Scaffold stability: ties, bracing and mobile scaffolds

A scaffold that is strong enough can still fall over. Tall, narrow scaffolds tip easily under wind, sideways pushes, uneven loading or a worker leaning on a guardrail. Stability comes from the right height-to-base ratio, bracing that keeps the structure square, ties or guys to the building, outriggers that widen the base, and careful loading. Mobile scaffolds add the hazard of movement. This lesson explains how scaffolds are kept upright and what users must never change.

Why scaffolds tip

A free-standing scaffold resists tipping only by its own weight and the width of its base. Anything that pushes sideways, or moves the center of gravity toward one edge, reduces that margin:

• wind, especially on scaffolds wrapped in sheeting, tarps or debris netting, which act like sails;
• workers pulling or pushing on the structure, for example hauling materials up the side with a rope;
• hoist arms, chutes or brackets that cantilever loads outside the base;
• uneven loading, such as stacking all the materials on one side;
• soft or settling ground under one side;
• removed braces, ties or outriggers;
• vehicles or equipment striking the scaffold.

Height-to-base ratio

The height-to-base ratio compares the height of the scaffold to its minimum base width. OSHA's 1926.451(c)(1) requires supported scaffolds with a height-to-base ratio of more than four to one to be restrained from tipping by guying, tying, bracing or equivalent means. Mobile scaffolds carrying workers while being moved have a stricter limit of two to one under 1926.452(w)(6).

Ties, guys and bracing

Ties connect the scaffold to the building or structure. They resist both inward and outward movement. Guys are cables or rods to ground anchors, used where there is no structure to tie to. Bracing, such as buttress frames, provides stability from the scaffold itself.

Under 1926.451(c)(1)(ii), unless the manufacturer specifies otherwise, the lowest tie is placed no higher than 4 times the minimum base width; vertical ties are repeated at no more than 20 ft for scaffolds 3 ft wide or less, and 26 ft for scaffolds more than 3 ft wide; the top tie is no further from the top than 4 times the base width; and ties are placed at each end and at horizontal intervals of no more than 30 ft. Ties must be attached where horizontal members support both inner and outer legs (c)(1)(i), and to parts of the structure strong enough to take the loads, never to window frames, downpipes or railings.

Sheeting, netting and signs

Enclosing a scaffold with plastic sheeting for winter work, debris netting for public protection or large advertising banners greatly increases the wind load. A sheeted scaffold may need several times as many ties as an open one. Never add sheeting, netting or banners without the approval of the competent person and, usually, an engineer's tie design. Remove or open sheeting before storms if the design requires it.

Never remove ties or braces without authority

Ties and braces are sometimes in the way: a window frame needs fitting, a brace blocks material delivery. Removing them without authority is one of the most dangerous things a scaffold user can do. Only the competent person may alter ties or braces, and only after installing an alternative tie or confirming that the scaffold remains stable. If a tie must come out, a replacement goes in first, nearby.

Mobile scaffolds and rolling towers

Mobile scaffolds are popular indoors because they are quick to move along the work. They also introduce specific hazards: rolling while someone is on them, tipping when a caster drops into a hole or hits an obstruction, and overturning when they are too tall for their base.

• Height: keep within the 4 to 1 height-to-base ratio when stationary unless outriggers or restraint are used, and within 2 to 1 if workers will ride while it is moved (1926.452(w)(6)(ii)).
• Casters: lock casters with positive wheel and swivel locks when the scaffold is stationary (1926.452(w)(2)), and pin or secure caster stems in the legs (w)(9).
• Moving: workers may ride only if the surface is within 3 degrees of level and free of pits, holes and obstructions, the ratio is 2 to 1 or less, outriggers are on both sides if used, power propulsion is no faster than 1 ft per second, and nobody is on any part extending beyond the wheels (1926.452(w)(6)). Manual force should be applied as close to the base as practicable and no more than 5 ft above the floor (w)(3). Many employers simply prohibit riding.
• Surface: use mobile scaffolds only on firm, level, smooth floors. Watch for floor openings, trenches, ramps, cables and edges of slabs.
• Platform: guardrails on all sides, internal access by ladder frame or stair, and hatches that close.
• Overhead: watch for beams, lights, sprinklers and power lines when moving.

Loading for stability

Keep loads within the rated capacity and spread them evenly. Do not stack materials against guardrails, lean ladders against guardrails, or hang loads from the outside of the scaffold. Hoist arms and gin wheels need to be part of the design, with ties near them to take the extra forces. Never use the scaffold as an anchor for fall protection unless the competent person and engineer confirm that it is designed for it.

Types of ties

Ties come in several forms, chosen by the competent person or engineer to suit the building:

• Through ties pass through an opening such as a window and clamp to a tube on the inside face, resisting push and pull.
• Reveal ties wedge a tube between the sides of an opening; they depend on friction and are used only where permitted and checked regularly.
• Anchor ties use bolts or anchors drilled into the building and tested to confirm their capacity.
• Box ties wrap around a column or structural member.
• Raker or buttress braces brace the scaffold from the ground where no tie point is available.

Anchors drilled into masonry or concrete are often proof-tested, with a sample pulled to a set load to confirm that the wall can hold them. A tie is only as strong as the weakest link: the anchor, the coupler and the part of the building it is fixed to.

Stability on uneven ground and slopes

On sloping ground, screw jacks level the scaffold within their limits, and stepped sills or engineered bases deal with larger slopes. The downhill legs carry more of the load when the scaffold leans, so foundations there need extra care. A scaffold that starts slightly out of plumb becomes more unstable as it gets taller, which is why the competent person checks plumb and level at the first lift before building higher.

Wind and stopping work

Wind loads increase with the square of the wind speed, so a gust twice as strong pushes four times as hard. A scaffold that is comfortable in a light breeze can be at its limit in a storm. Site rules usually set a wind speed above which work stops, and the competent person inspects after strong winds before work resumes. Loose items, such as sheeting, tarps, empty buckets and boards, should be secured or removed before a storm so they do not become missiles.

Stability is invisible until it is lost. That is why the rules on ties, ratios, outriggers and casters must be followed every time, even when the scaffold seems solid.