Problem - A power line tower is erected to support a high-voltage power line (see figure below), which includes six phase conductors and three Earth shield wires. The tower is fabricated as a steel structure resting on four square supporting feet bolted to a concrete foundation with four Anker bolts at each support (see figure below). The resultant tension force F1 from cables on each side of the tower is applied at point m, and can be at 60° to the horizontal plane (sagging downwards) if the tower is installed on a plane or at 30° to the horizontal plane pointed upwards if the tower is installed on a hilly landscape. The gusty wind load, which could be represented with a resultant force of lc is applied at the geometric centre of the tower n in any direction in the horizontal plane (see figure below) and tends to shift the tower sideways and topple it. In the case of an accident when all cables on one side of the tower are torn, the tension force F1 will be applied only at one side of the tower. Each foot of the tower is bolted to the concrete foundation with four Anker bolts (see figure). Because of safety considerations, ISO Grade 8.8 steel bolts and a safety factor of 5 (based on proof strength) are to be used. Assume that the tension force in cables on each side of the tower is FT = 12,000N, the resultant wind load is FW = 80,000N, and the weight of the tower is W = 120,000N.
For the given data carry out the following design calculations:
1. Discuss all possible combinations of external loads and identify the worst case. Consider additional loads that may appear, for example, if the tower is installed in seismic area subject to earthquakes.
2. Discus whether the Anker bolts are equally loaded.
3. Assess the frictional force in the flange on each foot created by the external forces. Assume frictional coefficient in the flange μ = 0.17.
4. Determine an appropriate bolt size to secure the joint and prevent the tower displacement.
5. Assess the tension force in a bolt under the maximum load for the worst case scenario.
6. Assess the pretension force in a bolt required.
7. Assess what torque T on the bolt head is required to develop this pretension force.
8. Recommend locking method for the bolted joint taking into account operating conditions.
