You CAN Weld to Watertanks
Michael L. Bohlinger, PE (Principal Engineer)

 

Wireless companies continue to search out new existing locations to mount the cell site antenna. Over the last ten years, water tanks have been a site favored by most carriers. The tanks have reasonable height, are located in populous areas, are very strong from a structural aspect, and most water tank utility companies enjoy the revenue earned from carriers locating on their tanks.

The only drawback occurs when a carrier wishes to attach the antenna and cable mounts to the tanks actual surface. If conventional arc stick welding is employed to attach the mounts, the transfer of heat to the interior metal surface will damage the special paint on the tanks inside surface. All drinking water tank interior surfaces are required to be painted and maintained with a special epoxy type paint. There are some exceptions to this paint where special liners or ceramic surfaces are employed, but for the vast majority of tanks, the interior metal is protected from rust by special epoxy paint.

There have been alternate approaches to attaching equipment to tank surfaces such as epoxy or silicone gluing. There have also been attempts to band around the tank or even screwing into the tank with self-threading screws. Most tank owners reject any conventional welding or screwing into the tank since it will violate the interior coating.

The increased demand to go onto existing tanks or to co-locate onto tanks has resulted in a challenge for engineers. In 1994, KM Consulting Engineers, Inc. was faced with the requirement to locate antenna and cable mounts on a water tank that already had multiple carriers attached to the tank's catwalk handrail. The handrails of tanks have been a popular mounting point since the tank surface problem goes away when attaching to the tank handrail. The problem resurfaces when the handrail is crowded with existing systems or the tank does not have a handrail.

KM Consulting Engineers, Inc. has had previous experience on field weld attachment using stud welding. Stud arc welding, similar to arc stick welding, was first utilized in the early 1950's when a ship welder named Nelson developed a method to attach wood decking to aircraft carrier's landing deck. The process of arc stud welding is similar to arc stick welding wherein the gap between the stick or stud is arced by electricity thus melting both the surface and the stick or stud. This melting of the surface and stick/stud results in considerable heat since to melt or liquefy metal requires heating to around 1600 degrees Fahrenheit. This high heat combined with a rather long duration, one or two seconds with stick arc welding and approximately 0.01 to 0.33 seconds with arc stud welding, transfers heat to the interior surface and thus damages the paint.

KM Consulting Engineers, Inc. contacted TRW Nelson stud welding to determine the heat transfer rate of stud welding. TRW Nelson informed KMCE of a new method of stud welding which does not transfer as much heat as stud welding. This new method, capacitor discharge stud welding, utilizes a welding machine that is basically a bank of capacitors. These capacitors store up a rather high electrical charge and then deliver this high charge to the weld application for a very short duration. An example would be a 1/4 inch stud taking 1/10 of a second for arc stud welding and 0.005 seconds for capacitor discharge stud welding. This high charge over a very short duration results in a complete fusion of stud and parent metals without the generation/transfer of high heat.

Further research by KMCE resulted in obtaining a copy of an independent laboratory test of arc and capacitor discharge stud weld heat transfer rates through the parent metal. The lab tests showed that a 1/4 inch thick A-36 steel plate would transfer 445 degrees Fahrenheit for arc stud welding and 150 degrees Fahrenheit for capacitor discharge stud welding.

The next step was to determine the capacity of the capacitor stud welds and to determine the limit that the interior paint could withstand. Technical data sheets were obtained that provided the limit of epoxy paint at the 275 degree Fahrenheit level. This is over 125 degrees Fahrenheit higher than the example of 1/4 inch stud on 1/4 inch plate. The process would work. The capacity of the studs was taken directly from the manufacturer's data sheets and was well within the loads applied from the antenna and cable mounts. This load is calculated by using the wind and ice factors from the EIA-222-F (Electronics Industry Association Standards for Steel Communication Tower and Antenna Mounting Structures).

The process was now feasible, however, KM Consulting Engineers, Inc. was still concerned about paper/data feasibility and actual welding and possible damage. A series of tests were performed on a 1/4 inch thick A-36 plate that was painted with the actual epoxy special paint. The plate was stud welded using the capacitor discharge method. No damage or discoloration to the paint was recorded on the side of the plate opposite the stud. The quality (fusion) of the stud to parent metal was excellent. A 1/4 inch stud can hold 2800 lbs. in tension and 2150 lbs. in shear. Next, a similarly painted plate was capacitor discharge stud welded with the welding machine set both too hot (higher amperage) and too cool to see if we could damage the paint on the other side of the plate. Again, no damage was observed. It was now fine for engineers to design and specify capacitor discharge stud welding on water tank surfaces.

Researching and discovering a way around a problem was exciting, however, developing and specifying demanding standards to ensure that the process is properly conducted was a project in itself. Consideration must be given to various factors when engineering a capacitor discharge stud welded mount. These factors include the loads on the mount from the weight of the systems, the wind load and ice loads. The height above ground of the mount and the thickness of the steel water tank plate must be determined, and the type of steel must be known. Another factor that has to be addressed is the experience and qualification of the contractor's personnel who are going to use the stud welding method. Preparation of the surface, weld machine settings, stud size, placement and spacing of the studs must be considered and specified. Consideration must be given to future moisture being trapped between the antenna mount attachment and the tank surface. The size and type of stud as well as the type of lock washer, hex nut and torque must be specified. In short, all aspects of the attachment must be accurately specified and adhered to if a proper attachment, without damage to the tank interior paint, is to occur.

This method of antenna and cable attachment has opened new sites to the wireless carriers and has given water tank owners the benefits of having carriers on their tank without worry of damaging the interior paint and subsequently fouling the water. When properly engineered and specified, this attachment method is a safe and secure construction technique for attaching wireless equipment to water tank surfaces.