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 fluctuating wind 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 increments with the majority 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 case missing 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.     

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.     

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.     

Theoretically estimated peak wind loads

Current estimation of peak pressure coefficients and peak wind loads on structures in the ASCE 7 Standard [American Society of Civil Engineers, ASCE Standard, Minimum Design Loads for Buildings and Structures, ASCE 7-02, ASCE, Reston, VA, USA, 2002] is based on the assumption that they are distributed normally. However, this assumption is erroneous in the case of low-rise structures because the time varying pressures and loads along roof edges and ridges have been observed to be generally non-Gaussian [H.W. Tieleman, Z. Ge, M.R. Hajj, T.A. Reinhold, Pressures on a surface-mounted rectangular prism under varying incident turbulence, J. Wind Eng. Ind. Aerodyn. 91 (2003) 1095-1115]. In this article, a new procedure is used to evaluate from one individual non-Gaussian sample record statistics of peak pressure or peak load coefficients. The initial step for the analysis requires the identification and evaluation of the appropriate marginal probability distribution. The results reveal that the distribution of the time histories of surface pressure and load coefficients is well represented by the gamma distribution, whose parameters can be adequately evaluated from the theoretical moment estimators. The corresponding distribution of the peaks of the sample records that represents either the pressure coefficients or the load coefficients, can then be obtained using a standard translation process approach. This information yields the mean and the standard deviation of the sample peaks, which are then used to determine the extreme coefficients associated at any selected probability level of non-exceedence. This latter step can be made by assuming that the distribution of the peaks follows the Extreme Value Type I (Gumbel) distribution. The analysis is applied to pressure measurements on the 1:50 scale model of the experimental building at the Wind Engineering Research Field Laboratory (WERFL), and executed in the Clemson boundary-layer wind tunnel over a range of incident turbulence intensities.     

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.     

论述中美风荷载的换算关系

结合工程实际应用,对于我国GB 50009—2001《建筑结构荷载规范》和美国ASCE 7—05《MinimumDesign Loads for Buildings and Other Structures》中关于风速、基本风压计算的差异与换算方法进行论述。     

Wind pressures and buckling of cylindrical steel tanks with a dome roof

An experimental/computational strategy is used in this paper to evaluate the buckling behavior of steel tanks with a dome roof under exposure to wind. First, wind tunnel experiments using small scale rigid models were carried out, from which pressure distributions due to wind on the cylindrical part and on the roof were obtained. Second, a computational model of the structure (using the pressures obtained in the experiments) was used to evaluate buckling loads and modes and to study the imperfection sensitivity of the tanks. The computational tools used were bifurcation buckling analysis (eigenvalue analysis) and geometrical nonlinear analysis (step-by-step incremental analysis). Geometric imperfections and changes in the buckling results due to reductions in the thickness were also included in the study to investigate reductions in the buckling strength of the shell. For the geometries considered, the results show low imperfection sensitivity of the tanks and buckling loads associated with wind speeds 45% higher than those specified by the ASCE 7-02 standard.     

A comparison of methods of extreme wind speed estimation

The paper presents a comparative assessment of methods for extreme value analysis of the US wind speed data using four different methods, namely Standard Gumbel, Modified Gumbel, Peaks-Over-Threshold (POT) and Method of Independent Storms (MIS). The analysis highlights the influence of methodological assumptions on the estimates of design wind speed corresponding to 50-year and 500-year return period. The results demonstrate that the MIS method leads to more stable quantile estimates than the POT method.     

