Trajectories of roof sheathing panels under high winds

A 1:20 scale, aeroelastic “failure” model of a house was used to evaluate the flight mechanics of a scaled 1.2 m×2.4 m (4 ft×8 ft) roof sheathing panel in a scaled, open country, turbulent boundary layer. The panel was held to the model house with electromagnets which applied scaled restraining forces. The initial failure mechanism and the trajectory of the panel in flight were captured with high-speed digital video. For nominally similar initial conditions, different modes of flight were observed including purely translational and auto-rotational modes. These two modes lead to distinctly different lengths of flight and to different flight speeds, with the auto-rotational mode yielding higher speeds over greater distances.     

Using buildings to harvest wind energy

Research and development by Altechnica into the applied use of wind energy suggests the novel and innovative solution of employing buildings as primary wind collectors. Buildings can be designed with the means of extracting wind energy advantageously. The roof design of a building can direct and accelerate the wind flow to turbines along the ridge. Dr Derek Taylor of Altechnica introduces the idea of a wind energy system for (new and existing) buildings that can augment solar power, work for a range of building typologies and settings, generate electricity for the building and contribute into the larger electric grid.     

Using buildings to harvest wind energy

Research and development by Altechnica into the applied use of wind energy suggests the novel and innovative solution of employing buildings as primary wind collectors. Buildings can be designed with the means of extracting wind energy advantageously. The roof design of a building can direct and accelerate the wind flow to turbines along the ridge. Dr Derek Taylor of Altechnica introduces the idea of a wind energy system for (new and existing) buildings that can augment solar power, work for a range of building typologies and settings, generate electricity for the building and contribute into the larger electric grid.     

Low-rise gable roof wind loads: Characterization and stochastic simulation

Mitigation of the damage caused by windstorms to low-rise buildings is a high priority in the wind engineering community. The development of cost-effective methods to withstand the effects of extreme winds can be advanced through improved modeling of wind loads acting on low-rise roof structures. This study explores the effects of the spatial and probabilistic characteristics of pressure fields on the aggregate uplift acting on roof panels of low-rise gable roof buildings representative of typical homes. Pressure time histories obtained at roof locations for buildings of varying roof slope at several angles of incidence in the boundary layer wind tunnel at Clemson University are used to characterize the correlation statistics between tap locations and model the marginal probability density function at individual tap locations. This information is incorporated into a multi-variate non-Gaussian simulation algorithm to study the effects of various levels of correlation on the aggregate uplift on sheathing panels. Comparisons are made between the simulated aggregate uplift and ASCE 7-98 provisions [Minimum Design Loads for Buildings and Other Structures, ASCE 7-98 Standard, American Society of Civil Engineers, New York [1]] as well as laboratory generated failure capacities for sheathing panels.     

Dynamic response and reliability analysis of tall buildings subject to wind loading

This paper explores the wind stochastic field from a new viewpoint of stochastic Fourier spectrum (SFS). The basic random parameters of the wind stochastic field, the roughness length z₀ and the mean wind velocity at 10 m height U₁₀, as well as their probability density functions (PDF), are obtained. It provides opportunities to use probability density evolution method (PDEM), which had been proved to be of high accuracy and efficiency, in computing the dynamic response and reliability of tall buildings subject to the wind loading. Principals and corresponding numerical solving algorithm of the PDEM are first presented. Then, the adopted model of the wind stochastic field is described briefly. The simulation method of the fluctuating wind velocity based on the SFS is introduced. Finally, as an example of the application of the PDEM, a 20-storey frame subject to wind loading is investigated in detail. The responses, including the mean value and the standard deviation, and the reliabilities of the frame are evaluated by the PDEM. The results demonstrate that the PDEM is applicable and efficient in the dynamic response and reliability analysis of wind-excited tall building.     

