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.
A companion paper reviewed the history of the development of the Canadian standard for towers, and demonstrated that the failure rate for guyed telecommunications towers designed to earlier editions of the CAN-CS A-S37 Standard "Antennas, Towers and Antenna-Supporting Structures" (S37) is generally unacceptably high. These towers are often reviewed for addition of new antennas, and upgrading to the strength levels of the most recent edition of the standard — as required in many cases — can be very expensive, due to the sizeable increases in strength requirements for guyed towers which have been introduced by successive revisions of S37. Clearly, there is a need to match strength requirements to the significance of the structure and the economic consequences of its failure. Uniformly high strength requirements will occasionally result in reinforcement costs that exceed the costs of the risk of failure. In answer to this, a rational approach has been used to develop a set of guidelines for the upgrading of existing towers. This methodology relies on the classification of towers in various reliability classes, having increased probabilities of failure attached, and correspondingly lower load factors for use in the analysis. Results from this research are presented herein, together with the recommended proposed guidelines. Key words: guyed towers, reliability classes, probability of failure, safety index, upgrading cost, failure rate, environmental loads, guidelines.
The Canadian standard CANICSA-S37-94 "Antennas, towers and antenna supporting structures" (S37) has introduced as an appendix a quasi-dynamic method of analysis. This patch load method yields a more realistic pattern of forces in a guyed tower, but S37 contains no firm guidelines as to when this new method is required over the conventional static gust factor approach. This paper describes various comparative studies that were made to determine the conditions under which the normally used gust factor approach does not give sufficiently acceptable results so that patch load analyses are required. Results of the parametric studies and the analyses of a large number of actual towers showed that leg loads in the upper few spans tended to be about 15% larger on average in a patch load analysis. Face shears at the guy supports were also about 15% higher on average in the patch load method. The studies revealed no strong trends between the patch load and gust factor correlation and the tower parameters, although there were slight trends suggesting that a patch load analysis was warranted if the tower height was greater than 250 m, the aspect ratio was greater than 120, or the drag area exceeded 2.0 m2/m. The presence of a cantilever accentuated the margin between the results of the two methods.RCsumC : La norme Canadienne CANICSA-S37-94 a Antennes, tours et structures supportant des antennes ,p (S37) a introduit en appendice une mCthode d'analyse quasi-dynamique. Cette mtthode de chargement selectif aboutit h un motif plus rCaliste de forces dans les tours 8 clbles. Toutefois, S37 ne contient pas de directives fermes h propos du moment oh cette mCthode est requise plutBt que l'approche conventionnelle du souffleur statique. Ce papier dCcrit diverses Ctudes comparatives qui ont Ct C effectuCes pour determiner les conditions dans lesquelles l'approche du souffleur statique normalement utilisCe ne donne pas des rCsultats suffisamment acceptables pour que des analyses par la methode des chargements sClectifs soient requises. Des rCsultats d'Ctudes paramttriques et des analyses de plusieurs vraies tours ont montrC que les charges aux pieds des quelques travCes suptrieures avaient tendance h Etre environ 15% plus ClevCes dans l'analyse par charges ~Clectives. Les contraintes de cisaillement aux supports des cibles ont Ct C aussi environ 15 % plus ClevCes en moyenne dans la mCthode de chargement sklectif. Les mtthodes n'ont pas rtvClC de fortes tendances entre la charge selective et la corrklation du facteur de coup de vent et les paramktres de la tour, bien qu'il y avait de 1Cgeres tendances suggCrant que l'analyse par charge stlective est justifite si la hauteur de la tour d$asse 250 m, si le coefficient d'Clongation est supCrieur a 120, ou si la zone de trainte dCpasse 2.0 m2/m. La presence d'un porte-8-faux a amplifit 1'Ccart entre les rCsultats des deux mkthodes.
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