ScotCalc · Back-to-Back Truss Designer — Design Report

Configurator note: geometry shown is the analytical centerline model of a back-to-back (strong-axis) cold-formed steel truss. Member sizing, loads, analysis, and AISI S100 checks are added in subsequent versions of this tool. This tool is provided as a free service; results are for preliminary configuration only and must be reviewed by a licensed design professional before use in construction.

Model assumptions: plane frame — chords are continuous beam-columns (axial + bending, gross A and Iₓ, ×2 for back-to-back), webs are pin-ended axial bars (C/T only); bottom chord is moment-released where it meets the top chord at the heels; on triangular trusses each top-chord face is a pin-ended continuous member from eave to ridge (moment-released at the ridge); on bowstring trusses each top-chord stick between panel points is pin-ended (simple-span flexure); E = 29,500 ksi. Loads are applied along the top-chord members per the bracing selection (purlin point loads at their true positions, or uniform member load with sheathing); member self-weight is included automatically when “Auto-add truss self-weight” is checked (chord weights as distributed member loads, web weights delivered to their joints); otherwise carry it in DL. Member capacity checks (P–M interaction) follow in the member design step. Positive axial = tension; positive moment = sagging.

Design basis: AISI S100-16 ASD. Compression per E2/E3: elastic flexural, torsional and flexural‑torsional buckling from catalog section constants (K = 1; Lₓ = panel spacing; L_y = L_t = purlin / fastener / bridging spacing, continuous board treated as fully braced); effective area per §1.1–1.3 at Fₙ; Ωᴄ = 1.80. Tension: AᵧFᵧ/1.67. Flexure: SFIA catalog allowables Mₐ (and distortional Mₐd where published), reduced for lateral–torsional buckling per F2 with Cᵇ = 1. Shear: catalog Vₐg. Interaction evaluated per load combination at each analysis station with concurrent axial and moment, including B₁ in‑plane amplification (Cₘ = 1). Back‑to‑back members assume interconnection per AISI I1.2 (composite Iᵧ; modified slenderness not applied). Web crippling, punchout deductions, connections and bearing are not checked. Results require review by a licensed design professional.

Envelope axial forces delivered by each web member end to its joint, from all ASD load combinations (webs are pin-ended axial members, so both ends of a web carry the same axial force; the joint they connect to differs). Each end carries a stable Connection ID = member key + joint node (e.g. V1-B1 = vertical V1 at bottom-chord node B1). Fastener design: screws are Hilti S-MD 10-16 HWH and S-MD 12-14 HWH self-drilling screws per ICC-ES ESR-2196 (fastener shear Pₛₛ/Ω from Table 5; select screw length and drill point for the joint ply stack per ESR-2196 Table 1) with connection tilting/bearing computed per AISI S100-16 J4 (Ω = 3.00, each ply at its own Fᵤ: 45 ksi for 33 ksi steel, 65 ksi for 50 ksi steel); bolts per AISI S100-16 J3 (bearing with C-factor, Ω = 2.50; bolt shear FₙᵥAᵇ/Ω with A307 = 27 ksi, A325 = 54 ksi threads-included, Ω = 2.40). Ply stack follows the member assignments: single webs between back-to-back chords act in double shear (2 planes/fastener, inner-ply mᶠ = 1.33 for bolts); back-to-back webs on back-to-back chords are mirrored single-shear laps (quantity split per face). Assumes minimum spacing 3d and edge distance 1.5d are provided, screw point/length suit the total ply thickness, and washers under bolt heads/nuts. Chord splices in a later phase. Member rupture (J6) at the connection is not yet checked.