A chase is an intentional opening in a floor truss created by omitting specific diagonal webs to provide space for HVAC ducts, plumbing lines, or electrical runs. For structural efficiency, chases should be located within the middle third of the truss span, where shear forces are lowest. Removing webs in high shear zones near the supports interrupts the intended load path, reduces truss stiffness, and significantly increases the potential for differential deflection. [For all images, See PDF or View in Full Issue.]
When mechanical chases are incorporated into a truss layout, their placement becomes critical to overall structural behavior. Because a chase naturally creates a more flexible region in the truss, evaluating vertical deflection across the opening is essential. Chases placed outside the middle third require explicit engineering review. Without supporting diagonal webs, these areas become more prone to vertical movement, which can result in noticeable sagging, increased floor vibration, and other serviceability issues.
In the example below, the chase located between joints 10–11 falls outside the middle third zone of the truss span. Its placement substantially increases the risk of excessive differential deflection and related field performance concerns.
To evaluate differential deflection, begin by analyzing the truss. In Search and View, locate the Vertical Deflection values for the joints of interest — in this case, joints 10 and 11. The differential deflection between these joints is: 0.50” – 0.05” = 0.45”.
A differential deflection of 0.45 inches across a span of only 2 feet is significant. Such movement is typically visible during inspection and may indicate potential structural or serviceability concerns.
If relocating the chase toward the center third of the truss is not feasible, the following measures may be considered to reduce differential deflection:
- Reducing the size of the chase,
- Upgrading the top and/or bottom chord lumber, or
- Doubling the top and/or bottom chords.
In the example below, doubling the top chord reduces the differential deflection to: 0.30” – 0.11” = 0.19”.
For additional information, or if you have questions, please contact the MiTek Engineering department.