Cold-formed steel truss

Cold-formed steel truss: pick a layout, span and pitch, and get C-section chords and webs sized and checked to AISI S100-16 and AISI S240-20 Chapter E, for roof and floor trusses, with ASCE 7-22 load combinations.

Inputs of the cold-formed steel truss #

• Roof or floor, truss type: roof trusses offer 18 layouts, floor trusses the parallel-chord ones.
• Geometry: span between the outer bearing lines, pitch (rise in 12), heel height, depth on parallel chords, flat length on a hip, rise on a bowstring, room width and height on an attic, overhangs, panels (Auto: about 6 ft on a pitched truss, about 1.5 depths on parallel chords; a count the layout cannot have snaps to the nearest one and the results say so).
• Supports: pin, roller or none at each end, intermediate supports (moved to the nearest chord joint, and the results say where), bearing on the bottom or top chord of a parallel-chord truss.
• Loads: area loads in psf times the truss spacing: top chord dead, roof live, snow, unbalanced snow, floor live, bottom chord dead and live. Live load on half the span as well (ASCE 7-22 4.3.3, on by default, because web forces reverse under it). Wind rows (pressures normal to the top chord from your wind analysis, each checked from both directions), total horizontal seismic, SDS for the vertical effect, and point loads anywhere on a chord, also between panel points.
• Members: Size the members automatically is on by default: at every calculation the lightest section of each group that passes is chosen. Turn it off to enter the sections and have them checked as they are.
• Bracing: what holds each chord out of plane, and the webs: as needed (a compression web that fails unbraced gets a lateral brace at mid-length before anything is upsized, and the results, schedule and sheet name it), none, or every web. These are your judgment as engineer of record, and the braces themselves are not designed.
• Members with their own section: a mark from the schedule and a section, for one heavy web or chord panel. The rest keep the section of their group.
• Serviceability: live and total deflection limits as span / n, and the horizontal movement limit at a roller.

Auto-design varies the top chord, bottom chord and web sections, lightest first, within the given families, and fixes the result (it turns automatic sizing off). It never changes the geometry, loads, bracing or code options.

Analysis. A plane frame, first-order, with chords continuous over the panel points and pinned at heels and pitch breaks, and webs pinned at both ends, or every panel point pinned if you choose. Loads between panel points are carried in chord bending, so a chord has axial force and bending together. Every load case is solved once and combined with ASCE 7-22 2.3.1 or 2.4.1 (and 2.3.6 or 2.4.5 with seismic). Deflections always use the ASD combinations.

• Sections: SSMA lipped C designations, depth in the plane of the truss, Fy and Fu. Net area An/Ag for screw holes is your input because connections are not designed.
• Effective lengths by AISI S240-20 E4.4.2.1: on by default (Kx, Ky, Kt and Cm = 0.85 as the standard prescribes). Off: the K values you enter and Cm = 1.0.
• C webs attached back to back with the chord's web: adds the eccentricity interaction of E4.4.4.1 to compression webs.

Checks performed #

The report prints the steps of the governing member of each group and lists every member in the schedule. Turn on Report every member step by step, under Serviceability, to print every member.

Limitations of the cold-formed steel truss #

• Connections are not designed. Screws, gusset plates (S240-20 E4.5), fastening and copes (E4.6), heel joints and anchorage. Proprietary truss sections need the manufacturer's values and tests. The forces in every member at every joint are listed for that design.
• One section each for the top chord, bottom chord and webs, apart from the members you give their own.
• One lateral brace per web at most, at mid-length.
• Work lines meet at the joints: joint eccentricity and secondary moments are not modeled. First-order analysis, with moments amplified member by member where the standard does so.
• Gable-end, gambrel, piggyback and unequal-pitch layouts are not generated. Use the 2-D truss analysis for any other geometry.
• Ponding, fire, floor vibration, system bracing design, wind pressure calculation and trusses carrying other trusses (use point loads) are outside this calculation.
• Not applied: S240-20 E4.4.6.1 joint eccentricity (the model is a single node without it). E4.4.6.3 chord shear between fastener groups: the shear check keeps kv = 5.34, which the stiffened-panel equations only raise. Connections, screws, gussets (E4.5), copes (E4.6) and proprietary sections stay out, as the owner decided for the truss designer.
• Added in wave 7: S240-20 E4.4.2.3 at compression chord panel points (the load on the flange is entered, and webs bearing on the flange add the step in the chord's shear; webs lapped back to back pass their force through fasteners and are not counted, which is our reading and not stated in the standard's text), S100-16 E4 distortional buckling in compression with no rotational restraint counted (it also limits the available axial strength in H1.2), and web crippling at the reactions, on one web without a stiffener. These are hand-checked only, and saved calculations can now show one more failing check.
• In Eq. E4.4.4-1, May takes local buckling only; global buckling about the minor axis is not evaluated.

Verification examples #

Verification status: 18 values compared in 8 examples (3 published, 5 hand calculations from the cited equations): 18 within tolerance, 0 recorded deviations. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.

These results carry the label "Preview: not yet verified, do not rely on these results" on the screen, in the report and on the sheet. The frame solver is checked against closed-form beams and the pin-jointed solver. Published values are SSMA's Mal and Vag only; compression, the combined checks and S240 are hand calculations. No published cold-formed truss or effective-width compression example is on file.