Wind-Borne Debris Impacts on Façades

INTRODUCTION

The evidence of climate change and its consequential increase in the frequency and intensity of extreme weather events is well-documented (IPCC 2022; Munich RE 2022). This trend is reflected in rising economic losses and human casualties from natural disasters (Lu et al. 2019; Munich RE 2021; Eckstein et al. 2021). World Economic Forum surveys emphasize extreme weather as a top global risk, alongside concerns about climate action failure (World Economic Forum 2022). Extreme weather events, including hurricanes, typhoons, tornadoes, floods, and wildfires, have widespread impacts on human life, ecosystems, and economies globally. Climate action failure, characterized by inadequate measures to address climate change, exacerbates these effects. Notably, United States and Canada identify extreme weather events and climate action failure among their top risks, underscoring the need for strict carbon emissions control (World Economic Forum 2022).

Wind-related extreme weather events, such as hurricanes, cyclones, and tornadoes, pose significant threats globally. Different geographical areas use various terms to denote these events. For example, tropical cyclones are hurricanes in the Atlantic but cyclones in the South-West Pacific. Typhoon is used in the Pacific on the West side of the International Date Line. The frequency of extreme wind events, including tornadoes, is not confined to specific regions but spans multiple countries (Goliger & Milford 1998; Piscitelli et al. 2022). In this paper a case study is presented for Salt Lake City. The impact of extreme wind events on the urban environment is substantial, causing damage to buildings and posing risks to life and property, among the impossibility of accessing essential facilities most of the times. Notably, the vulnerability of building envelopes to wind-borne debris during extreme events is evident in incidents like the Salt Lake City tornado in 1999. On August 11, 1999, an F2 tornado made landfall in the Salt Lake City metropolitan area. Lasting for ten minutes, the tornado resulted in one fatality, over 80 injuries, and incurred damages exceeding $170 million (National Weather Service 1999). This event marked the most destructive tornado in Utah’s history, serving as a wake-up call for the entire state’s population, highlighting that tornadoes can indeed occur in Utah. Despite extensive research and codes on preventing building failures in major wind events, recent occurrences in non-traditional regions, like Salt Lake City, underscore the need for further research and improvements in building design standards (ASCE 2022).

The current lack of consideration for wind-borne debris impact resistance in building codes highlights a gap in building design standards. In contrast, some countries have implemented guidelines and certification systems to enhance resistance to wind-borne debris (National Building Code of India, 2016). The International Standard Organization has been actively working on introducing testing requirements, emphasizing the need for a comprehensive approach to building envelope resilience (Laboy et al. 2012; Henderson et al. 2018; Herseth et al. 2012; ASTM 2019, 2023) and the ISO/DIS 16316 is in the progress of being published in 2024. This standard aims to provide a tool for “Windows, doors and curtain walling — Impacted by windborne debris in windstorms — Test method and classification.”

To address these gaps, an international effort is underway to develop a standard for testing the impact of wind-borne debris on building envelopes. This initiative aligns with Sustainable Development Goals related to resilient infrastructure and sustainable cities (International Organization for Standardization 2018; UN 2022). A performance-based design tool for façade designers is presented in this paper, incorporating trajectory and velocity analysis of a case-specific debris. The outcomes can inform testing standards and design code requirements, as demonstrated in a case study on roof component’s failure and its impact on surrounding buildings during extreme wind events.

Currently, there are building codes such as the International Building Code (2021) that support designers to identify the risk category of a building. Essential facilities such as hospitals have usually a higher risk associated with design load estimation. Structures such as nuclear reactors, hurricane shelters, and tornado shelters currently have the most stringent requirements for performances during extreme events (IBC 2021). This is especially related to their façade impact resilience since the building envelope should remain intact to guarantee contaminating chemicals protection or people safety. The Performance-Based Wind Design (Scott et al. 2023) workshop report is introducing wind-borne debris impact performance as a requirement in all essential facilities in the US, since building envelope breakage linked to wind-borne debris impacts has been identified as one of the major damage causes in extreme wind events.

BACKGROUND

The impact of Tropical Cyclone Tracy in 1974 prompted regulatory responses in Australia, leading to the Darwin Area Building Manual (1975) and subsequent amendments. The manual addressed the vulnerability of buildings in cyclonic regions, emphasizing the role of roof cladding and windows in contributing to damage and debris generation. Australia’s response influenced testing standards, including Technical Record 440 (1978), and eventually led to revisions in the Australian Standard 1170.2 (1989) mandating building envelopes capable of withstanding wind-borne debris impact. Similar developments occurred in the United States after Hurricane Andrew in 1992, particularly in Florida. The Florida Building Code introduced regulations (TAS 201, 202, 203 - 1994) requiring impact-resistant façade systems. Florida’s standards, including those set by Miami-Dade, became some of the most stringent in the U.S. This eventually led to the creation of ASTM standards, such as E1996 (2023) and E1886 (2019), which cover a wider range of temperature conditions than Miami-Dade requirements.

