- FIRM PROFILE
The reemergence of our core cities as more active and vibrant communities brings pressures and challenges to those who design. The density of buildings, traffic, the scarcity of land, and a competitive spirit among developers are all factors that work together to push modern buildings higher.
Sometimes, especially in motion pictures, we envision high-rise buildings as towering skyscrapers. While this is the romantic and not always incorrect vision, a “high” rise can be as short as eight to 10 floor levels. The National Fire Protection Association (NFPA) defines a high-rise building as a building with an occupied floor that is 75 feet above the level where the firefighting apparatus would stage firefighting operations. That low threshold requires several specific features to be designed into buildings to promote life safety and allow for emergency responders to safely and quickly access the higher levels of the building, thereby saving lives and considerable invested resources. With that fairly simple definition, all high-rise design challenges should be the same, right? Perhaps some additional discussion is warranted before we make that determination!
High-rise design and construction present more than a few special challenges, especially regarding the design of plumbing systems. Some of the biggest challenges to high-rise plumbing design relate to controlling pressure. Pressure is both friend and foe in plumbing systems. Plumbing engineers learn early on that as you lift water above a datum, you lose one pound per square inch for every 2.3 feet of elevation. While this may seem a reasonable incremental loss, it can be a significant penalty when the water is raised 75 feet; then, a requirement is added to maintain a high minimum pressure at the top of the column. Many designers answer this challenge daily.
For instance, a common condition in a water riser serving a toilet group in an office building supplied with flush valve fixtures requires 25 psi at the most remote fixture. You add a pressure boost system to meet that demand on the top floor. A common complication begins when you begin to stack floors. The combined head pressure causes the pressure at the bottom may exceed the allowable safe level as limited by code and materials. This too is a fairly routine condition that often is solved by either placing pressure-reducing valves on each level where pressure exceeds code maximum or branching from the higher pressure riser to make a pressure zone. This pressure zone uses a central pressure-reducing valve and sub-riser to meet the minimum pressure required at the highest level and the maximum pressure allowed at the lowest level. This particular method has been used successfully in many high-rise building designs.
Supplying adequate water pressure at all levels of the building is critical for building occupants, although economics, basic building functions, and overall heights have significant impact on methods of water supply distribution. Numerous intermediate-height and even very tall high-rise buildings use various pumping schemes. One early method used elevated storage tanks at the top of the building with fill pumps at the bottom of the building, a classic gravity downfeed arrangement. This method evolved into direct pumping systems using multiple pump packages with constant-speed, constant-pressure controls. Both of these methods proved to be reliable and affordable through the years, and many such designs are still active today or still are used in current design practices. Continuing improvements and development of variable-frequency electric drives and an ever-increasing emphasis on reducing energy consumption and costs make the variable-speed, direct-pumped package a modern workhorse of the industry.
The critical need to provide adequate flow and pressure gives the high-rise plumbing engineer ample opportunity to practice their craft. A thorough understanding of pumping basics is critical to start with, and one of the most widely recognized sources is the Fluid Handling Training and Education Department of ITT Industries, better known as Bell and Gossett’s Little Red Schoolhouse. From this fundamental training, more advanced texts could include the Pumps and Pumping Systems Handbook, published by ASPE, as well as training brochures published by all reputable pump manufacturers and system packagers. Even the seasoned professional can benefit from occasional review of these texts to refresh some of the basics and rediscover some of the subtleties of pressure booster systems.
Pressure control on the drainage side presents other challenges. True, water is essentially the same in either system; however, drainage theory holds that considerable air travels downward with the water flow. This theory asserts that water flowing in a vertical pipe tends to adhere to the pipe walls, acting very much like a sleeve of water with a hollow core of air, all sliding down the pipe walls until it reaches a ratio of approximately 6/24 full of the pipe cross-sectional area. This watery sleeve travels at very nearly 15 feet per second (fps), propelled by gravity but restricted by friction. When the piping remains vertical, the entrained air is relatively simple to control, but when piping offsets from the vertical, the fluid flow velocity drops considerably, filling the entire pipe diameter. Horizontal, sloped drainage piping should flow in the 4–8 fps range, so it is easy to see that a large slug of water can quickly develop. This can lead to compressing air in the path of the fluid and/or lowering air pressure on the leaving side of the fluid flow. The impact of these fluid and air fluctuations can be controlled by effective use of yoke vents, relief vents, and vent connections at the bases of stacks. Here again, the solutions are largely not unique and have been used successfully on many intermediate-height and even extremely tall high-rise buildings. (For those who are just beginning in this type of plumbing design, a recommended reference is High Rise Plumbing Design, by Dr. Alfred Steele.)
