Performance of Structures Under Successive Hurricanes: Observations from and the US after D.O. Prevatt1, D.B. Roueche2, L.D. Aponte-Bermúdez3, T. Kijewski-Correa4, Y. Li5, P. Chardon6, M. Cortes7, C. López del Puerto8, A. Mercado9, J. Muñoz10, A. Morales11 1 Engineering School for Sustainable Infrastructure & Environment, University of Florida, 365 Weil Hall, Gainesville, FL 32611; email: [email protected] 2 Department of Civil Engineering, Auburn University, 334 Harbert Center, Auburn, AL 36849; email: [email protected] 3 Department of Civil Engineering and Surveying, University of Puerto Rico Mayagüez, Call Box 9000 Mayagüez, PR 00681; email: [email protected] 4 University of Notre Dame, Department of Civil & Environmental Engineering and Earth Sciences and Keough School of Global Affairs, University of Notre Dame, 2130F Jenkins Nanovic Halls, Notre Dame, IN 46556; email: [email protected] 5 Department of Civil Engineering, Case Western Reserve University, 10900 Euclid Ave, Cleveland, OH 44106, Email: [email protected] 6 Department of Engineering Sciences and Materials, University of Puerto Rico Mayagüez, Call Box 9000 Mayagüez, PR 00681; email: [email protected] 7 Department of Engineering Sciences and Materials, University of Puerto Rico Mayagüez, Call Box 9000, Mayagüez, PR 00681; email: [email protected] 8 Department of Civil Engineering and Surveying, University of Puerto Rico Mayagüez, Call Box 9000, Mayagüez, PR 00681; email: [email protected] 9 Department of Marine Sciences, University of Puerto Rico Mayagüez, Call Box 9000, Mayagüez, PR 00681, email: [email protected] 10 Department of Civil Engineering and Surveying, University of Puerto Rico Mayagüez, Call Box 9000 Mayagüez, PR 00681, email: [email protected] 11 Department of Civil Engineering and Surveying, University of Puerto Rico Mayagüez, Call Box 9000, Mayagüez, PR 00681 email: [email protected]

ABSTRACT Hurricane Maria has been termed the worst natural disaster on record in Puerto Rico, as the third consecutive major hurricane to threaten the in a two week period. The storm caused catastrophic damage and numerous fatalities across the northeastern Caribbean, creating hurricane force on St. Croix in the US Virgin Islands, less than two weeks after caused extensive damage in St. John and St. Thomas. Maria made landfall its US landfall on September 20 near Yabucoa, Puerto Rico as a strong Category 4 hurricane. As part of a wider coordinated effort for the 2017 season, a regional node was established in Puerto Rico to organize local reconnaissance efforts between October 6 and November 18, 2017. A separate reconnaissance campaign was organized for the US Virgin Islands November 9-14, 2017 to document the impacts of Irma and Maria. Investigations on all four islands employed primarily door-to-door building Damage Assessments using a customized

Fulcrum smartphone application. At select locations, additional unmanned aerial surveys were conducted. This paper will introduce the overall reconnaissance effort, followed by case studies highlighting common failure modes with a particular emphasis on topographic effects.

INTRODUCTION Maria, the third consecutive major hurricane to threaten the Leeward Islands in a two week period during the 2017 season, had devastating impacts to the Caribbean. experienced the brunt of the storm, as Maria made landfall as the island’s strongest storm on record (Cat 5) on September 18, 2017. The storm generated hurricane force winds on and on St. Croix in the US Virgin Islands (USVI), which had already been impacted significantly by Hurricane Irma. Maria then made landfall again on September 20, near Yabucoa, Puerto Rico (PR) as a strong Category 4 hurricane with sustained winds of 155 mph, making it one of the strongest hurricanes to landfall in the US. Puerto Rico had not experienced a storm of this intensity in over 80 years (Masters 2017). Maria’s strong winds were accompanied by heavy rainfall, second highest on record for a in Puerto Rico. With over 80% of river gauges reporting flood stage levels, flooding and landslides were prevalent, especially in west-central Puerto Rico (NWS 2017). Sadly, over sixty lives were directly lost in US territory as a result of the Maria (mostly in Puerto Rico), with indirect deaths likely much higher, possibly upwards of 1000 (Robles et al. 2017).

