Sunday, January 28, 2018

Bridges of Mexico: Puente Xiotepec Carrying Highway 95D across Rio Apatlaco in the State of Morelos

September 2017 (18.7625, -99.2343) Xiotepec Bridge
While my team investigated earthquake damage in the State of Puebla, another team was studying damage in the State of Morelos. That team found quite a bit of damage related to weak soils. A single span 125 ft long highway bridge on the Mexico City-Acapulco Highway 95D, south of Cuernavaca (and 50 miles south of Mexico City) collapsed during the 9/19/2017 Mexico Earthquake.

In the photo above, we can see that the span moved away from the south abutment and fell onto the banks of the Rio Apatlaco. By the time the second reconnaissance team got to the site, the bridge had been removed (see photo below) in preparation for building a replacement bridge on the site.
Fortunately, there was another bridge less than 200 yards away. Although Puente Xiotepec collapsed, Puente Xochitepec continued to carry traffic on Highway 95D across the river. Also,  Highway 95 (a few hundred yards to the west) remained undamaged (see Google Earth photo below).
Recorded ground motion in the area was too low to damage engineered structures but the soil was weaker and could cause damage to structures for peak ground accelerations (pga) as low as 0.10g. (see USGS ShakeMap below). The team that went to Moreles found a lot of soil-related earthquake damage.
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Sunday, January 21, 2018

Bridges of Mexico: Undercrossing over Calzada de la Viga in Mexico City

September 2017 (19.3696, -99.12248) UC Eje 2 Ote Calzada de la Viga
My next few blogs include photos taken by other earthquake reconnaissance team members and collaborators. Professor Eduardo Miranda of Stanford shared some photos of an undercrossing (where a highway goes over a road) that had some damage from the 9/19/2017 earthquake. This is where the Circuito Interior Avenida Rio Churubusco goes over Eje 2 Ote Calzada de la Viga.
I really like all the information In Prof. Miranda's photos. The photos show the direction of the camera and whether it is level, the latitude, longitude, azimuth, and bearing, the elevation above sea level and the time. The only problem with so much information is that it's hard to see the bridge! If we look closely we can see the undercrossing is two parallel continuous box girder superstructures supported on wide piers with shear keys at the ends of the drop bent caps.
In the photo above we see that the ground is torn up, most likely from rocking of the foundations that support the piers.
The photo above shows a masonry abutment which was damaged due to banging of the superstructure against the shear walls. It's not a good idea to built masonry elements on bridges unless they are well reinforced. I don't see any reinforcement in this abutment.
In the photo above we can see that the undercrossing has a long center span over the roadway and two very short spans with masonry abutments at the ends. It seems like the piers and the abutments couldn't laterally support the long center span, which resulted in rocking of the piers and damage to the abutments.
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Sunday, January 14, 2018

Bridges of Mexico: Pedestrian Overcrossing across Blvd. Adolfo Ruiz Continues in Mexico City

September 2017 (19.2913, -99.1105)
There were just a couple of bridge collapses during the 9/19/2017 Mexico Earthquake. A Pedestrian Overcrossing across Blvd. Adolfo Ruiz Cortines (taken from Google Earth) is shown before the earthquake. The supports are slender single column bents with slotted bent caps to support the single girder spans.

The superstructure must have been made up of simple spans supported on a narrow seat. This allowed the bridge to come apart when it was shaken during the earthquake, unfortunately landing on a passing cab. Bridge designers must provide continuity or very large seats in order to protect human lives at locations where earthquakes can occur.

Sunday, January 7, 2018

Bridges of Mexico: Metro Viaduct in Mexico City

September 2017 (19.3017, -99.0520) Metro Viaduct
About 10 km to the northeast of the Tlapan Viaduct is another viaduct that carries light rail trains. The Metro Viaduct is about 20 km long, it was recently built, and it had some surprising damage during the 9/19/2017 earthquake.
The Metro Viaduct has a two steel girder superstructure (and three girders at the elevated stations) supported on single column bents. At the first site we saw evidence that a column foundation had rocked during the earthquake (above photo). Looking up at the superstructure we could see damage resulting from this movement (photo below). Steel keeper plates had busted out of the bent cap during the earthquake (although still restrained by the reinforcement). Also note the vertical slot in the column, meant to hold a drainage pipe. Like the previously studied Tlapan Viaduct, the drainage for these viaducts was never built.
Walking west along Avenue Tlahuac we continued to see signs of banging between the simply supported steel girder spans and the reinforced concrete bent caps (see figure below). However a few hundred meters to the west we saw some surprisingly serious damage.
At this location the column didn't rock because the column base was weaker and broke instead. After the earthquake the superstructure was supported on steel shoring while engineers made plans to repair the column.
In the photo below we can see that not only had the column's cover concrete spalled off but the cracks extended into the core concrete. Looking at the damage, we noted the lack of transverse reinforcement in the column. Also, the main column reinforcement had bundled bars with splices, which shouldn't be located in the plastic hinge zone.  Fortunately the viaduct didn't collapse at this location. However, there was only moderate shaking in Mexico City from the 9/19/2017 earthquake. Imagine the damage that could occur if strong shaking were to occur. Even with only moderate shaking, about 40 buildings collapsed.
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