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.     

The UWO contribution to the NIST aerodynamic database for wind loads on low buildings: Part 2. Comparison of data with wind load provisions

Wind tunnel test data on generic low buildings have been obtained at the University of Western Ontario (UWO) to contribute to the NIST aerodynamic database. In Part 1 the basic data and archiving format are presented. In the present paper, data from two models of different roof slope (1:12 and 3:12) at four eave heights each (4.9 m (16 ft), 7.3 m (24 ft), 9.8 m (32 ft), and 12.2 m (40 ft)), in open and suburban terrain conditions, were examined in detail. The data were compared to the historical data obtained by Stathopoulos in the late 1970s from which current North American code provisions were developed. Structural response coefficients were calculated on an assumed structural system and these data were compared with the current wind load provisions for low buildings in the ASCE 7-02 (ASCE Standard, Minimum Design Loads for Building and other Structures, ASCE 7-02, ASCE, New York, USA, 2002), the AS/NZS 1170.2 (Australian/New Zealand Standard Structural Design Actions, Part 2: 2002—AS/NZS 1170.2:2002, Standards Australia International Ltd., Sydney, AS and Standards New Zealand, Wellington, NZ, 2002), the Eurocode (Eurocode 1: Basis of Design and Actions on Structures. Part 2-4: Wind Actions, ENV-1991-2-4-1, CFN, Brussels, 1995), and the NBCC (National Building Code for Canada 1995 (NBCC(1995)); Includes User's Guide—NBC 1995 Structural Commentaries (Part 4), NRCC, Ottawa, Canada, 1995. The peak response coefficients from the current data set were found to increase with eave height. The ASCE 7-02 and the NBCC (1995) underestimated the peak response coefficients calculated for the current data set by ∼15% for the lowest eave height; and were worse for larger eave heights. Generally, the Eurocode (ENV, 1995) wind load provisions match the peak response coefficients from the data set at all eave heights. The response coefficients calculated using the AS/NZS (2002) provisions were generally a good match for the interior region only, however they significantly underestimated the wind tunnel response coefficients for the end bays.     

The UWO contribution to the NIST aerodynamic database for wind loads on low buildings: Part 1. Archiving format and basic aerodynamic data

Wind tunnel testing of generic low building models has been carried out at the University of Western Ontario (UWO) in support of an initiative by Texas Tech University (TTU) and the United States National Institute of Standards and Technology (NIST) to create an aerodynamic database for low building design. This paper describes the background of the project, the basic models, testing configurations, the wind simulation, the standard archival format for distribution of the data, and a basic analysis of the data. Part 2 presents a detailed comparison of the data with existing wind load provisions in building codes.Basic quality checks of the data are made via limited comparisons among the data obtained during this study. Parametric comparisons based on roof slope, building height and building plan dimension show that the data obtained within this study are consistent with the expected aerodynamic behaviour. Comparisons with full scale TTU data show that the wind tunnel tests match the full-scale reasonably well, but cannot reproduce the largest of the peak point suctions near roof edges.     

Wind tunnel tests for mean wind loads on partially clad structures

Many structures, such as buildings under construction, industrial process structures, and manufacturing and storage facilities, are temporarily or permanently partially clad. Scant wind load data for these structures are available in the literature and wind codes. An investigation was undertaken at the L.S.U. Wind Tunnel Laboratory to study the aerodynamic behavior of these structures. Mean force coefficients for various cladding arrangements are reported for models of a 10-storey structure for two plan aspect ratios (1:1 and 3:1) in smooth, grid turbulent, and boundary layer flow. Three flow regimes were tested to validate results against published data and to investigate the possibility of reducing the number of key variables. Results for the fully clad and unclad cases compared well with available data for ground-mounted prisms and lattice frames, respectively. It was found that force coefficients for some of the cladding configurations significantly exceeded those for a fully clad structure of the same overall geometry. Additionally, the manner in which the force coefficients varied with wind direction was found to differ dramatically from the fully clad cases for some of the cladding arrangements. The conditioning of the approach flow was found to be less important for unclad models than for partially or fully clad configurations.     

