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WRK provides comprehensive Seismic Resiliency Planning Programs to protect your full inventory of physical assets and to prepare immediate backup and recovery plans. WRK has led teams on a multitude of seismic strengthening and earthquake engineering related projects for public and private utility clients and multi-national corporations.

An effective Seismic Resiliency Planning Program will address the need for reduction of outages when restoration of water and electric services is critical to post-earthquake response and recovery for industrial, commercial and residential customers.

Components of a successful Seismic Resiliency Planning Program include:

  • Seismic evaluation of all existing facilities, substations, transformers and other equipment within the agency
  • Strengthening of existing structures
  • Seismic hardening (anchorage) of necessary equipment
  • Post-Earthquake Building Safety Evaluation training for staff and personnel
  • Development of a Building Seismic Inventory Database within which to prioritize the timing of upgrades
  • Creation of a Seismic Design Manual for all new and existing buildings and equipment

Education in the wake of an earthquake

WRK president, Brian Knight, explains to the DJC Oregon how the Pacific Northwest can learn from the Central Mexico Earthquake.

On Sept. 19, a 7.1 magnitude earthquake rocked central Mexico. With a major seismic event expected to hit the Pacific Northwest in coming decades, engineers visited Mexico City recently to see what they could learn to apply here.

“One of the biggest challenges is to get people prepared and make it a priority for the earthquake that is coming,” said Brian Knight, a principal of WRK Engineers in Vancouver, Washington.

Knight traveled to the quake zone Sept. 28 to Oct. 5 as part of a team. He was joined by Erica Fischer, an assistant professor of structural engineering at Oregon State University; David Swanson, principal/director for structural engineering at Reid Middleton in Everett, Washington; Darin Aveyard, a Reid Middleton project engineer; and Kenny O’Neill, another Reid Middleton project engineer…

…read the rest with this link to the Oregon DJC article.

Education in the wake of an Earthquake article in the DJC

 

 

Mexico City Earthquake Reconnaissance – Day 5

Central Mexico Earthquake Reconnaissance – Day 5

 

 

Today was my last day in Mexico City, but Team 2 (Infrastructure focus) from Reid Middleton would be staying for another few days.  There were also several other teams of engineers from the U.S. that were actively doing earthquake reconnaissance as well.  Teams from EERI (Earthquake Engineering Research Institute), GEER (Geotechnical Extreme Events Response), and Degenkolb Engineers.  We met for dinner last night with the Degenkolb group to trade notes, share information and learn from each other.  It was a great experience for several reasons. First of all, everyone spoke English and, as a non-Spanish speaker, this was great!  But more importantly, collaborating with other professionals who are passionate about learning from earthquakes and observing the recovery process is energizing to me.  There is simply too much geography to cover in Mexico City and surrounding areas in just 5 days for any one team, so sharing our experiences helped to fill in the information gaps we had.  Through EERI, this process continues even as we are all back in the US.

 

Reflections

As I look back on my time in Mexico City, there are numerous take-aways for me, both technical and non-technical.  What struck me the most was the response of the people.  The resilience, determination, stamina and gratitude.  For example, we visited a building collapse site in the La Condesa neighborhood earlier in the week.  There were hundreds of people involved with the recovery efforts at the collapse site.  It looked very organized, but I was surprised to learn that nearly everyone there was a volunteer that had come to help.  The response was organic and grew from concerned neighbors, friends, family and fellow Mexicans.  People from across Mexico City, as well as people from other parts of Mexico and even other nations (France, Israel, Brazil), came to help.  For each of the recovery workers actively searching the collapse site, there were many more people in support roles.  We were there nearly two weeks after the collapse and some people we spoke with had been there the entire time.  Relentless.

 

There were also several instances of people thanking us just for showing up.  From the group of school children handing out cookies (see Day 2 report) to an elderly lady who thanked us for just looking at her building, these brief encounters will stay with me.  Even though we were not actively part of the search, recovery or repair process, I was surprised at the gratitude and friendliness the local people expressed when they saw us in hardhats and neon orange vests.  Maybe our presence simply gave them reassurance that “official” looking engineers were there to help.

 

Signs of Resilience

To me resiliency is a time measurement of how quickly systems (such as cities) impacted by external forces (such as earthquakes) absorb, respond and restore life to “normal” (i.e. pre-event conditions).  Resiliency includes pre-disaster mitigation (i.e. building/infrastructure strengthening), pre-disaster preparation (i.e. plans for responding) and post-disaster response (i.e. rescue, recovery and rebuilding).  Ideally, the built environment has high enough resilience such that the time required to restore life to “normal” is very short (days or weeks).