The rightful attention on Puerto Rico after Maria overshadowed the harsh realities facing the US Virgin Islands. Hurricane Irma’s passage at Category 5 intensity devastated much of St. Thomas (STT) and St. John (STJ) (Cangialosi et al. 2015), with Maria’s fierce winds and torrential rains, also at Category 5 strength, then impacting St. Croix (STX) less than two weeks later (see Figure 1). While the financial toll of the disaster is still being determined, the governors of Puerto Rico and the US Virgin Islands have respectively submitted official disaster recovery aid requests of $94.4 billion and $7.5 billion (the latter cumulative between Irma and Maria), making Maria the third costliest US hurricane after Katrina and Harvey (NHC 2018). The majority of these funds will be directed to the reconstruction of homes, with the remainder funding the re-establishment of the islands’ utility infrastructure, which left millions without power and communication for months.

In response to this unprecedented sequence of major hurricanes, the authors participated in sustained field reconnaissance across these affected US territories. This not only afforded the opportunity to document the performance of regional construction practices but to also examine the intensification of loads by the islands’ topography. The following sections will introduce the overall reconnaissance effort and methodologies engaged, with a specific focus on building damage assessments via case studies highlighting common failure modes.

METHODOLOGY As part of a larger, multi-storm reconnaissance effort managed by the fourth author’s coordination node (Pinelli et al. 2018, Kijewski-Correa et al. 2018), a regional node was established at the University of Puerto Rico Mayagüez from which local reconnaissance was organized to document damage across Puerto Rico between October 6 and November 18, 2017. A separate reconnaissance campaign was organized from the mainland to document damage across the US Virgin Islands on November 10- 14, after which the USVI team briefly joined the local teams in Puerto Rico. The following survey classes were employed:

● Damage Assessments conducted door-to-door for a detailed evaluation of building condition, including primary structural typologies and component damage levels, accompanied by geotagged photos. Municipalities assessed in Puerto Rico included Aguadilla, Añasco, Arecibo, Barranquitas, Bayamón, Cabo Rojo, Canóvanas, Carolina, Cayey, Comerío, Corozal, Hatillo, Humacao, Isabela, Juana Díaz, Las Marías, Mayagüez, Naranjito, Rincón, Río Grande, San Juan, Santa Isabel, Toa Baja and Yabucoa; Anna’s Retreat/Tutu in St. Thomas; Frederiksted and Christiansted in St. Croix; Cruz Bay in St. John. ● Unmanned Aerial Surveys (UAS) were conducted at select locations to generate additional aerial imagery and 3D point clouds. Surveys were executed in Corozal (PR), Yabucoa (PR), Humacao (PR), Anna’s Retreat (STT), Frederiksted (STX), Christiansted (STX) and Cruz Bay (STJ). ● Coastal Surveys established impacts to coastal infrastructure, high water marks and inundation extent in St. Thomas and St. Croix.

HARDWARE & INSTRUMENTATION The second author served as Data Standards Lead to ensure uniformity in data collection, processing and curation standards, leading the development of a custom multi-level Fulcrum mobile application for the Caribbean. The App was based upon that developed for Irma reconnaissance in Florida (Pinelli et al. 2018) but expanded to support seamless classification of a wider range of infrastructure damage and hazard observations. This included submenus for assessments of buildings with typical concrete and masonry typologies, power infrastructure, bridges, and dams (collecting basic dimensional data and overall damage ratings). The App supports in-line capture of geotagged photos, video and audio directly from the user’s mobile device and is capable of operating and geolocating in regions without cellular coverage -- an important functionality in this context. The resulting Fulcrum database underwent the same quality assurance/quality control process developed for the wider 2017 Hurricane season campaign (Pinelli et al. 2018) and was curated in NHERI DesignSafe along with other collected data.

OBSERVATIONS While the assessments included other infrastructure and coastal surveys, emphasis herein is placed upon building damage. Figure 1 geospatially visualizes the Damage Assessments conducted on buildings relative to the two storm tracks. Assessed building totals are as follows: 334 in Puerto Rico, 86 in St. Croix, 10 in St. John and 39 in St.

