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Recommended guidelines on shared responsibilities of the designer, ductwork manufacturer, and installing contractor to make sure duct systems are properly reinforced
Duct reinforcement is an essential element of proper duct system design that is often overlooked, especially for negative pressure systems. Even when reinforcement specifications are addressed, they are often so vague that the designer, fabricator, and installer each assumes that the others have taken reinforcement into consideration. On most projects, reinforcement becomes a major consideration only if there is a problem in the field. The consequences of inadequately reinforced ductwork are rarely noticed in commercial building applications. Positive pressure supply systems rarely exceed 6 in. WG, and few negative pressure return air systems exceed - 3 in. WG. Duct systems generally meet approval if the design volume of air gets from the fan to the diffuser with no structural failure and within the budget.
However, problems that avoid detection initially can result in costly retrofits in the future. Over time, inadequately supported positive pressure duct can experience serious leakage and noise problems. Duct walls that continually pressurize and depressurize in variable air volume (VAV) systems can eventually increase leakage at duct joints, thereby requiring the fan to push more air through the system to meet the original design criteria. How much more air? That depends on the quality of workmanship in fabricating, installing, and sealing the ductwork. The "oil-canning" effect can also cause excessive noise problems that could require installation of expensive noise abatement equipment.
Reinforcement specifications are intended to minimize duct wall deflection, thus preventing potential leakage and noise problems. Recent problems in the commercial/institutional arena involve negative pressure systems exceeding the common return air system pressures of-2 to -3 in. WG. These systems, when constructed of common commercial gauges, will experience structural failure if not properly reinforced. Herein lies the problem. The construction standards referenced by most commercial specifications do not properly address all reinforcement issues. Potential disaster awaits duct systems when reinforcement issues are not addressed during the design stage.
The most prominent construction standards found in specifications today are published by SMACNA. The premiere commercial standard is SMACNA's 1985 HVAC Duct Construction Standards. The following paragraph comes from Section 1, "Basic Duct Construction," under the subsection titled "Reinforcement Arrangements."
"Fabricators and installers are obligated to select features from among the joint, seam, reinforcement, and support options that will result in a composite assembly that will be serviceable within the express and implied performance criteria identified herein. Experience in construction is valuable; no representation is made that all detail and knowledge necessary to select, fabricate, and install a workable assembly is implied. Indiscriminate selection and poor workmanship compromise construction integrity. Conversely, the obligation to make suitable selections does not constitute an obligation to compensate for a designer's negligence in specification application. A construction standard must be applied by a designer to the requirements of the individual project within the range of its limits."
So who is responsible? Designers, fabricators, and installers! The above paragraph implies that the fabricator and installer are responsible for selecting reinforcement and other construction details from among SMACNA's options to meet specific performance criteria. Gauge/reinforcement options allow the contractor and fabricator to select reinforcement combinations that offer the best price advantages, providing they are within their manufacturing capabilities. Together, the contractor and fabricator can evaluate reinforcement, joint connector, and support options to reduce costs further. Selection and workmanship are the combined responsibility of the fabricator and installer.
However, designers cannot totally dismiss themselves from duct construction responsibilities. Construction standards are available for various applications; one manual does not cover them all. Therefore, the designer is responsible for choosing the construction standards according to the specific application.
The following example typifies how vague wording in commercial specifications can cause severe problems for all parties concerned when responsibilities are not clearly stated.
A specification states: "Provide 80 ft of 14 by 41 flat oval exhaust air duct for the animal cage room ventilation system. Ductwork is to be of galvanized steel able to withstand an operating pressure of - 6 in. WG. The duct must be in conformance with SMACNA's HVAC Duct Construction Standards. Four-bolt flat oval duct connectors are required."
According to SMACNA's 1985 HVAC Duct Construction Standards (pp. 3-11, Tables 3-4), flat oval duct with a major axis dimension of 41 in. should be constructed of 22 gauge galvanized sheet metal. The proposal submitted by the fabricator to the installing contractor includes pricing for the sheet metal duct and fittings. The fabricator offers two four-bolt flat oval connector price options: shipped loose and shop installed.