 

During our trip we observed clear signs of low resiliency, such as damaged or collapsed buildings and broken infrastructure.  These items are easy to spot and broadcast on news media very quickly.  But there were also many signs of high resiliency too.

 

After the 1985 earthquake, Mexico invested in a network of earthquake detection sensors and an early warning system to provide real-time alerts when earthquakes happen.   Our understanding is that early warning sirens in Mexico City blasted for at least 20 seconds before the strong shaking started.  For the buildings that suffered either partial or total collapse, I assume this warning system likely saved lives, particularly people who were close to building exits and could make it out safely before the strong shaking started.

 

But we learned that several bodies were recovered in the stairwell at the La Condesa building collapse (mentioned in Day 2 report).  Perhaps if these stairwells were designed and constructed as stronger “areas of refuge”, these lives could have been saved even as the building around them collapsed.  The early warning system gave these people enough time to reach the stairwell, but sadly they couldn’t make it any further.  One possible mitigation idea is, if seismic strengthening of buildings is too costly, then just strengthening the stairwells with intent to provide “area of refuge” that occupants could get to prior to strong shaking might be feasible.

 

Another sign of high resilience was the action taken to provide fresh water.  SACMEX mobilized a multitude of water trucks for delivery of water to the millions of Mexico City residents left without a fresh water supply after the event.  The Director General of the Water System of the Mexico City, Ramon Aguirre Diaz, reported that SACMEX was distributing over 6 million gallons of fresh drinking water per day to Mexico City residence.  This is amazing.   Although the water system network is not highly resilient, the post-earthquake response seemingly was.

 

 

There are many areas of Mexico City that have inconsistent water supply on a regular basis.  This is primarily due to the very poor soil conditions in Mexico City.  In some areas of Mexico City, the ground is actually sinking at a rate of close to 1 foot per year!  It’s easy to understand why buried systems, such as water and sewer, have areas of unreliable supply.  However, as I mentioned in my Day 3 report, SACMEX is working 24 hour shifts to restore water service.  The activation of manpower and materials in response to this event is impressive.

 

Technical Take-Aways

For me the most interesting aspect of this earthquake is the soil amplification of ground shaking and the impact on building response.  Going back to the building damage map published by UNAM (National Autonomous University of Mexico) which shows on outline of the ancient Lake Texcoco shoreline along with reported damaged buildings.  In addition, the map identifies approximate fundamental periods for the ancient lakebed soils.   I assume the soil period is primarily proportional to the depth of soil column above bedrock.

 

It’s interesting that a significant number of the collapsed buildings occur at shallow lakebed soil locations with soil fundamental periods between 1 sec & 2 sec.  This is reinforced by plots of spectral accelerations vs. period developed by UNAM for various stations across Mexico City.  These plots show spikes in the spectra response in the 1 to 2 second period domain.

 

Further, we found information, published by Crowley and Phiho (2006), that estimates the period of vibration (in sec.) for reinforced concrete frames with cracked masonry infill to be T = 0.055H with H = total building height.  Running the simple calculations, assuming a typical 10-foot story height, buildings with a fundamental period between 1 sec & 2 sec occur in the 6 to 12 story range.  Our understanding is that most of the collapsed buildings were in this mid-rise range.

 

Essentially, in the period range of 1 to 2 seconds, the soil vibration was in resonance with the fundamental building period, causing significant building structural response.  For an example, go look up the Tacoma Narrow Bridge Collapse for an example of what happens when structures are excited in their fundamental, or resonance, period for many cycles.  Not good. Due to these amplification effects, the Mexico City basin is a truly unique geological setting and results in high seismic demands on the built environment.

 

Additional take-aways included confirmation that good earthquake design should avoid the use lateral force resisting system layouts that include structural irregularities, such as discontinuous shear walls or braces, highly torsional layout of shear walls, soft stories, weak stories and short/captured columns.  It was readily apparent to me the need for ductile detailing requirements of reinforced concrete members with tight spacing of confining reinforcement (i.e. column ties) and adequate lap splice lengths and locations.

 

Why go look at earthquake damage?