Computational fluid dynamics simulations and wind tunnel measurements of unsteady wind loads on a scaled model of a very large optical telescope: A comparative study

Thorough theoretical and experimental investigations were performed to analyze the main characteristics of unsteady flows past a 1:100-scale wind tunnel (WT) model of a very large optical telescope housed within a spherical enclosure. The investigations were focused on the prediction and measurements of unsteady pressures on the inner and outer surfaces of the enclosure and on the telescope primary mirror. The WT measurements were performed essentially to provide aerodynamic data on the telescope structure and also to build a database for correlation with numerical simulation of the flow using computational fluid dynamics (CFD). Unsteady viscous flow solutions were computed for different telescope orientations using the lattice Boltzmann method coupled with the RNG k-ε turbulence model. For WT testing, unsteady pressure measurements were performed in an open jet WT for different telescope orientations and wind speeds, using a number of pressure taps distributed around the inner and the outer surfaces of the enclosure and on the primary mirror surface. A smoke stream visualization technique was also used to study the flow behavior around and inside of the telescope enclosure. The flow solutions were computed using the WT flow conditions. Correlations were obtained between CFD and WT data in terms of the mean pressure coefficients on the enclosure and the primary mirror surfaces, and for their standard deviations. Power spectral density analyses were also carried out for a number of pressure signals collected on the primary mirror surface. Both CFD solutions and WT measurements demonstrated that the flow inside and outside the enclosure was unsteady and massively separated on the back of the enclosure. The mean values and standard deviations of the pressure coefficients on the enclosure and the primary mirror surfaces correlated well with the experimental data. Using the WT Mach number in the simulation, the shear layer over the enclosure opening and the resulting acoustic wave effects were well captured, and there was excellent agreement between the CFD results and the WT measurements.     

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.     

On the propagation of uncertainty in inflow turbulence to wind turbine loads

When stochastic simulation of inflow turbulence random fields is employed in the analysis or design of wind turbines in normal operating states, it is common to use well-established standard spectral models represented in terms of parameters that are usually treated as fixed or deterministic values. Studies have suggested, though, that many of these spectral parameters can exhibit some degree of variability. It is not unreasonable to expect, then, that derived flow fields based on simulation with such spectral models can be in turn highly variable for different realizations. Turbine load and performance variability would also be expected to result if response simulations are carried out with these variable flow fields. The aim here is to assess the extent of variability in derived inflow turbulence fields that arises from the noted variability in spectral model parameters. Simulation of these parameters as random variables forms the basis of this study. A commercial-sized 1.5 MW concept wind turbine is considered in the numerical studies. Variability in turbulence power spectra at field points on the rotor plane and in turbulence coherence functions for separations on the order of a rotor diameter and smaller is studied. Using time domain simulations, variability in various wind turbine response measures is also studied where the focus is on statistics such as response root-mean-square and 10-min extreme estimates. It is seen that while variability in inflow turbulence spectra can be great, the variability in turbine loads is generally considerably lower. One exception is for turbine yaw loads whose larger variability arises due to sensitivity to a coherence decay parameter that is itself highly variable. Finally, because reduced-order representations of turbulence random fields using empirical orthogonal decomposition techniques allow useful physical insights into spatial patterns of flow, variability in the energy distribution and the shapes of such empirical eigenmodes is studied and a simplified model is proposed that retains key variability sources in a limited number of modes and that accurately preserves overall inflow turbulence field uncertainty.     

Analysis of hurricane wind loads on low-rise structures

Time series of pressure coefficients collected on the roof of a house by the Florida Coastal Monitoring Program during landfall of Hurricane Ivan on the Florida panhandle in 2004 are analyzed. Rather than using peak values, which could vary due to the stochastic nature of the data, a probabilistic analysis is performed to characterize extreme values of pressure coefficients and associated wind loads. It is shown that the pressure coefficient time series follows a three parameter Gamma distribution, while the peak pressure follows a two-parameter Gumbel distribution. The analysis yields a probability of non-exceedance of a given threshold of the pressure or load coefficients. For this specific house and specific storm, the 80 percentile load coefficient value of the probability of non-exceedance is −1.7. This is discussed in the context of ASCE 7 GCp values.     

