings. Our team – from Cal Poly, UC-San Diego, Purdue, University of Nebraska, and University at Buffalo – all landed in Mexico City hoping to get some interesting data and some ideas of what was happening.
However, if there was one thing that I came away from this first day talking to local structural engineering experts is that Mexico City is literally constructed upon a huge wildcard: its soil. Over the course of the day the soil was described using many analogies, some of them partially lost in translation (not many of us on the research team, including myself, understand more than a handful of words in Spanish) but the effect was that the soil acts like a giant bowl of Jell-O. I’m sure all of us have at one point played with a bowl of Jell-O by shaking it back and forth. This seems to be the exaggerated version of Mexico City in the earthquake, and a lot of these damaged structures went along for the ride. In fact, it came up in one of the meetings that the soil has its own resonant frequency, and that we would have to be aware throughout our studies that the soil itself exhibits a dynamic response to ambient vibrations and may influence our structure greatly through soil structure interactions.
While our local colleagues made a few other mentions about earthquake performance of certain building techniques such as soft-story buildings (apparently these are more common in Mexico due to less strict code regulations for soft story structures than we see in the United States), as well as concerns about older structures being built without too much concern for ductility. However, as the day progressed it all seemed to come back to how the city’s structures were interacting with the soil, and we were left with ominous warnings to not underestimate the effects of the underlying soil that each building was constructed on and how that would impact our ambient vibration measurements.
However, we left our many meetings with local faculty and engineers excited at the prospects of getting access to a range of damaged and undamaged buildings, including buildings that had been retrofitted post-1985 as well as damaged buildings with plans for retrofit in the works. For the next few days, we plan to start by instrumenting two to four story school buildings, before hopefully getting our hands on some taller structures.