Aerodynamic Load Coefficients of Lattice Telecommunication Masts

With the continuing expansion of mobile telecommunication throughout the world there is a growth in demand for putting up more antennas on new and existing telecommunication structures. The height of these structures causes wind load in the atmospheric boundary layer (ABL) to be one of the main critical loads for carrying capacity design. Because of their high strength and low wind resistance, steel lattice towers and masts are widely used as telecommunication structures. The relatively small number of mast failures during strong storm events in the last years suggested a potential overdimension of the installed masts. This inspired the Section for Masts and Towers of the Danish construction and consultancy company Rambøåll A/S to search into the background of aerodynamic loading of lattice mast towers. The here presented thesis project is based on an initiative by Mogens G. Nielsen (Rambøll) to re-investigate the accurracy of wind load coefficients for lattice masts given by structural design codes such as the Eurocode.

The empirical data basis for Eurocode, ESDU and other standards is a series of wind tunnel tests on scaled models of lattice structures conducted in the 1970’s in UK. The issue of model-scale effects on the measured aerodynamic loads, e.g. Reynolds effects, has always risen questions regarding the comparability to full-scale conditions. Testing a section of a real mast structure, i.e. at full size, in a wind tunnel might give an idea on how far or close model scale data are deviating from full-size results. The work of this thesis focuses on the aerodynamic drag force coefficients by investigating the influence of turbulence and the effects of testing at full size instead of model scale. Due to the size of the required test set-up the experiments were performed at the VELUX wind tunnel facility in Østbirk.

Test set-up to measure the aerodynamic forces (drag, lift and moment) on a full-size mast section. The mast section was 3m long and mounted on both sides on rotating discs allowing for simulating different wind directions. The entire test rig was placed inside the test chamber of the VELUX wind tunnel.

In order to avoid scaling effects full size wind tunnel tests are conducted on four variations of an equilateral triangular lattice mast. The effect of turbulence is examined through tests in both smooth and turbulent flow at different incidence angles and various velocities up to approximately 45 m/s. Furthermore a study is made of the wind resistance provided by Eurocode and ESDU for the same four configurations.

The results of the experiments in this project show first of all a relatively consistent difference between the drag coefficients found in smooth and turbulent flow. The wind resistance in turbulent flow is lower than in smooth flow which is a very different result than what was found in the scaled tests in the 1970’s. In comparison with Eurocode the full size wind tunnel tests provide significantly lower drag values in the situation of turbulent flow, while the smooth flow shows better correlation. The test results are closer to ESDU than Eurocode in both flow types, but there are also discrepancies in relation to the method of this document.

The conclusion to the work of this thesis is that the available data basis for design approaches used in Eurocode and other standards is insufficient. The scaling and turbulence effects both have an influence on the drag coefficients of the tested lattice structures pointing towards a lower wind resistance than provided by the Eurocode. The results of this project show a need for further investigation of among others the effect of surface roughness, more configurations and higher turbulence intensities.

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