Top chord restraint by span
BCSI-B1 (BCSI booklet, SBCA/TPI) sets the maximum spacing of top chord temporary lateral restraint by truss span. Shorter spacing as spans grow:
- Up to 30 ft: 10 ft o.c. max.
- 30-45 ft: 8 ft o.c. max.
- 45-60 ft: 6 ft o.c. max.
- 60-80 ft: 4 ft o.c. max., and spans over 60 ft call for a registered design professional.
Find the band, read the spacing: 28 ft gives 10 ft, 40 ft gives 8 ft, 52 ft gives 6 ft, and 70 ft gives 4 ft plus a registered design professional for the restraint and bracing.
The trap: carrying the 10 ft figure onto longer spans. It applies only up to 30 ft. On a 70 ft span, 8 ft is wrong twice: wrong band, and it leaves out the design professional required over 60 ft.
Lateral restraint material and lap
For spans up to 60 ft, BCSI-B1 specifies lateral restraints of 2x4 × 12 ft or greater, lapped over two trusses.
The traps:
- 8 ft pieces butted end to end at a truss have no lap and are shorter than 12 ft.
- A 1x4 is not the specified 2x4, and a lap over one truss misses the two-truss lap.
- A 10 ft piece with no lap fails both the length and the lap.
Diagonal bracing
For spans up to 60 ft, BCSI-B1 calls for diagonal braces every 10 truss spaces, 20 ft max., in the top chord plane.
Lateral restraint ties the trusses together so they move as one. The diagonals keep that restrained group from racking over together.
The trap: bracing only at the gable ends leaves the interior restraint lines free to rack, which is exactly the failure diagonals prevent. The 5-space/10 ft and 20-space/40 ft figures are not the BCSI spacing.