Roof Gravel Research Articles

Analysis of Damage in Laminated Architectural Glazing Subjected to Wind Loading and Windborne Debris Impact

Wind loading and windborne debris (missile) impact are the two primary mechanisms that result in window glazing damage during hurricanes. Wind-borne debris is categorized into two types: small hard missiles; such as roof gravel; and large soft missiles representing lumber from wood-framed buildings. Laminated architectural glazing (LAG) may be used in buildings where impact resistance is needed. The glass plies in LAG undergo internal damage before total failure. The bulk of the published work on this topic either deals with the stress and dynamic analyses of undamaged LAG or the total failure of LAG. The pre-failure damage response of LAG due to the combination of wind loading and windborne debris impact is studied. A continuum damage mechanics (CDM) based constitutive model is developed and implemented via an axisymmetric finite element code to study the failure and damage behavior of laminated architectural glazing subjected to combined loading of wind and windborne debris impact. The effect of geometric and material properties on the damage pattern is studied parametrically.

Wind tunnel measurement of the parapet height role on roof gravel blow-off rate for two dimensional low rise buildings

A new experimental technique has been developed to measure the rate of removal of aggregate from built-up roofs in severe storms. The technique was used to quantify the rate of removal of aggregate from a two-dimensional building as a function of the building parapet height and the wind speed. The experimental results indicate that the mass flux varies over time. Initially the mass flux is quite high, and there is significant movement of aggregate within the roof cavity formed by the parapet. After this initial adjustment, the mass flux reduces by a factor of four and then remains steady. The adjustment time was found to be independent of the parapet height but strongly dependent on the wind speed. This suggests that the primary role of the parapet is to prevent removal of aggregate from the roof cavity, but that parapets are less effective at preventing aggregate motion within the roof cavity. Increasing the parapet height significantly reduces the mass flux. A parapet height equal to 15% of the building height reduces the blow-off rate by a factor of 100 compared to a roof without a parapet. Further, the addition of a parapet always reduced the mass flux. This stands in contrast to studies of the wind speeds required to initiate motion in which small parapets have been observed to reduce the critical wind speed required to initiate blow-off. Although the experiments were run at relatively small scale, the results were not significantly influenced by viscosity, indicating that small scale wind tunnel studies of mass blow-off rate may, with care, be scaled up to full-size buildings. Again this is in contrast to blow-off initiation studies, in which the wind speed required to start mass removal is strongly dependent on the Reynolds number, and only full scale experimental results should be used in design.

Analysis of Laminated Architectural Glazing Subjected to Wind Load and Windborne Debris Impact

During windstorms and hurricanes, architectural glazing is subjected to wind loading and windborne debris impact. Wind-borne debris is categorized into two types. One is small hard missile like roof gravel and the other is large soft missile representing the lumber from wood-framed buildings. Laminated architectural glazing (LAG) is the commonly used glazing in buildings where impact resistance is needed. The prefailure stress response of the LAG due to the combined loading due to wind and windborne debris impact is studied. Following the ASTM standards (E1886 and E1996), a steel ball with an impact velocity of 39.62 m/s and a wooden cylinder with an impact velocity 12.19 m/s were chosen to be representative of small and large missiles, respectively. A lateral pressure that corresponds to a wind speed of 58.11 m/s was used to represent wind loading on LAG. The effect of geometric and material properties on the stress response of a rectangular LAG is studied parametrically. Thinner outer ply would result in better prefailure stress pattern than a thicker outer ply, while thicker interlayer generally results in lower stresses in failure critical areas. The contribution of wind loading to the principal stress is between 5–10% of the combined stress with small missile case having higher percentage.

Full-scale validation of vortex suppression techniques for mitigation of roof uplift

Widespread damage to roofs in hurricanes and other high wind events has highlighted the need for improving the wind performance of roofs, including both new construction and retrofit on existing buildings. Effective and economical methods to mitigate wind effects on roof components are sought to reduce accumulated losses from frequently recurring low- and moderate-intensity events. An experimental study was carried out in the 6-fan Wall of Wind (WoW) testing apparatus to assess the effectiveness of aerodynamic edge devices at full scale in reducing wind effects over the roof corner and edge regions. Two sets of tests, roof gravel scour testing and pressure testing, were carried out to visualize the development of edge vortices and compare roof uplift pressures with and without aerodynamic edge shapes, thus allowing the estimation of the effects of such shapes in alleviating wind damage of roofing material as well as high uplift pressures on roofs. Probabilistic estimates of peak pressures indicated that the use of such mitigation devices resulted in significant reductions in uplift within the instrumented area, which according to the US wind load specifications (ASCE 7-05) represents a corner section of the roof that will experience the highest suction. The largest reduction was achieved with the Flat Roof AeroEdge Guard (FRAG1). This research demonstrates the potential for substantial reduction of roof uplift pressures through the development and application of mitigation techniques to lessen or prevent hurricane-induced damage to new and existing roofs.

