Stress analysis and joint design of ETFE membrane structure of air pillow
1. Stress analysis of ETFE membrane structure with air pillow
Select a typical air pillow, and calculate the external load when the air pillow is in different stress state according to the elastic modulus tested in the tensile test, as shown in figure 7.2.5. It can be seen that when the stress of ETFE membrane material is less than 22.5n/mm2, the mechanical performance of the air pillow is basically linear. With the further increase of load, the mechanical performance of the air pillow is nonlinear, but there is still a large safety reserve, which generally will not lead to the immediate damage of the structure. To sum up, the membrane stress of air pillow structure design can be controlled within 22.5n/mm2. To verify the software calculation results and the material data measured in the laboratory, an air pillow model of - 2250mm × 1265mm (Figure 7.2.6) can be established, which is fixed on the aluminum alloy frame and inflated at normal working air pressure (200 ~ 250pa), and then gradually pressurized to simulate different external load conditions. During the test, the air pillow maintains its normal shape under normal air pressure. When the pressure increases to a certain extent, it begins to deform slowly. As the pressure continues to increase, the deformation increases faster and faster. Finally, the membrane material is not damaged under high load. This is in good agreement with the result of software calculation, so the structure calculated by the above method has enough safety.
The air pillow mainly bears the wind load and snow load. The wind load is determined according to the wind tunnel test. The maximum positive wind pressure is 1.3kn/m2, the maximum negative wind pressure is - 5.31kn/m2, and the gust coefficient is reduced to 0.85 times of the original value; the snow load is taken as 0.45kn/m2 according to the 100 year return period. In the design, by adjusting the thickness of membrane material, the internal pressure of air pillow is controlled within 200 ~ 250pa, the rise span ratio is controlled within 1 / 8, and the membrane material stress under the most unfavorable load combination is not more than 22.5n/mm2. Figure 7.2.7 shows the deformation diagram and stress diagram of the upper membrane of an air pillow under the maximum negative wind pressure, with the maximum deformation of 147mm and the maximum stress of 18.6n/mm2. Under the maximum negative wind pressure, the boundary horizontal reaction force is 7.33kn/m and the vertical reaction force is 4.11kn/m.
2. Joint design of ETFE membrane structure with air pillow
The air pillow structure of the stadium canopy is arranged along the curved surface spirally. It is difficult for purlin or gutter to provide four coplanar edges for each air pillow, that is, the supporting edges of each film are difficult to be coplanar. If each gutter is lofted, the construction difficulty and construction period will be greatly increased. In this project, the method of adjusting the height of gutter side wall is adopted to solve the problem, i.e. gutter is still standard specification, but an adapter with adjustable height is added to the standard gutter side wall (Fig. 7.2.8). During field installation, the four supporting edges of air pillow can be installed by adjusting the adapter. The basic structure of the air pillow node is to fix the gutter on the purlin through the gutter connector, and the air pillow is fixed on the upper edge of the gutter through the air pillow connector. In addition, bird dropping lines are set along both sides of the gutter to prevent the birds from scratching the ETFE membrane surface when they fall. Under the air pillow, there are also facilities for air pillow inflation and air pressure monitoring.