Thanks, Lazypup. But how about the copper (not steel) plumbers tape? I've used it by attaching one end to the ceiling, coming down about 8" and around the pipe and back up to the ceiling. (It you saw all the nearby ABS, you would understand why I didn't use something that attaches to the 2 joists.) Does this count, or must I fashion something more rigid?

By Tape i presume you mean you used the 1/2" wide copper or copper-clad perforated hanger strapping. If so, that is fine. If you desire to make something more stable you can extend a piece of wood down from the joist, then use the strapping to attach the pipe to the bottom of the wood.

While this is not required on supply or distribution piping, when running DWV pipe we are required to use hangers that will keep the pipe held down as well as hanging it. This insures that the pipe will not shift upwards from thermal expansion and change the required pitch on the drain. The manufactured plastic J-hooks for PVC or ABS have a built in stop device to hold the pipe down, but when hanging with strapping we are required to install the blocking as described. Also, you may not use metal strapping or metal hangers on plastic pipe.

Copper tubing used in my baseboard heating system was fastened to subflooring in the evespace with what I think are called "bell brackets." These brackets have a bell shape about 2 inches in diameter with the opening of the bell sitting on the subfloor. At the tapered end of the bell is a hole to receive a screw that fastens the bell to the floor. The pipe lies over this screw in a metal cradle, and a top piece to the cradle goes over the pipe and is bolted to the lower cradle.
Over a period of seven years from installation, a hole was worn in the copper pipe where it rested on the bracket over the screw fastening the bracket to the floor. I presume that the expansion and contraction of the pipe against the screw,probably set at somewhat of an angle, created a gouge in the pipe sufficient over the seven years to gouge through the pipe to create a pinhole and serious flooding.
Questions: Is my assumption correct that the bracket was incorrectly installed? What is the industry standard for installation of these bell brackets for copper tubing? Was the cradle bracket fastened too tightly so that combined with the angled screw head the sharp edge of the screw eventually wore through the pipe? Is this condition a risk that might affect many installations and liable to cause considerable damage claims for insurance companies?

While seldomly used in Plumbing the Bell offset brackets are commonly used in the heating industry to run copper for Hydronic or saturated steam heating applications.

The base section of the bell offset has a counterbore where the screw seats so that if the proper screw is used and installed properly the head of the screw is recessed in the counterbore and cannot come in contact with the pipe.

It should also be noted that due to the problems associated with dissimilar metals the offset bracket should be mounted with a copper or brass flat head screw. (No steel screws).

In Plumbing we have to be concerned with pipe movements that could result from water hammer, but water hammer can only occur when there is a substantially sharp rise from dynamic head pressure to static head pressure when a fast acting valve is closed. A hydronic heating pipe is a closed loop so the actual line pressure is equal at all points in the system and water hammer cannot occur.

While it might be extremely remotely possible that your problem resulted from thermal expansion the likelyhood of that being the significant cause is slim to none.

A hydronic heating system operates at approximately 180degF thus the normal temperature differential in ordinary use would be 70degF room temp to 180degF operating temp for an operating differential of 110degF.

For a 110degF differential the linear expansion of copper pipe is 1.8" per 100 linear feet. This would be a prime consideration on a commercial or institutional heating system where the runs can be very long but as a rule in residential service straight runs would only be a fraction of that length.

Now taking into consideration that we are required to install hangars at 4' intervals the actual linear expansion at any given hanger is only .048".

Thanks lazypup. It sounds to me that your key statement is:
"The base section of the bell offset has a
counterbore where the screw seats so that if the
proper screw is used and installed properly the
head of the screw is recessed in the counterbore
and cannot come in contact with the pipe."

In my case the gouge perfectly fits the screw head and the gouge is angled indicating that the screw head was not recessed but gradually worked into the copper tubing until it penetrated the tube wall. I should also mention that this section of tubing was in an unheated eve-space so that the temperature variation would range from 180 degrees down to winter outdoor temperatures (zero and below on occasion).

Another question: What is the code for how tightly the cradle straps on the bell are fastened together? If those straps are bolted tightly together is that a factor? Should the straps be lose enough to allow for expansion with minimum resistance against the bracket?

In that section of pipe in the evespace there are gouges at many bracket sites, but none as deep as the one that penetrated the tubing, and none at such a sharp angle.

I am not aware of any specific torque specification although I am certain if we were to examine the complete ASTM (American Society of Testing & Materials) specifications we would probably find one. As a rule we simple tighten them down snug.

If the potential for movement is a consideration you might consider applying two or three wraps of vinyl electrical tape around the pipe before tightening the clamp. The tape would serve as a bearing surface.