Australia revised its design actions related to wind in 2002, leading to the creation of the unified Australian and New Zealand design wind code (AS/NZS 2002). Subsequent modifications in 2011 (AS/NZS 2011) incorporated impact loading criteria from wind-borne debris, influenced by U.S. studies on wind profiles in tropical storms and hurricanes. The U.S. and Australian approaches to impact resistance testing differed, with Florida’s requirements including pressure cycling after the impact phase. In Australia, the choice of a 2-gram steel ball as a small projectile for testing was influenced by observations of wind-borne gravel. While pressure cycling testing was present in Australian roofing requirements for cyclonic areas, it was not specified for wind-borne debris resistance.

The focus on tornado-prone regions, including North America, Europe, South Africa, and Asia, emphasizes the importance of understanding non-synoptic winds. Studies cover meteorological, probability, and engineering perspectives, with attention to building skin design to resist wind-borne debris impacts. The potential damage includes the breakdown of building envelopes, leading to air infiltration, water penetration, internal pressurization, and the failure of roofs and walls. Urban resilience is enhanced through building envelope design based on risk assessments that consider wind-borne debris and impact characteristics. Various studies, including Cui & Caracoglia (2020), identify wind-borne debris impacts on target building envelopes in extreme winds, considering the urban setting, wind field, and case study typology.

OBJECTIVE

The objective of this paper is to present a performance-based design approach to identify impact test requirements that are relevant to case-specific projects, for façade design. An alternative performance-based design framework is presented, to identify case-specific impact test requirements, to mitigate the effects of wind-borne debris, to design and verify the wind-borne debris resistance of façades. Building aerodynamics, the trajectory, and the velocity of specific debris elements are considered to deploy performance-based façade solutions. The design framework can be implemented both for tornado-, hurricane-, typhoon-, and tropical cyclone-prone areas.

The author has implemented a design framework (Mejorin 2022) to enhance building envelope resilience, finding ad-hoc impact test requirements linked to wind-borne objects in hurricane- and tornado- prone areas. The strength of the performance-based design tool is the almost infinite cases it can address. In particular, the author is interested in:

• the identification of object in the urban environment that can be critical because of their failure and wind-borne behavior in extreme wind events;
• the introduction of a guided wind engineering assessment for wind-borne debris;
• the identification of performance-based requirements for wind-borne debris impact resistance of façades;
• the enhancement of building skin impact resilience to wind-borne debris impact;
• the serviceability in the post-event scenario of primary importance facilities and buildings that followed the design framework presented in this paper.

A case-study for Salt Lake City is presented, for the University of Utah Hospitals and Clinics, and the debris type assessment focuses on roof shingles.

METHOD

The proposed design tool utilizes existing literature on wind-borne debris trajectory analysis to assess the impact performances of building façades. It focuses on two key investigation areas: failure mechanisms and flight assessment, aiming to calculate the impact energy on the target façade. The tool employs a probabilistic approach, incorporating a Monte Carlo simulation to estimate the distribution of impacts on the downstream target building façade and their specifications.

The analysis seeks to determine the failure wind speed of the case study building component, flight features, and velocities of the wind-borne debris for various wind speeds. Probability estimates are derived based on technical specifications or failure capacity assumptions. The analysis considers the likelihood of debris leaving the source building and impacting the target façade over the building’s lifespan, factoring in return periods and the building’s importance level.

Through analytical assessments, the tool addresses various aspects, including minimum wind speed for debris failure, debris travel distance and speeds, and the relative positions of the debris source and target building envelope. The gathered data informs the development of standard impact test proposals for target building envelopes, tailored to specific wind-borne debris characteristics and flight simulations using numerical models. The impact performance considerations encompass diverse building envelope typologies such as windows, walls, curtain walls, and double-skin façades, aiming to enhance debris resistance in construction cases, considering the diversity of debris types.

CASE-STUDY

The case-study considers the façade resistance for the University of Utah Hospitals and Clinics in Salt Lake City. It is an essential facility, with risk category IV, in which “enhanced protection” is required. The design focus is on the target façade of this hospital, which is located in Exposure category C, according to ASCE 7-22 (2022).

Wind Zone

The target building location analysis is the first step for the designer to identify the code and standard requirements that are in place for the design and specifically for wind speed identification. Therefore, this case-study is located in an area with a basic wind speed of 49 m/s.

