Horizontal diagonal brace
A brace laid diagonally in the horizontal plane of a lift, keeping the scaffold from twisting or shifting in plan.
Also called: horizontal brace, plan brace, horizontal bracing, plan bracing, base ledger
A scaffold has to be stable not only when seen from the front but also from above. Look down on a bay: four uprights with ledgers all round. Push hard enough and that rectangle skews out of square. The horizontal diagonal places a slope into this horizontal plane and turns the rectangle into two triangles, so the lift can no longer twist.
On an ordinary facade scaffold the system decks often do this job, because they brace the lift as they hook in. But as soon as lifts stay undecked, loose timber boards are used, or a freestanding tower, a birdcage or a falsework structure is built, separate plan bracing is needed. Without it the structure can rotate under wind or one-sided load, at worst until it collapses.
On frame scaffolds the term often means the longitudinal ledger directly above the base jacks. It links the feet of the frames in the bay that holds the vertical diagonal, so its force is taken cleanly into the base.
Design and variants
A horizontal diagonal is a zinc-coated 48.3 mm tube with a connector at both ends – on rosette or perforated-disc systems these are wedge heads. Its length depends on bay length and width: a 2.57 × 0.73 m bay needs a different brace from a square 2.07 × 2.07 m tower bay. Hence the many combinations, and some manufacturers offer adjustable versions.
For rectangular bays the heads are welded on at an offset so they meet the node at the right angle. A brace made for a square bay will not simply fit a rectangle.
On frame scaffolds the horizontal brace is a straight tube of bay length, fixed with wedges, drop-lock pins or half couplers depending on the system. Where no system part fits, plan bracing can also be made from tube and couplers.
Fitting
- Check in the instructions which lifts and bays need plan bracing – often at least every fifth bay, on towers frequently every tied lift.
- Close the bay with ledgers first so the nodes are fixed.
- Pick the brace for that length and width and hook it in at two opposite corners.
- Drive the wedges until they stop giving; on frames close the pins or tighten the couplers.
- Check the bay is square and nothing is strained.
Typical mistakes and used parts
- Leaving it out because decks are down. Only system decks that the instructions declare as bracing can replace it; loose boards never do.
- Forcing the wrong length into place.
- Loose wedges, which make the bracing useless. Re-driving them is part of every inspection.
- Bent tubes, crushed or opened heads and cracked welds mean the part is rejected.
- Used: straight tubes, free-moving wedges, undamaged heads. Some white rust is normal, deep pitting at the connection is not.
Used horizontal braces are listed under diagonal braces.