Wind-Borne Debris Hazards

This chapter provides an overview of the damage resulting from wind-borne debris. It starts by reviewing the wind environment in which debris can become wind borne and field observations of debris found in post-storm surveys. This is followed by field data on the vulnerability of commercial and residential buildings to wind-borne debris, code responses, and the risks debris breaches pose to the building envelope. The vulnerability of buildings depends significantly on the building envelope’s integrity. Whenever missile debris penetrates the building envelope, the building becomes susceptible to further damage due to rain infiltration and increases in pressure inside the building. Hence, maintaining the integrity of the envelope is important to reduce chances of significant damage. Finally, the chapter reviews field observations of building envelope damage and damage mitigation methods.

This chapter explains the current understanding of how wind-borne debris becomes airborne. Wind-borne debris is either naturally occurring (e.g., tree limbs, sand, and loose stones) or engineered (e.g., roof sheathing, shingles, and roof ballast such as pavers and aggregate). The chapter discusses the basic theoretical modeling of motion initiation, regardless of source; the key nondimensional parameter that controls motion initiation; and the scaling of laboratory studies to full scale. It also briefly reviews building aerodynamics, including air flow and pressure distributions; the role of parapets in mitigating extreme suction pressures; and potential scaling issues associated with laboratory-scale measurements of pressure coefficients and debris motion initiation. Wind-borne debris from buildings is broadly categorized as either loose laid (e.g., roof pavers and gravel ballast) or fixed (e.g., sheathing and shingles). The chapter further describes the motion initiation of these materials.

This chapter discusses the equations of motion for the flight of wind-borne debris, focusing on flight trajectories and debris speeds for the three classes of debris. It also describes the effects of turbulence and the local flow field around building surfaces. Many experiments on and analyses of the flight of wind-borne debris have focused on uniform, smooth flow with well-defined initial conditions such that the initial forces and moments at release closely relate to the static aerodynamic coefficients. For plate-like debris, these experiments show that initial conditions have a significant effect on flight trajectories. The aerodynamic coefficients for plates usually use quasi-steady models that include static force coefficients, which depend on the angle of attack, ζ, and auto-rotational coefficients, which depend on the rotational speed, (dθ/dt), of the object. Using these models, the trajectory can be solved if the wind field is known or assumed.

Impact from wind-borne debris and the consequences of a breach in the building envelope during windstorms are design load criteria to consider in addition to other design actions such as wind pressure. Observations from past windstorm events reveal impact damage to buildings caused by structural members. These observations highlight the importance of testing building envelope components for resistance to wind-borne debris impacts. This chapter refers to wind-borne debris as missiles and the impacted wall or roof system as the target. The potential for damage depends on the missile impact load and the resistance provided by the building envelope. Missile impact loading can be analyzed in terms of missile impact velocity, mass of the missile in terms of kinetic energy and momentum, and missile impact area and nose shape.

This chapter discusses the failure risk of the building envelope due to wind-borne debris impacts in high wind conditions. It provides a brief history of wind-borne debris simulation studies, the minimum modules required for computational modeling of wind-borne debris, and the current state-of-the-art wind-borne debris trajectory models utilized in wind-borne debris risk and loss assessments. J. D. Holmes provides an assessment of wind-borne debris risk modeling that includes the essential elements of damage risk models, results from wind-borne debris trajectory models that are relevant to damage risk models, the merits of impact momentum versus kinetic energy, the use of data from generic impact tests in damage risk models, and the variation of debris impact damage with wind speed. Holmes concludes that damage from wind-borne debris impacts can be almost as costly, if not as costly, as damage attributed to direct wind loading.

Initial efforts to quantify and design for debris impact involved high-risk facilities associated with the nuclear power industry. After substantial tornado damage to domestic construction and consequent loss of life, an industry developed around providing shelters, which required impact resistance design criteria. Multiple associations have their own criteria for wind-borne debris testing, including ASCE, FEMA, DOE, ASTM International, ICC, and SAA. All but SAA directly or indirectly specify ASTM standards for wind-borne debris impact tests. The Southern Building Code Congress International (SBCCI) proposal recommends that buildings either be designed for full internal pressure, or have external protection devices (shutters) for windows or impact-resistant glazing. In wind-borne debris regions, door and window assemblies must be designed to resist test missile loads specified in ASTM E1996-14a, and garage doors should conform to the requirements of ANSI/DASMA 115.