Pipe Riser Design Consideration
Riser isolation is simple in buildings up to 6 stories or so since thermal expansion or contraction is a minor issue. It is often possible to solve the problem by providing a single basement support and guides at the midpoint and roofline. If thermal motion is larger, an isolator in the center of the structure will allow for half motions in both directions. The application is completed with a few resilient guides.
In high rise work the situation is entirely different. We often work with massive risers where a single 20 story length is common. Single risers that run 40 stories or more are not that unusual. The thermal effects become very important and the risers become so heavy that it is necessary to distribute the weight on a number of floors. When the isolation is not critical, there is still the structural problem. Once an anchor is established, the supports must be able to follow the motion. As the pipe expands, it will lift off simple devices, such as pads and shift the load to the lowest or lower stories. Should the pipe contract, it will lift off pads in the lower sections and transfer the load to the top. Not allowing or considering this phenomena, has resulted in structural failure of the riser supports.
A riser may be resiliently supported in the basement or on a column that is bolted to a basement pier. Assuming branch off connections are not a problem, and the horizontal run at the roof has allowed for expansion, when the system expands, there is no structural problem as none of the load rested on the intermediate floors to begin with. The same is true if the system contracts.
In another example the pipe rises through a building where pipe clamps or brackets are used to transfer the load to successive floors. These attachments may or may not rest on isolation pads. When the system was installed at ambient temperature, all of the floors assumed part of the load. When the system is filled with water, there might be minor deflection in some of the floors to adjust to the slightly shorter or longer length, but this higher load is still evenly distributed.
However, if this is a hot water system with 2" (50mm) expansion, the top clamp would rise 2" (50mm) above structure. At the mid point 1" (25mm), at the quarter point 0.5" (12mm), etc. If there are no pads, the total load shifts to the lower 1 or 2 clamps immediately. If there are pads, the load shifts completely as soon as it exceeds the small initial deflection of the upper pads and this leads to the structural problem.
We are suggesting a safer system. Starting with the same parameters, it is always better to cut the motion at the two ends in half by establishing a neutral central resilient anchor. This anchor assumes no load. It is only there to direct the movement in the two directions.
Regardless of the length of the riser, there are an equal number of supports points, both above and below the anchor. An extreme design would be a support on every floor, but commonly they are on every third floor. These supports are selected with a minimum deflection of 0.7" (20mm), but four times the pipe travel at any given location. Since the ends are moving 1" (25mm) up at the top and 1" (25mm) down at the bottom, these mountings would have 4" (100mm) static deflection. Half way to the anchor, both top and bottom, the movement would be 0.5" (12mm) and the isolator deflection 2" (50mm). In this example these four mountings would have enough capacity for the entire riser weight. That is why the neutral central anchor is not statically loaded.
When the expansion takes place, the top hanger would lose 25% of the 4" (100mm) deflection or 25% of its load and the mounting midway to the top would experience the same proportionate phenomena. The two mountings below the anchor point would be compressed an additional 25% each, so there is a load shift of minus 25% above and plus 25% below the anchor. Therefore the anchor location remains neutral. This load change of plus or minus 25% per support location is minor and causes no difficulty.
For a complete discussion, please refer to our Bulletin Riser-112 in the "Hangers, Piping Anchors, etc." section of the catalog. The specification reflects this system.