Lattice girder (bridging beam)

A steel or aluminium truss that spans a gap where no uprights may reach the ground, carrying the scaffold built above it.

Also called: bridging girder, ladder beam, truss beam, beam, bridging truss

Sometimes no upright may stand under the scaffold: the yard entrance has to stay passable, the shop on the ground floor needs its door, or a canopy is in the way. That is what the lattice girder is for. It spans a gap in which no frames or standards run down to the ground and carries the scaffold continued above. The load of the upper lifts travels through the girder sideways into the neighbouring uprights and from there via the base jacks into the ground.

It also serves as a bearer for fans and temporary roofs or for cantilevers.

Design and sizes

A lattice girder is a small truss: two parallel tubes – top and bottom chord – linked by vertical posts and inclined struts. The chords usually have the common 48.3 mm scaffold tube diameter, so couplers fit. On standard girders the chord centres are often around 40 cm apart; heavy-duty girders are much deeper.

• Lengths: based on two bays, for example 4.14 m, 5.14 m or 6.14 m (about 13.6, 16.9 and 20.1 ft); shorter and longer versions exist.
• Material: galvanised steel or aluminium. Aluminium is far lighter but needs attention in salty air, because it can corrode unnoticed under steel couplers.
• Connections: on frame systems, end plates or welded half couplers on the top chord fit the spigots or frame uprights, while the bottom chord is fixed with special or right-angle couplers. Rosette systems have girders with welded wedge heads. A bridging beam is essentially the same – usually a girder with ready-made connections for one system.

Building a bridge

1. Plan position, girder length and extra measures from the instructions, approval or a structural calculation. Bridges usually sit directly under the first or second lift.
2. Build the scaffold either side of the gap up to girder height, align and tie it.
3. Hook or clamp inner and outer girders in place – top chord first, then bottom chord at every upright.
4. Lay temporary boards across the girders.
5. Restrain the top chords against sideways movement: by ties to the building or by plan bracing of tube and couplers, typically at mid-span and at the bay centres.
6. Fit transoms, lay decks, set frames or standards and complete the side protection.
7. Add the extra diagonals and ties required in the bridging zone.

What goes wrong

• A free compression chord. The top chord is in compression and wants to buckle sideways; missing restraint is one of the most common bridging faults.
• Overlooked loads beside the gap. The uprights left and right carry much more, so check their sole boards and jacks carefully.
• Home-made girders. Use only beams with known, verified capacity.
• Used: on steel look for rust at the nodes, on aluminium for corrosion where couplers sat and for dents in the chords. A bent girder cannot be repaired.

More on bridging doorways and canopies is in getting the scaffold past obstacles. Used girders are listed under lattice girders.