Steel Gusset Plates: Design, Failure Modes and Checks

Wherever several members meet at a joint, something has to gather their forces and pass them from one to another. In steel trusses, bracing systems and many bridges that job falls to a flat plate known as a gusset. Steel gusset plates look like the least interesting part of a structure, but they carry the full force of every member framing into them, and when they are undersized the consequences can be catastrophic.

This article explains what gusset plates do, how engineers check them, the failure modes that matter, and what the 2007 collapse of the I-35W bridge in Minneapolis taught the profession about these small but critical components. Timber gusset plates exist too, as plywood or punched metal plates in roof trusses, but the focus here is structural steel.

What a gusset plate does

In a truss, members are arranged so that they carry mostly axial force: tension or compression. At each node, the members’ centrelines are ideally arranged to meet at a single point so that no secondary bending is introduced. A gusset plate is the physical device that makes this possible. Each member is bolted or welded to the plate, and the plate transfers force from one member to the others through in-plane tension, compression and shear.

The same component appears in vertical bracing, where a diagonal brace connects to a beam-column junction; in horizontal bracing under roofs; in steel trusses for roofs and long-span floors; and in the main trusses of bridges. In a truss bridge, gusset plates at the main nodes may be large, multi-layered and heavily bolted or riveted.

The plate is part of the load path, not an accessory. A gusset that yields, tears or buckles disconnects the members it joins, and in a determinate truss the loss of a single node can lead to collapse.

Failure modes engineers check in steel gusset plates

A gusset plate has to be checked for each way it can fail. The principal modes are:

Yielding of the gross section — the plate stretches excessively across a section near the end of a tension member.
Fracture of the net section — the plate tears across a line of bolt holes, where material has been removed.
Block shear (block tearing) — a block of plate is torn out along a combination of a tension plane and shear planes around a bolt group.
Bolt bearing and tear-out — the plate deforms or splits locally around individual holes.
Buckling — the plate’s free edge or the zone ahead of a compression member buckles out of plane.
Shear on critical sections — sections through the plate between members must carry the combined forces passing across them.

The fastener group and welds themselves must also be checked, and connection eccentricities can add bending to the plate. In Eurocode practice, steel connection design follows EN 1993-1-8, published locally as SS EN 1993-1-8; our page on Eurocode 3 outlines the wider steel code.

The Whitmore section

A member force does not spread uniformly across the whole width of a gusset. It enters through the bolt group or weld and fans out gradually. In 1952, R. E. Whitmore published tests on a scale model of a truss joint and proposed a simple rule that is still widely used: the effective width of plate resisting the force is bounded by lines spreading at 30° from the first row of fasteners to the last row.

For a bolt group with an outer gauge width g between the outermost bolt lines and a length L from first to last bolt row, the Whitmore effective width is approximately:

w_w = g + 2 L tan 30°

For example, with two bolt lines at 100 mm gauge and four rows at 70 mm pitch (L = 210 mm), w_w = 100 + 2 × 210 × 0.577 ≈ 342 mm. The tension yield resistance of the plate on that section is then the effective width × plate thickness × yield strength, divided by the relevant partial factor, and part of the width is discounted where it extends beyond the edge of the plate or into an adjacent member. The Whitmore method is an engineering idealisation; later research has shown that stress redistributes once the plate begins to yield, but it remains a practical and widely accepted basis for design.

Buckling of gusset plates in compression

Where a compression member frames into a gusset, the plate ahead of it behaves like a short column. A common approach treats the Whitmore section as the column cross-section, with an unbraced length taken as the distance from the end of the member to the nearest supporting edge, and an effective length factor chosen to reflect how the plate is restrained. Thin plates with long free edges are particularly vulnerable.

In bracing systems designed for seismic energy dissipation, gusset detailing becomes more specialised: plates are sometimes designed to allow the brace to buckle out of plane by providing a deliberate clear distance for hinge formation. That level of detail is unusual in Singapore, where seismic actions are low, but it shows how much the plate’s behaviour depends on its geometry rather than its thickness alone.

Practical detailing helps. Keeping free edges short, stiffening long edges with angles or plates, and keeping the gusset compact all improve buckling resistance, as explained more generally in our article on buckling.

Lessons from the I-35W bridge collapse

On 1 August 2007, the I-35W highway bridge over the Mississippi River in Minneapolis collapsed during the evening rush hour, killing 13 people. The US National Transportation Safety Board found that the probable cause was the inadequate load capacity of gusset plates at the U10 nodes of the main deck truss, resulting from a design error in the original 1960s design. Some gusset plates had been specified at about half the thickness they needed.

The board also found that the bridge had become heavier through later modifications, including added deck concrete, and that on the day of the collapse construction materials and equipment were stockpiled on the deck. Inspectors had photographed bowing of the gusset plates in earlier years without recognising its significance, partly because gusset plates were generally assumed to be stronger than the members they joined.

The investigation changed practice. Gusset plates on existing truss bridges were reassessed, guidance was issued for load rating them, and inspection regimes began to look specifically for distortion and corrosion of plates. For any engineer, the lesson is general: connections must be checked with the same rigour as members, and changes to a structure’s loading over its life must be traced back to every component in the load path.

Detailing and fabrication considerations

Good design of steel gusset plates is as much about detailing as calculation. Plate edges should be kept clear of the member ends so that fasteners can be installed and inspected. Bolt holes need adequate edge and end distances. Welded gussets need access for welding and inspection, and the choice between bolted and welded connections affects fabrication, site work and quality control. Corners where water and debris collect should be avoided or drained, since corrosion at section loss locations is a common defect in older steelwork.

Gusset plates should also be sized with the whole structure in mind. Where member forces are large, a thicker plate may be lighter overall than a thin plate needing many stiffeners. Where connections are repetitive, standardising plate thickness and bolt patterns simplifies fabrication and reduces the chance of errors on site.

Frequently asked questions

What are steel gusset plates used for?

They connect several members at a joint, transferring axial forces between truss chords and diagonals, bracing members and beam-column junctions. They are found in roof and floor trusses, building bracing systems and the main trusses of steel bridges.

What is the Whitmore section?

It is the effective width of a gusset plate assumed to resist a member force, bounded by lines spreading at 30 degrees from the first to the last row of fasteners. It was proposed by Whitmore in 1952 from model tests and is used to check tension yielding and compression buckling of the plate.

Why did gusset plates cause the I-35W bridge collapse?

The NTSB found that some gusset plates at the U10 nodes were about half the required thickness due to an original design error. Added dead load from earlier modifications and heavy construction loads on the day of the collapse pushed the undersized plates past their capacity.

How thick should a gusset plate be?

There is no single answer. Thickness follows from the forces in the connected members, the bolt or weld arrangement, the effective width, the unsupported free-edge lengths and buckling, and is checked against yield, fracture, block shear, bearing and stability. A structural engineer must design it for the specific joint.

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