Extreme value modelling of hurricane wind speeds

Estimating return levels of extreme wind speeds due to hurricanes presents both practical and analytical difficulties. The practical difficulty of collecting data has been resolved in the past by modelling simulated data--we adopt such an approach in this paper also. The analytical difficulties concern the problem of estimating the probabilities of events which are more extreme than those simulated. We follow common practice here also, using standard extreme value models to describe extreme tail behaviour. We differ from previous analyses of hurricane data in two respects. First, we use a model parameterisation which enables models fitted at different thresholds or at different sites to be easily compared. Second, we use maximum likelihood as the method of inference. This is found to produce results similar to those of previous studies, but enables the development of a spatial analysis which exploits similarities in the behaviour of the data from one site to another in order to improve the precision of estimation, and facilitates prediction at coastline locations other than those with simulated data.     

Effects of wind exposure on roof snow loads

This paper presents results from an investigation of the suitability of the exposure coefficient as defined in ISO 4355 “Bases for design of structures—Determination of snow loads on roofs”, based on thorough analyses of weather data from 389 weather stations in Norway for the reference 30-year period 1961–1990. First, the background of the exposure coefficient is examined. Historical field investigations of snow loads on roofs are also evaluated. Next, values for the exposure coefficients in Norway are calculated according to ISO 4355. Finally, possible approaches aiming at improving calculations of wind exposure on roof snow loads are suggested. It is shown that the exposure coefficient as defined in ISO 4355 does not reflect the actual effects of wind exposure on roof snow loads in Norway, the main reasons being oversimplifications in the definition of the coefficient and the extreme variations of the climate in Norway. The definition is based on coarse simplifications of snow transport theories, and must be revised and improved to serve as an applicable tool for calculations of design snow loads on roofs in Norway.     

Response of pierced fixed corrugated steel roofing systems subjected to wind loads

The low-cycle fatigue response of corrugated metal roof cladding to fluctuating wind loads was studied by subjecting cladding specimens to a series of static, cyclic and simulated “real” cyclonic wind loads using a Pressure Loading Actuator (PLA), and measuring fastener response using a x–y–z load cell. The overall performance of cladding including crack initiation, propagation and patterns, and cycles to failure was found to be similar to previous tests that used line-loads to simulate wind pressure. The reaction at a fastener to spatially varying pressures was assessed by analysing the influence coefficients, to show that it is predominantly influenced by local loads acting along the screwed crest. In addition, the response of roofing specimens subjected to fluctuating cyclonic wind pressures replicated failures observed in the field. The fastener response varied with the load level and the response spectrum followed the wind load spectrum up to 5 Hz even with deformation and cracking of the cladding showing that these higher frequency wind “load cycles” were transferred into the supporting structure via the fastener.     

Reliability of woodframe residential construction subjected to earthquakes

Most housing in the United States is light-frame wood construction (90% nationally, and 99% in California). Residential construction in the United States typically has received little or no structural engineering. The performance of wood residential construction to earthquake effects was apparent in the 1994 Northridge Earthquake, and underlines the need for new approaches to enhance building performance through improved prediction, evaluation and design methods. In this paper, methods of stochastic nonlinear dynamic analysis are used to simulate the behavior of lateral force-resisting shear wall systems typically found in residential construction subjected to earthquakes. The probability that shear wall drift limits are exceeded for uniform hazard earthquake ground motions with various return periods and intensities can be estimated from this analysis and can be related to performance levels for residential occupancy that have been suggested in concurrent research on performance-based engineering. The probability of failure under a spectrum of possible earthquakes is determined by convolving the structural fragility derived from the above analysis with the seismic hazard specified by the US Geological Survey.     