Wind loads on curved roofs

Curved roofed buildings are increasingly used in the modern built environment because they offer aerodynamically efficient shapes and provide architects and designers with an alternative to regular rectangular building forms. However, there is little information available on the wind loads on these roof forms. The Eurocode for wind actions (EN1991-1-4) includes pressure coefficients for a limited range of aspect ratio cylindrical roofs from measurements in low-turbulence conditions but only for wind blowing normal to the eaves. There is some concern regarding the reliability of these data, consequently EN1991-1-4 allows National Choice (National Determined Parameter) for wind loads on these roofs. This paper describes a series of parametric wind tunnel studies undertaken at BRE to measure wind pressures on a wide range of curved roof models in a properly scaled atmospheric boundary layer simulation and gives an alternative to the EN1991-1-4 recommended procedure.     

The UWO contribution to the NIST aerodynamic database for wind loads on low buildings: Part 3. Internal pressures

Wind tunnel tests of generic low buildings have been conducted at the University of Western Ontario for contribution to the National Institute on Standards and Technology (NIST) aerodynamic database. Part 1 provided the archiving format and basic aerodynamic data. In Part 2, the data of external pressures were compared with existing wind load provisions for low buildings. This paper, Part 3, deals with an investigation of wind-induced internal pressures of low-rise buildings with realistic dominant opening and leakage scenarios. Data from one building model with four different opening sizes were compared with numerical simulations. The existing theory, using the unsteady orifice discharge equation, works well for the building models used in this study, given the external pressures near the openings, irrespective of shifts of wind direction and upstream terrain. Numerical simulations can capture the temporal variations of the internal pressure fluctuations, as well as mean values.The internal pressure fluctuations for the building with leakage (nominally sealed building) are attenuated as they pass through the openings, while mean values are consistent with spatially averaged external pressures. Internal pressure resonance occurs for the dominant opening (3.3% open ratio) with building leakage. Effects of oblique wind angles on internal pressure dynamics are not significant, at least for the openings in the centre of wall, as is the case herein. Peak internal pressures occur for a wind direction normal to the wall with a dominant opening. Measured internal pressure coefficients are compared with current wind load provisions. Some peak values were found to exceed the recommended design values for the dominant windward wall opening cases.     

Wind loads on residential and large-scale solar collector models

Previous studies on solar water heaters have mainly focused on the thermal efficiency of various types of solar collectors, and there has been limited work done on the aerodynamic characteristics of solar collectors or their supporting structure. However, typhoons are among the natural hazards that have a costly impact on residential construction and solar water heaters are usually installed on the flat roofs of buildings for hot water production. Thus, the safety of solar water heaters under severe wind load during typhoon season is a critical issue in the promotion of solar water heaters. In this study, an experimental program was conducted to examine the aerodynamic characteristics of solar collector models (residential and large-scale solar water heaters). The unit sectional uplift coefficient with or without a guide plate is also addressed. Measurements of mean longitudinal and spanwise surface pressure were performed. The data show that the presence of a water storage tank (or a horizontal cylinder) tends to reduce the suction force on the upper surface within the first half of the tilt flat panel (solar collector). Stronger negative longitudinal differential mean pressure is observed for the test case of a tilt flat panel only, which corresponds to strong wind load. The inverted U-shape of mean spanwise pressure distributions is also noted. With a guide plate, less wind load and more uniformity of spanwise differential mean pressure distributions are associated with a larger projected area of a guide plate on the lower surface of a tilt flat panel. This is essentially attributed to the attenuation of corner vortices and a decrease in uplift force.     

