Bridge terms explained: a truss glossary with labelled diagrams

Bridge terms for the parts of a truss

Point at any triangle in a truss bridge and every side has a name. Bridge terms start with the parts, and the diagram below labels them on a 12 m Warren truss.

TensionCompression

Truss. A structure built from straight members joined at their ends so that the members form triangles. A triangle cannot change shape without a member changing length, which is why trusses are stiff for their weight.

Member. One straight bar of the truss, joined at both ends. In the idealised truss each member carries a force along its own length and nothing else.

Chord. One of the long members along the top or bottom edge of the truss. The top chord and bottom chord together carry the bending of the whole bridge, one in compression and the other in tension under a normal load.

Web member. Any member between the chords: a diagonal or a vertical. The web carries the shear, the tendency of one part of the bridge to slide past the next.

Joint (node). A point where members meet. Truss analysis treats a joint as a frictionless pin, so loads and supports act there and the members meeting at it must balance.

Panel. The section of truss between two neighbouring sets of web members. A 12 m bridge with four panels has a panel length of 3 m.

Diagonal. A web member set at an angle. Its direction decides whether it pulls or pushes, which is the difference between a Pratt and a Howe truss.

Vertical (post). An upright web member. In a Pratt truss the verticals carry compression, and in a Howe truss they carry tension.

End post. The inclined member at each end of a truss that carries the support reaction up into the top chord. It is the most heavily compressed member in many designs.

Gusset plate. A flat plate, usually steel, that connects several members at a joint. In a card or wood model, a small patch of card or wood glued over the joint does the same job.

Deck. The surface that carries traffic, together with the beams under it. The deck passes its load into the truss at the joints.

Span. The distance between the two supports. Every preset in the truss bridge designer is a 12 m span.

Forces and behaviour: bridge terms for what members do

A member does only two things, so the bridge terms for forces are few. The cases are worked out in the guide to truss analysis.

Tension. A pulling force that tends to lengthen a member. Tension is written as a positive number by the usual sign convention, and a member in tension fails by yielding or by pulling out of its joint.

Compression. A pushing force that tends to shorten a member. Compression is written as negative, and a slender member in compression usually fails by buckling rather than by crushing.

Axial force. A force along the length of a member. A pin-jointed truss member carries axial force only.

Yield. The stress at which steel begins to stretch permanently. The designer on this site uses 250 MPa.

Buckling. Sideways bowing of a member in compression, which happens suddenly and well below the yield load when the member is slender. For a pinned column the Euler load is P_cr = π²EI/L². The tension and compression page works through it.

Slenderness. How long a member is compared with the width of its cross-section. The more slender a compression member is, the sooner it buckles. Tension members do not buckle at all.

Deflection. How far a bridge sags under load. A bridge can be strong enough and still deflect more than a design allows.

Utilisation. The force in a member divided by its capacity, usually as a percentage. The member with the highest utilisation fails first.

Supports and loads

Reaction. The force a support pushes back with to keep the structure still. For a symmetric load on a simply supported truss, each support takes half the total.

Pin support. A support that stops the joint moving in both directions but lets the truss turn. It supplies two reaction components.

Roller support. A support that stops only vertical movement, letting the joint slide along the abutment as the bridge changes length. It supplies one reaction component and lets the bridge change length as the temperature changes.

Simply supported. Resting on one pin support and one roller support. This is the standard set-up for a single-span truss.

Determinate. A truss whose forces can all be found from equilibrium alone. The test is m + r = 2j: members plus reaction components equal twice the number of joints.

Load path. The route a load takes through the structure, from the deck into the joints, along the members and down into the supports.

Factor of safety. Capacity divided by the expected load. In the designer, the same idea appears as design margin: failure load divided by applied load. The factor of safety page shows the arithmetic.

Truss layouts

Four layouts cover most of the bridge terms a student meets for whole structures, and the four truss types compared lay them side by side.

TensionCompression

Warren truss. Diagonals alternating in direction, forming equilateral or isosceles triangles, with no verticals in the basic form. It was patented in England in 1848.

Pratt truss. Diagonals slope so that they pull, and the verticals push, except the hip vertical beside the inclined end post. It was patented in 1844 by Thomas and Caleb Pratt.

Howe truss. The reverse of the Pratt: diagonals push and verticals pull. William Howe patented it in 1840.

K truss. Verticals split at mid-height, with pairs of diagonals meeting at the split to form a K. The shorter members buckle less easily.

Model-building terms

Glue joint. The bond between two model members. In small models the joints, not the sticks, are the usual place where a build gives way.

Jig. A template or fixture that holds the pieces in position while the glue sets, so that both sides of a model come out the same.

Efficiency. In model contests, the load a bridge holds divided by the weight of the bridge itself. A lighter model that holds the same load is more efficient.