Effect of Bolt Slip on Tower Deformation

Abstract

The analytical deformation of a transmission line (TL) tower computed using software is less than the test deformation. A TL tower consists of many splice joints in the leg member. A small rotation owing to bolt slip in the joint may cause additional deformation in the tower, which is difficult to predict and cannot be accounted for in the analysis. Experimental studies are conducted on variation of bolt force with the applied torque and on bolt slip in butt-jointed specimens and the load at which it occurs. The actual behavior of the joints is studied on seven towers recently tested at the Tower Testing and Research Station of the Structural Engineering Research Centre, a national laboratory under the Council of Scientific and Industrial Research, Government of India. The bolt slip occurs when the axial force in the leg member exceeds the clamping force at that particular joint. On the basis of the studies conducted, a factor that gives the relationship between experimental and theoretical deformation is suggested to modify the analytical deformation. Rotations of 0.057° at stub level and 0.034° at intermediate levels for TL towers with double-cover butt joints and 0.125° at all levels for towers with single-cover butt joints are suggested for computing the actual deformation. The proposed factor is useful in estimating the exact deformation of a communication tower and can also be used as a predictor for monitoring the tower deformations during testing.