Database-assisted design methodology to predict wind-induced structural behavior of a light-framed wood building

This study investigates the applicability of the database-assisted design (DAD) methodology to predict structural reactions in a light-framed wood structure subjected to fluctuating wind pressures. Structural influence functions were determined on a 1/3-scale light-frame wood structure, which was then subjected to a wind flow, while the surface pressures and structural reactions at roof-to-wall and wall-to-foundation connections were simultaneously recorded. There was a good agreement between the DAD-predicted structural reactions and experimentally measured reactions, confirming that the DAD method is suitable for predicting the structural reactions in light-frame wood buildings. Subsequently structural reaction time histories at several connections within the building were generated using a 1:50 scale wind tunnel model of the structure and the peak structural reactions determined using the DAD method and previously obtained influence functions. When the DAD-estimated reactions were compared with reactions predicted by the ASCE 7-05 main wind force resisting system (MWFRS) method, they showed the ASCE 7 reactions were highly non-conservative(i.e. smaller than the DAD method predictions), by as much as 39% at the gable end truss. The components and cladding method showed reasonable agreement with the DAD method for the gable end and first interior truss reactions but it too underestimated the reaction loads at the second and third interior trusses by 30% and 12% respectively.     

A novel approach to estimate the wind uplift resistance of roofing systems

Roof wind design consist of three parts: determination of wind loads, evaluation of wind uplift resistance and correlating the resistance with the design load such that the resistance is higher than the load requirement. Wind uplift resistance of a system with its respective components is evaluated in laboratory testing. This paper presents a novel approach to estimate wind uplift resistance when components are substituted during field application. Wind dynamics, on a mechanically attached single-ply roofing assembly, lift the membrane and cause fluttering, introducing stresses at the attachment locations. In such assemblies, the fastener–deck interface is a critical design factor. First, by taking steel deck as a component this paper systematically characterizes the various steel decks that are commonly used in low slope application. Second component, namely the fasteners and its engagement strength with deck have been quantified for variations of its design, size and sources. Based on this component characterization, fastener pullout resistance (FPR) is identified as a verification factor for system wind resistance estimation. When variations occur in the fastener–deck interface between the proposed and the existing configurations, the present research through case studies has proved that: “as long as the FPR of the proposed configuration is higher than the existing configuration then wind uplift ratings can be maintained”. This is valid as long as both the configurations have all the remaining roofing components similar with comparable layout. Based on this verification, the study recommends that the testing lab should report the FPR along with the wind uplift resistance such that FPR can be used as a verification factor to accept design/field alternatives.     

Simplified design approach for cold-formed stainless steel compression members subjected to flexural buckling

The design criteria of stainless steel compression member are more complicated than those of carbon steels due to the nonlinear stress strain behavior of the material. In general, the tangent modulus theory is used for the design of cold-formed stainless steel columns. The modified Ramberg–Osgood equation given in the ASCE Standard can be used to determine the tangent modulus at specified level of stresses. However, it is often tedious and time-consuming to determine the column buckling stress because several iterations are usually needed in the calculation. This paper presents new formulations to simplify the determination of flexural buckling stress without iterative process. Taylor series expansion theory is utilized in the study for numerical approximations. The proposed design formulas are presented herein and can be alternatively used to calculate the flexural buckling stress for austenitic type of cold-formed stainless steel columns. It is shown that the column strengths determined by using the proposed design formulas have good agreement with those calculated by using the ASCE Standard Specification. A design example is also included in the paper for cold-formed stainless steel column designed by using the ASCE Standard equations and the proposed design formulas.     

2010年版美国建(构)筑物荷载规范有关风荷载部分修改简介

从2010年1月1日起,美国土木工程师协会编写的建(构)筑物最小设计荷载规范ASCE 7-10开始正式实施.其中风荷载部分有了较大的修改.风荷载内容由原来的1个章节分解为6个章节.基本风速的重现期也由原来的50年改为根据不同建(构)筑物风灾害类别分别取为300年、700年或1 700年.为与地震作用一致,荷载组合中的风...     

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.     

