Many broadcasters go through their entire careers without having the joy of building a new tower. For many others, this particular activity only occurs on a few occasions and doesn't allow for the build up of enough bad experiences to help make the best choices. Even if a new tower isn't necessary, the need to modify an existing structure can add problems to one's job that are not true joys.
For this column, the problems of zoning, FAA clearances, building permit, etc. will be ignored. This time the emphasis will be simply on the structure itself. That alone can create enough worries to last until next month.
First, the issue that often comes up is whether a new structure is really needed or if the existing tower can be modified in a reasonable manner. If changes are to be made to the tower loading, as when adding more antennas and lines, it is usually necessary to have a structural engineer or the tower manufacturer analyze the tower to see if the additions can be made without endangering the tower. One problem in this case is determining the standard that must be used in evaluating the structure.
The current standard for towers is ANSI/EIA/TIA 222F. A new standard, 222G, is in the works and may be issued by the end of 2002 to become effective in early 2003. For new towers, the applicable standard is always that which is in effect at the time of the design. That is the standard that represents the latest thinking in the design of a reliable structure.
If the existing structure was designed to 222C and it is desired to change it slightly, it is not always necessary to bring it up to 222F standards. This will be dependant upon local requirements and the insurance carrier. If a building permit is required for the changes proposed, local regulations may require that any changes comply with the latest version of the standard. In a like manner, the station's insurance carrier may prohibit any modifications that do not bring the tower into compliance with the applicable standard. Upgrading the tower to comply with the latest version of the standard is often prohibitively expensive to the point that it may be more economical to simply replace the old tower with a new one.
On the other hand, if the old tower is in good condition, maintaining compliance with the standard that was in effect at the time of its construction may be a viable option. After all, there are a lot of towers standing that were designed to older versions of the standard that have a lot of usable life remaining. This is a decision that needs to be made with the help of the structural engineers doing the study.
If the decision is made to replace the tower, the first step is to fully determine the anticipated loads. That is, just how many antennas and lines are to be installed and the loads presented by each load. Don't make the mistake of only considering the loads to be installed at the present time. Towers offer valuable vertical real estate for rental income. Even if no additional broadcast antennas are anticipated, there is usually a market for communications systems with multiple small antennas and lines. It is highly advisable to plan on loading up the tower with such systems. It doesn't cost much for the additional steel and even less for additional construction costs if they are included at the time of the initial fabrication and installation. On the other hand, the costs can be great to modify the tower later by adding loads.
After the initial loading is determined, contact the desired bidders for the project. They will need to be provided with those loads and the guy radius for the tower. If the tower is in an area where ice can be a problem, an allowance should be made to accommodate that problem. Again, the amount of ice loading to be requested should be determined with the aid of either your consulting engineer or a structural engineer. Simply identify what you want the tower to hold and the structural engineers at the manufacturer will determine how best to hold those loads.
There is a long-standing argument as to whether solid steel legs are preferred over hollow structural tubing. Either will provide a good tower with a long life expectancy if done properly. If hollow tubing is used, demand that it be galvanized inside and out. The galvanizing should be done before the ends of the tubing are sealed by plates or flanges. In addition, long runs of hollow tubing should be provided with drain holes to avoid the buildup of water inside the tubing.
Before accepting the tower for installation, require that a complete set of the design calculations showing compliance with the applicable standard be furnished and stamped by the registered structural engineer responsible for the design. If there still is any feeling of uneasiness about the design, have those calculations checked by an independent engineering firm so that there won't be any questions about the design in the case of a failure. In addition, those calculations can then be furnished to your insurance carrier.
The contract with the tower installer should clearly show that the tower manufacturer and/or installer, if separate, are totally responsible for all liability associated with the erection. The station should be shown on the builders insurance to be held harmless from all claims associated with the construction in any way. It is usually a good idea to have the station listed as co-insured on the builder's insurance policy. In addition, require that the builder waive all rights of subrogation. This is also not a bad time to have the station's attorneys involved.
The intent here has been to bring up a few points that should be considered when dealing with even the best manufacturers. If it is decided to avoid those companies and have your tower built by the guy out at the edge of town who built a windmill once, you are on your own.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
Send questions and comments to:email@example.com