Low Building Roofs Research Articles

Effect of roof-edge parapets on mitigating extreme roof suctions

Very high intermittent suctions have been found to occur near corners of low rise building roofs. This paper investigates the effectiveness of parapets mounted at the roof-edge on mitigating these high suctions. The study included full perimeter parapets, both solid and perforated and parapets placed only at the ridge and the corner zones. The parapets used in the study had a 0.2 m full scale height which is more feasible for installation on typical homes than the taller parapets that have been considered in previous studies. The peak pressure coefficients in the corner zone were reduced by over 50% with the solid and perforated parapets placed along the perimeter of the building. Perforated parapets with a perforation ratio of 33% and having a length of 10% of the building dimension at the corners and at the ridge performed best resulting in about a 60% reduction in the peak pressure coefficients at the corners.

Simulated tropical cyclonic winds for low cycle fatigue loading of steel roofing

Low rise building roofs can be subjected to large fluctuating pressures during a tropical cyclone resulting in fatigue failure of cladding. Following the damage to housing in Tropical Cyclone Tracy in Darwin, Australia, the Darwin Area Building Manual (DABM) cyclic loading test criteria, that loaded the cladding for 10000 cycles oscillating from zero to a permissible stress design pressure, and the Experimental Building Station TR440 test of 10200 load cycles which increased in steps to the permissible stress design pressure, were developed for assessing building elements susceptible to low cycle fatigue failure. Recently the ‘Low-High-Low’ (L-H-L) cyclic test for metal roofing was introduced into the Building Code of Australia (2007). Following advances in wind tunnel data acquisition and full-scale wind loading simulators, this paper presents a comparison of wind-induced cladding damage, from a “design” cyclone proposed by Jancauskas, et al. (1994), with current test criteria developed by Mahendran (1995). Wind tunnel data were used to generate the external and net pressure time histories on the roof of a low-rise building during the passage of the “design” cyclone. The peak pressures generated at the windward roof corner for a tributary area representative of a cladding fastener are underestimated by the Australian/New Zealand Wind Actions Standard. The “design” cyclone, with increasing and decreasing wind speeds combined with changes in wind direction, generated increasing then decreasing pressures in a manner similar to that specified in the L-H-L test. However, the L-H-L test underestimated the magnitude and number of large load cycles, but overestimated the number of cycles in the mid ranges. Cladding elements subjected to the L-H-L test showed greater fatigue damage than when experiencing a five hour “design” cyclone containing higher peak pressures. It is evident that the increased fatigue damage was due to the L-H-L test having a large number of load cycles cycling from zero load (R=0) in contrast to that produced during the cyclone.

Generation of local wind pressure coefficients for the design of low building roofs

This paper presents recent research on the experimental evaluation of wind loads on low buildings and the recommendations provided in the form of traditional codification. These mainly include the wind loads on buildings with geometries different from those examined in previous studies. This is followed by the evaluation of simulated wind loads on low building roofs. The overall application of a recently proposed simulation methodology for codification purposes is discussed in detail. The traditional codification provides for a group of roof geometries a single peak design pressure coefficient for each roof zone considering a nominal worst-case scenario; this may often lead to uneconomical loads. Alternatively, thernpresented methodology is capable of providing peak pressure coefficients corresponding to specific roof geometries and according to risk levels; this can generate risk consistent and more economical design wind loads for specific roof configurations taking into account, for instance, directional design conditionsrnand upstream roughnesses.

Wind Loads on Low Building Roofs: A Stochastic Perspective

A comprehensive experimental study of local wind pressure fluctuations on low building roofs has been carried out, by testing a variety of building models exposed to open and suburban boundary-layer flows, for various wind directions, to determine the overall stochastic characteristics of wind loads on roofs. The analysis of pressure data indicates that the pressure fluctuations are sensitive to tap location, roof geometry, and wind direction. Both Gaussian and non-Gaussian fluctuations have been found in all roof types; however, non-Gaussian fluctuations are generally found in flow separation zones. Zones of Gaussian and non-Gaussian pressure fluctuations are presented. Relevant stochastic characteristics of wind pressure fluctuations in time, frequency, and amplitude domains are discussed.