Trajectories of Wind-Borne Debris in Horizontal Winds and Applications to Impact Testing

This paper considers the trajectories of compact and rod-type wind-borne debris in horizontal winds, using a combination of experimental and numerical studies. These types are representative of the roof gravel and timber members currently used for testing building facades and storm shutters. These studies indicate that the ratio of horizontal debris speed to wind gust speed is primarily a function of the horizontal distance traveled by the debris as it accelerates toward the wind speed. Empirical expressions to approximate the horizontal speed of these debris as a function of travel distance and time are developed, and may be used to establish rational debris impact criteria.

Impact Resistance of Laminated Glass Using “Sacrificial Ply” Design Concept

Experimental results of low velocity, small steel ball impact tests on laminated architectural glass units are presented. These tests are part of an ongoing effort to develop a design method for laminated architectural glass units to resist windborne debris from sources such as roof gravel, as required in several current and proposed U.S. building codes and standards. A design concept known as “sacrificial ply” permits the exterior-facing, outer glass ply of a laminated glass unit to fracture during windborne debris impacts, but prevents fracture of the inner glass ply. This concept also depends on an inner glass ply designed to resist lateral wind pressures for the remainder of the windstorm so that the integrity of the building envelope is preserved. In these experiments, inner and outer glass ply thicknesses and polyvinyl butyral interlayer thickness were varied to determine their effects on the impact resistance of the inner glass ply of laminated architectural glass when impacted on the outer glass ply. Results show that inner glass ply thickness and PVB interlayer thickness have stronger effects on the impact resistance of the inner glass ply than does outer glass ply thickness.

Dynamic strains in architectural laminated glass subjected to low velocity impacts from small projectiles

An experimental validation of a mechanics-based finite element model for architectural laminated glass units subjected to low velocity, two gram projectile impacts is described. The impact situation models a scenario commonly observed during severe windstorms, in which small, hard projectiles, such as roof gravel, impact windows. Controlled experiments were conducted using a calibrated air gun to propel a steel ball against simply supported rectangular laminated glass specimens. Dynamic strains on the inner glass ply were measured using foil strain gages and a high speed data acquisition system. Impact speed, interlayer thickness, glass ply thickness, and glass heat treatment conditions were varied. Dynamic strains predicted by the finite element model were in close agreement with those measured in the laboratory.

Low Velocity Impact Resistance of Laminated Architectural Glass

Recently developed finite element algorithms are used to study the effect of glass ply geometry on low velocity, small missile impact resistance of laminated architectural glass. The impacts studied are those typically found during severe windstorms, in which debris such as roof gravel can attain sufficient velocity to break windows in large high rise buildings. Outside ply fracture and interlayer debonding are both incorporated in the finite element analysis. It is shown that glass ply geometry can significantly affect the impact resistance of laminated glass. Three-layer laminates are shown to offer better impact resistance than five-layer and seven-layer laminates of the same overall thickness. For three-layer laminates of the same overall thickness, those with thin outside glass plies provide better impact resistance than those with thick ones.

Comparative study on reduction of cooling loads by roof gravel cover

The increase in demand for power to meet the ascending energy used for air conditioning in arid areas calls for new techniques to lower the cooling loads. In this paper a simple inexpensive method is suggested, where the thermal resistance of the building roof is increased by adding a layer of loose gravel. The gravel properties as well as the intergranular trapped air reduces the heat transfer through the roof. This technique is investigated experimentally for actual outdoor weather conditions. The results show that the effect of the gravel mass intensity is more pronounced than the size of the gravel. A factor that relates the heat transfer through the gravel covered layer to that of blackened concrete roofs is suggested on the basis of the present experimental results.

