Written by: Briar Goldwyn, Cole Velasquez, Yarelis Gonzales, and Polly Murray
When we first set out to study the multi-hazard performance of informally constructed housing in Puerto Rico, the island had not been majorly affected by an earthquake for over 100 years. We began our study in July 2019 by asking household members, local builders, engineers, and hardware store employees about the expected risk of an earthquake to their housing and were met with statements about tsunami damage, without many references to shaking or damage to concrete housing. There was a major shift in the conversation after a series of earthquakes, the largest of which was a moment magnitude (Mw) 6.4, occurred along the southwest coast of Puerto Rico in late 2019 and early 2020. In response to earthquake-induced damage, which mostly affected reinforced concrete (RC) and masonry housing in the island’s southwest, safety perceptions shifted, resulting in distrust of the existing construction sector and a growing interest in consultation of engineers and architects. As part of a larger project, we sought to assess construction methods and structural performance of houses in an earthquake event.
To evaluate the performance of Puerto Rico’s housing in earthquakes, we analyzed the structural performance of RC houses, including those with masonry walls. To begin this assessment, we first documented the wide variety of housing designs, even between houses built by the same builder on the same street. We identified a set of archetype houses, and assessed the impact of construction and design decisions on seismic performance, using collapse risk in a future event as a measure of life safety. These archetypes include elevated houses with open ground stories, and houses with masonry infilled or confined masonry walls. We varied material strength, reinforcement detailing, wall openings, and construction method, among other building characteristics. We also looked at the ways households and builders could implement retrofitting techniques to reduce safety risks.
Image: Confined vs. infill masonry (Source: https://emiworld.org/emi-tech/same-materials-different-buildings)
Through this study, we have identified several recommendations to improve seismic safety of RC construction in future earthquake events. We have provided recommendations for new and existing construction. For new houses, we suggest builders:
Use lighter weight construction for roofs and floor slabs
Build housing through the process of confined rather than infilled masonry construction
Build columns with closer traverse tie spacing, larger cross sections, and with reinforcing bars that are not corroded.
If open-ground story construction is preferred due to potential flooding, ensure columns provide adequate strength and deformation capacity, with adequate cross-sectional area, and longitudinal and transverse reinforcing bars for expected earthquake loads
For existing structures, we found:
Vulnerable columns in open ground story can be strengthened using RC jackets that enlarge the cross-section, and provide additional longitudinal and transverse reinforcement
Image: Reinforced concrete jacket
Builders and households would ideally construct horizontal expansions (side rooms added to structure) with concrete and vertical expansions (second stories) with wood (considering hurricane risk, of course)
These findings are important, however, simply knowing that specific construction decisions may be safer does not lead to a change in local construction practices. The critical and the final aspect of our project is to recognize and understand why certain construction decisions are made. Across three fieldwork trips, and with the vital help and guidance from eight different research assistants and countless local organization staff, builders, architects, and structural engineers, our research team collected surveys and data to study the perceptions surrounding construction decisions and their effect on multi-hazard performance. After interviewing over 50 people and collecting more than 300 surveys of local builders and hardware store employees across Puerto Rico, we are currently analyzing the results to holistically understand housing safety perceptions.
To date, we have found a couple of housing safety perceptions within our surveys that do not necessarily align with our structural performance assessments. So, our team is left with a lingering question: how do we merge the results of the structural analysis and the findings about safety perceptions from the local perspective?
Recognizing the vital and helpful role that community-based and grassroots organizations play across Puerto Rico, we seek to turn to these groups for their wealth of knowledge and influence. In the hope to improve housing safety, local community-based organizations can help disseminate our findings in a memorable, clear, and impactful way.