Murray J. Morrison

Analysis of Wind-Induced Clip Loads on Standing Seam Metal Roofs

The current investigation extends the previous work in calculating components and cladding loads for standing seam metal roof clips. It was found that the ASCE 7-05 wind loads for these clips are conservative, while several other studies have shown that the ASCE 7-05 is unconservative when compared to integrated wind tunnel pressure data. The current note resolves this contradiction, showing that the observed conservatism is primarily due to the fact that the critical clips are not located in the worst aerodynamic region of the roof.

The response of the dines anemometer to gusts and comparisonswith cup anemometers

The Dines pressure tube anemometer was the primary wind speed recording instrument used in Australia until it was replaced by Synchrotac cup anemometers in the 1990s. Simultaneous observations of the gust wind speeds recorded using both types of anemometers during tropical cyclones have, however, raised questions about the equivalency of the gust wind speeds recorded using the two instruments. An experimental study of the response of both versions of the Dines anemometer used in Australia shows that the response of the anemometer is dominated by the motion of the float manometer used to record the wind speed. The amplitude response function shows the presence of two resonant peaks, with the amplitude and frequency of the peaks depending on the instrument version and the mean wind speed. Comparison of the gust wind speeds recorded using Dines and Synchrotac anemometers using random process and linear system theory shows that, on average, the low-speed Dines anemometer records values 2%–5% higher than those recorded using a Synchrotac anemometer under the same conditions, while the high-speed Dines anemometer records values 3%–7% higher, depending on the mean wind speed and turbulence intensity. These differences are exacerbated with the adoption of the WMO-recommended 3-s moving average gust wind speed when reporting the Synchrotac anemometer gust wind speeds, rising to 6%–12% and 11%–19% for low- and high-speed Dines anemometers, respectively. These results are consistent with both field observations and an independent extreme value analysis of simultaneously observed gust wind speeds at seven sites in northern Australia.

Damage to residential construction from the tornadoes in Vaughan, Ontario on August 20, 2009

The results of a detailed damage investigation of the two tornadoes in Vaughan, Ontario, which were part of the 19 tornadoes in the outbreak of August 20, 2009, are presented. The bulk of the damage in both tornadoes consisted of structural roof failures categorized as F1 on the Fujita Scale and EF1 on the Enhanced Fujita Scale. However, multiple houses in both tornadoes suffered total roof failure, which is F2 (or EF2). It was observed that the total roof failures were all due to either internal pressurization and or weak (or non-existent) roof-to-wall connections. Since internal pressures are likely to be larger than external pressures for these single family residences, it is argued that the total roof failure occurred under wind speed conditions associated with a lower degree of damage, such that actual wind speeds are like those in EF1 in these particular tornadoes.

A Multichamber, Pressure-Based Test Method to Determine Wind Loads on Air-Permeable, Multilayer Cladding Systems

Current standardized testing on air-permeable multi-layer cladding is inaccurate as one of the primary parameters for setting the peak net wind loads across the cladding is the external pressure gradient which is removed from all standardized testing. After implementing a latex barrier system, and examining the control strategy for the Pressure Loading Actuators, a multi-chamber airbox system for testing air-permeable multi-layer cladding is developed and compared to a benchmark study performed by the Insurance Research Lab for Better Homes. The results indicate that using multi-chamber, pressure-based, testing to obtain wind loads for air-permeable, multi-layer wall systems with flexible cladding is sufficiently accurate. This method also shows that eliminating temporal variations from the external pressures for easier standardized testing would provide fairly accurate results.

Performance of Toe-Nail Connections under Realistic Wind Loading

Using recently developed pressure loading actuators (PLA), ramp and realistic fluctuating wind loads are applied to toe-nail connections which are typically used in wood-frame residential construction. The failure capacity from the ramp and fluctuatingwind load tests are found to be similar, and are comparable to capacities reported in the literature. However, under realistic wind loading, the toe-nail connections are found to fail in incrementswith themajority of the damage to the connection occurring intermittently at the peak pressures so that it takes many peaks for a connection to fail. In addition, the effects of construction defects, in this casemissing nails, were also examined in order to determine the reduction in capacity. Considering the wind loads on a typical house, estimates for failure wind speeds were obtained, assuming a factor of safety, and compared to ASCE 7-05 wind regions.