At WRK Engineers, our corporate purpose is “To make a difference worldwide by designing resilient structures that protect lives”.  As such, we are passionate about understanding seismic performance of buildings and infrastructure.  We follow this passion through life-long learning so that we can be the best resource for our clients and engineering community.   Part of our mission is to not just make buildings safer, but better than safe. Resilient.

 

To me, one of the best ways to learn about earthquake behavior of the built environment is to see first-hand how engineered systems performed.  Essentially, earthquakes result in a giant laboratory of test subjects that are each unique in size, shape, configuration and construction.  Each building is different and unique and provides a learning opportunity in terms of figuring out how it was damaged and why.

Learning from earthquakes has formed the backbone of modern seismic design practice and will continue to do so with each subsequent event.  So in essence, we visit earthquakes to learn, to improve, and to grow.

 

We also visit earthquake damaged area to help.  As professional engineers we have a duty to serve public with our skills, abilities and expertise.  The NSPE (National Society of Professional Engineers) Code of Ethics includes “…engineers…must be dedicated to the protection of the public health, safety, and welfare”.  Showing up at earthquake damaged areas and assisting with building safety assessments is critical to the local restoration efforts.  Giving people assurance that their home, school or place of business is safe to re-enter after a major earthquake is essential to beginning the process of returning life to “normal”.

 

Finally, I would like to thank the people at Reid Middleton for allowing me to join their earthquake investigation team.  Their willingness to include me on their team and share information, resources, knowledge in a collaborative manner was great.  The Reid Middleton team included David Gonzalez, David Swanson, Erik Bishop, Nicole Trujillo, Darin Aveyard, Kenny O’Neil, Kevin Galvez, and Drew Nielson.  Also, part of our group was Dr. Erica Fischer (Asst. Professor, Oregon State University) and Mark Pierepiekarz (President, MRP Engineering).   These professionals are all passionate about earthquake safety and without these great people I would not have been able have this experience. Thank you.

 

 


Brian Knight, P.E., S.E.

Founder and Principal of WRK Engineers, a Structural & Seismic Engineering consulting firm that is in the business of solving structural and seismic problems with adaptive solutions that protect and preserve people and assets.

 


Thank you for reading my observations from Mexico City.  If you have questions or comments, please go to our Contact page.  I would love to get your input.

 

Mexico City Earthquake Reconnaissance – Day 4

Central Mexico Earthquake Reconnaissance – Day 4

 

Today we drove 2.5 hours southeast of Mexico City to the city of Puebla. With a population of 1.5 million, Puebla is located 30 miles closer to the epicenter of the September 19th, M7.1 earthquake than Mexico City. The published USGS (U.S. Geological Survey) shake map showed higher levels of ground shaking intensity for Puebla as compared to Mexico City.

We read a September 28th article in the Mexico News Daily with the title “State’s largest hospital has to be demolished”. This article was referring to the Regional General Hospital Number 36, San Alejandro in Puebla. This hospital is a 323,000 square foot, 641-bed facility with more than 2,500 staff members and was constructed in 1976. The hospital, one of the largest in Latin America, reportedly had suffered substantial damage and would need to be completely rebuilt. Based on the article, we assumed there would be other damage to buildings and infrastructure to look at.

 

When we arrived in Puebla we went directly to the hospital. We noticed the hospital was closed and staff was actively moving medical equipment from the facility. However, we saw no real evidence of significant structural damage from the outside, except for what appeared to be some confined masonry infill damage near the top of the building. After walking around the facility perimeter, we were told by hospital staff that only nonstructural systems were damaged and that the hospital did not require demolition.

 

 

 

 

We did notice that tucked inside the perimeter building skin were steel braced frames in a chevron configuration. Based on the age of the building and the heavily reinforced bracing connections, this looked like a seismic retrofit.

 

After leaving the hospital, we ventured to the historic downtown area of Puebla in search of building damage. Our team split into two groups of 4 and walked for about 1.5 hours through the neighborhoods. To our surprise there were very few buildings that experienced structural damage. The primary extent of damage was cracked exterior concrete plaster or damaged parapet cornices, all very minor. However, there were a few buildings that experienced significant cracking of exterior walls and were stabilized with temporary shoring. All the buildings are generally two stories in height with no seismic joint between them. Our thinking is that these building likely all laterally supported one another and rode out the shaking as one big raft of buildings. Also, the ground accelerations reported by UNAM for the city of Puebla turn out to be only 141.7 cm/s2, or 0.14g.

