Base connection retrofits for high mast towers and pole luminaries used for roadway and bridge lighting

Abstract

In-service experience has clearly demonstrated that the base welded connection of the high mast tower or pole luminaire structure is extremely fatigue sensitive. In addition, laboratory research has shown that the commonly used base welded connection details provide a fatigue resistance aligned with either Category E for the full penetration weld type connection or category E′ for the fillet welded socket type connection. These base type connection details limit the service stress range levels from 2.6 ksi (17.9 MPa) (E′)to 4.5 ksi (31.0 MPa) (E). State Departments of Transportation with these common transportation structures for roadway and bridge lighting have experienced significant base connection cracking and total failure after short service lives. One State Department of Transportation had installed eighteen 140 ft (42.7 m) tall high mast towers in 2000. In late 2003, one of these towers failed, and 17 others exhibited base cracks. All eighteen towers were removed with less than 3 years of service. Based on the extent of the problem, a field-installed, mechanically fastened steel strengthening jacket has been designed and installed for several State Departments of Transportation. Base retrofit criteria were established for the design and installation. These criteria included the following: (a) significantly increase the service life of the shell to base plate connection; (b) field install without tower or pole removal; (c) no damage to existing electrical wiring or internal mechanisms; (d) installation accomplished using a standard bridge maintenance or ironworker crew; and (e) retrofit cost limited to about 15% of the cost of a new installed replacement pole or tower. Six high mast tower base retrofits and 800 pole luminaire base retrofits have been completed. Instrumentation and field testing measurements of in-service stress ranges have been recorded over a continuous 77 day interval to access the performance of the retrofitted tower bases. Base retrofits of this type are a practical and cost-effective repair solution that should be considered when base cracking problems occur or if a redundant load path is warranted. The implementation of these base retrofits will significantly increase the service life of these structures.