Scaffold types and components

A scaffold is a temporary structure that supports workers, tools and materials at height. Unlike a ladder, it gives a stable, guarded platform where people can work with both hands for long periods. Scaffolds range from a single rolling tower in an office to multi-story systems wrapped around a building for months. This lesson introduces the main types of scaffold, the names of their parts and what each part does, so that you can recognize a scaffold that is complete and one that is not.

Supported and suspended scaffolds

Scaffolds fall into two families.

• Supported scaffolds stand on the ground or a structure. Their legs, frames or posts carry the load down to base plates and sills. Most construction scaffolds, rolling towers and bracket scaffolds are supported scaffolds.
• Suspended scaffolds hang from above on ropes or cables, from roof outriggers, davits or parapet clamps. Swing stages, multi-point suspended platforms and boatswain's chairs are suspended scaffolds. Lesson 12 introduces them at an awareness level.

Common types of supported scaffold

• Frame scaffold, also called fabricated frame or tubular welded frame scaffold: prefabricated welded steel or aluminum end frames joined by cross braces. Quick to erect and very common for masonry, painting and general work.
• Tube-and-coupler or tube-and-clamp scaffold: individual tubes joined with couplers or clamps at any angle. Very flexible, used around complex shapes and in industrial plants, but it needs skilled erectors and, in Ontario, engineering above 10 m.
• System scaffold, also called modular scaffold: vertical posts with rosettes, cups or wedges at fixed intervals, into which horizontal ledgers and diagonal braces lock. Fast, strong and adaptable, common on large industrial and commercial projects.
• Mobile scaffold or rolling tower: a frame or system scaffold on locking casters, used indoors and on flat surfaces. Lesson 10 covers its special rules.
• Bracket scaffold: platforms supported on brackets attached to a wall or structure, common in residential framing and siding.
• Pump jack scaffold: platforms raised on vertical poles by a foot-operated jack, used for siding and roofing on light structures.
• Ladder jack scaffold: planks supported by brackets hooked onto two ladders, limited to light work at low heights.
• Mast climbing work platform: a powered platform that climbs a mast tied to the building, used for façades. It has its own standards and training.

Parts of a frame scaffold

• Mudsill or sill: a timber plank or pad under the base plates that spreads the load over soft or uneven ground.
• Base plate: a flat steel plate at the bottom of each leg that transfers the load to the sill. Every leg needs one.
• Screw jack or adjustable base: a threaded leg extension used to level the scaffold. It may only be extended within the manufacturer's limits.
• End frame: the welded frame that forms each end of a frame scaffold bay.
• Cross brace: diagonal braces between frames that keep the scaffold square and stiff. Missing braces are a common defect.
• Coupling pin and lock: pins that join frames vertically, with locks or pins to stop uplift.
• Platform: planks, decks or boards that form the working surface.
• Guardrail system: top rail, midrail and toe board on the open sides and ends.
• Access: an attached ladder, a stair tower or built-in ladder frames.
• Ties, guys and braces: connections to the building or structure that keep a tall scaffold from tipping.

Parts of system and tube-and-coupler scaffolds

System scaffolds use standards (the vertical posts), ledgers or runners (horizontal members along the length), transoms or bearers (horizontal members across the width that support the platform) and diagonal braces. Tube-and-coupler scaffolds use the same names, plus couplers: right-angle couplers that join tubes at 90 degrees, swivel couplers for other angles, sleeve couplers to join tubes end to end, and putlog couplers. Mixing components from different manufacturers or systems is generally not allowed unless a competent person, and often an engineer, confirms that they are compatible.

Load classes

Scaffolds are designed for a duty or load class. OSHA's non-mandatory Appendix A to Subpart L uses light-duty (25 lb per square foot), medium-duty (50 lb per square foot) and heavy-duty (75 lb per square foot) classes, with the scaffold able to support its own weight and at least 4 times the maximum intended load. Light duty suits workers with tools only, such as painting; medium duty suits plastering and light masonry; heavy duty suits stone and brick masonry with stockpiled material. The rated load is shown on the manufacturer's data or the qualified person's design.

Engineered scaffolds

OSHA requires scaffolds to be designed by a qualified person and constructed and loaded in accordance with that design (1926.451(a)(6)). Some configurations need a registered professional engineer, for example tube-and-coupler scaffolds and fabricated frame scaffolds over 125 ft (1926.452(b)(10) and (c)(6)). Engineered scaffolds come with drawings that show the layout, ties, loads and erection sequence, and they must be built exactly as drawn.

Choosing a scaffold type

Choosing a scaffold depends on the height, length and shape of the work, the loads, the ground, the duration, access for the public, and the skills of the erection crew. A competent scaffold supplier or erector usually helps choose and design the system. The user should know what the scaffold is rated for, how to access it, who inspects it and what not to change.

Specialty scaffold types

Beyond the common types, several specialty scaffolds appear on construction and industrial sites:

• Birdcage scaffolds: wide, multi-bay scaffolds that cover a large area under a ceiling, used for work on high ceilings in halls, atriums and theaters.
• Hanging or suspended-from-structure scaffolds: tube-and-coupler scaffolds hung from overhead steel, used in industrial plants and under bridges. They are engineered.
• Cantilever and outrigger scaffolds: scaffolds that project from a building or structure where the ground below is not available.
• Chimney and silo scaffolds: specialized systems for tall cylindrical structures.
• Shoring towers: these look like scaffolds but are temporary supports for formwork and structures, not work platforms. They are designed and used under different rules and must never be used as access scaffolds unless designed for both purposes.

Aluminum compared with steel components

Scaffold components are made of steel or aluminum. Steel is heavier but stronger and more resistant to rough handling. Aluminum is lighter, which helps manual erection and mobile towers, but it is more easily dented and must not be mixed with steel components unless the manufacturer allows it, because the two materials have different strengths and can corrode where they touch. Aluminum planks and decks are common on both. Components are designed as part of a system, and the manufacturer's data sheet gives the allowable loads for each.

Scaffold drawings and data sheets

Users rarely see the scaffold drawings, but they should know that they exist and what they control. The manufacturer's data sheet shows the allowable leg loads, bay sizes, platform ratings and tie requirements for standard configurations. An engineer's drawing shows a specific scaffold: plan, elevations, tie layout, load class, foundation details and erection notes. Anything not shown on the drawing, such as an extra bracket, a hoist arm or sheeting, needs the engineer's approval before it is added.

When in doubt about any component, ask the competent person before using the scaffold. A scaffold is a system, and every part has a job.