 

 

For the most part, Puebla felt like “business as usual” with normal activities of pedestrians on the sidewalks and vehicle traffic in the streets. Very few businesses appeared closed and it was hard to tell a major earthquake happened less than two weeks prior. On our way out of town we noticed a 14-story residential building with significant “X” cracking of the perimeter walls. Like many other buildings in this region, it appeared to be a concrete frame building with masonry infill walls (i.e. confined masonry). The building had two exterior walls with ample amount of solid wall over most of the building height, but the other two building sides had very little shear wall.

 

 

We could not enter the building, but noticed on one side of the exterior there was only a small segment of exterior wall that was heavily damaged. There is likely a confined masonry elevator or stairwell inside the building, but the building response may have been primarily torsional since only one wall was heavily damaged. One interesting thing we observed was the adjacent building and lack of damage. The building next door, connected at the roof level, was identical, but oriented 90 degrees in plan from the damaged building. Building orientation with respect to the direction of strong shaking likely had some influence on the difference in damage between these two buildings.

 


Brian Knight, P.E., S.E.

Founder and Principal of WRK Engineers, a Structural & Seismic Engineering consulting firm that is in the business of solving structural and seismic problems with adaptive solutions that protect and preserve people and assets.

 


Check back for more daily posts as I share my observations from Mexico City.  If you have questions or comments, please go to our Contact page.  I would love to get your input.

 

Mexico City Earthquake Reconnaissance – Day 3

Central Mexico Earthquake Reconnaissance – Day 3

 

On day three we turned our attention to the performance of lifelines in Mexico City, specifically water and power. The previous days were mostly spent surveying building damage in the area of La Condesa. During that time we ran into a group from GEER (Geotechnical Extreme Events Reconnaissance) performing reconnaissance work as well. They mentioned the neighborhood of Col Del Mar in the Municipality of Tláhuac (southern Mexico City) had experienced areas of significant ground settlement with damaged buildings and buried water line damage. It appeared to them that settlement occurred in old channels or river beds that meandered through the area. We also learned there is a large wastewater treatment plant in the same area. We decided to check it out on day three.

 

We arrived in Col Del Mar to be greeted by a myriad of closed streets that turned the area into a maze of dead ends. After getting close enough to our drop point, we didn’t have to walk very far to observe why so many streets were closed. As the GEER team reported, it appeared there were very distinct “channels” of PGD (Permanent Ground Displacement) running through the neighborhood. We noted vertical ground displacement in excess of 13” at one location. Later we would see vertical displacements closer to 24”.

 

Some areas of PGD seemed to align with older buried utility trenches, while other settlement looked like wide channels or streams. According to Wikipedia, Tláhuac has been experiencing urbanization since the 1960s when it was transformed from a mostly rural area. It is highly likely the distinct areas of settlement we observed were drainage channels that were infilled with relatively loose materials in order to build roads and houses.

 

We learned the three municipalities experiencing the most damage to the buried water and wastewater systems were Xochimilco, Tláhuac, and Iztapalapa. Col Del Mar is right in the middle of this area, so we were hoping to see evidence of pipe damage and repair activities.

 

As we moved on we noticed both reinforced concrete pipe and corrugated plastic lined piping staged at various locations, we assumed it waiting to be used to repair pipe breaks. We encountered repair work for sewer piping in one street, but also saw evidence of potable water piping (6” HDPE) repair as well.

 

We eventually came across a long street where active repair of potable water pipes was occurring. Repair was underway on a 24” diameter asbestos concrete pipe. Repairs were already complete on the larger 48” diameter aqueduct (i.e. transmission) pipe. We were told there were a total of 10 breaks along these two lines over just about a mile stretch. We were also told that SACMEX, the government agency owner/operator of the potable water and wastewater in Mexico City, was working 24-hour shifts to restore water/sewer service. Note, as of October 4th, SACMEX reported fixing over 1,000 leaks in the water transmission and distribution network with still more to be completed. That’s 67 repairs per day, which seems like a lot considering all of the breaks need to be excavated before repairs can begin.

Historically, larger diameter segmented concrete pipe typically fails at joints, tees, or elbows. We were told the pipe joints at these locations pulled apart, which is a common failure mode with bell and spigot joints subject to axial tension. It appears the pipe repairs included cutting out the failed joint in preparation for installing a shorter repair segment with bolted collars.