Level of Protection

A preliminary step, in the design procedure proposed in this design framework, is to identify the target building level of protection, which is related to the Risk Category of the building (Table 1604.5, IBC 2021). The identification of the risk category of the target building to be designed is therefore a starting point, for further considerations that lead to the performance-based impact-resistance of the building envelope design. A hospital is an essential facility and belongs to “Risk Category IV buildings”.

Urban Environment

In the hospital design location, there are currently no wind-borne debris impact requirements for façades (hurricane regions, ASCE 7-22 2022). The buildings belonging to the University of Utah Hospitals and Clinics are not located in an area in which the basic wind speed is larger than 67 m/s, and the building location is not within 1.6 km from the coastline. Therefore, for Risk Category IV buildings in this area, no wind-borne debris impact performances are required for façades (ASCE 7-22).

Analyzing the surrounding environment plays a pivotal role in the alternative design framework for wind-borne debris façade design. It is essential to conduct a comprehensive preliminary assessment that combines engineering and building technology within the surrounding environment. From an assessment, it can be noted that in this area there are a considerable number of low-rise houses, and their roof cover technology is mostly represented by shingle elements.

Basic Wind Speed

Considering the Wind Zone and the Level of Protection of the building, the designer can calculate the minimum requirements in terms of Basic Wind Speeds (ASCE 7-22) to be considered for the design wind loads. In general, the outcome of the analysis for target building design should consider:

• Wind-borne debris impact requirements for façades if already set in place;
• Building design wind speeds;
• Upward trends and factors linked to climate change (if available).

For the analyzed case-study, the basic wind speed is equal to 49 m/s.

“Other Missile”

In ASTM E1886 (2009) the “Other Missile” is defined as “any other representative missile with mass, size, shape, and impact speed as a function of basic wind speed determined by engineering analysis”. The alternative projectile identified through analysis of the environment could, therefore, be different from the standard large (2 × 4 in. lumber) and small (8 mm diameter steel ball) projectiles.

The Urban Environment analysis identifies objects that could fail, become airborne, and, as wind-borne debris, strike the target façade. The case study specifically discusses roof shingles. This design step takes into account the surrounding buildings at the target location, as well as objects present in the urban environment. The characteristics to be collected are:

• Mass, Size, Shape: The potential wind-borne debris identified for design purposes within the performance-based design framework are analyzed based on their geometric features, weight, constituent material density, restraint systems, and position on the source building or in the urban environment, focusing on their uplift resistance.

TABLE 1 – MASS, SIZE, SHAPE OF ROOF SHINGLES

“Other Missile” Failure Analysis

Utilizing information gathered in preceding stages, Newton’s Second Law, and the aerodynamic force definition outlined in ASCE 7-22 (2022), the analysis of failure for the “other missile” can be undertaken.

For the presented case-study there is an existing database available for roof shingle flight assessment (Kordi 2009). Current analysis considered just failure wind speeds of the experimental database that were smaller than the design wind speed (49 m/s). 205 experimental data were considered and to proceed with the flight assessment through Monte Carlo simulation 10,000 initial failure wind speeds have been calculated. These values are reported in Fig. 6 (a).

In this design step, the façade designer can assess whether the wind speed required for debris failure exceeds the reference wind speed for the target building design, which may differ from the code’s basic wind speed. If the object’s failure does not meet the criteria, the façade designer should abstain from advancing to subsequent design steps, as there is no risk of the object becoming wind-borne. Conversely, if failure occurs at wind speeds lower than the reference wind speed for the target building design, the analysis of flight should be pursued. From experimental data (Kordi 2009) we have the confirmation that roof shingles can fail and fly in Salt Lake City.

Two “Other Missile” Failure wind speed datasets are used in the following sections:

• Failure wind speeds based on experimental database developed by Kordi (2009);
• Failure wind speeds based on ASCE 7-22 (Fig. 5), with a mean value of 49 m/s.

“Other Missile” Trajectory Analysis

Based on the “other missile” characteristics, on its Failure Analysis and case-study related failure wind speeds, on the Basic Wind Speed for the design location, a trajectory analysis can be conducted, through Monte Carlo simulation. The flight analysis is based on numerical calculation and on equations of motion that have been developed in the Eighties (Tachikawa 1983). Therefore, using a 4th-order Runge-Kutta scheme, and aerodynamic coefficients for lift, drag, and momentum, debris trajectories can be defined. Through the simulation, a range of trajectories and terminal velocities can be estimated. Through this design step, it is also possible to estimate the maximum height to have debris impact on the target façade for a specific initiation, besides the impact velocity.