97/00583 Comparative study on reduction of cooling loads by roof gravel cover

97/00583 Comparative study on reduction of cooling loads by roof gravel cover

Windborne debris and the building envelope

Twenty-three years of documented windstorm damage experiences illustrate the importance of protecting the building envelope from windborne debris and fluctuating pressures that occur during windstorm events. A 2 × 4 timber and roof gravel are defined as representative objects for use in standard tests to qualify products for use in the building envelope. A 9000 cycle pressure spectrum that represents fluctuating pressures which occur in a hurricane is also specified for product qualification tests. The evolution of these test standards is traced and building codes that have adopted these criteria are referenced. Current design practices that are responsive to these windstorm effects and standard tests are described. Windows and architectural glazing systems that can protect openings in the building envelope from being breached by windborne debris are illustrated.

Impact resistance of common building materials to Tornado Missiles

Objects and debris picked up and transported by tornado winds range in size from roof gravel to railroad cars. The objectives of this project were to identify the most probable missiles transported by tornadoes, to estimate the speeds achieved by these missiles, and to determine the speeds required to perforate walls used in ordinary construction. The two most relevant missiles were identified as a 2 × 4 in. timber plank weighing 15 lbs and a 3 in. diameter steel pipe weighing 75 lbs. The two missiles are typical of a wide variety of objects and debris found in tornado paths passing through residential neighborhoods. The plank and pipe can be accelerated to speeds up to 170 mph and 90 mph, respectively in a tornado rated F5 on the Fujita Scale. Impact tests using an air-actuated missile cannon revealed that residential stud walls consisting of exterior materials other than masonry could not stop the plank at speeds above 50 mph. Most wall configurations defined in Architectural Graphic Standards were tested. Both 8-in. and 12-in. reinforced concrete masonry walls were able to stop the 2 × 4 in. plank, if the cells in the block were reinforced and grouted. At speeds above 100 mph, the planks disintegrated, but the walls remained intact. The 2 × 4 in. plank perforates the face shell of unreinforced concrete masonry at about 65 mph. There-in. diameter pipe impacts have not been tested to date.

Impact resistance of common building materials to tornado missiles

Objects and debris picked up and transported by tornado winds range in size from roof gravel to railroad cars. The objectives of this project were to identify the most probable missiles transported by tornadoes, to estimate the speeds achieved by these missiles, and to determine the speeds required to perforate walls used in ordinary construction. The two most relevant missiles were identified as a 2 × 4 in. timber plank weighing 15 lbs and a 3 in. diameter steel pipe weighing 75 lbs. The two missiles are typical of a wide variety of objects and debris found in tornado paths passing through residential neighborhoods. The plank and pipe can be accelerated to speeds up to 170 mph and 90 mph, respectively in a tornado rated F5 on the Fujita Scale. Impact tests using an air-actuated missile cannon revealed that residential stud walls consisting of exterior materials other than masonry could not stop the plank at speeds above 50 mph. Most wall configurations defined in Architectural Graphic Standards were tested. Both 8-in. and 12-in. reinforced concrete masonry walls were able to stop the 2 × 4 in. plank, if the cells in the block were reinforced and grouted. At speeds above 100 mph, the planks disintegrated, but the walls remained intact. The 2 × 4 in. plank perforates the face shell of unreinforced concrete masonry at about 65 mph. There-in. diameter pipe impacts have not been tested to date.

Performance of Cladding in Hurricane Alicia

The performance of building cladding in the Houston‐Galveston area during Hurricane Alicia is described. Following an overview of the damage based on the observations made by the writer, various possible mechanisms of cladding damage are discussed. These mechanisms include wind pressure exceeding the design value, missile impact from windborne debris, and performance of glass under dynamic wind loading. Analysis of these mechanisms suggests that impact from windborne debris, e.g., loose sheet metal, roof gravel, broken glass, and parts of the appurtenances from roofs, was the main source of glass damage. In some cases faulty installation practices, e.g., missing jamb blocks, insufficient glass bite, and improper installation, contributed to glass damage. The failure of cladding components other than glass can be attributed to poor connections between cladding components and the main structural system, either by design or by loss of strength in the fastening system as a result of corrosion. Most of the cladding damage from Alicia in the Houston‐Galveston area was caused by lack of hurricane‐resistant construction, rather than by the severity of the storm. The provision of adequate fastenings and anchorage, and control of the availability of windborne debris, would have substantially reduced the damage caused by Alicia.