 

We then traveled to the Cerro De La Estrella Wastewater Treatment Plant to inquire about damage suffered during the September 19th event. However, when we arrived we were turned away by the security guard. The response of “No damage, come back later” was similar to other buildings we visited earlier in our trip.

 

Later that day we learned about damage to the electric power system in Mexico City and further south in the State of Morelos. Damage to the power grid in Mexico City appears to have been limited to the distribution system, primarily pole mounted transformer and disconnect switches. It is uncertain if “wire slap” played any part in the loss of power. CFE (Comisión Federal de Electricidad) reported 4.1 million customers lost power in Mexico City during the earthquake. Power was restored to nearly 100% of Mexico City customers within four days.   This indicates there was likely limited damage to the electric substations in Mexico City. Note that most electric substation equipment consists of high frequency (i.e. very stiff) systems and it is likely the Mexico City substations did not experience strong shaking in the high frequency domain. However, pole mounted equipment—based on the mounting height above grade—may have been subject to stronger shaking due to pole amplification of the ground motions. I could not find out if these distribution transformers experienced anchorage failures or damage due to other factors like short circuit.

 

We also learned the Yautepec Substation in the State of Morelos, closer the epicenter, experienced significant damage, knocking the station out of service. The damage appears to include primarily 500kV equipment. Based on information published by CFE, the equipment that suffered damage includes power transformer bushings, surge arresters (ground mounted), instrument transformers, circuit breakers (live tank), and flexible bus. Damage to the substation control building and contents was not available. The time needed to restore the substation to fully operational was unknown, but based on the extent of damage documented, it will likely take many weeks (perhaps 6+) to replace the damaged equipment and restore this station to full operations.

Additional damage at the CFE Oaxaca Substation (in southern Mexico) was also reported. Damage included anchorage failure of a power transformer and collapse of live tank circuit breakers. The equipment appears to be no greater than 230kV and the damage was reported to be the result of the September 7th, magnitude 8.1 event.

 

 


Brian Knight, P.E., S.E.

Founder and Principal of WRK Engineers, a Structural & Seismic Engineering consulting firm that is in the business of solving structural and seismic problems with adaptive solutions that protect and preserve people and assets.

 


Check back for more daily posts as I share my observations from Mexico City.  If you have questions or comments, please go to our Contact page.  I would love to get your input.

 

Mexico City Earthquake Reconnaissance – Day 2

Central Mexico Earthquake Reconnaissance – Day 2

We wrapped up day one at 11:00 PM, but were back at Starbucks by 7:30 AM again to formulate our plan of attack for day two. After a quick coffee and breakfast sandwich, we headed out again toward the La Condesa area to continue our search for damaged buildings.

 

We learned more about the earthquake motions this morning and how the September 19, 2017 earthquake was very different than the 1985 event. On September 19, 1985, a magnitude 8.0 subduction zone earthquake occurred roughly 230 miles away, but this event shook Mexico City for nearly 1½ minutes. (Note this event was similar to the expected Cascadia Subduction Zone event). The seismic waves that reached the city were very damaging to tall buildings with high fundamental periods. In essence, the soils were excited to a period of oscillation that matched the period of many tall buildings, causing them to be heavily damaged and even collapse. An estimated 10,000 people died in 1985.

 

By contrast, the September 19, 2017 earthquake was a deep intraplate event (similar to the 2001 Nisqually Earthquake) with a magnitude of 7.1. The epicenter was only 76 miles away from Mexico City and strong shaking lasted less than 30 seconds. However, as every earthquake motion is different due to many factors (such as depth, frequency content, duration, etc.), this event did not excite the tall buildings in Mexico City, but rather the low-rise to mid-rise buildings which have lower fundamental periods.

 

Accelerometers stations located around Mexico City recorded the ground shaking and revealed higher accelerations were felt by these low- to mid-rise buildings at locations primarily in alignment with the ancient Lake Texcoco shoreline. The shoreline has thinner layers of lakebed deposits, which respond differently than areas with thicker deposits. This earthquake excited the thinner layers, which in turn amplified the earthquake motions at the ground surface and resulted in devastating effects on many low-rise and mid-rise buildings.

