 | Duct openings
Maximum clear openings for ducts
This article discusses the dimensions required for duct openings in joists and joist girders, depending on the construction project and Canam practices with regard to the integration of ducts through web members.
Canam’s Joist Catalogue (p. 23) provides the dimensions of clear openings in a joist or joist girder with respect to the geometry of round, rectangular or square ducts.
Dimensions of openings
According to the clear opening dimensions in the tables below,1 it is more economical to position the opening according to the geometry of the joist to avoid having to remove one or more web members. The maximum clear opening for the duct and the location of the duct must be specified in the design criteria, as stated in item C.6 of the “Joist Design Essential Information Checklist” in Canam’s Joist Catalogue (p. 70).
It is recommended that you contact our estimating team at the preliminary design stage for assistance in accommodating the integration of the duct according to the geometry of the joist.
DIMENSIONS OF FREE OPENINGS FOR VARIOUS JOIST AND JOIST GIRDER CONFIGURATIONS
METRIC
IMPERIAL
NOTE: Final dimensions of free openings should be verified with Canam's joist design sheet.
1 - CANAM, Joist Catalogue, 2006
Opening dimensions above the limits
A web member must be removed when the dimensions of an opening exceed the limits shown in the tables above.1 The shear force is then transferred to adjacent web members through the top and bottom chords. Since the chords will need to be reinforced, the maximum height of the clear opening will be limited.
Different types of sections can be used to reinforce the joist shoes:
- Back-to-back "C" channels
- HSS tubes
- Reinforcing angles
Basic rules
Canam must know the dimensions of the clear opening required and the exact location of the duct. These criteria must be clearly shown on the design drawings.
The maximum height of the opening should not exceed the joist depth minus 200 mm (8 in.). In the case of a joist girder, the maximum height may have to be less than this, in which case it is recommended that you contact a Canam sales representative.
The length of the panel at the duct location will be calculated to ensure that a gap of approximately 76 mm (3 in.) is available at the construction stage.
Generally, when the dimensions of an opening exceed those shown in the previous tables, Canam recommends a “Pratt” type joist which, because of its geometry, provides the joist designer with greater flexibility.
Although the panel points of a joist girder generally coincide with the joist bearings, it is still possible to provide a girder geometry that will accommodate the integration of a special duct. Please contact a Canam sales representative for technical assistance in this regard.
For economical reasons, it is best to position the duct opening at the mid-span location of the joist since there is generally no shear force at this location of a uniformly-loaded member. Consequently, the location of the opening is more critical near the bearings since the shear force transferred by the web members increases as it moves towards the ends of the joist.
It should be noted that at least two panels are required between the duct and the end of the joist to make sure that the shear force is evenly distributed. Therefore, the location of the center of an opening must be greater than 2.5 x H.
Integration of more than one duct in a joist or joist girder
When there is more than one duct to be integrated in a joist or joist girder, at least two panels are required between the openings to ensure that the shear force is evenly distributed. Please contact a Canam sales representative for assistance in providing the best solution for your construction project.
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