Thomas. The majority of these assessments focused on residential construction. Prevatt (1994) documented the likelihood that Caribbean residential structures would suffer the worst damage if their construction did not change. The typologies identified in that study regrettably were observed during field reconnaissance and unsurprisingly manifested the same failure mechanisms. In some instances, the authors noted adaptations of newer, more resistant wind design provisions in the US Virgin Islands. The following case studies provide additional context.

Figure 1. Visualization of Damage Assessments and UAS locations across the four islands relative to the two storm tracks reported by the National Hurricane Center.

PERFORMANCE OF BUILDINGS: PUERTO RICO CASE STUDIES Figure 2 illustrates Puerto Rican residential construction using mixed typologies and the subsequent damage observed. The first story is a concrete and/or masonry typology such as a reinforced concrete frame with masonry infill or load bearing masonry with varying levels of reinforcement/confinement, topped with a reinforced concrete slab roof intended to accommodate vertical expansion. In this mixed system, the vertical expansion employs light-framed wood construction that does not necessarily rely upon robust vertical load paths. A very weak connection occurs between the base of the wood columns to the reinforced concrete slab. Exposure to the corrosive air hastens the deterioration of these connections making the structure even more vulnerable over time. A common feature in many homes is a covered open porch extending continuously along one or more faces of the structure. These light-framed second floors often have a reduced footprint, creating a large overhang. Wind loading to the underside of the patio roof acting concurrently with the high suctions from above often result in complete detachment of the roof system.

Figure 2. (a) Map of damage assessments in Yabucoa (PR) with severity notionally increasing with elevation, both due to increased wind speeds as well as construction quality as lower-income families are pushed up into the hillsides; (b) typical residential construction with light-framed second floor atop concrete/masonry lower level [site D2 in (a)]; (c) another example of mixed residential typologies with minimal damage to lower (masonry) portion while wood-framed second story is severely damaged [site D1 in (a)]; (d) close up of this same home reveals significant setback between upper floor’s exterior walls and edge of slab, resulting in large overhang facing the high wind direction. PERFORMANCE OF STRUCTURES: US VIRGIN ISLANDS Topographic Effects at Anna’s Retreat Unlike the relatively flat topography of the hurricane-exposed Gulf coast, there were several locations in St. Thomas and St. John where elevation changes, escarpments and rolling terrain factored into the performance of building systems. The first is Anna’s Retreat, a town in the administrative sub-district of Tutu, on the northeast side of St. Thomas, with a population of approximately 8,000. As is common on St. Thomas, the topography of Anna’s Retreat is diverse, with the ground rising from a base elevation of approximately 50 m near the Tutu Park Mall to almost 200 m just north of the mall, giving an average vertical gradient of approximately 20 m per 100 m of horizontal length. The area was impacted previously by Hurricane Marilyn in 1995, which caused widespread damage due to wind gusts reported as high as 140 mph (Marshall & Schroeder, 1997). Due to the local topography (an east-west mountain ridge splits the island), Hurricane Marilyn primarily impacted homes in the north and east sides of Anna’s Retreat as it passed just south of the island. Hurricane Irma passed just north of St. Thomas, and many structures sustained heavy damage on the west, south and east sides of Anna’s Retreat. The assessment team focused on an area sloping upward from northwest to southeast (18.345°, -64.894°), as shown in Figure 3, aligned with the strongest winds as measured by a weather station on Rupert Rock near the capital city Charlotte Amalie. Within this area, the team conducted 34 Damage Assessments, with UAS to generate 3D models, digital elevation models, and orthomosaics of the area.