 

Preliminary data developed by the University National Academia Mexico (UNAM) shows acceleration data from stations scattered around the city. Most of the stations show low ground accelerations with very little amplification of ground shaking due to soil conditions. However, there were two stations (stations LEAC and SCT2) that show higher levels of strong shaking occurring in the one to two second period range. This is attributed to soil conditions at these locations, which are both on shallow Lake Texcoco materials. It is interesting to note that for confined masonry systems, the estimated fundamental period for these buildings with heights between four and twelve stories roughly corresponds with the one to two second period range. Based on the buildings we observed that were heavily damaged or collapsed, they seem to fit within this range of four to twelve stories in height.

 

We resumed our “walk about” in the La Condesa area and came across an eight-story building that suffered complete collapse, or “pancake failure”, of the sixth story. We were told one person perished in this building at the collapsed sixth level. This confined masonry building is sandwiched between four-story buildings on either side with no apparent seismic joints between them. Similar to the building we entered yesterday with column failures, this building has virtually no shear walls on the front and rear sides of the building, but has solid walls on the longitudinal sides.

 

It appears the building failure may be attributed to the fact that its lateral strength in the transverse direction was very weak, but it is also due to the lack of seismic joints between adjacent buildings. It appears the adjacent buildings may have helped to laterally restrain the lower five stories and resulted in significant earthquake force and displacement demands being concentrated at the sixth story. The lower stiffer buildings may have “locked” the base of the taller building in place, resulting in a “flag pole” effect at the sixth story. These amplified demands appear to have overwhelmed the available lateral strength at this level, leading to collapse. Without adequate seismic joints, building designers must take into account the adjacent buildings to truly understand the response impacts these adjacent buildings will have.

 

While we were on the street taking photos of the partially collapsed building, we were approached by a group of schoolchildren and parents handing out cookies and paper origami in the shape of hearts. They thanked us many times for coming to Mexico City to help with building inspections after the earthquake. It was inspiring to see such warmth and hospitality after their city and neighborhoods were scarred with building damage and loss of life. The resiliency of the people after this earthquake was apparent. These children wanted to help their community in any way they could, and for them it meant handing out cookies and paper hearts to the response and recovery workers they encountered. Very cool!!

 

After a quick lunch break we learned of another collapse site several blocks away and started to meander our way towards it. Along the way we saw numerous examples of damaged confined masonry buildings. These buildings, for the most part, fit within the four to eight story range and appeared to have experienced significant shaking.

 

We arrived at the street intersection where a building collapse occurred. On the morning of September 19th, an eight-story building occupied this corner. However, when we arrived all that was left of the residential building was the first level. Since the earthquake, many buildings that collapsed have been removed. We were not certain why the upper seven stories collapsed while the first level did not, but it may have been due to the very thick first level slab, which appeared to be close to 18” thick. We could not find out if anyone perished in this building, but demolition only occurs after all rescue and recovery is completed.

As we wound down the day, I was reflecting on the “take-aways” for the past two days. Damage ranged from broken glass and delaminated concrete plaster, to adjacent building pounding damage, heavily cracked confined masonry shear walls, concrete column failures, and out-of-plane masonry infill failures. A common feature noticed in nearly every heavily damaged building was the presence of lateral force resisting system irregularities, such as discontinuous shear walls or braces, highly torsional layout of shear walls, soft stories, weak stories, and short/captured columns. The damage witnessed only reinforces to me the need for strong seismic design standards and thoughtful building lateral system layout skills to avoid “bad actor” buildings subject to strong ground shaking.

 


Brian Knight, P.E., S.E.

Founder and Principal of WRK Engineers, a Structural & Seismic Engineering consulting firm that is in the business of solving structural and seismic problems with adaptive solutions that protect and preserve people and assets.

 


Check back for more daily posts as I share my observations from Mexico City.  If you have questions or comments, please go to our Contact page.  I would love to get your input.

 

Mexico City Earthquake Reconnaissance – Day 1

Central Mexico Earthquake Reconnaissance – Day 1

I arrived in Mexico City on Wednesday afternoon, having never been to Mexico City before, what immediately struck me while flying in was the enormity of the Greater Mexico City area.  As one of the largest metropolitan areas in the world, with a population of 21.3 million, the sheer size of the urban area was impressive.  It looked like a carpet of concrete stretching for miles in all directions.

 

On day 1 I joined a group of structural engineers from Seattle, San Diego & Honolulu who arrived earlier in the week.  Our day began at 7:30 AM with a meeting at Starbucks to grab coffee and plan out our day.  We spent a few hours trying to identify the locations where damaged or collapsed buildings were reported by the Mexican government, Ciudad de Mexico (CDMX), and other agencies.  Most of the reported building collapses were reported in a swath of the city that coincides with the edges of the old Lake Texcoco lake bed, which most of greater Mexico City is built upon.  We spent time pouring over maps of Mexico City to match up reported damage with the lake bed “shoreline”.  We would use this as a guide to direct our path in looking for more damaged buildings.

