Characteristic wind actions on large flat roofed porous canopies

Osborn, Eric Peter (2016) Characteristic wind actions on large flat roofed porous canopies. Masters (Research) thesis, James Cook University.

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Abstract

Large porous protection canopy construction has evolved in Australia over the past 30 years from modest small orchard canopies to large canopies over essential water storages to reduce evaporation and pollution, canopies over large numbers of vehicles for car importers and exporters, and horticultural canopies of over 40 hectares in area. The canopies have proved to be an effective economical method to protect increasing numbers and types of assets from exposure to sun, hail, wind, birds and insects.

Canopy design and construction has evolved from the grass roots with initially no structural engineering design input. As the value of assets protected has increased, the request for structural engineering certification of the canopies has become common. To be able to certify the canopy's structure, the certifying engineer needs to confidently be able to predict the wind actions that may occur. In the past there has been limited structural engineering research undertaken into wind loads on porous structures. The aim of this study is to research the characteristic wind actions normal to large flat roof porous canopies and derive design pressure coefficients. Surface friction actions from wind drag across the surfaces are not researched in this thesis and remain a subject for future research.

Four scale models of a typical porous protection canopy were constructed for testing in the wind tunnel at the Cyclone Testing Station (CTS), James Cook University, Townsville. The models are of identical geometry, but each of different porosity, 0%, 19%, 38% and 58%. The Models were placed in the CTS boundary layer wind tunnel and rotated through 360° at increments of 15°. At each 15° increment, three sets of pressure readings, each for 30 seconds, were taken at a series of pressure taps located on the Model externally and internally. The pressure readings were processed by the wind tunnel transducer into non dimensional pressure coefficients and then adjusted for the boundary layer speed at the height of the Model.

The pressure coefficient results were imported into the analysis software Matlab and Excel and then plotted against the geometry of the canopy. This thesis presents these results graphically with pressure coefficients being plotted against distance. The distance is shown non dimensional as ratios of model width and length to model height.

It is evident from the external pressure results, that the introduction of porosity into the canopy's roof surface causes significant reduction in the magnitude of the wind actions acting on the roof when compared to the non-porous canopy. In contrast, the introduced porosity does not modify the magnitude of the wind actions on the walls greatly, but does alter the coefficient distribution. Internal pressure coefficients were found to decrease in magnitude across the model away from the windward edge.

To predict the resultant wind actions on the canopy surfaces, the simultaneous external and internal pressure coefficients were summed and adjusted using a gust factor in accordance with the Standard AS/NZS 1170.2. The net surface pressure coefficient was then plotted for four wind directions in increments of 15° from 0° to 45°. The results were then combined to find the peak maximum and minimum pressure coefficients for a ±45° wind direction sector orthogonal to the structure as required in AS/NZS1170.2.

It was found that the introduction of porosity to the models caused significant reduction in the magnitude of normal roof wind actions. Increasing the magnitude of the porosity caused only minor further reduction. The wind actions on the porous walls do not decrease significantly from the magnitudes of the coefficients on the equivalent non-porous walls, but are distributed differently due to the flow of the wind in and out of the canopy interior through the porous wall and roof surfaces. The derived net pressure coefficient results are summarised in tabular form and provide values for the design of large porous flat roofed canopies under wind load.

Item ID: 44649
Item Type: Thesis (Masters (Research))
Keywords: asset protection; bird netting; net protection; porous canopies; prediction; protective canopies; shade canopies; structural design; sun canopies; wind actions; wind loads; wind pressure
Date Deposited: 11 Aug 2016 04:52
FoR Codes: 09 ENGINEERING > 0905 Civil Engineering > 090506 Structural Engineering @ 100%
SEO Codes: 87 CONSTRUCTION > 8702 Construction Design > 870299 Construction Design not elsewhere classified @ 100%
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Last 12 Months: 36
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