The installer gives the fabricator a contract for the ductwork with four-bolt flat oval duct connectors installed in the shop. The job is fast track, so after receiving approved submittals from the designer, the installer releases the fabricator to send material to the job site. Eight months later, the installer informs the fabricator that the duct collapsed and wants to know what the fabricator is going to do about it. The fabricator claims no responsibility because the exclusions page in the submittal package plainly notes that reinforcement would not be provided.
The installer claims no responsibility, having assumed that the four-bolt oval duct connectors selected by the designer were the reinforcement. The designer informs the installer that four-bolt flat oval duct connectors were selected for their attractiveness as connectors, not for their reinforcement value. The designer will not take responsibility for the misunderstanding because the specification plainly states that the installer is responsible for providing ductwork that meets SMACNA standards, including reinforcement. The designer also points out that the installer is responsible for the reinforcement because the fabricator stated that it would not be provided. The installer approaches the fabricator and demands help in solving the problem, claiming that it was the fabricator's responsibility to detail the reinforcement required even though it was not supplied. Who is responsible for the reinforcement?
After researching SMACNA's 1985 HVAC Duct Construction Standards to determine what reinforcement is required, the fabricator finds no guidance for reinforcing round, flat oval, or rectangular systems exceeding -3 in. WG. The installer requests an add from the designer for the unspecified reinforcement. The designer finds that SMACNA's Rectangular Industrial Duct Construction Standards and Accepted Industry Practice for Industrial Duct Construction both address reinforcement for systems exceeding-3 in. WG and demands that the installer pay for fixing the problem.
The fabricator informs the installer that following the reinforcement guidelines in Rectangular Industrial Duct Construction Standards is impossible because 16 gauge sheet metal is the minimum allowed. The 14 by 41 spiral duct is 22 gauge. Accepted Industry Practice for Industrial Duct Construction (p. 8, Table 2-A) requires that 22 gauge duct be reinforced every foot, which is very expensive. The fabricator and installer both inform the designer that these other publications are not referenced in the specification and are for industrial applications outside the construction standards used for this application.
Fortunately, there is no other flat oval duct on the job. Ultimately, the fabricator agrees to supply reinforcement, provided that the installer assumes installation expenses. But there is still the problem of paying delay back charges. The installer and fabricator join forces, demanding that the designer assume these expenses due to the weak specification and the fact that the designer approved the submittals. If this had been a larger project involving more money, there would probably have been litigation.
The only win-win scenario is for the owner, designer, installer, and fabricator to work out a solution together. This rarely happens, especially on larger projects, and usually all parties think they are in the right and the lawyers are brought in. Who is responsible? There is no guaranteed after-the-fact solution because someone stands to lose profits and respect for admitting fault. The best solution is prevention! Consider the following suggested responsibilities:
* The designer is responsible for choosing construction standards for the job, specifically detailing exceptions or modifications thereof.
* The installer is responsible for working with the fabricator to determine what gauge/reinforcement option offers the best price and still meets the designer's performance criteria. Usually, the installer can save money by purchasing and installing the reinforcement and by properly coordinating the hanger/support layout with the reinforcement spacing.
* When the installer and fabricator provide price options for the engineered ductwork systems, the designer should keep those prices in confidence and not shop around for a better price before or after the bid.
* The fabricator should write a letter to the installer, copying the designer, calling attention to the need for reinforcement and the fact that it is not included. However, the fabricator should provide the installer with reinforcement requirements for the duct provided. the fabricator may provide an option price for reinforcement at the installer's request.
* The designer should review the submittals and verify that the duct construction conforms with the performance criteria selected. The designer should not allow the installer to release the fabricator to supply material to the job site until all concerns about the submittals have been resolved and approval has been given.
The new SMACNA HVAC Duct Construction Standards was published in October 1995 and covers gauge/reinforcement guidelines for round, flat oval, and rectangular duct for +10 to -10 in. WG in standard commercial gauges. The designer, fabricator, and installer should all share the responsibility for duct construction. Accountability, cooperation, and coordination among all parties are essential.
TODD TALBOTT, Airflow Group Engineering and Marketing Manager, United McGill Corp., Groveport, Ohio
Talbott, Todd. "Duct system reinforcement: whose responsibility?" Heating, Piping, Air Conditioning 67.11 (1995): 73+. General OneFile. Web. 27 Nov. 2010.
Gale Document Number:A17911588