Duct Spacer Specifications


The following five paragraphs outline a suggested specification for Formex Interlocking Module Spacers to be used for underground distribution systems. For each suggested specification, the reasoning behind it is stated. On many projects, underground duct bank configurations are complex. May we suggest that when designing a duct system utilizing several sizes or types of duct, Formex could possible help keep costs down if consulted in advance. Any questions concerning the following should be directed to your Formex representative or to the factory.

  1. Spacers shall be made from high density polyethylene (not less than .96 specific gravity). Spacers shall be double wall construction and shall be of Formex Manufacturing, Inc. make, or approved equal.
    REASON: High density polyethylene (.96 specific gravity) has been chosen by Formex for the manufacture of all interlocking module spacers because of its flexibility, strength, and resiliency. These characteristics afford at least two important advantages, i.e. (a) under compressive stress (ex: workmen walking on duct bank or when concrete is poured) there is sufficient give in the material to prevent the spacer from causing a kink in the duct wall, and (b) the spacer modules can be freely handled on the job site without breakage. Since many utilities install underground systems year round, and most large projects carry through more than one year, it is important to use a formulation of plastic for spacers that will not be adversely affected by the extremes of summer or winter temperatures. Finally, double wall construction makes it possible to design a spacer which will hold safely the vertical loads normally encountered when men work atop large duct banks, especially when concrete is poured.
  2. Spacers shall consist of interlocking modules, i.e. bases, intermediates, and caps. Base pads shall be used to assure specified dimensions between trench floor and bottom of first tier of ducts. The interlocking modules shall include an internal vertical channel on both side edges of the spacers. When the duct bank is assembled, a No. 3 reinforcing rod shall be passed through the internal vertical channel on one side of the spacer bank and driven into the trench floor. At the next spacer location, the No. 3 rod shall be inserted on the opposite side, etc. The reinforcing rods shall be bent inwardly at the top of the spacer bank sufficiently to squeeze spacer cap so duct assembly will not float or move in any direction during the concrete pour. When soil conditions negate the use of reinforcing rods for this purpose, then an alternate method shall be to place a trench jack directly above the spacer location and use adjusting wedges between the spacer cap and the trench jack to hold the duct assembly in position during the concrete pour.
    REASON: The features specified in this sub-paragraph have been designed into the Formex spacers mainly to do the best job at the lowest possible cost. A duct bank erected to specification and firmly anchored against floating will (a) save concrete, and (b) assure a straighter duct line thus making cable pulling easier.
  3. The interlocking module spacers shall provide independent support for each duct.
    REASON: If each duct is not independently supported, it will rest upon the ducts directly underneath. In larger duct banks this often causes the ducts in the bottom tier to become oval and many times this condition prevents the use of the bottom tier of ducts.
  4. When spacer modules are locked together the openings shall provide approximately 1/16″ clearance over the O.D. of the duct. The interior surface of the duct spacer opening shall be convex to prevent kinking of the duct.
    REASON: The suggested clearance is desirable because duct is not always perfectly round. The additional clearance in the spacer opening permits the spacers to snap into locking position with a minimum effort. The surface in the spacer opening upon which the duct rests should be slightly convex because the spacers are bound to tilt to some degree under pressure of the concrete pour. The convex profile eliminates sharp edges that could kink the ducts.
  5. The openings of the spacer modules shall be positioned to assure the horizontal and vertical separations for each duct as specified on the project drawings.
    REASON: For power distribution, the specified separations for ducts is important to adequate heat dissipation. For all duct lines specified separations are important for a variety of reasons, i.e., cables and/or conductors, when pulled through duct, may cut into duct walls at bends, sweeps, and changes in elevation. Concrete encasement is recommended even if the duct is direct buried; with certain soil conditions it is necessary to use reinforcing rods and sufficient concrete.
  6. Spacers shall be installed at ____ foot intervals.
    REASON: Soil conditions, types and sizes of duct to be used, foreseeable loads, etc. all have bearing on the frequency of spacer locations. As a general guide, for each 20 foot length of plastic duct up to 6″ diameter, there should be a minimum of three spacer locations

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Interlocking Module
Spacers Specifications
 Duct Spacers
Common Features

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