Scaffolds: Ties, Duty and Plank Spans

A scaffold tips before it breaks

A supported scaffold is a tall, light frame on a narrow base, and the first way it fails is not a tube snapping but the whole frame overturning. OSHA's rule therefore sets a proportion rather than a height: once a scaffold stands more than four times as tall as its narrowest base width — outrigger supports counted, where they are fitted — it must be restrained by guys, ties, braces or an equivalent. A frame 5 feet (1.52 m) wide reaches that at 20 feet (6.1 m); a frame half as wide reaches it at half the height.

The rule then says where the restraint goes. The first ties sit at the horizontal member closest to the four-to-one height. They repeat up the face at horizontal members no more than 20 feet (6.1 m) apart for a scaffold 3 feet (0.91 m) wide or less, and no more than 26 feet (7.9 m) apart for a wider one, until the highest is within the four-to-one height of the top. Along the face they go at each end and at intervals of no more than 30 feet (9.1 m), measured from one end. Because every tie sits on a horizontal member, the real interval is the largest whole number of lifts that fits inside the rule's figure, and the count is the number of levels times the number of positions.

The pattern is a default. The same paragraph allows the scaffold manufacturer's recommendations instead, a designed scaffold follows its design, and a scaffold that is sheeted, netted or carries a hoist takes wind and loads the default was never meant for. In the UK a tube-and-fitting scaffold is tied to the NASC's TG20 guidance or a design, neither of which the site reproduces.

Duty is a load spread over the whole platform

A scaffold is built for a duty, and the duty is a load per unit of platform area. OSHA's scaffold specifications define light, medium and heavy duty as 25, 50 and 75 pounds per square foot (1.2, 2.4 and 3.6 kN/m²) applied uniformly over the entire span area. BS EN 12811-1 defines six service load classes, from 0.75 to 6.0 kN/m² (about 16 to 125 psf). Comparing a job with either is a matter of adding up the people and materials on one bay and dividing by the area of its platform.

The comparison is an average, and that is its weakness. A pallet of blocks puts most of its weight on the boards directly under it, which is why BS EN 12811-1 gives every class a concentrated load and a partial-area load as well as the uniform one, and why a bay whose average sits comfortably inside its class can still be overloaded where the stack stands. Separately, OSHA's rule requires each scaffold component to support its own weight and at least four times the maximum intended load without failure — a margin built into the parts, not an allowance to load the platform beyond its duty.

Plank spans: the cube of the thickness

A plank bridging two supports behaves as a beam, and its stiffness rises with the cube of its thickness. That is why OSHA's scaffold specifications give a full-thickness 2 × 10 plank a longer span than a dressed one of the same nominal size: 2 inches (51 mm) against about 1½ inches (38 mm) makes the thicker plank more than twice as stiff, and the table allows it 10, 8 and 6 feet (3.05, 2.44 and 1.83 m) at light, medium and heavy duty against 8 and 6 feet (2.44 and 1.83 m) for the dressed plank at light and medium duty. The table lists no heavy-duty span for the dressed plank, and a 1¼ × 9 inch plank only 4 feet (1.22 m) at medium duty.

The table is non-mandatory. It is offered as one way of meeting the rule, it applies only to solid sawn planks graded and stamped by a recognised grading association or agency, and a platform built to it must still not deflect more than 1/60 of its span under load. The rule also governs the ends: a plank passes at least 6 inches (15 cm) beyond the centre of its support unless it is cleated, overhangs by no more than 12 inches (30 cm) — 18 inches (46 cm) for a platform longer than 10 feet (3.05 m) — unless the overhang is designed or guarded, and overlaps only over a support, by at least 12 inches (30 cm), unless the planks are fastened.

Planking the full width

OSHA requires each platform to be fully planked between the front uprights and the guardrail supports, with no gap wider than 1 inch (2.5 cm) between planks or at the uprights. The number of planks across follows from the platform's width and the plank's actual width, with every gap held to that limit: a platform 5 feet (1.52 m) wide takes six dressed 2 × 10 planks, which are 9¼ inches (235 mm) wide, where five would leave gaps over 2 inches (51 mm).

What none of these pages decides

Each page applies a published rule: a tie pattern, a set of duty ratings, a span table. None of them designs a scaffold. The force in a tie, the capacity of its anchor in the wall behind it, the bracing, the effect of sheeting in the wind and the bearing of the ground under the base plates belong to the scaffold's manufacturer, its designer and the competent person who inspects it — and in the UK to TG20 or a design. So each page reports the count or the comparison, names the rule it came from, and stops there.