Wood truss
Wood truss: pick a layout, span and pitch, and get chords and webs sized and checked to ANSI/TPI 1-2022 (the default, for metal-plate-connected trusses) or to NDS 2024 alone (heavy timber), for roof and floor trusses, with ASCE 7-22 load combinations.
Inputs of the wood 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.
- Design standard: ANSI/TPI 1-2022 for metal-plate-connected trusses, or NDS 2024 alone. Design values and load duration come from the NDS and its Supplement in both.
- Sections: visually graded sawn lumber and timbers by species and grade; plies on chords; chords laid flat for the 4x2 floor truss. Bearing length.
- Heavy timber truss: choose NDS 2024 alone with sawn timbers, or Material = Glulam with a class and sections listed as width x depth. Glulam members use Fbx+ for sagging and Fbx- for hogging, and the lesser of CL and CV in bending, with L taken as the member's length (the conservative end of Eq. 5.3-1). A balanced lay-up suits a chord. Bolted joints, steel gussets, and heavy timber as a construction type (IBC 602.4, fire) are not checked here: the forces at every joint are listed, for Wood connection or your own design.
- Repetitive member increase, Kcr, person load: on by default where the standard sets them; yours to change.
Checks performed #
| Check | ANSI/TPI 1-2022 | NDS 2024 alone |
|---|---|---|
| Effective lengths | 7.2: in-plane K of a chord from its neighbours, overhangs, webs at 0.8 | K entered |
| Repetitive increase | 6.4.2, chords | 4.3.9, bending |
| Tension | 7.3.1 | 3.8.1 |
| Compression | 7.3.2 | 3.7 |
| Bending | 7.3.3 | 3.3 |
| Axial + bending | 7.3.4, 7.3.5, panel point checks without the buckling terms | 3.9.1, 3.9.2 |
| Slenderness | 7.3.6 | 3.7.1.4 |
| Shear | 7.3.7 | 3.4.2 |
| Bearing | 7.3.9, with its buckling limit | 3.10.2 |
| Person load | 6.2.2.5 | Not applied |
| Deflection | 7.6: live, live + creep, horizontal movement, narrow-face floor multiplier | Your limits |
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 wood truss #
- Connections are not designed. Metal plates (ANSI/TPI 1-2022 Chapter 8) are normally the truss manufacturer's delegated design with that plate maker's tested values; gussets, bolts, heel joints and anchorage. 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.
- The factors Km (6.4.3), CT (6.4.5) and Cs (7.3.2) stay 1.0, which gives up a benefit the standard permits.
- Not applied from ANSI/TPI 1-2022: rolling shear, heel shear and moment rules, extended chord bearing, reaction limits for top chord bearing, girders, and the deflection limits for a top chord panel and for overhangs.
Verification examples #
Verification status: 18 values compared in 18 examples (7 published, 11 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 sawn-member values are read through a chord for compression and bending (AWC E1.4 and Aghayere & Vigil through secondary tabulations, WoodWorks DE-2025, HUD). The ANSI/TPI 1 rules are hand calculations only: no published wood truss or TPI example is on file. ANSI/TPI 1 was read in place in part, and several of its provisions are not applied: see the limitations.