| Portal frame has many advantages such as light-weight, low cost, rapid construction and so on. based on these advantages, portal frame is widely used in industrial plants. With the frequency of extreme snow storm increasing, more and more light steel portal frame industrial plants were destroy and even collapsed in heavy snowstorms, causing huge economic losses. Under this background, this paper presents the mechanism of collapse of portal frames in blizzards. The detail works go as follows:(1)This paper mainly analyzes four factors of over snow load, including the large natural snowfall exceeding the basic snow pressure of the current snow load codes, the interaction of water and snow, the influence of wind-induced snowdrift and the influence of the building shapes. By contrasting the computing methods about the snow loads on single-span gable roofs in these five snow load codes, including the Chinese snow load code(GB 50009-2012), the American snow load code(ASCE 7-10), the Canadian snow load code(NBC2005), the European snow load code(EN 1991-1-3:2003), and the snow load code of International Organization for Standardization(ISO 4355:1998), it shows that the basic snow load requirements in Chinese code are too simplistic to deal with major factors that control the complex roof snow load pattern. Through a numerical example about the snow loads in the five codes under normal condition and the most unfavorable condition, it's found out that the snow load of Chinese code under normal condition is equal to that under the most unfavorable condition, which means that the estimation of Chinese code for the most unfavorable snow load is so optimistic that it cannot accurately predict the over snow load.(2)Based on the design theory on portal frame structure, programs on portal frame plane model are written by MATLAB. To analyses the mechanics performance of portal frame under the different snow loads introduced by the five snow load codes, the 14 kinds of snow loads are combined into the 11 kinds load combinations introduced by Chinese load code, which is perspective imposed on the portal frame. As the analysis results show, when the snow load is relatively large, the most unfavorite load combination is the case 1 with the snow load being the first controlling load; the largest strength stress happens on the section 4 on the left column and the largest stable stress happens on the in-plane stability design of the left column; under all kinds of snow load, the displacement on the top of column in case 1 is the largest of that, and so is the displacement on the top of beam. As the comparison shows, for the Chinese snow load code, the analysis result of mechanics performance of portal frame under the general condition is too conservative to evaluate the structural safety, while the result under the most unfavourite condition is too optimistic. In addition, the displacement on the top of beam is found to be in direct proportion to the total amount of snow load, while the displacement on the top of column is closely related to the distribution of snow load.(3)Based on the design theory on portal frame structure, programs on portal frame plane model are written by MATLAB. The following two kinds of load combination are considered: the first is that considering both live load and snow load, the second is that only considering snow load. With the method of increasing the basic snow load by steps, this paper contrasts the mechanical behaviors of portal frame under the first load combination with those under the second load combination. As the results show, under the circumstance with normal snowfall, the displacement on the column top, the displacement on the beam top, the stability stress and the strength stress under the first load combination increase by 44.08%, 29.97%, 34%, and 33%, respectively, compared with those under the second load combination; under the circumstance with heavy snowfall, the difference between these two load combination change as 19.41%, 10%, 9% and 9%, respectively. Compared with the latter load combination, the former load combination is closer to the real mechanical behavior of portal frame. Without considering the purlin tension applied to portal frame when purlins are destroyed, the displacements and the stresses of portal frame are lower than that of realities of the situation, which weakens the safety-alarming ability.(4)By established a portal frame model with ANSYS, the mechanical behaviors of portal frame when purlin tension applied on different location is analyzed. As the results show, when the purlin tension imposed on the location next to the ridge of the roof, the structure is in the most unfavorable state. Compared with the results without purlin tension, the displacement on column top, the displacement on beam top and the comprehensive stress with purlin tension respectively increase by 35%, 14%, 41%, respectively, and the longitudinal displacement sharply increases from 0 to 1m. By fixing one purlin tension of one side of portal frame to the location next to eave and changing the location of another purlin tension of the other side of portal frame, the mechanical behavior of portal frame with double-side purlin tensions is analyzed. As the result shows, because of the moment induced by double-side purlin tension, the loading effects decrease in some degree. When the unfixed purlin tension applies on the location next to eave, the displacement on the column top, the displacement on the beam top, the longitudinal displacement and the comprehensive stress with double-side purlin tensions decrease by 7%, 17%, 2% and 16%, respectively, compared with those with singe-side purlin tension. |
PDF Full Text Request