Cold-formed steel girt

A C- or Z-section wall girt spanning between columns, its outside flange through-fastened to the wall panel: the wind toward the wall and away from it, the gravity load carried by sag rods, the girt's strength in each region and its deflection. To AISI S100-16 (with S2-20) Chapters F, G, H and I6.2.1, with the wall pressures of ASCE 7-22 Chapter 30.

Inputs of the girt #

• Section: a listed Z- or C-section (named as the AISI Design Manual names them; confirm the dimensions against the maker's data) or custom dimensions; Fy and Fu (55 and 70 ksi are usual for girts).
• Spans: one to four equal spans; more than one is continuous and prismatic. Lapped girts are modelled by the cold-formed steel purlin type. The girt spacing is its tributary height.
• Wind: the net components and cladding pressures you enter (strength level, toward and away from the wall), or computed for wall zone 4 or 5 of ASCE 7-22 Fig. 30.3-1 (h at most 60 ft) from V, exposure, h, Kzt, Ke and the enclosure. The effective wind area is the span times the larger of the spacing and a third of the span (ASCE 7-22 26.2). The 0.9 factor of note 5 (roof slope 10° or less) is an option, off by default.
• Suction, free flange in compression: the R method of S100 I6.2.1 (the default) or discrete bracing of the free flange at the sag rods. With the R method each condition (a) to (o) of I6.2.1 is a check: those this type can see from the section, spans and steel, and those it cannot (laps, panels, insulation, fasteners) as inputs you confirm. Insulation thickness reduces R on a simple span (Eq. I6.2.1-2).
• Rotational restraint kφ of the panel on its flange for F4; 0 is conservative.
• Gravity: carried by sag rods (the default: the girt is checked about its weak axis between rods, and the rod in tension), or by the wall panel in its plane, which is your judgment and switches the weak-axis check off. The panel weight, the rods per span, the girts hung from one rod, the rod's diameter and Fu.
• Deflection: the wind factor (0.42, IBC Table 1604.3 footnote) and your limit. L/90 is prefilled, the IBC value for girts supporting metal siding (L/120 for flexible finishes).

Auto-design varies the section (listed sections of the profile entered, lightest first) and the sag rods per span. Spans, spacing, wind, the panel conditions and Fy (unless unlocked) are kept.

Checks performed #

Wind governs the strong axis alone: LRFD 1.0W, ASD 0.6W, in both directions. The weak-axis check pairs each combination's dead load factor with its wind.

Limitations #

• Equal spans, no laps, uniform wind. Openings, jambs and point loads are not modelled.
• The weak-axis strength is the gross weak-axis section modulus at Fy, and the moment between sag rods is taken as simple (w a²/8). This is a simplification to confirm against an effective-section method for weak-axis bending, which this type does not build.
• The girt is taken as bolted through its web to clips at the columns: web crippling is not checked, and the clips are not designed.
• Axial load from wall bracing (S100 I6.2.3) is not included.
• The plan for this type cited S100 "I6.3" for discrete bracing; in S100-16, I6.3 is the standing seam panel system. Discrete bracing here is C2.2 with F2.

Verification examples #

Verification status: 12 values compared in 4 examples (0 published, 4 hand calculations from the cited equations): 12 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 worked example of a wall girt is in the free library. The section properties and Se come from the same Z-section library the purlin type verifies against the MBMA Roof Framing Design Guide and AISI RP20-5 (Se of 8ZS2.75x085 at 55 ksi, 2.84 in³, Guide p. 83). The moments, R, the wall pressure and the sag rod are hand calculations from the cited equations. An AISI D100 or MBMA girt example would replace them.