Understanding the dynamics of riser design in skyscrapers
By Marty Rogin, PE; Engineering Manager, Metraflex
The modern skyscraper has been around for over a century. Like other elements of our built environment, the skyscraper can only exist because of other innovations in building technology, namely steel frame construction and safe elevators. Even though we have figured out how to build strong, tall structures and safely move the people inside, there are still the challenges of heating and cooling the building, moving fresh water in and dirty water out, providing fire protection and electricity. Defying gravity adds another twist to the challenges of providing services within tall buildings. This article will introduce some basics of pipe riser design and performance, explain some considerations in using different expansion joints in pipe risers, and briefly describe some of the codes and standards regarding guiding and supporting risers.
Thermal Expansion Basics
While the pipe is nothing special, gravity will make things way more interesting. Consider the riser pipe (Figure 1). The pipe runs the entire height of the building, 50 stories. If the slab-to-slab height is 10 feet, our pipe is 500 feet tall. A typical support for this pipe may be a riser clamp, maybe on every other floor. With no temperature change, the riser weight is distributed evenly between all the riser clamps.
Let’s heat the water in the pipe (Figure 2). Now the pipe will expand against the supporting riser clamps. But the riser clamps are only restricted to move in one direction – down. There are no restrictions to upward movement. The clamps will just move up with the pipe. Any clamp above the bottom floor will now be floating above the slab. All the weight of the pipe, insulation and media is on the bottom clamp. Most pipe clamps are not designed to support the full weight of a tall riser.
There are solutions. A pipe anchor at the bottom of the riser, designed to support the full riser weight, will solve this problem. But let’s look at how much the pipe moves. Let’s say our pipe is made of steel and the liquid medium is hot water at 180°F. Like gravity, thermal expansion (thermal strain) of steel will not disappear in a riser. If we assume an ambient temperature of 50°F, the pipe will want to expand according to the equation:
ΔL = Length change (inches)
∝ = Coefficient of thermal expansion (for steel, 6.33×10-6 inch/inch/°F)
Lo = Starting length (6000 inches)
ΔT = Temperature change (180°-50° = 130°F)
ΔL = 4.9 inches
The very top of the riser will move 4.9 inches up. Is this a problem? It could be. Can the takeoffs at the upper levels move about 5 inches without breaking? Maybe, if there is enough runout length to the equipment connections. Will the field conditions allow the pipe to move this much before colliding with structure or equipment? Maybe, but then, who can answer these questions prior to construction? Usually they can’t be answered until the structure is up and the pipefitters install the pipes at the ceiling with all the unplanned bends and modified runout lengths.
One solution may be to move the anchor to the center of the riser (Figure 3). The anchor is a hard connection from the pipe to the structure and a point of zero movement. The riser is now divided into two sections, each 250 feet. Now the maximum pipe movement will be half of the entire riser, or 2.45 inches. The previous questions may be asked regarding 2.45 inches of movement. If they can be answered during the design stage of a project, great! On to the next project!
But wait. What about those riser clamps? Above the anchor they will ride on the pipe, rising above the floors. But below the anchor, the riser clamps will try to restrain the pipe from moving downward. The likely outcome will be that the clamps will slip along the pipe as it moves. If the riser clamps are welded to the pipe, something will break – either the clamp or the pipe. Hopefully the clamp, but then the anchor will be carrying the load of the entire riser.