Day 3: Successful Retrofitting following 1985 Earthquake & Lessons on Cracks
Successful Retrofitting & Lessons on Cracking
October 30, 2017
Today we focused our efforts on a different area of Mexico City, near Colima and Calle de Durango (shown in the map below). One of our goals was to investigate retrofitted concrete buildings that Dr. Breña had previously investigated after the 1985 Mexico earthquake.
Halfway through the day we were joined by one of Dr. Breña’s former students, Alex, who lives in the vicinity. He explained that after last month’s earthquakes the task of assessing damage and tagging buildings fell to local architects, including university students. These investigators were instructed to tag damage based off crack width/extent which led to over-tagging damage.
The rest of this blog post will discuss two case study buildings that had been retrofitted since the 1985 earthquake as well as lessons I learned about different crack patterns in concrete structures.
Above: map of Reinforced Concrete buildings
How did the retrofitted buildings perform?
The first retrofitted building we investigated was the local police station which performed rather well with the addition of external steel bracing (shown in the images below). Still, the interaction between the original concrete structure and steel frame was apparent in hairline cracking in concrete beams/ columns where steel bracing was introduced. Based on our observations, we concluded that: (1) the location of more numerous or wider concrete cracks in the columns indicated which member (beams vs. columns) controlled in the concrete system, and that (2) connections between the concrete and steel systems at every other floor created a collector in the concrete beam on the intermediate story that was not tied into the steel bracing (as evidenced with the lack of visible cracking in these beams).
Local Police Office: retrofit with external steel frame
We also visited a local hospital building retrofitted with the addition of steel rods/anchorages for the beams up to the 6th floor and widening of columns to result in strong column behavior, per the strong column-weak beam seismic design approach. In the images shown below, the thin line (highlighted in red) are the steel rods and the end anchorage of these rods are housed in square box (highlighted in yellow) on the exterior face of columns. This building performed exceptionally well. The on-duty security guard informed us that during the earthquake there was some shaking of the building and slight uplift in the foundation shifting; however, overall the columns exhibited no visible signs of damage. This was probably the most successful retrofit seen today.
ABOVE: Performance of Steel Rod/Anchorage Retrofit between Beams and Columns (left) & Slight Uplift of Foundation (right)
What do different types of cracking mean?
Today’s major lesson was what different cracking patterns can reveal about a damaged member. Typically, horizontal hairline cracks on a column result from tension or elongation due to building rocking or tilting. However, in some instances these cracks may just be movement or expansion of paint/plaster which requires further investigation to determine if the underlying concrete member experienced structural damage. Vertical cracking along a column can be an indicator that there is insufficient vertical reinforcement and that some movement in that direction has occurred. Diagonal cracking in columns is most likely a sign of shear failure, but again this can be misleading as cracks may only appear in the paint/plaster and be non-structural in nature.
Examples of Column Damage : Concrete Cracking and Spalling
Beam cracking is slightly different. Horizontal cracking can provide insight into the load path for a concrete frame system: cracks along the length of beam may continue through the column-beam connection into the adjacent column and down to the foundation. Vertical cracks in beams generally result from bending due to flexural demands, while diagonal cracks are indicative of significant shear demand. Cracks that are initially vertical and become inclined suggest a combined shear-flexure response. Similar to column cracking, beams covered in paint/plaster that exhibit visible cracking may be non-structural with limited-to-no damage of the underlying concrete member, or they can be critical.
Additionally, a crack can be pre-existing from a past earthquake, these are normally epoxied and painted over. The following images show the difference between a large pre-existing crack that has been epoxy injected (left) and new crack which occurred on a similar wall (right, note black dots placed along crack).
Examples of Pre-existing Epoxy Injected Cracks and New Cracks
Many buildings we have seen in the Mexico City area have a concrete frame system with unreinforced masonry (URM) infill. Significant failures of URM components are easy to distinguish as the plaster finish usually has significant cracking or has completely detached and is falling from the building. Concrete components tend to fare better because they have a more ductile response, such that they can carry significant loads at greater deformation demands than brittle URM walls. The images below illustrate an example of structural damage to concrete wall with exposed rebar (left), and non-structural damage to a cracked URM infill wall (right).
Examples of damage to concrete wall (left) and urm infill wall (right)