4.2　Free-standing Steel Tower

4.2.1　The cross section of free-standing steel tower is generally triangle and square，but square angle steel tower is generally preferred.To adapt to site conditions or decoration effects，rectangular angle steel tower or triangle steel tube tower with small foot distance is also available.

4.2.2　The structure of free-standing steel tower shall be calculated as a whole with whole space steel frame method，and the stress is analyzed with 3D space procedure，and the connection between major member and web member and between web members may be deemed as rigid connection or hinged connection based on actual situation.

4.2.3　When the section of free-standing steel tower is a quadrangle，the following two action directions under the wind load or seismic action shall be taken into account（Fig.4.2.3）.

Fig.4.2.3　Action direction of towers horizontal force

4.2.4　When the section of free-standing steel tower is a triangle，the following three action directions under the wind load or seismic action shall be taken into account（Fig.4.2.4）.

Fig.4.2.4　Action direction of towers horizontal force

4.2.5　The internal force of diagonal member is the internal force of pylon multiplying coefficient α providing the ratio of shear force acting on the diagonal member of quadrangular steel tower and the shear force acting on the pylon on the same layer is

Fig.4.2.5　Calculation of the minimum internal force limit value of diagonal member

As shown in Fig.4.2.5，V and M are top shear force and bending moment respectively；b is top width；θ is the angle between pylon and vertical line；h is the height of tower body above section calculated.For rigid diagonal member，μ=1；for flexible diagonal member，μ=2.

4.2.6　The bearing capacity of towers secondary member shall not be less than 2% and 5% of internal forces of major member supported and diagonal member respectively.