 

After our planning session was complete we set out on foot to survey building damage in the La Condesa area of Mexico City where a number of buildings collapsed.  This area is made up of primarily residential, office and retail buildings.  The buildings vary in height from one-story to 15 stories.  Many of the taller buildings (i.e. more than 10 stories tall) that survived the 1985 Mexico City Earthquake appeared to have been seismically strengthened. Many “creative solutions” were used, but most utilized a scheme that included adding exterior steel braced frames, or braced frames tucked just inside the building perimeter envelope.  One of the more creative seismic strengthening schemes used large diameter tension cable X-bracing on the building exterior. This 12-story building was seismically strengthened with high-tensile strength cables, post-tensioned in place and attached to the existing exterior steel columns.

 

The predominate construction type in Mexico City consists of reinforced concrete buildings with unreinforced brick masonry infill, often referred to as “confined masonry”.  The walls typically finished with a thin layer, or skim coat, of concrete.  The floors and roof typically consist of reinforced concrete slabs while the interior partition walls are constructed with unreinforced masonry brick or hollow clay tile block.  Like the US, many buildings are separated from one another with seismic joints (i.e. air gaps) that vary from 4” to 12”, depending upon building height.

 

The buildings are very heavy and often have very little, if any, solid wall on the perimeter side facing the street.  Buildings that do have perimeter walls on the street side often have very little wall at the street level to accommodate garage entrances, retail shops or residential lobbies.  The other three building perimeter walls are often solid with little or no openings.  As a result, these buildings have lateral force resisting systems that have torsional irregularities and contain “soft stories”.  Buildings with these types of lateral force resisting system deficiencies often experience significant damage or collapse during strong shaking, as was the case for many buildings in Mexico City.

 

As mentioned earlier, the buildings varied in height and it was very common to see a 4 story building next to an 8 or 10 story building.  Adjacent buildings will respond and sway back and forth differently based on the building height and construction materials.  There were many instances of building damage due to inadequate seismic joints, which resulted in pounding effect between buildings.

 

As we moved through the streets, many buildings had yellow “caution” tape or red “do not enter” tape in front of the building or blocking the sidewalk to restrict access.  We found damage scattered throughout the neighborhood that ranged from damaged parapets to total collapse.  It was interesting to note that damage was not consistent from building to building, even if adjacent buildings appeared to be very similar.  One was damaged, the other was not.

 

One of the more dramatic instances of building damage we observed was at a 7-story office building.  The building has a partial basement parking garage that is depressed 5 feet below exterior grade.  The building is rectangular shape in plan with solid confined masonry walls at each side of the building in the longitudinal direction.  However, in the transverse building direction, there were only two short walls surrounding the stairwell to resist earthquake demands.  The front and rear sides of the building were open with glass at the rear of the building and balconies at the front.

 

Outside the building was a large board showing pictures of the interior building columns that “exploded” from the inside out.  The photos showed the columns had lost nearly all vertical load carrying capacity.  The people who entered the building to install shoring were very lucky no aftershocks occurred while they were inside the building as it would have likely experience partial, if not total, collapse.

 

The building owner’s representative was stationed outside to prevent entry, but after showing him our credentials as structural engineers, he agreed to take us on a limited and rapid tour of the building.  We entered the building through the basement and noticed the interior columns were shored with 4×4 wood posts and cribbing.

 

They had shored the columns over the lower 3 stories, where the columns were damage. It appears during the earthquake, due to lack of shear walls in the transverse direction, the building story drifts were significant enough in the transverse direction that the gravity columns in the lower 3 stories failed due to lack of confining transverse reinforcement.  While inside the building, I observed the transverse reinforcing consisted of either #3 rebar or #2 tie wires at +/- 12” o.c., with non-seismic hooks.

 

At each column location we visited, the vertical column reinforcing bars had buckled and the concrete core had fractured due to lack of transverse reinforcing.  While standing on the 2nd Floor, it was noticeable that the 3rd floor had dropped approximately ½” due to column failure.  This was evident in the nonstructural wood framed partition walls that were still in place, but heavily bowed outward by having to support the slab weight.  Essentially, these partition walls became load bearing walls and assisted in holding up the floors.

