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February 2022 #14271 Page #105
Recommendations and Limitations for Depth and Deflection of Parallel Chord Trusses
By MiTek Staff
n addition to allowable lumber stress limitations, parallel chord truss designs are also regulated by maximum permissible deflection-to-span and depth-to-span limitations. All of the following recommended limitations should be achieved to provide a quality roof / floor system and assure complete customer satisfaction. Trusses falling outside these limits may be sealable as well but should be reviewed by an experienced truss engineer before fabrication.
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Maximum span to depth ratio is 24 for parallel chord roof trusses and maximum span to depth ratio of 20 for floor trusses. The following is a summary of some clear span limits: Floor Truss Depth 12” 14” 16”
Maximum Floor Truss Span 20’ 0” 23’ 4” 26’ 8”
Limit attic room bottom chord L/d to max 23. The following is a summary of some attic room widths: Attic Room Bottom Chord 2x8 2 x 10 2 x 12
Maximum Attic Room Width 13’ 10-3/4” 17’ 8-3/4” 21’ 6-3/4”
It is possible to go beyond limits shown above, but the truss may exhibit increased deflection and/or vibration. Roof truss deflection limits cannot be less than L/240 LL (live load) + L/180 TL (total load). Floor truss deflection limits cannot be less than L/360 LL (live load) + L/240 TL (total load); the higher the number on the bottom of the fraction, the stiffer the floor truss.
For additional information, or if you have questions, please contact the MiTek Engineering department. PHONE: 800-289-5627
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