15 to 30m. This stage spans nine meters. Steel tube trusses are set at 15 to 30m, and the column spacing is 6 to 12m, which are hinged at the top of concrete columns. From the point of view of economy and rigidity, the mid-span height of trapezoidal steel truss is preferably one tenth to one sixth of the span. According to the transportation conditions, it should not exceed 3.85m, and the height of the end of trapezoidal truss is usually1.8 ~ 2.1m. The slope of the upper chord of trapezoidal steel truss should be 1/8 ~ 1/20, and most of it should be110.