Effect of building length on wind loads on low-rise buildings with a steep roof pitch

A wind tunnel model study was carried out on long, low-rise buildings with a steep roof pitch to determine the effect of the length-to-span aspect ratio on the external wind pressure distributions. The study showed a significant increase in the magnitude of the negative pressure coefficients on the leeward roof and wall, with an increase in aspect ratio, for oblique approach winds. These large suction pressures also generate large design wind load effects on the frames near the gable-end. The 1989 edition of the Australian standard for wind loads, AS 1170.2-1989 was found to underestimate the wind loads on steep pitch gable-roof buildings of aspect ratio greater than 3, on areas near the windward gable-end, and hence the critical bending moments in the supporting structural frames. The current Australian/New Zealand wind load standard, AS/NZS 1170.2-2002 specifies increased negative pressure coefficients on the leeward half of high pitch roof buildings, and critical bending moments in the supporting frames calculated from these distributions agree quite well with values obtained from the wind tunnel study. However, other major standards severely underestimate the critical bending moments, and the effective pressure coefficients producing those bending moments, especially on the leeward roof slope.     

预应力大跨度屋架梁的施工要点及注意事项

本文以永安市粮食储备库工程为例,介绍和总结其中一个单位工程粘结预应力在大跨度屋架梁上应用的主要施工过程及其注意事项。     

Reliability analysis of a glulam beam

The present case study is an example of the use of reliability analysis to asses the failure probability of a tapered glulam beam. This beam is part of a true structure built for a super market in the town of Kokemäki in Finland. The reliability analysis is carried out using the snow load statistics available from the site and on material strength information available from previous experiments. The Eurocode 5 and the Finnish building code are used as the deterministic methods to which the probabilistic method is compared to. The calculations show that the effect of the strength variation is not significant, when the coefficient of variation of the strength is around 15% as usually assumed for glulam. The probability of failure resulting from a deterministic design based on Eurocode 5 is low compared to the target values and lower sections are possible if applying a probabilistic design method. In fire design, if a 60 min resistance is required, this is not the case according to Eurocode 5 design procedures, a higher section would be required. However, a probabilistic based fire analysis results in bounds for the yearly probability of failure which are comparable to the target value and to the values obtained from the normal probabilistic based design.     

Reliability-based design of wind turbine blades

Reliability-based design of wind turbine blades requires identification of the important failure modes/limit states along with stochastic models for the uncertainties and methods for estimating the reliability. In the present paper it is described how reliability-based design can be applied to wind turbine blades.For wind turbine blades, tests with the basic composite materials and a few full-scale blades are normally performed during the design process. By adopting a reliability-based design approach, information from these tests can be taken into account in a rational way during the design process. In the present paper, a probabilistic framework for design of wind turbine blades are presented and it is demonstrated how information from tests can be taken into account using the Maximum-Likelihood method and Bayesian statistics.In a numerical example, the reliability is estimated for a wind turbine blade in both ultimate and fatigue limit states. Information from tests is used to formulate the stochastic models used in the limit state equations. Partial safety factors for use in traditional deterministic design are estimated using the stochastic models.     

Evaluating and modelling the response of an individual to a sudden change in wind speed

The response of an individual to a sudden increase in wind velocity is important in terms of wind comfort and wind safety. This paper is concerned with the latter issue and outlines a series of physical and numerical experiments undertaken in order to evaluate the response of an individual to a sudden change in wind speed. The physical experiments were undertaken in the dynamic circuit of the Jules Verne Climatic Wind Tunnel at CSTB in Nantes, France and subjected 31 people to wind speeds up to 20 m/s. In all cases the wind speed increased from a mean value of zero to the target value in approximately 0.2 s. The wind speed required to cause loss of balance is shown to be a function of orientation and weight.     

