A parametric, experimental analysis of conical vortices on curved roofs of low-rise buildings

Different methods to reduce the high suction caused by conical vortices have been reported in the literature: vertical parapets, either solid or porous, placed at the roof edges being the most analysed configuration. Another method for alleviating the high suction peaks due to conical vortices is the use of some non-standard parapet configuration like cantilever parapets. In this paper the influence of roof curvature on the conical vortex pattern appearing on a curved roof (Fig. 1) when subject to oblique winds is experimentally analysed by testing the mean pressure distribution on the curved roofs of low-rise building models in a wind tunnel. Also, the efficiency of cantilever parapets to reduce mean suction loads on curved roofs is experimentally checked. Very high suction loads have been measured on curved roofs, the magnitude of these high suction loads being significantly decreased when cantilever parapets are used. Thus, the suitability of these parapets to reduce wind pressure loads on curved roofs is demonstrated.     

Wind tunnel study on the influence of different parapets on the roof pressure distribution of low-rise buildings

High suction loads appear on roofs of low-height buildings. The use of parapets with appropriate height at the roof edges alleviates these loads. The performance of six parapet configurations to decrease the suction loads induced on roofs by oblique winds has been studied in a low speed wind tunnel. The studied parapet configurations include vertical wall parapets, either solid or porous, and cantilevered parapets formed by a small horizontal roof close to the building roof. Low-height parapets with a medium porosity and cantilevered parapets are more efficient than solid parapets to reduce the wind suctions generated on the roofs by conical vortices.     

Modified method for rainwater ponding on flat roofs

A method for the analysis of rainwater ponding on flat or nearly flat one-way and two-way roofs, published in 2007, is modified. The existing method appears accurate for roofs of continuous secondary members and profiled steel sheeting, but is far too pessimistic for roofs in which all composing parts are simply-supported. After summarizing the formerly published method, a modification is proposed in order to fit better with FEM-packages, considered in this context to be exact solutions. In essence, the stored water in the deflected secondary members and steel sheeting elements is estimated more properly. An easy way has been found how to adapt the existing spring-piston model in order to better account for the correct volume of accumulated water in the secondary members and profiled steel sheeting. An application example and discussion of the results complete the article.     

Influence of an upstream building on the wind-induced mean suction on the flat roof of a low-rise building

The effect of an upstream building on the suction forces on the flat roof of a low-rise building placed in the wake of the former is analyzed. The analysis has been performed by wind tunnel testing of a flat roof, low-rise building model equipped with pressure taps on the roof and different block-type buildings (only configurations where the upstream building is as high or higher than the downstream one are considered in this paper). The influence of the distance between both buildings on the wind loads on the downstream building roof is analyzed, as well as the height of the upstream one and the wind angle of incidence. Experimental results reveal that the wind load increases as the relative height of the upstream building increases, the wind load being highest for intermediate distances between buildings, when a passage between them is formed.     

倒角化迎风前缘对平屋盖表面风压特性的影响

通过刚性模型风洞测压试验,针对分离泡和锥形涡作用情况,研究了不同风向下倒角化迎风前缘对平屋盖表面风压幅值和脉动特性的影响。对比分析采用倒角化迎风前缘前后,平屋盖表面风压分布以及角部面积平均风压的变化。通过本征正交分解法,给出了平屋盖表面风压脉动的特征值和特征向量。从时域和频域角度,分析了倒角迎风前缘部位测点的风压特性。结果表明：采用倒角化迎风前缘后,分离泡和锥形涡作用区内风吸力单调递减,但迎风前缘附近风吸力可能增大;分离泡作用下,倒角化迎风前缘将增大屋盖角部面积平均风压均值;锥形涡作用下,其可减小屋盖角部面积平均风压的均值和极值,最大降幅分别为68%和82%;屋盖表面风压脉动区域减小至迎风前缘附近,且风压脉动能量降低,最大降幅出现在锥形涡作用下倒角半径较大的平屋盖表面;在倒角迎风前缘部位,极值风吸力和脉动风压谱峰值可超过其邻近区域;增大倒角半径,该部位的极值风吸力和低频风压脉动能量将有所降低。     

