Cold-formed steel truss bracing
Permanent bracing of cold-formed steel trusses: a continuous lateral restraint (PLR) across a run of trusses at one or more points of a compression web or chord, anchored by diagonal bracing every m trusses. The brace force and stiffness follow AISI S100-16 (with S2-20) C2.3, which AISI S240-20 B4.5 names for the bracing of roof framing; the restraint and diagonal members and their screws are checked to S100.
Inputs of the truss bracing #
- Braced member: a web or a chord, its SSMA section, Fy and Fu, its length, the in-plane unbraced length Lx (the web, or the chord panel), the number of equally spaced restraint rows n (Lb = L/(n + 1)), Kx, and its compression by load type from the truss design (the truss designer's forces). The combinations of ASCE 7-22 give the required compression Pr.
- Maximum restraint spacing: 10 ft prefilled, the bottom chord suggestion of the S240 Commentary to E6; 0 turns the check off.
- Restraint and diagonals: the truss spacing, the trusses in the run, the trusses per diagonal m (your layout; diagonals at about 20 ft is common practice in the CFS-BCSI guide), the brace force as a percentage of Pr (1 % by S100 Eq. C2.3-1; 2 % is the S240 B3.4.1.2 practice for wall studs), the restraint and diagonal sections and Fy, the diagonal's run along the restraint (in truss spacings) and its rise, the brace effective length factor K and Fu.
- Screws: the size, the count at each truss and at each end of a diagonal.
Auto-design varies the restraint and diagonal sections (lightest first) and the screws. The member, its forces, the spacing, the diagonal layout and the brace percentage are kept.
Checks performed #
| Check | Basis |
|---|---|
| Member compression with the restraint (Lb for y and torsion) | S100 E2, E3 |
| Restraint spacing against your limit | S240 Commentary E6 (limit entered) |
| Brace force Prb | S100 Eq. C2.3-1 |
| Restraint stiffness at the farthest truss against βrb | S100 Eq. C2.3-2a / -2b |
| Restraint compression and tension (m Prb) | S100 E2, E3, D2; S240 Commentary B3.4 |
| Diagonal compression and tension (m Prb Ld / run) | S100 E2, E3, D2 |
| Screws at each truss and at the diagonal ends | S100 J4.3.1, J4.3.2 |
The brace forces of the m trusses between diagonals add along the restraint. The restraint and the diagonal take all m of them, the safe side. The stiffness the system gives the farthest truss is Prb / δ with δ = Prb [m / kd + s m (m − 1) / (2 E Ap)], kd = E Ad cos²θ / Ld being the diagonal's stiffness along the restraint and the second term the shortening of the restraint segments. This system model is ours (to confirm against a published bracing design); screw slip is not in it.
Limitations #
- The truss and its connections are the truss designer's; this calculation takes the member's force as entered.
- The diaphragm or wall that the diagonals deliver to, laps and splices of the restraint, and temporary installation bracing are not designed.
- AISI S202 (truss responsibilities) and the CFS-BCSI guide are not in the library: the bracing layout is your input.
Verification examples #
Verification status: 7 values compared in 2 examples (0 published, 2 hand calculations from the cited equations): 7 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: not yet recorded for this module.
No published cold-formed truss bracing example is in the free library. The values are hand calculations of Eqs. C2.3-1 and C2.3-2 (LRFD and ASD, one and two rows), the accumulation over m trusses, the diagonal force and the system stiffness.