Evolution of design standards and recorded failures of guyed towers in Canada

Abstract

The information contained herein constitutes the foundations for a companion paper in which a set of guidelines for the upgrading of existing guyed towers is developed, following a rational approach. Large increases in strength requirements for guyed towers have been introduced by successive revisions of the CAN-CSA-S37 Standard "Antennas, Towers and Antenna-Supporting Structures" (S37). Up to now, there has been a perception among tower owners that tower failures were few and, in consequence, that the added strength requirements are not needed, and further, that existing towers should not be forced to comply with the latest edition of the S37 standard. This paper demonstrates that the failure rate for guyed towers designed to earlier versions of S37 is generally unacceptably high. General comments on standard developments and various design philosophies are presented. The evolution of the Canadian standard for the design of guyed towers is also examined, as deficiencies in earlier versions of S37 are partly accountable for the high observed failure rate. Other international standards for the design and analysis of guyed towers are also reviewed for their approach towards the upgrading of existing towers. Since guyed telecommunication towers are often reviewed for addition of new antennas, upgrading to the strength levels of the most recent edition of the standard — as required in many cases — can be very expensive. Yet, in many cases, much of this expense is unjustified for a variety of reasons. This paper proposes the need for the development of upgrading guidelines and further development work on S37. Key words: guyed towers, failure, failure rate, working stress design, limit state design, environmental loading, existing structures, strength upgrading.