A related concern is the impact of the hydraulic jump on the piping itself. The mass of water and the rapid change of velocity from vertical to horizontal cause this jump. While the pressure associated with this jump is significant, it does not destroy the fitting at the base of the stack. Rather, the movement of the pipe stresses the frictional forces that hold the joint to the pipe, leading to eventual coupling failure. Good design must compensate for the strong thrust that occurs at this change of direction. Successful methods include increasing the horizontal drain size and/or slope, using thrust blocks, or using restraining joints with threaded rod or similar arrangements that mechanically anchor the fitting to the entering and leaving piping.
Once the water is raised and used, it is discharged to a drainage system that includes an attendant venting system, which is responsible for the flow of air in the drainage piping network. Air is critical to the drainage process because drainage flow is caused by sloping pipes, and the motive force is gravity. Absent air, the drainage would range from erratic to nonexistent. When the water in a pipe flows to a lower area, air must be added to replace the water, or a negative pressure zone will occur. If this zone is near a fixture, air will be drawn into the drainage system through the fixture trap with an easily identified gulping sound and very slow drain performance. This condition will lead to poor performance throughout the drainage system and trap seal loss due to siphoning or blowout. The remedy for this condition is venting. At the individual fixture level, this consists of a fixture vent. As the number of fixtures increases, venting needs do as well, evolving into a venting system, with branch, circuit, and loop vents at the appropriate locations. When dealing with high-rise drainage stacks, a vent stack should be attendant, allowing for pressure equalization and relief along the height and breadth of the system. Aside from relieving pressure in the drainage system, the vent system allows air to circulate in both directions in response to the fluctuating flow in the drainage system. In many high riser vent designs, where stacks need to offset horizontally on a given floor, a relief vent is required. Although not often highlighted, the building venting system also serves to supplement the vent for the municipal sewer, relieving noxious or even hazardous gases and allowing the sewer to drain without pressure limitation.
Plumbing engineers must consider the impact of plumbing systems on general construction practices. Most experienced engineers and contractors will agree that vertical piping systems are generally more effective than horizontal piping systems in multilevel projects. Vertical piping uses fewer supports, hangers, inserts, etc. and requires less horizontal space in ceiling plenums for sloping to achieve drainage. Altogether, vertical piping is a pretty good bargain; however, it is not without penalty. The penalty of vertical piping is multiple penetrations through structural slabs. Each of these penetrations must be sealed or protected to prevent vertical migration of fire and smoke (i.e., turning the tall building into a tall chimney). Not only is the sealing of penetrations an issue, but the sheer number of penetrations can be equally difficult. The location of these multiple penetrations is critical to the integrity of the structure and the function of the fixtures even more than the aesthetics of the built environment. Higher buildings require more robust structures, further limiting the allowable spaces for penetrations. Other structural practices, such as post-tensioned beams and slabs, which serve to lighten the overall building structure, can limit even further the available locations for slab penetrations. Successful high-rise design requires the entire design team to take extra effort to read, understand, and interpret the impact of building systems on one another, as well as be open to discuss, coordinate, and adjust each individual system to suit the needs of the building. A well-executed high-rise design is an integrated and complex assembly, and each component should be treated as a part of that integrated whole.
One area that should not be overlooked in any high-rise design is the fire protection systems. As a minimum, all high-rise buildings should have sprinkler systems on each floor and standpipe systems in each stairwell. These systems have proven themselves throughout the years to significantly save both life and property. The specific type, coverage density, and outlet placement all vary based on the building type, height, and location and local fire authorities. All high-rise buildings containing #re protection systems have large, dedicated fire pumps to provide the flows and pressures required for the individual system. While not always tasked with these system designs, plumbing engineers need to know that these systems are an integral part of the building and must account for their presence regarding equipment space, riser locations, and ceiling cavities.