Figure 3. Looking south from approximately mid-slope near Anna’s Retreat in St. Thomas, USVI. The structures in this middle-income neighborhood consisted primarily of one-story, single-family homes with concrete block exterior walls (some reinforcement observed but they generally at inconsistent spacings and with variable diameters), wood-frame interior walls and metal-plate connected wood roof trusses. Most of the roofs were gabled and had corrugated metal roof panels overlaid on wood battens atop the roof trusses. A few homes, primarily near the crest of the hill, were two-story and scattered throughout the area were a few hip roofs (possibly rebuilt after Hurricane Marilyn). Where roofs had been removed, it was possible to see two primary styles of roof-to- wall connections: (1) straps embedded in a concrete tie beam with 2-4 nails to connect the roof truss against the side of the strap (Figure 4a); and (2) a 2x4 wood member anchored to the concrete tie beam along the top of the wall, with metal clips connecting roof trusses to the wood top plate. Many windows did not have glazing. Rear entry doors were often sliding glass doors. Structural performance observed in this region varied from no observable damage to the complete removal of the roof. A few homes with damage to the roof structure had sliding glass door openings facing southwest that had been breached by the wind pressure or debris as shown in Figure 4b. An obvious trend in the observed damage was the improved performance of hip over gable roofs. Most of the hip roofs observed in this region appeared to be undamaged (and many appeared to be more recent construction) after the event. Comparison with pre-hurricane imagery showed that in every case, complete roof removal only occurred with gable roofs.

Figure 4. (a) Partial view of failed roof showing wood purlins extending over gable end, sitting on wood trusses. The roof-to-wall connections consist of metal strap embedded in concrete tie beam; (b) failed sliding glass door on windward wall causing failure of the roof structure and exterior masonry wall; (c) and (d) hip roofs showing improved performance over adjacent gable roofs.

It is important to note that after the Hurricane Marilyn damaged structures throughout the islands in 2005, many changes were made to the building code that upgraded the wind resistance of future construction. In some cases, houses that were undamaged in Marilyn remained occupied with in their original form. The authors believe that the houses with the dramatic gable roof loss employed this pre-Marilyn construction. Striking contrasts can be noted with immediately adjacent post-Marilyn construction with hip roofs that were untouched or experienced minimal damaged from Irma’s winds (Figures 4c and 4d). Topography also likely contributed to the extent of observed damage on east, south, and southwest slopes of Anna’s Retreat. As illustrated by Figure 5, damage appears to increase in magnitude and frequency with distance up the slope. Walmsey et al. (1989) provide a simple model for estimating the increase in wind speeds for basic topographic shapes:

Up=(1+ΔS)U0(Z) (1) where ΔS=(BH/L1)exp(-AZ/L1) (2) and Up is the flow over the topography, U0(Z) is the upstream wind speed at height, Z, H is the height of the topographic feature, L1 is the distance from the crest to the upstream point where the elevation is 0.5H, and A and B are empirical constants, taking on values of 3.5 and 1.55, respectively, for 2D rolling terrain. Applying these constants with Z=10 m, H=200 m, and L1=375 m, results in Up/U0(Z)=1.56, roughly a 50% increase over the upwind wind speed.

Figure 5. (a) Ground-based overall damage ratings and elevation contours in 10 m increments overlaid on pre-hurricane aerial imagery of Anna’s Retreat (STT); (b) overview of the east-central portion of St. Thomas with elevation contours in 50 m increments. The assessment area is highlighted by the red box. Impact of Simple Design Choices in St. John On the island of St. John the team assessed damage to a neighborhood of luxury houses located on an escarpment about 200 ft above the sea, facing towards the south west. Among three houses located side-by-side (see Figure 6a), the home on the left survived with very little structural damage while the adjacent home’s roof was completely destroyed. It appeared that owners of the surviving house employed common sense design solutions, such as using very small eaves and no roof extending over the exposed patios to catch the wind. All the doors and windows were also protected with storm shutters during the hurricane. It was remarkable to observe that these measures and the use of a flexible awning over the porch contributed in large part to the survival of this house.