Investigating the reliability of RC beams of tall buildings designed based on the new ACI 318‐05/ASCE 7‐05

The level of safety achievable in reinforced concrete (RC) beams designed based on the new ACI 318‐05/ASCE 7‐05 is investigated in this study. The study makes use of the more recent statistical data on flexure and shear provided by Szerszen and Nowak. Due to the importance of using real ratios of lateral/gravity loads in any such reliability analysis, a set of RC buildings was chosen, loaded and designed and the actual nominal wind‐to‐dead load ratios were derived. Using these and the statistical data mentioned above, reliability indices for moment and shear were calculated. The resulting reliability indices for moment and shear are then presented in comparison. In addition, the reliability index variations along the beam at the controlling stations are compared to each other. The results of the study indicate that reliability indices at the controlling stations obtained for different limit states are not consistent for low values of nominal wind‐to‐dead load ratios, but converge consistently for high ratios. It seems that the use of older shear strength factors provides a reliable safety margin and that the use of the more recent statistical data for material combined with the older shear strength factors leads to more consistency. Copyright © 2010 John Wiley & Sons, Ltd.     

Wind effects of parapets on low buildings: Part 1. Basic aerodynamics and local loads

This is the first paper in a series on the effects of parapets on the wind-induced loads on low buildings. Part 1 focuses on the basic aerodynamic effects of parapets and the local (components and cladding) loads. Wind tunnel data were obtained from about 700 pressure taps in the area of a corner panel of 3.7 m×7.6 m (equivalent full-scale dimensions) for several parapet heights and configurations. Significant downward loads were observed which exceed code values for all parapet heights. This may be significant when combined with other loads (such as snow or water). It was also found that parapets alter the suction loads on the roof by changing the location of the corner vortex relative to the roof, for continuous perimetric parapets, and the type of vortex formed, for isolated (single wall) parapets. In the ASCE-defined interior region, the measured coefficients for component and cladding loads exceed those in the code for all parapets and areas examined. For the edge zone, the experimental coefficients for areas less than 1 m² exceed the code values (except for tall perimetric parapets). However, it was found that the component and cladding loads in the ASCE 7 adequately envelope the uplift caused by perimetric parapets in the corner zone for H=4.6 m, but not for isolated parapets, in particular for areas less than 1 m². It was also discussed that the ASCE 7 will be unconservative for larger eaves heights since H² is the correct normalizing factor for roof areas beneath the separated flow. Furthermore, the use of edge zone coefficients in the corner zone for h ?0.9 m should be changed to h/(H+h)?0.23 in the ASCE 7.     

Performance based design extreme wind loads on a tall building

This paper presents a procedure for the calculation of wind loads on a proposed 385 ft tall building located in strong wind and mixed strong wind and hurricane wind regions. The procedure for the computation of design wind loads uses mixed distribution and Monte Carlo simulation. The results of a probabilistic analysis of hurricane wind speeds are combined with the probability distribution of recorded extreme wind speeds (excluding hurricane data) at the site. A 50‐year sample of extreme wind speeds is created and the maximum 50‐year wind (from the hurricane and the recorded data) is noted. The simulation is repeated for a large number of samples (>10000) and the probability distribution of the 50‐year wind speed is computed for use in establishing the design wind speed Copyright © 2001 John Wiley & Sons, Ltd.     

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.     

美国输电线路风荷载计算介绍

了解《美国输电线路结构荷载指南》(ASCE74-2009)对从事涉外工程和提高工程师设计水平都有着重要意义。风荷载是输电线路设计的控制荷载,了解ASCE74-2009风荷载计算方法非常有必要。本文介绍了ASCE74-2009输电线路风荷载的计算方法,并对基本风速、风压高度变化系数、体型系数、风荷载调整系数、地形影响因子和斜向风荷载计算方法及其计算参数进行了说明。     

美国规范风荷载浅析及计算

美国规范作为世界主流标准之一,被越来越多的涉外工程所要求采用.以算例为基础,针对美国建筑最小荷载规范ASCE/SEI 7-05中的风荷载进行分析和研究,以期对采用美国规范的涉外工程有所帮助.