Steel Truss Design: Forms, Members, Joints and Forces

A steel truss is a framework of straight steel members connected at their ends to form triangles. Under load, each member is mainly pulled or pushed along its length rather than bent, which lets a truss span distances that would demand a very heavy solid beam. Steel trusses carry the roofs of factories, warehouses, sports halls and airports, the decks of footbridges and, inside tall buildings, the transfer structures that redirect columns over large openings.

This article explains why steel suits trusses, the common forms, how members and joints are chosen, how an engineer quickly estimates the forces, and what governs the design in practice. For the underlying principle of triangulation, see our introduction to what a truss is.

Why steel suits trusses

Steel is equally strong in tension and compression, it can be cut and joined accurately, and its members are slender enough to keep the truss light. Hollow sections, angles, tees and rolled H-sections are all available in a wide range of sizes, and joints can be welded in the shop and bolted on site. Compared with a solid steel beam of the same span, a truss uses far less steel because its material is concentrated in the top and bottom chords, as far apart as practical, with only light diagonals between them.

The trade-offs are depth and fabrication. A truss needs more height than a beam, typically set by span, loading and stiffness requirements, and every node is a connection that must be detailed, fabricated and inspected. Trusses are therefore most economical for long spans or where the depth is available anyway, such as within a pitched roof.

Common steel truss forms

Pratt: verticals in compression and diagonals sloping towards the centre in tension under gravity load. Efficient in steel because the long diagonals are in tension. See our article on the Pratt truss.
Howe: diagonals sloping the other way, so they are in compression and the verticals in tension. Historically suited to timber with steel tension rods.
Warren: diagonals alternating in direction, forming a series of equilateral or isosceles triangles, often without verticals. Simple, with fewer members and repetitive joints.
Fink and fan trusses: pitched roof forms that subdivide the rafters into short panels, common for shorter roof spans.
Bowstring: a curved top chord following the bending moment diagram, giving nearly uniform chord forces.
North-light or saw-tooth: asymmetric trusses for factory roofs with glazing on one face.

The roof truss types article compares these in roofing applications. A Vierendeel girder, which has rectangular panels without diagonals, is sometimes called a truss but works quite differently: its members are in bending, and it is much less efficient.

Members and connections

Member sections

Light roof trusses often use angles or tees, connected through gusset plates. Medium and heavy trusses increasingly use circular or rectangular hollow sections, which are efficient in compression, clean in appearance and easy to protect from corrosion. Heavy trusses, such as transfer trusses, use rolled H-sections or fabricated box chords. Chords are often continuous through several nodes, while web members run between nodes.

Joints

Joints may be bolted, welded or a mixture. Angle trusses traditionally use steel gusset plates with bolts or welds. Hollow-section trusses are typically welded directly, with web members profiled to fit around the chord, and the joint capacity depends on the chord wall’s ability to resist local deformation. Eurocode 3 provides specific rules for these hollow-section joints, and the joint, not the member, often governs. Large trusses are fabricated in transportable pieces and spliced on site, usually with bolted end plates or cover plates.

Estimating the forces: a worked example

Engineers analyse trusses in detail with software or by the method of joints and method of sections, covered in our article on truss analysis. But a quick estimate using beam theory is invaluable for checking output and choosing initial sizes.

Consider a simply supported, parallel-chord truss spanning 20 m, 2 m deep, with panels 2 m long, carrying a total design load equivalent to 10 kN/m applied at the nodes.

Maximum bending moment at midspan: M = wL²/8 = 10 × 20² / 8 = 500 kNm.
The chords form a couple resisting this moment, so the chord force ≈ M / h = 500 / 2 = 250 kN, compression in the top chord and tension in the bottom chord.
Maximum shear at the supports: V = wL/2 = 10 × 20 / 2 = 100 kN.
With 2 m panels and 2 m depth, the end diagonals are at 45°, so the diagonal force ≈ V / sin 45° = 100 / 0.707 ≈ 141 kN.

The pattern is general. Chord forces follow the bending moment, largest at midspan; web forces follow the shear, largest near the supports. A deeper truss reduces chord forces in proportion. A bottom chord in S355 steel carrying 250 kN of tension needs a net area of roughly 250,000 / 355 ≈ 700 mm² before partial factors and connection effects. The top chord carries the same force in compression, but its size will be set by buckling, not by yield.

What governs steel truss design

Buckling of compression members

Compression chords and struts usually govern. Their capacity depends on slenderness, the ratio of effective length to radius of gyration, so the distance between restraints matters as much as the section. A top chord may be well restrained in the plane of the truss by the web members but only restrained out of plane where purlins or bracing connect, giving a much longer effective length in that direction.

Wind uplift and load reversal

Lightweight roofs can experience net upward wind load. The forces then reverse: the bottom chord, normally in tension, goes into compression. Since it is often restrained only at the supports, it can buckle unless bracing is provided. This reversal is a classic oversight in light roof trusses.

Bracing and stability

A truss is stiff in its own plane but floppy out of it. Roof bracing, purlins, bottom-chord ties and end bracing give overall stability and restraint. During erection, before the bracing is complete, individual trusses are vulnerable and need temporary support.

Deflection and camber

Long trusses are often fabricated with a camber so that they settle to a level or desired profile under dead load. Joint slip in bolted connections adds to deflection and should be allowed for.

Secondary effects

Loads applied between nodes, such as purlins not located at panel points, cause bending in the chord in addition to axial force. Rigid welded joints and eccentricities at nodes also introduce secondary moments, which Eurocode 3 addresses for hollow-section trusses.

Protection and practicalities

Steel trusses in Singapore’s humid climate need durable corrosion protection, usually galvanising or a suitable paint system, and details that avoid trapping water at gussets and bases. Where fire resistance is required by the SCDF Fire Code, trusses can be protected with intumescent coatings or boards, though roof trusses in some building types may not require protection; this depends on the building and its use. Transport limits often set the maximum length of each prefabricated piece, so splice positions should be decided early.

Frequently asked questions

What is a steel truss used for?

Steel trusses are used to span long distances efficiently, most often for roofs of industrial buildings, sports halls and transport terminals, as well as footbridges and transfer structures in tall buildings.

Which steel truss design is most efficient?

For gravity loads, a Pratt arrangement is often efficient in steel because its long diagonals are in tension and its shorter verticals in compression, but Warren trusses with fewer, repetitive members are also widely used.

How deep should a steel truss be?

Depth depends on span, load and stiffness requirements, and since chord forces fall as depth increases, a deeper truss uses lighter chords, so the engineer balances steel weight against the height available.

Why do steel trusses need bracing?

A truss is stiff only in its own plane, so bracing and purlins are needed to hold compression chords against sideways buckling, to resist wind and to keep the roof stable during and after erection.

Are steel truss joints welded or bolted?

Both are used, with members often welded together in the fabrication shop and large pieces bolted together on site, and the choice depends on the section type, the forces and the transport and erection plan.

Planning a long-span roof or assessing an existing truss? Talk to our consultancy or get in touch.