Interpretation of full-scale strain data from wind pressures on a low-rise structure

This paper presents a study on the structural performance of a low-rise building under high-wind action, derived from the data analysis of a long-term full-scale monitoring program (1997–2000). Experimentation and full-scale measurements are critical in the study of wind-pressure-induced loading associated with complex phenomena such as three-dimensional flow fields and non-stationary winds. Recorded pressure data and corresponding deformations (strains) on selected portions of the structural frame in the proximity of the pressure transducers are evaluated and compared. The system under investigation is unique since it is located in a complex topography (coastal) environment, for which even the interpretation of basic wind and pressure data becomes challenging.The correspondence of the observed data with the United States wind load specifications (ASCE-7) is investigated both from the perspective of the loads (pressures) and of the deformation response predictions. The structural performance is evaluated through the comparison of the recorded strains with predicted deformations, derived from the application of the corresponding external wind pressure loading to a simplified structural model simulating the behavior of an instrumented region of the building.Results show good agreement between measured pressure coefficients and selected values estimated through the ASCE-7 specifications, especially in areas of positive pressures where the predictions are consistently larger than the recorded data. Reasonable and consistent correspondence was also found from the analysis of the strains in structural members.     

Field measurement and wind tunnel simulation of hurricane wind loads on a single family dwelling

During landfall of Hurricane Ivan on the Florida ’panhandle’ in 2004, pressure time-history data were recorded on multiple pressure sensors installed on the roofs of six single-family homes. An analysis approach was developed to determine the peak negative, mean, peak positive, and standard deviation of pressure coefficients for these datasets. This paper presents a comparison of the full scale pressure coefficients from one of these homes, which experienced sustained hurricane force winds, with the results of wind tunnel experiments on a 1:50 scale model of that home. It was determined that the wind tunnel and full-scale mean and rms pressure coefficients matched very closely at almost every monitored location on the roof, while the peak negative pressure coefficients in the wind tunnel study generally underestimated the full-scale values, consistent with observations from previous full-scale/wind tunnel comparative studies. Field-measured hurricane wind loads may prove useful for evaluating existing wind load provisions. However, recommendations in that regard are premature without the analyses of multiple homes in multiple storms, performed by more than one wind tunnel facility. Future work will focus on building such a joint study.     

Safety of glass panels against wind loads

Safety of glass panels under wind loads is investigated. Noting that strength of glass displays significant size dependent scatter, the theory of brittle fracture statistics is applied to find the probability of failure of brittle rectangular plates under uniform lateral loads. The effects of plate size, plate thickness, intensity of lateral load and strength of glass on probability of failure are studied. The results are presented graphically and a specific example is studied in detail.     

Automated extraction and classification of thunderstorm and non-thunderstorm wind data for extreme-value analysis

Design wind loads are partly based on extreme value analyses of historical wind data, and limitations on the quantity and spatial resolution of wind data pose a significant challenge in such analyses. A promising source of recent wind speed and direction data is the automated surface observing system (ASOS), a network of about 1000 standardized US weather stations. To facilitate the use of ASOS data for structural engineering purposes, procedures and software are presented for (a) extraction of peak gust wind data and thunderstorm observations from archived ASOS reports, (b) classification of wind data as thunderstorm or non-thunderstorm to enable separate analyses, and (c) construction of data sets separated by specified minimum time intervals to ensure statistical independence. The procedures are illustrated using approximately 20-year datasets from three ASOS stations near New York City. It is shown that for these stations thunderstorm wind speeds dominate the extreme wind climate at long return periods. Also presented are estimates based on commingled data sets (i.e., sets containing, indiscriminately, both non-thunderstorm and thunderstorm wind speeds), which until now have been used almost exclusively for extreme wind speed estimates in the US. Analyses at additional stations will be needed to check whether these results are typical for locations with both thunderstorm and non-thunderstorm winds.     