大尺度平屋盖聚类最优风压分区研究

以大尺度平屋盖为研究对象,针对其风压分布变化梯度较大的问题和GB 50009-2012《建筑结构荷载规范》关于大尺度平屋盖风压分区规定的不完善,围绕大尺度平屋盖风压分区问题,在得到各种风向下风压测点最不利极值风压的基础上,利用最短距离聚类法对风压测点分区,得到不同分区类数下的分区方案,用基于质量系数的聚类有效性评价法确定最佳聚类数及最佳分区方案,并采用面积权重法给出大尺度平屋盖分区风压系数。研究结果表明：大尺度平屋盖聚类最优风压分区类数为3;大尺度平屋盖角部大致在10%屋盖跨度范围内,属于风敏感部位,设计、施工时需特别注意;整个屋面平均风压系数会导致大尺度平屋盖中部偏于保守设计,角部低估了风荷载;在进行大尺度平屋盖抗风设计时,应先确定极值风压分区,并针对不同部位分别进行设计和施工。通过与规范平屋盖分区方式、常用分区方式确定的分区风压系数对比显示,聚类最优风压分区较其他方式确定的分压风压系数结果更为合理。     

女儿墙对平屋面低矮建筑风荷载特性的影响

对平屋面低矮建筑进行1∶25缩尺刚性模型测压风洞试验,研究了无女儿墙工况和4种不同高度女儿墙的平屋面低矮建筑的风荷载分布规律。无女儿墙的平屋面主要承受风吸力作用,斜风向锥形涡诱导的最不利吸力区域为屋面迎风边缘角部区域,为全风向下最不利区域。女儿墙的存在可明显减小屋面的平均风吸力和极值风吸力,平均风吸力减小幅度可达150%,同时最不利平均风压系数和极小值风压系数的出现位置逐渐远离了屋面角部区域;随着女儿墙高度的增加,极值风吸力进一步减小,极值风压力增大,最大的极大值风压系数出现在尾流区;采取分区的方式给出了不同女儿墙高度的屋面体型系数建议取值。     

Experimental study of wind loads on gable roofs of low-rise buildings with overhangs

Gable roofs with overhangs (eaves) are the common constructions of low-rise buildings on the southeastern coast of China, and they were vulnerable to typhoons from experience. The wind pressure distributions on gable roofs of low-rise buildings are investigated by a series of wind tunnel tests which consist of 99 test cases with various roof pitches, height-depth ratios and width-depth ratios. The block pressure coefficients and worst negative (block) pressure coefficients on different roof regions of low-rise buildings are proposed for the main structure and building envelope, respectively. The effects of roof pitch, height-depth ratio, and width-depth ratio on the pressure coefficients of each region are analyzed in detail. In addition, the pressure coefficients on the roofs for the main structure and building envelope are fitted according to roof pitch, height-depth ratio and width-depth ratio of the low-rise building. Meanwhile, the rationality of the fitting formulas is verified by comparing the fitting results with the codes of different countries. Lastly, the block pressure coefficients and worst negative pressure coefficients are recommended to guide the design of low-rise buildings in typhoon area and act as references for the future’s modification of wind load codes.     

平屋盖围护构件设计风荷载研究

GB 50009-2012《建筑结构荷载规范》中未给出复杂体型且重要建筑物的风荷载局部体型系数,此类建筑物的风荷载需通过风洞试验确定。基于此,提出了基于风洞试验的围护构件设计风荷载计算方法,将规范中阵风系数与局部体型系数的乘积修改为局部体型系数与脉动风压系数极值之和的形式,称为风压系数极值。提出的围护构件设计风荷载计算方法不仅适用于迎风面围护构件设计风荷载的计算,也适用于气流分离区围护构件设计风荷载的计算。在脉动风压系数极值的计算中,考虑了气流分离区非正态风压时程的特性,采用非正态峰值因子的简化计算式,可简便确定非正态风压时程的峰值因子。以平屋盖围护构件设计风荷载的确定过程为例,对比了我国规范方法与文中方法的异同,提出了平屋盖围护构件风压系数极值的设计建议值。结果表明,采用文中提出的围护构件设计风荷载计算方法,基于风洞试验数据可确定气流分离区围护构件的设计风荷载,采用日本风荷载规范的屋盖风荷载分区方法是合理的;采用风洞试验得到的局部体型系数,套用GB 50009-2012规范方法确定气流分离区围护构件的设计风荷载,可能严重低估风荷载取值。