Efficient models for wind turbine extreme loads using inverse reliability

The reliability of wind turbines against extreme loads is the focus of this study. A procedure to establish nominal loads for use in a conventional load-and-resistance-factor-design format is presented. The procedure, based on an inverse reliability approach, permits inclusion of randomness in the gross wind environment as well as in the extreme response given wind conditions. A detailed example is presented where three alternative nominal load definitions are used to estimate extreme bending loads for a 600 kW three-bladed horizontal-axis wind turbine. Only operating loads—here, flapwise (out-of-plane) bending moments—at a blade root are considered but the procedure described may be applied to estimate other loads and response measures of interest in wind turbine design. Results suggest that a full random characterization of both wind conditions and short-term maximum response (given wind conditions) will yield extreme design loads that might be approximated reasonably well by simpler models that include only the randomness in the wind environment but that account for response variability by employing appropriately derived “higher-than-median” fractiles of the extreme bending load conditional on inflow parameter values.     

Development of a shrouded wind turbine with a flanged diffuser

We have developed a wind turbine system that consists of a diffuser shroud with a broad-ring flange at the exit periphery and a wind turbine inside it. The flanged-diffuser shroud plays a role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the flange at the exit of a diffuser shroud. Namely, the flange generates a low-pressure region in the exit neighborhood of the diffuser by vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form of the flanged diffuser, such as the diffuser open angle, flange height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a flanged diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 4-5 compared to a standard (bare) wind turbine. In a field experiment using a prototype wind turbine with a flanged diffuser shroud, the output performance was as expected and equalled that of the wind tunnel experiment.     

Performance analysis of a simple roof cooling system with irrigated soil and two shading alternatives

The use of passive cooling on roofs holds a significant unfulfilled potential in hot-arid regions. In this study, the contribution of a watered soil with two types of shading for roof cooling was assessed. Two test cells of approximately 4 m² in area and 2.5 m high were monitored during the summer season. Both cells were covered with a 16-cm layer of soil. One was untreated while the other was watered and shaded consecutively by means of an overhead shading mesh and a layer of lightweight gravel. Temperature profiles were measured across the section of each roof, from the top surface of the soil to the ceiling inside the chamber, and embedded heat flux plates were also used to evaluate the cooling effect. A comparison of the two shading strategies demonstrated that while the mesh provided more cooling over a daily cycle, the daytime cooling potential, which is crucial in a desert climate, was higher with lightweight gravel.     

The role of frame members and sheathing in partition wall panels subjected to compression

This paper presents an experimental study on the role of frame members and sheathing by means of strain/stress distributions in each of the constituents of a cold-formed steel wall panel. Vertical compression loading was applied, and strain gauges were extensively used during the tests. At a given cross-section of the middle stud, strains/stresses experienced by its flanges are found to be significantly different from those on the web, indicating that the middle stud is not balanced in simple compression mode. However, the averaged stress on the flanges and that on the web follow the same trend as that recorded by the load cell, and can be used to work out the load carried by the stud. Screw connections between the stud and the board not only restrain the lateral displacement of the stud, but also support and re-distribute a portion of the load to the board and then to the bottom track. Upon loading, the axial force experienced by a typical screw is negligibly small initially, increasing slowly with load until substantial stud buckling occurs, resulting in often its pulling-out from the board or the stud/track. The board acts as a shearing member to steady the whole panel, a supporting member to enhance the overall/local buckling performance of the middle stud and sidetracks, as well as a structural member to support part of the vertical load.     

Multi-approach back-analysis of a roof bed collapse in a mining room excavated in stratified rock

Roof bed deformation mechanics is a complex topic in mining practice. There are a number of empirical, analytical and numerical techniques that can be used to analyze and design underground excavations in stratified rock masses. Each has particular drawbacks, however, so the use of just one approach in order to obtain consistent and economic designs is not practical. To deepen understanding of this topic, it is useful to study real failure in mines. In this paper we describe how a roof bed failure in an underground mine, excavated in a stratified rock mass, was back-analyzed using different approaches, namely, the empirical stability graph method, the analytical voussoir techniques, and the numerical discrete element method. The study enabled us to better understand the conditions of failure and to define the most significant parameters affecting the failure. It has also been shown how relaxation or loss of confinement stress can play an important role in roof bed deformation.