Interference effects on wind loading of a row of closely spaced tall buildings

Interference effects on a row of square-plan tall buildings arranged in close proximity are investigated with wind tunnel experiments. Wind forces and moments on each building in the row are measured with the base balance under different wind incidence angles and different separation distances between buildings. As a result of sheltering, inner buildings inside the row are found to experience much reduced wind load components acting along direction of the row (x) at most wind angles, as compared to the isolated building situation. However, these load components may exhibit phenomena of upwind-acting force and even negative drag force. Increase in x-direction wind loads is observed on the upwind edge building when wind blows at an oblique angle to the row. Other interference effects on y-direction wind loads and torsion are described. Pressure measurements on building walls and numerical computation of wind flow are carried out at some flow cases to explore the interference mechanisms. At wind angle around 30° to the row, wind is visualized to flow through the narrow building gaps at high speeds, resulting in highly negative pressure on associated building walls. This negative pressure and the single-wake behavior of flow over the row of buildings provide explanations for the observed interference effects. Interference on fluctuating wind loads is also investigated. Across-wind load fluctuations are much smaller than the isolated building case with the disappearance of vortex shedding peak in the load spectra. Buildings in a row thus do not exhibit resonant across-wind response at reduced velocities around 10 as an isolated square-plan tall building.     

金属吊顶抗风性能的研究

目前金属吊顶广泛应用于轻轨站、敞开式走廊等半户外环境,会受到自然风的影响。本论文通过自行研发的测试设备,研究了金属吊顶系统在均布荷载与风荷载下的强度,建立了风载荷强度检测方法,并将其写入了最新的《金属与金属复合材料吊顶板》国家标准。     

广州西塔风荷载研究

广州西塔是广州双塔的西塔楼,目前为华南地区第一高楼,高宽比超过6.5,主塔楼结构采用钢管混凝土斜交网格外筒+钢筋混凝土内筒的筒中筒体系。由于广州为台风多发地区,风荷载为其结构设计的控制荷载,故对其抗风设计进行了详尽的介绍。通过风气候分析确定了区域风况和设计风参数,以及与风响应有关的结构参数,并通过模型风洞试验确定了大楼的等效风荷载取值,并对大楼舒适度进行了判别。     

Wind effects of parapets on low buildings: Part 3. Parapet loads

As part of the study on the effects of parapets on wind-induced loads on low buildings, measurements of the pressures on parapet surfaces have been carried out. Pressures were measured on both the exterior and interior for several parapet heights, h=0.46, 0.9, 1.8, 2.7 m, and building heights, H=4.6, 9.1, 18 m, for both a uniform perimetric parapet and an isolated parapet on one wall. These data were used to quantify the local (component and cladding) and structural wind loads on the parapets. It was found that the worst structural load coefficients over all wind angles are approximately constant with h and H because of opposing trends of the pressures on the interior and exterior parapet surfaces. That is, the loads increase on the interior surface with H (as they do for roof loads), while decreasing on the exterior surface. The current structural load coefficients prescribed by the ASCE 7-02 capture this well for the building configurations considered. However, the suction component and cladding loads on the interior surface of isolated parapets are not well captured by the code.     

Wind effects of parapets on low buildings: Part 2. Structural loads

The present paper, Part 2 in a four part series, focuses on the effects of solid, perimetric parapets on the wind-induced structural loads on low-rise buildings. Roof and wall pressures were measured at more than 500 locations simultaneously for five parapet heights (h=0, 0.46, 0.9, 1.8 and 2.7 m in equivalent full-scale dimensions) and three building heights (H=4.6, 9.1 and 18.3 m) with plan dimensions 31.1 by 61.6 m and a on 12 gable roof slope. The data were obtained in simulated open country and suburban terrain conditions, at a scale of 1:100, in a boundary layer wind tunnel. It was observed that the distance from the eaves edge to the reattachment point for winds normal to the wall increases from x/H∼0.4 for h/(H+h)=0 to x/H=1.8 for h/(H+h)=0.23. While mean and fluctuating point pressure distributions tend to decrease in magnitude with h, the increased areas of separated flow lead to increased loads for interior frames with the taller parapets.