Highly intensive tower upgrades require a depth of knowledge in a variety of areas including structural steel and concrete design, foundation engineering, soils, construction engineering, and manufacturing/fabrication methods. Proper installation should be thought out well in advance prior to execution. The ability to fabricate components on site is an exceptional capability that offers enormous versatility. Finally, a qualified infrastructure development team prepared to do what it takes to make the system work is essential.
Successful upgrades are highly dependent on good designs. Engineering groups who clearly understand loads involved and proper methods to improve the structural characteristics are highly critical. “So many of the upgrade designs are not cost effective and don’t appear warranted. Does 40 yards of additional concrete being poured at the base of a 195’ guyed tower make sense? I find it challenging to understand how an engineer can specify such a sizable foundation improvement for a guyed tower” according to James Tinsley a Project Manager with GlenMartin. We see large variations in design recommendations. A lack of structural governing codes and standards leave these structural upgrades largely up to the design engineer’s interpretation of the current standards. As a general rule most design teams err on the side of conservatism rather than explore innovative answers to common upgrade requirements.
Structural upgrades generally tend to be very evasive in nature. Evasive methods include drilling, excavating, cutting and or welding to existing structural steel. According to Tide, R. H. R., “Basic Considerations When Reinforcing Existing Steel Structures,” in Conference Proceedings, National Engineering Conference and Conference of Operating Personnel, April 29-May 2, 1987, New Orleans, LA; “When existing steel members are in good condition and their composition is known, the welding requirements are straightforward. However, for older unknown or corroded steel, other factors are involved, such as weld-ability, contaminants, and deep pitting.” These factors must always be taken into consideration during the installation process.
Please note Mr. Tide’s recommendations:
- Are the current and future loads static or cyclically applied?
- What is the ratio between the in-situ load and the original design load?
- What is the type and condition of the steel?
- Is local buckling a possibility?
- How does the stability of each individual compression member affect the overall stability of the whole system?
- What safety factor must be maintained during the reinforcing operation
- Generally not recommended to weld transversely on loaded members.
- It is a general rule in accordance with AISC standard practices that welding on an existing structural member is not permitted unless provisions are made to unload the member first.
- Welds must not degrade the properties of the material.
As Dave Ricker, P.E. of Payson, Arizona stated on 12/01/2001, “Despite the desirability of such a rule (to unload members first), in the real world it is rare that an existing beam can be shed of both its live and dead load and rarer still for a column. However, there are proven procedures for field welding to existing load carrying members. Additional recommended reading includes David T. Ricker article entitled “Field Welding to Existing Steel Structures” in the Engineering Journal, First Quarter 1988.
Mr. Tide’s and Ricker’s recommendations are very applicable to tower upgrades as primary leg members remain under a compression loading condition during the upgrade process. Notice both Mr. Tide and Ricker vary on their opinions on what is actually going on with loaded structural members. This is not uncommon within the telecommunication engineering community. A lack of good research and testing leaves most of this up to interpretation and theoretical opinion. This presents a unique set of challenges that must be considered as tower upgrades. As the norm it is always best to air on the conservative side but of course not always practical side.
If you have questions about a tower upgrade, contact our expert infrastructure management team. You can reach us by phone or email. Click below to contact our team.
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