In the projects touched on in this article, piping systems have been specified and installed using very “standard” piping and fittings. Sanitary and vent piping and storm water piping within the building are mostly hubless cast iron, selected primarily for availability and quiet operation. Underground sanitary and rainwater is hub and spigot cast iron with gasket joints. In some instances, particularly horizontal, large-diameter drainage below grade, the piping is ductile iron with mechanical-type joints. This is a routine installation for this type of piping system, used widely because its suitability to flow and pressure, availability, and quiet operation, and largely made of post-consumer product, so it is very “green” in application. Water systems for these buildings are typically Type L copper. Tubing sizes 2 inches and smaller are typically assembled using 95-5 solder; for larger diameter tubing, we usually leave the contractor the choice to braze or use mechanical joints with roll groove fittings. Medical gas distribution is typically Type L copper with brazed joints as outlined by NFPA. Except for extremely tall buildings, these materials generally give good service over a wide pressure range and are within maximum pressure limits by significant amounts. As buildings get taller, many water systems can exert pressures that exceed the safe working pressure of copper tubing. In some areas, stainless steel light wall pipe (Schedule 10) or standard pipe (Schedule 40) is a reasonable alternative to increase safe working pressures. Both of these materials can be joined using roll groove mechanical joints.
Moving from the very general discussion about basic concepts of design and system coordination, one must consider pressure piping in the water supply and distribution system, as well as general drainage and venting approaches. Finally, plumbing engineers must recognize the impact of plumbing installation on the building structure. All of these discussions apply, in various degrees, to any type of high-rise building: office, condominium, hotel. These challenges multiply when plumbing engineers design buildings that are more complex because of function, such as hospitals. Typically, hospitals have a higher density of plumbing fixtures than most other types of buildings, leading to more penetrations to serve them.
Hospitals offer a challenge because they require so many more systems. Aside from the routine rainwater, sanitary drain and vent, and cold water systems, hospitals often have other special piping needs, such as laboratory waste, medical gases, or multiple water temperatures to serve patient care or cleaning and sanitizing purposes. Each of these additional systems must be complete and follow the general requirements of the systems already discussed.
Many hospitals have laboratories, and some other types of institutional buildings may have drainage systems to serve chemical- or acid-using fixtures or equipment. Where this occurs, it is important to define acceptable piping materials, in both suitability to the medium being piped as well as acceptability to the local authority. High silicon iron, borosilicate glass, polypropylene, and PVDF are all commonly used. Different materials have different strengths and weaknesses. Iron and glass piping are almost universally suitable for use with most acids, bases, and similar chemicals. Both are heavy and require more space for installation, but they are not easily attacked by flame or generate heavy fumes and smoke. Simple penetration protection is adequate in most locations. On the other hand, plastic products can be somewhat troublesome for both chemical drainage systems in general and high-rise buildings in particular. They have a narrower list of chemicals that they resist well, and they are more fragile as well as susceptible to failure by flame exposure. Plastics also may cause smoke-generation issues that must be addressed to maintain life safety. Resolution of these installations may vary by location and authority having jurisdiction. Regardless of the material and approval received, chemical, acid, and laboratory drainage and vent systems must be separate from the domestic drain and vent systems used throughout the building.
In one recently completed high-rise laboratory building, biological research labs were on the upper four floor levels. Each of these lab spaces was served by an acid- and chemical-resistant drain and vent system, separate from the domestic drain and vent systems, that extended to connection at a monitoring station at the junction with the building sewer. In this case, glass piping was selected as the proper material, offering the benefits and longevity of that material. On the highest level, a bio-safety containment facility was added for critical research in a fully secured environment. Even though this floor used products and materials identical to the adjacent lower floors, the piping circuits were segregated and protected from potential discharge to the environment until passing through a sterilization facility. Even the vents were filtered to prevent uncontrolled discharge to the environment. This containment facility also housed a small population of research animals, which were appropriately safeguarded and cared for, including cage-washing and autoclave equipment to protect against infection. Drainage from this equipment is a high-temperature waste, which often causes difficulty with leakage when using one of the available plastic products.
Multiple water temperatures required for different operations lead to another increase in piping and penetrations. This is not only for the supply side, such as cold water distribution, but also for the circulating hot water piping. Usually each water temperature must circulate independently, but occasionally multiple risers or multiple-temperature circulating piping can be combined to return to the heater or mixing valve. Finally, there are the medical pipeline gases. Codes require distribution for patient uses to be horizontal, on each floor, with zone valve boxes and area alarm panels. These distribution systems must be fed from sources that are usually remote, thus requiring another set of supply risers.
