Serving western Canada's sheet metal industry

By: Eli Howard

The newly released ANSI/SMACNA HVAC-DCS 3rd Edition will be presented during the 2006 SMACNA Annual Convention in Scottsdale, AZ this Fall. Last revised in 1995 the new HVAC-DCS was a major undertaking by SMACNA to enhance the components and technology used in today’s sheet metal industry.

In 1997 the SMACNA Technical Resources Committee embarked on a process to revise the existing HVACDCS 2nd Edition 1995 and obtain ANSI approval. One of the primary purposes of this process was to re-affirm the duct construction tables within the HVAC-DCS and establish a methodology through modeling to define an engineering calculation for the development of all future duct construction development. And added benefit to the development of a model was to significantly reduce the need for physical duct specimen testing.

Finite Element Analysis (FEA) modeling was determined to be the logical choice for the development of an HVAC duct model. The FEA for rectangular steel ducting was a step-by-step process, in which a combination of theoretical analysis and experimental testing were used to verify the numerous finite element models. Over 800 finite element models were generated, including both non-linear stress analysis and non-linear buckling analysis, along with experimental tests of over 20 full scale ducts of various sizes. Additionally translational/ rotational restraint; internal/external pressure (+/-); total displacement; surface and membrane stress (panel, joints, seams) were analyzed.

This extensive FEA modeling enabled the HVAC-DCS Task Force to expand the efficient use of stiffened and un-stiffened sheet metal panels. Furthermore, the duct construction tables in the existing HVAC-DCS 2nd Edition 1995 precluded the use of certain duct thicknesses for the larger duct panel sizes. Through FEA, it is now possible to expand the duct construction tables for panel sizes previously not rated that have now been included in the new ANSI/SMACNA HVACDCS 3rd Edition.

THE FOLLOWING IS A LIST OF MAJOR CHANGES FROM THE SECOND EDITION THAT WILL BE PRESENTED:

• A new engineering and design chapter directly primarily to design professionals
• New double-wall construction details for round, oval and rectangular ductwork
• New casing construction details • Included the application of framing channel (strut) for use as trapeze supports and duct reinforcement
• Expanded rectangular pressure class tables to include selections previously not designed
• New figures highlighting the conditions of use and limitations for transverse joints, longitudinal seam and reinforcements
• New detailed tables for joints T-25a and T-25b for 4, 5 and 6 ft. stock—coil or sheet
• Expanded mid panel tie rod application to include 6” wg negative pressure and additional panel sizes
• Expanded round duct construction to 96˝ diameter
• New 6˝ wg pressure class for round duct
• Included minimum gage table for flat oval duct
• New tables for longitudinal and spiral seam round duct for negative pressure at reinforcement spacing of 20, 12, 10, 6 and 5 ft. plus un-stiffened duct

SMACNA will be publishing four additional documents this Fall, that include the fourth edition of the HVAC Systems Duct Design manual which is designed to offer both the HVAC system designer and installer detailed information on duct design, materials and construction methods; third edition of the Sheet Metal Welding Guide reflecting the latest welding technology and designed to assist contractors understand and manage their welding operation more profitably, it includes updated coverage of welding processes, flux cored wire guidelines and a safety chapter covering such items as health effects, hazard controls and safety procedures; first edition of the Seismic Restraint Manual – Guidelines for Mechanical Systems – OSHPD Edition. This document complies with requirements as mandated for the State of California hospital projects which fall under the California Office of Statewide Health Planning and Development (OSHPD); first edition of the Rectangular Industrial Duct Construction Standard (SI) hard metric version available for PDF download.