Random number sensitivity in simulation of wind loads

Recently, an efficient and practical method has been developed for the generation of univariate non-Gaussian wind pressure time histories on low building roofs; this methodology requires intermittent exponential random numbers for the simulation. On the other hand, the conventional spectral representation scheme with random phase is found suitable for the generation of univariate Gaussian wind pressure time histories on low building roofs; this simulation scheme requires uniform random numbers. The dependency of these simulation methodologies on the random number generator is one of the items affecting the accuracy of the simultion result; therefore, an attempt has been made to investigate the issue. This note presents the observed sensitivity of random number sets in repetitive simulations of Gaussian and non-Gaussian wind pressures.

Synthesis of non-Gaussian wind pressure time series on low building roofs

This paper presents an efficient and practical method for the digital generation of univariate non-Gaussian wind pressure time series on low building roofs. The method, based on the Fast Fourier Transform (FFT) approach, essentially inverts the Fourier coefficients which are a linear combination of Fourier amplitude and phase. In this study, the Fourier amplitude part is assumed to be known. The Fourier phase capable of inducing non-normality to the time series is carefully modelled and a simple stochastic model with a single parameter is suggested for its simulation. The computation of this single parameter is accomplished by minimizing the sum of the squared errors in higher order statistics such as skewness and kurtosis. The simplicity and effectiveness of this methodology have been demonstrated using several measured non-Gaussian pressure data from various low building roofs under different conditions.

Modeling of wind-induced pressure fluctuations on roofs of low-rise buildings

A systematic study on the modeling of wind-induced pressures on low building roofs with application to extreme value and fatigue analysis is described in this paper. Extensive wind tunnel measurements form a basis to carry out the modeling. Based on the Fourier representation of time series, a general approach for simulating Gaussian as well as non-Gaussian wind pressure fluctuations has been presented. Both Fourier amplitude and phase required for the simulations are modeled individually. A simple stochastic model is proposed for the generation of Fourier phase of non-Gaussian time series. An empirical model has been suggested for the synthetic generation of normalized spectra; synthetic spectra are utilized for the generation of Fourier amplitude part. Towards the generalization of the simulation scheme, the standard spectral shapes associated with various zones of each roof and their parameters are established. The efficiency of this simulation methodology is illustrated with several examples. Applications of the simulation methodology have also been discussed. The established simulation scheme can be used to generate fluctuating wind pressures on low building roofs in a generic fashion not only for the evaluation of extreme pressures but also for fatigue design purposes.Key words: low-rise building, modeling, roofs, wind pressure.

Modeling of wind-induced pressure fluctuations on roofs of low-rise buildings

A systematic study on the modeling of wind-induced pressures on low building roofs with application to extreme value and fatigue analysis is described in this paper. Extensive wind tunnel measurements form a basis to carry out the modeling. Based on the Fourier representation of time series, a general approach for simulating Gaussian as well as non-Gaussian wind pressure fluctuations has been presented. Both Fourier amplitude and phase required for the simulations are modeled individually. A simple stochastic model is proposed for the generation of Fourier phase of non-Gaussian time series. An empirical model has been suggested for the synthetic generation of normalized spectra; synthetic spectra are utilized for the generation of Fourier amplitude part. Towards the generalization of the simulation scheme, the standard spectral shapes associated with various zones of each roof and their parameters are established. The efficiency of this simulation methodology is illustrated with several examples. Applications of the simulation methodology have also been discussed. The established simulation scheme can be used to generate fluctuating wind pressures on low building roofs in a generic fashion not only for the evaluation of extreme pressures but also for fatigue design purposes.Key words: low-rise building, modeling, roofs, wind pressure.