I had not observed concrete column failures like this, only through photos of previous earthquake reconnaissance.  I probably should have been more nervous than I was, but I didn’t feel uncomfortable in the building as all the damaged columns had been shored and we were only inside the building for what felt like 10 minutes.   Seeing this damage only reinforces the need to design earthquake resilient buildings that protect lives and property.

 

We continued to walk the area looking for building damage.  Several blocks away we arrived at one of the 44 collapsed buildings in Mexico City.   This building was a soft story structure with only concrete columns at the street level and confined masonry walls above.  Recovery operations were underway with an army of volunteers from all over the globe helping.  We observed teams from France, Israel, Brazil and the US, in addition to the dozens of Mexican nationals.

It was very sobering to know that many people, including children, lost their lives in this tragedy.  I can only wonder if seismic strengthening of this soft story building may have prevented this needless loss of life.  Being witness to the aftermath of needless building collapses only makes me more determined to be a strong advocate for seismic safety and resilient buildings worldwide.


Brian Knight, P.E., S.E.

Founder and Principal of WRK Engineers, a Structural & Seismic Engineering consulting firm that is in the business of solving structural and seismic problems with adaptive solutions that protect and preserve people and assets.

 


Check back for more daily posts as I share my observations from Mexico City.  If you have questions or comments, please go to our Contact page.  I would love to get your input.

 

High Voltage Substation Seismic Anchorage

Nevada

Designed seismic retrofit anchorage of existing high voltage substation equipment for over 70 NV Transformers throughout their service territory. This highly specialized work included extensive field investigation time in order to capture the in service condition. All anchorage designs called for installation and construction activities to occur while the substations remained fully-energized. Knowledge of the substation environment and operation aspects of the effected equipment is essential for successful completion of this work.

Power Generations Station, Seismic Risk Assessment

Greater Los Angeles, California

Served as part of team that provided preliminary seismic risk assessment for three power generation stations in the Greater Los Angeles, California area. Power stations at Huntington Beach, Los Alamitos and Redondo Beach were included in the Seismic Risk Assessment. Project included site walk-down at each station and identification of likely seismic deficiencies which would affect power generation operations during a major seismic event. Provided recommendations for addressing identified seismic hazards.

System Earthquake Risk Assessment (SERA)

Washington, Oregon, Idaho, Montana & California

Worked with BPA to collect field data and provide computer input for nearly 130 substations across the BPA transmission system grid. The computer program SERA is used to identify seismic hazards on a system wide basis by utilizing estimated fragility values to calculate expected damage levels of substation equipment when subjected to scenario earthquake simulations.

Seismic Building Mitigation Program

Various Locations

Serving as prime consultant for BPA, led a multidisciplinary design team to provide building seismic evaluation and develop seismic strengthening construction documents for 38 mission critical buildings including a Control House all in the BPA system. The design team, which included architects, mechanical engineers, electrical engineers, cost estimators & hazardous materials consultants, was assembled to address identified seismic vulnerabilities for both structural and non-structural building systems. All strengthening measures considered the highly-sensitive building functions and allowed for construction activities to occur during continuous, 24-hour operations of the Control House. These buildings were strengthened to provide Immediate Occupancy Performance following a major seismic event.

"It is good to have a resource with experience to help establish a proper and accurate analysis of the equipment so as to limit cost by getting the design right the first time… WRK is not simply asking us what we want them to do, but they are trying to understand what we want to do, and then use their experience to determine the best way to get there."

Wesley Wills, Southern States LLC.

"The benefit of working with WRK is that they are creative. I work with many engineers that are very conservative–they prefer to stick to standard details. WRK brings innovative solutions and enjoys the challenge of using new technologies that might be less costly and easier to construct. Brian was trained to be innovative, and his firm operates the same way."

Jeff McGraw, MWA Architects

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Casey Scoggins, Tennessee Valley Authority

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Matt Weisensee, PacifiCorp

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Casey Wyckoff, LSW Architects

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Leon Kempner, Bonneville Power Administration

"I value WRK’s nuts-and-bolts approach with their ability to communicate in the field and provide constructable solutions. Historic projects can be convoluted, which can cause structural engineers to jump through hoops. This can be difficult for many engineers, but not WRK. They are great at considering and explaining all of the options with the understanding that structural isn’t driving the design."

Eric Philps, SERA Architects

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