The steps of this analysis, in a general formulation that can be applied to multiple cases, follow:

1. Identification of reference wind speed for target building design;

2. Identification of distance between the source of debris and the target;

3. Identification of debris element geometric and weight characteristics;

4. Classification of the debris element and identification of rotational and static aerodynamic coefficients for lift, drag, and momentum;

5. Identification of GCp distribution on the assessed debris element;

6. Sample debris element failure capacity probabilistic distribution;

7. Calculation of failure wind velocities;

8. If debris failure wind velocity (Step 5) is smaller than reference wind speed (Step 1) failure occurs, and debris trajectory is calculated by solving Equations of motion developed by Tachikawa (1983), through a 4th-order Runge-Kutta scheme;

9. Step 7 is repeated with the trajectory calculation for the equivalent 10-min mean wind speed instead of the previously used 3-s gust failure wind speed;

10. Calculate impact characteristics (speed and height) distribution on target façade.

“Other Missile” Speed and Max Height

The information about impact features is achieved through a wind engineering analysis of the wind-borne “other missile” that has been identified as a vulnerable element in specific wind speeds. The achievement of these data about specific debris trajectories and speeds leads designers to weigh façade performances, encouraging site-specific solutions. Experimental databases available for roof shingles (Kordi 2009) and building code requirements (ASCE 7-22) for wind loads are adopted in to calculate the results. With reference to Fig. 9, the impact velocities, calculated through Monte Carlo simulation, of roof shingles are respectively:

• 33 m/s following experimental databases (Kordi);
• 49 m/s following ASCE 7-22.

The maximum height depends on the initial conditions of the "other missile." In this example, this height is considered as the initiation height. An initial height of 10 meters has been adopted to represent a typical two-story house. From the trajectory analysis, the highest trajectory in distance has intersected the façade of the target building with a maximum impact height of 7.8 meters. Additionally, the maximum flight velocity has been assessed at the final trajectory. For the impact characteristics, the standard definition of kinetic energy has been employed. This can be calculated based on the projectile's velocity and weight. Kinetic energy, which is the energy an object possesses due to its motion, can be determined by multiplying half the object's mass by the square of its velocity.

TABLE 2 - MAX IMPACT SPEEDS (M/S), ENERGIES (J), AND HEIGHT ON TARGET FAÇADE (250 M RADIAL DISTANCE FROM DEBRIS SOURCE), FROM MONTE CARLO SIMULATION

LIMITATIONS AND FUTURE WORK

This paper focuses on a specific roof element within the target façade design. To fully assess the wind-borne debris resistance impact performance for both the case-study and target building locations, it is crucial to conduct additional studies encompassing all building components and urban objects identified in the environmental assessment. Furthermore, since this study pertains to a particular wind zone, it is necessary to extend the evaluation to include the same type of “other missile” used in the case study but with various impact characteristics dictated by different wind conditions. Additionally, for the same projectile type and design location, but with varying mutual distances, the impact characteristics on the target façades are expected to differ.

In future studies, the author intends to enhance the understanding and reliability of trajectory analyses for wind-borne debris. While there are existing studies, such as those by Abdelhady et al. (2022), that improve wind-borne debris models by incorporating the effects of surrounding structures, these models do not specifically focus on trajectory accuracy. Moreover, they lack strategies to precisely determine impact characteristics, particularly in relation to turbulence effects caused by the aerodynamics of surrounding buildings.

CONCLUSIONS

Considering the escalating global impact of extreme winds attributed to climate change, safeguarding essential facilities against windstorms becomes imperative. Among the various elements of a building, façades serve as the primary defense mechanism, shielding individuals and internal property during severe wind events. Notably, wind-borne debris emerges as a predominant catalyst for façade damage in such scenarios. To address this concern, this paper introduces a performance-based design framework aimed at formulating impact test requirements specifically tailored to façades, to mitigate the destructive effects of wind-borne debris. Through the adoption of a performance-based design approach, opportunities arise to enhance the identification of façade impact requirements in diverse contextual settings.

A case study exemplifying the implementation of building envelope design for enhancing façade resilience in essential facilities within Salt Lake City, Utah, further underscores the practical application of the proposed framework. This comprehensive approach seeks to fortify façades, promoting resilience against wind-borne debris and contributing to the overall structural robustness in the face of increasing global wind challenges. It is shown that roof shingles, that are typically adopted in Salt Lake City, can fly for distances up to 200 m, following both ASCE-7 requirements for design Basic Wind Speed and experimental databases that are available. From the Monte Carlo simulation, arises that University of Utah Hospitals’ and Clinics’ façades, to guarantee performance-based resilience to the impact of roof shingles that can fail and fly in tornadoes, should with stand impact tests up to 1,920 J.

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