Steel Truss Calculator

No — and this is the misconception this page exists to break. In a statically determinate truss (one pin, one roller, joints that satisfy the determinacy check), member forces are pure geometry: the same span, pitch, bays, and loads produce exactly the same force table whether the members are steel, wood, or unobtanium. Material enters through E (elasticity) and A (area), which govern displacement, not internal force. The comparison links above load the identical 32 ft Pratt frame twice — once with E = 29,000 ksi steel sections, once with E = 1,700 ksi timber — and the force columns match line for line while the displacement column changes by a factor near 17. Where material does change forces: indeterminate frames (extra supports or redundant members), where the stiffness distribution actually carries load share — and the solver on this site handles those too.

Open the 'Advanced: Cross-Sectional Properties' panel under the calculator (it's collapsed by default). You'll see three rows — top chord, webs, bottom chord — each with an area (A) and an elasticity (E) input. For steel, type 29000 into each E field (the unit is ksi: 29,000 ksi equals 29,000,000 psi, the standard value for structural steel). Areas default to 5 in²; for steel members 1–3 in² is more realistic, so change A to match the shape you're considering — the section table on this page lists typical values. The force table updates live as you type, which makes it easy to feel the difference: changing E moves the displacement numbers, changing A barely moves anything on a determinate frame (it moves stresses, not forces).

The crossover is span and environment, not strength on paper. Below roughly 40 ft of clearspan with ordinary roof loads, wood prefabricated trusses are cheaper per truss, lighter per piece, and connectable with nails — which is why about 90% of residential roofs are wood. Steel takes over when: spans push past 40 ft where wood chord sizes and splice complexity compound; concentrated equipment loads (HVAC platforms, crane monorails, storage) hang from bottom chords; the environment chews material — salt air, ammonia-rich livestock barns, and pool enclosures demand galvanized or painted steel or heavily treated wood; or when the headroom budget is brutal, because a 4-inch steel member can do a 9.25-inch wood member's job. Each condition has a number attached — pick your condition, and the decision tree at the top of this page does the routing.

Same geometry workflow as wood: pick the profile (Pratt and Howe roof presets cover gable steel buildings; parallel chord covers flat), set span, rise, and panel bays, then apply loads at the purlin joints. The differences are all in the Advanced panel and in your load table: steel purlins typically land at 5–6 ft spacings, so joint loads are larger and fewer — 4–6 kip at a top-chord joint is a normal snow-zone number versus 0.5–1 kip for wood trusses at 2 ft spacing. Set E to 29,000 ksi and A to your intended HSS area, then read the force table. One watch-out: light-gauge cold-formed steel behaves differently at connections than hot-rolled HSS — the solver models member forces, and it's your job to confirm the connection hardware matches the force in each member.

Budget it as a fixed insurance premium on the material decision. Hot-dip galvanizing runs roughly 20–35% of bare steel material cost for truss-sized members; paint systems are cheaper up front but re-coat on a 10–15 year cycle. Against that, wood in the same corrosive environment needs treatment plus more careful detailing, and even treated lumber has a service life ceiling in ammonia barns and salt spray. The honest comparison: steel with one good galvanize layer often ends a 40-year lifecycle cheaper than the second and third re-treatment of wood — but only if the environment truly demands it. In dry inland air, uncoated steel trusses inside an enclosed building routinely outlive the building's mortgage, and the coating money was unnecessary.

No — a rafter is a single sloped beam, a truss is a triangulated frame. 'Steel rafters' usually means one of two things: either sloped HSS or I-section members that carry purlins between frames (common in metal buildings, spanning 20–40 ft), or the top chords of a steel truss doing double duty as the rafter plane. The design question differs: a rafter is checked as a beam (bending, shear, deflection) and needs a ridge beam or bearing wall under its high end, while a truss converts the roof load into axial member forces and can clearspan without interior support. This calculator models the truss version; a single steel rafter needs a beam calculator instead. The overlap is vocabulary — in steel building slang, the whole frame (rafter columns + rafters) is sometimes called a 'rafter frame', which is not a truss at all.

The table is a sizing reference for modeling, not a fabrication certificate — and the distinction matters legally as well as practically. What you can do safely: model the truss, read the force in every member, pick an HSS whose capacity comfortably exceeds the force with a healthy margin, and understand which members carry tension versus compression. What you should get help with: compression-member buckling (an HSS that's fine at 5 kip of tension can buckle at 2 kip of compression depending on its length), connection design at joints, and anything that carries people, livestock, or storage. Many fabricators will take your modeled geometry and force table and turn it into a stamped drawing for a modest fee — arriving with the member forces already computed makes that conversation shorter and cheaper.