Design of the towers shall comply with British Standard (BS) 5950. These shall cover the following:
Contractor is obliged to provide the foundation design for approval based on the supplied information.The foundation shall be designed for each site specifically & the site specific foundation design shall be submitted to for approval prior to the commencement of the civil works at sites.
Temperature degreeC, High Humidity climate , Exposed to Sand storm
Heights of Square self support towers shall be in an increment of 5 meters(i.e. 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65,70, 80 & 90m.
Heights of Triangular self support towers shall be in an increment of 5 meters(i.e. 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 & 60m
If specific site conditions demand deviation from the above-mentioned height increment, detailed technical justification must be specified.
The towers shall be designed for at least for a life time of 30 years.
Since the wind force on a structure depends on the square of the wind speed, the specification of wind speed is critical in determining the weight, rigidity, and cost of a mast or tower. Every effort should be made to specify the realistic maximum wind speed for the location based on a 3 second gust value.
There are two wind speeds which are important for the design: the maximum wind speed in which twist and sway shall be maintained within the specified limits (operational wind speed); and the survival wind speed in which the twist and sway specification shall be temporarily exceeded during extreme gusts but the structure shall return to normal afterwards. These correspond to the 10 and 50 year maximums, respectively. In this case, the towers shall be capable of sustaining survival wind speed of 55.56 m/sec or 200 km/h at 3 second gusting and the operational wind speed shall be 44.44 m/sec or 160 km/h.
Wind Pressure (Loads)
The wind load on a Self-supporting Communication Towers or mast is the combined effect of the wind pressure on its structural components and on the antennas which it supports. Antenna manufacturers will normally indicate the wind load for their products at an indicated wind speed. A grid has approximately 65 to 70% less exposed area than a solid dish of the same diameter whilst a Radom fitted to a solid dish improves the dynamic wind flow and hence reduces wind pressure.
The contractor is responsible for combining the total load and for any recalculation of the antenna load at the specified wind speed according to the antenna loading criteria for the structure. Wind pressure on the structural members, ladders, antenna mounts, etc. shall be calculated in accordance with British Standard (BS) 8100 on open areas without obstruction.
Consider Topography category 2 with minimum 10 mt crest height.
Expose category D as per ANSI / TIA / EIA / RS -222-G
Factored load should be considered at survival wind speed for Design check
All structures move in the wind to some extent. The directivity of the antennas shall determine the allowable twist and sway. In the absence of specific requirements it is normal to limit these values to ±0.5 degrees which should give adequate performance of most radio systems under extreme wind conditions.
Twisting of guyed masts can be reduced by using spreader bars or anti-torque devices between the mast structure and the upper end of the guy wires.