Maximum Allowed Duct Sizes & Vierendeel Openings
Maximum Allowed Duct Sizes
See the table below for approximate allowable duct sizes based on joist depths.
•Reduction in floor to floor height
•Allows larger ducts through the joist
1. As a “general rule of thumb” vierendeel openings can be fabricated with a horizontal width up to 2 times the depth of the steel joist.
2. It is desirable, to locate the vierendeel opening near the mid-span of the steel joist. Doing so reduces the required chord bending moments from transferring the vertical shear forces across the vierendeel opening with no diagonal web members. This decreases the penalty to the top and bottom chord size for the opening.
3. One critical load case is live loading on only half the joist span.
4. It is desirable to avoid multiple vierendeel openings within a joist. Where multiple vierendeel openings are required, the minimum distance between vierendeel openings shall be no less than two times the joist depth.
5. When determining the net available opening dimensions, consideration must be given to fire proofing thicknesses (were it occurs).
•Call out locations of openings on structural framing plans.
•In addition to width of opening, call out required height of opening so joist chords or joist depth can be adjusted accordingly.