Scaffold Tie Spacing and Height-to-Base Ratio

THE 4-TO-1 RATIO IS A THRESHOLD RATHER THAN A DESIGN.

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It is the point at which a scaffold can no longer be relied on to stand on its own, and everything above it depends on ties. The first tie therefore goes at or BELOW that height, and a scaffold erected past the ratio with its first tie somewhere higher has an unbraced length below it that is exactly the failure geometry -- the tie pattern above is irrelevant to a column that has already gone. OUTRIGGERS WORK BY WIDENING THE BASE, which is the denominator, so they raise the free-standing height directly and proportionally. A five foot wide frame is limited to twenty feet free-standing; outriggers taking the effective base to ten feet double it to forty. That is often cheaper and faster than tying, and it is why outrigger frames exist -- but the arithmetic has to be run, because a scaffold that still exceeds the ratio with outriggers on needs both. SHEETING CHANGES THE PROBLEM ENTIRELY. A scaffold covered in netting or shrink wrap becomes a sail, and the lateral load on the ties can multiply several times over. A tie pattern adequate for open frames does not carry it, and wrapping an already-erected scaffold without re-evaluating the ties is a recognized cause of collapses. The flag here is a reminder, not a wind calculation: the re-evaluated pattern comes from the manufacturer or a qualified person. AND THE TIE ITSELF HAS TO GO TO SOMETHING. A tie anchored to a window frame, a gutter, a downspout, or a piece of architectural trim is not resisting a lateral load, and that is a common finding -- the tie pattern is correct on paper, every tie is present, and every one of them is anchored to something that cannot hold it. Each tie has to reach structure capable of the load, and a tie pattern correct on the drawing and anchored to cladding is a scaffold with no ties at all. A ratio check and a tie count at spacings the reader supplies. IT IS NOT A SCAFFOLD DESIGN. Tie spacings come from the manufacturer's instructions and the applicable OSHA requirements for the scaffold type and height, and they vary; the tie's own capacity and the anchor's capacity are separate and are usually what governs. It does not compute wind load, sheeted or open; it does not address the foundation, the leg loads, the planking, the guardrails, or the access, each of which has its own requirement; and it does not cover suspended, mast climbing, or system scaffolds, whose rules differ. Scaffolds are erected and altered under the supervision of a competent person, and one is required here. OSHA 29 CFR 1926 Subpart L, the scaffold manufacturer's instructions, and the competent person govern.

height-to-base ratio = height / the minimum base dimension, with outriggers substituting their effective base; the maximum free-standing height is the limit ratio times that base, so the first tie goes at or below it; tie rows and ties per row follow the entered spacings.

The OSHA 1926.451(c)(1) height-to-base rule by name -- a supported scaffold whose height exceeds four times its minimum base dimension must be restrained from tipping. Tie SPACINGS are entered from the manufacturer's instructions and the applicable requirements; the tie's own capacity and its anchorage are separate and usually govern. Sheeting multiplies the lateral load and requires a re-evaluated pattern. OSHA 29 CFR 1926 Subpart L, the scaffold manufacturer's instructions, and the competent person govern.

One ratio and two divisions; no manufacturer tie table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: scaffold_height_ft, base_width_ft, outrigger_base_ft, max_ratio, vertical_tie_spacing_ft, horizontal_tie_spacing_ft, scaffold_run_ft, sheeted, effective_base_ft, height_to_base_ratio, bare_ratio, max_free_standing_ft, max_free_standing_bare_ft, tie_rows, ties_per_row, tie_count, base_to_eliminate_ties_ft

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