The variation of peak loads with tributary area near corners on flat low building roofs

Abstract This paper discusses the effect of spatial averaging of roof pressures on peak loads over various tributary areas near the corner of flat-top low-rise buildings. Simultaneous time series of pressures measured at locations within the corner region were used for analysis to form new time series of up-lift loads by instantaneous spatial averaging, for various building heights and plan dimensions. The variations of effective peak loads with location, shape and area, as well as with building dimensions, are presented and compared with those currently recommended for the corner zone by building codes, such as NBCC 1995 and ASCE 7-93. For areas including the corner point, the load reduces rapidly with increasing tributary area from the very high local peak value at the corner. The results demonstrate that the relationship between the effective load and tributary area is insensitive to the building dimensions if area is normalized by H2 where H is the building height, and is only slightly dependent on the shape of the areas that include the corner point. From an aerodynamic point of view, for buildings with plan dimensions larger than the height, the roof zoning by building codes should be based on H, and the specified load should be related to tributary areas normalized by H2.

Power spectra of wind pressures on low building roofs

Wind pressure spectra were measured at various tap locations on the roofs of several low building models placed in two types of terrain in order to determine their characteristic shape and derive a suitable analytical model for their representation. The results show that though spectra vary under different conditions, appropriately normalized spectra are found to be identical in many situations. On this basis, a suitable empirical form has been derived for the synthetic generation of normalized spectra. An attempt has also been made to classify complex spectral patterns, and suggest standard spectral shapes associated with various zones of each roof and their parameters. The synthetically generated pressure spectra can be utilized for the simulation of wind pressure fluctuations on roofs not only for the evaluation of extreme pressures in a risk-consistent way but also for fatigue design purposes.

Computer simulation of fluctuating wind pressures on low building roofs

Efficient simulation techniques are required to generate fluctuating wind pressures on building surfaces in a generic fashion not only for the evaluation of extreme pressures but also for fatigue design purposes. Within this context, a general approach for representing Gaussian as well as non-Gaussian wind pressure characteristics using the fast Fourier transform (FFT) algorithm is presented. The simulation procedure, which is developed based on several wind tunnel measurements, requires the knowledge of both Fourier amplitude and phase in order to generate pressure time histories. The amplitude part is constructed from measured pressure spectra or fitted empirical expressions for subsequent simulations in a synthetic manner. A simple stochastic model with a single parameter is suggested for phase simulations. The computation of the model parameter has been carried out by minimizing the sum of the squared errors in higher-order statistics such as skewness and kurtosis. The efficiency of this methodology for generating fluctuating wind pressures on low building roofs has been demonstrated. Applications of the methodology have also been briefly discussed.

Pressure factors for edge regions on low rise building roofs

Point and area-averaged pressures are presented for enclosed low rise (height ( h)/breadth (b) = 1 3 building roof, based on 1:100 scale wind tunnel measurements obtained in a simulated suburban atmospheric surface layer. Large magnitude mean and peak suction pressures were measured close to the leading edges under the separating shear layer. Significantly larger magnitude mean and peak pressures were measured on 0.2 h (wide) by 1.0 h (long) rectangular edge strips compared to the pressures on 1.5 h by 1.0 h roof sections. Pressure factors of the order of 3.0 for suction pressures were obtained for these regions which are larger than code values. However, measured peak area-averaged suction pressures were significantly smaller than code values.

Fluctuating Wind Pressures on Low Building Roofs

Quasi-steady theory can be used for the prediction of fluctuating wind pressure coefficients acting on the edge strip of flat or low-sloped roofs and the ridge strip of steeper roofs of low buildings. Comparisons with experimental data indicate that the quasi-steady model, expressing the root mean square pressure coefficients as functions of mean pressure coefficients and the turbulence intensity at eave height, predicts the fluctuating wind pressures for wind directions normal to the roof edge; for oblique wind directions the model holds for flat or low-sloped roofs but only for the edge strip away from the corners. Many older test results dealing only with mean pressures may now be used with the model suggested to yield root mean square pressure coefficients. The latter can lead to the prediction of expected peak pressures, which are necessary for design, if an appropriate load probability distribution is assumed.