Many of the conditions already described could apply to any number of projects. The general discussion about pressure and control is based on practices used successfully in office building, hotel, and condominium designs throughout the United States. Each of these projects has major similarities and only subtle differences depending largely on construction methods, and less so on decisions of the local authority. These are very basic practices, and variations are currently being used in active design projects including condominium, office, hotel, and museum projects. The examples used to illustrate the hospital conditions are also real and are actually some of the design limitations encountered on projects that are still in construction at the time of this writing.
A particular new hospital has a number of additional plumbing design opportunities beyond those associated with high-rise construction. First, this project is an infill project, constructed between two wings of an existing high-rise hospital, one of which is also involved in a vertical expansion and facility upgrade to the ICU floors. A second interesting task was the relocation of several active drainage systems serving the hospital and exiting through this project’s site, which included primary and secondary storm drainage, sanitary drainage, relocation of the grease waste drainage from a significant food preparation area, installation of a new passive-type interceptor, relocation of acid-resistant drainage from a major laboratory function, and installation of a new acid neutralization basin. The new interceptor and neutralization basin and outfalls are located in the private perimeter roadway that surrounds the building. Another area of coordination with the underground systems is the addition of a new branch from the central utilities on campus, designed and installed as a separate contract by a separate engineering and contracting team. This included high-pressure steam and condensate, chilled water supply and return, emergency power duct bank, primary high-voltage power supply, telephone, and fiber optic. All of these modifications were required to be completed before the first floor slab was poured.
Even after the underground adventures were covered, the building continued to present creative opportunities to the design team. The slab spacing was determined to copy those in the existing hospital, which were very short intervals. This led to an approach that is commonly used for hotel-type construction, using multiple vertical risers placed in the toilet chases to serve multiple floors. Of course, this approach was required to be modified because of the irregular stacking of like fixture groups from floor to floor and the relatively large floor plates (varying between 22,000 and 24,000 square feet per floor). Additional complexity was provided by the modern HVAC requirements for medical facilities and the impact of ceiling plenums, high-density communication and data systems, and high ceiling elevations for more spacious aesthetics on typical patient care floors. Interspersed throughout the building were specialty areas, such as isolation rooms, patient preparation and patient step-down recovery, and ADA-accessible patient rooms. Each of these areas required a unique solution or a variant of those heretofore unique solutions. The ultimate solution for the project was a combined system using large, centrally spaced main waste and main vent stacks that allowed each smaller fixture riser to extend to the main stacks individually or as a building drain. The riser diagram that resulted has a distinctive fan- or brush-shaped outline where all piping funnels together into the main stack. In the final configuration, this building ended with three main soil, waste, and vent stacks, two main rainwater stacks, one main water supply riser, and one main medical gas riser.
As this discussion illustrates, modern high-rise design is often a series of design concepts that must be tested through analysis and coordination, and then adjusted during the coordination period to maximize flexibility and constructability. This exercise is critical for all building trades but especially so for plumbing systems, for which piping must be accurately placed or accounted for in the early construction phases, while the fixture mounting and finishing connections are made much later after the piping systems are concealed. It also highlights the need for designers and engineers to have a familiarity with the work of their peers in other trades. This allows for a certain amount of anticipation between trades, which should be beneficial to the overall project.
In summary, I have quickly reviewed the process of high-rise plumbing design, particularly focusing on pressure control and on the impact of piping systems on the general construction of the building. You can see that although many solutions are routine and similar in application, each approach has trade-offs that must be identified, evaluated, and committed to on each unique project. This understanding supports the notion that good engineering is thoughtful and proactive and that good engineers are open to frank discussion and understanding pertaining to their own trade work, as well as that of other trades that are involved in the building. All high-rise buildings, in design and construction, are significant undertakings for everyone involved. All buildings are unique in form and specific design solutions. It takes a collaborative effort and a determined outlook to achieve success in high-rise design and construction. Good high-rise plumbing design makes even the tallest of structures more comfortable and safer for all building occupants, and good engineering and design practices and experiences turn the most daunting high-rise design into a matter of scale.
In the final analysis, I believe the answer to the question is “yes.” It is all the same—all high-rise buildings are such complex organisms that they require close scrutiny and evaluation to maximize the project’s potential for the owner and to create a design that is robust enough to serve the needs of the building for years to come, and still provide for affordable construction.
“High-Rise Plumbing Design: It’s all the same, right?” was published in the May/June 2007 edition of the Plumbing Systems & Design Magazine by the American Society of Plumbing Engineers, Inc.
Author: Dennis M. Connelly, CPD