6.1. ERMUSR 11-17-2009 - Laserfiche WebLink

Home Browse Search

Storage Tank Integrity Want to Know the Best Antenna and Cable Attachment Methods? ~ttaavhing t~l~~®n~~ni~ati®ns ant~nna~ and. gable l~>ra~ket~ t®a water t®~r~>r pan d.anlage the t®wer's metal st>r~et~.><;re and. ~®ating systen~.1~ regent s~dy sheds light ®n which attach>rment meth®d.5 w®rk best. BY DANIEL J. ZIENTY HE APPROPRIATE METHOD for attaching telecommuni- cations antennae and coax- ial cable support brackets to water towers is based on numerous factors, including the brack- ets' purpose, water tank design, and tank structural integrity .requirements. Connec- tion fatigue caused by wind vibration can affect a tower's base metal as well as the connected component. Although attach- ment by seal welding is the method most widely used, other common attachment methods can also be used. The follow- ing are the three most common attach- ment methods: v Stud Welding. In the stud-welding method, bolts are welded to the base metal of the tower or tank with a capac- itor discharge or drawn-arc on a small spot where the protective coating has been removed. Aftet• welding, the sur- face directly around the stud should be cleaned and the coating reapplied to the affected area- Tlais method doesn't damage the coating on the other side of the tank's surface, so it's typically used on standpipes and ground stor- age reservoirs. However, stud-welding application has a high tendency for fatigue failure. Torque testing per- formed according to the said manufac- turer's specification is recommended to verify the weld strength. ^ Bolting. This method involves fasten- ing ashop-fabricated attachment (base plate) to the tower with bolts through holes drilled in the tower's metal sur- face- Bolting can be applied to water towers with support columns, includ- ing pedestal and fluted designs. Bolt- ing results in the lowest structttrai fatigue for a tower's base metal and the attached component. ^ Sea! Welding. This method .involves direct fillet welding of the attachment base plate to the towet• and requires removal of the surface coating .from the .heat-affected area of the attached component and tower before placing a full fillet weld around the perimeter of the base metal. Heat damage caused by welding usually makes cleaning and repainting the mounting surface and the reverse side necessary. Water on the backside of the tower (immersion surface) can dissipate the welding heat and prevent coating damage. Welding the attaclnnent plate to the tower pro- vides glow fatigue-prone method for the tower's base metal and attachment. It's recommended that attached plate corners be radiused to reduce stress concentration. OWNER AND TENANT CONCERNS A tenant's engineer usually selects the attachment method based on the integrity of the tower attachment after installa- tion. However, this doesn't consider how the attachment method affects the tank's coating integrity or installation schedul- ing and costs. A tower owner's concerns rest in maintaining the structural integrity of the facility and long-term serviceabil- ity of its protective coating. Winter con- struction usually .limits repairs to surfaces damaged by welding, which leaves sur- face areas unfinished and exposed. This can lead to coating damage outside the initial area, requiring additional time and costing the contractor more to make nec- essary .repairs. .Aside from structural considerations, the decision to use bolting or stud-welding can aeate a positive situation for the owner and tenant. iJpon application, these methods create the least amount of initial damage to a tower and help the contrac- tor and tenant save time and money. Mak- ing these .issues a primary concern can result in a win-win proposition. Many of these issues can be addressed by level- aping comprehensive specifications and requiring review by a qualified inspector. STUDY PREPARATION With these issues in mind, a study was conducted to determine if stud-welding or bolting can be used without compro- mising an owner's facility and still meet tenant scheduling and budget goals. The 20 OpFlocv October ?009 www.awwa.or;/opFlow