Figure 6. (a) Comparative performance of luxury houses in St. John demonstrating impact of simple design choices; Arthur A. Richards Junior High School in St. Croix: (b) ground-level perspective of roof damage, (c) example of UAS photo detailing roof damage from above, (d) 3D point cloud of complex illustrating full extent of roof damage. Impacts to Schools in St. Croix: UAS Perspectives Unmanned aerial surveys proved to be a particularly versatile and effective tool in the survey of the Arthur A. Richards Junior High School in St. Croix. This complex was located near Frederiksted and was exposed from the southwest to Maria’s strongest winds, estimated at approximately 140 mph on site. It was obvious upon arrival that there was extensive roof damage to a two-story building (Figure 6b), but ground-based surveys offered no vantage points to swiftly assess such a large complex. As a result, UAS was used to acquire a pre-programmed overlapping image sequence at specific grid locations (see example image in Figure 6c) that can be post-processed to generate 3D point clouds. The resulting color-enhanced 3D point cloud of the high school is shown in Figure 6d. This recreation of the structure can then be freely manipulated (zoomed and rotated on multiple axes) to offer a rich perspective and even can be used to extract dimensional data. CLOSING THOUGHTS In contrast to the disruption communities experienced in Florida following Hurricane Irma, the US Virgin Islands and the Puerto Rico suffered greatly, largely due to the wind vulnerability of their electrical grids. At the time this paper was authored, power had just been restored in the Virgin Islands, but not yet fully in Puerto Rico. This is a powerful illustration of the amplified effect hurricanes of this size and intensity can have on isolated small islands. The damage to the US Virgin Islands (as well as Dominica) was nearly three times its annual GDP, while Puerto Rico’s damage was 93% of its 2016 GDP. In contrast, post-Irma losses in Florida only accounted for about

5% of the state’s GDP, making recovery a far easier proposition. Thus what may be catastrophic damage for the US Virgin Islands or Dominica is relatively minor damage in Florida. As noted by Prevatt et al. (2010), “Hurricane risk models for [the Caribbean must consider] the consequences of damage in these small islands where the economic cost of hurricane disasters can easily exceed the annual GDP of an island’s economy. Special attention is needed to improve resilience of the informal housing sector, which will suffer the most from wind damage.” In many ways, the hurricanes of 2017 have regrettably shown this to be true. Thus disaster risk reduction strategies for a small island that can be 100% affected by a single extreme event, and are logistically more difficult to support in response and recovery, differ considerably from strategies in the mainland United States. It is hoped that this reconnaissance effort is only the beginning of the ongoing effort to increase small island resilience. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Science Foundation (CMMI-1761461) and in particular the efforts of Dr. Joy Pauschke for her commitment to mobilize teams so quickly after this event. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation (NSF). The authors wish to thank their other NSF-supported collaborators in the field, including the NSF Coastal Team led by Dan Cox (Oregon State University), as well as the Coastal Team affiliated with the Japanese Society of Civil Engineers. The Puerto Rico team thanks, Mr. Ernesto Díaz, Director of the Coastal Zone Program and Climate Change Office of the Department of Natural and Environmental Resources of PR,and local meteorologist Ada Monzón for their collaboration in the field assessments. The team also expresses its gratitude to Dr. Gillian Marcelle, Executive Director of the RTPark, the USVI specialist economic development agency and her team whose office provided tangible, in-kind support during their time of extreme personal hardships. In particular, we are grateful for the dedicated transportation they arranged for us through the services of a local taxi company owned by Mr. Elroy Hall, with drivers Mitch and Edwin, who ably ferried us throughout the three days of our stay in St. Croix. We are grateful to USVI President David Hall for meeting with us in St. Thomas as well as for the tremendous logistical help and friendship of Dr. Greg Guannel of the University of the Virgin Islands.

The team also acknowledges a number of graduate students: Andrew Bartolini (University of Notre Dame) for logistics coordination throughout the missions, Mr. Requel O. Petersen (University of the Virgin Islands) who assisted with the data collection in St. Croix, and the following students at University of Puerto Rico Mayagüez who assisted with data collection in Puerto Rico -- Graduate Students: Edgar Albandoz, Francisco Villafañe and Undergraduate Students: Fernando Benitez, Gabriella Buono, Diego Delgado, Oscar Lafontaine, and Samuel Montalvo. Finally, the team recognizes Kwasi Perry of UAV Survey Inc. for his UAS support across the islands. All data and data products are curated in the NHERI Data Depot at www.designsafe-ci.org/.

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