Scaffold Tower Stability and Ties
Height to base ratio against the 4 to 1 limit, the maximum height at your base, wind pressure on the tower, the stability factor with and without ties, and the tie schedule.
A 24 ft tower on a 5 ft base is 4.8 to 1, over the 4 to 1 free-standing limit, so it needs outriggers, ties, or a shorter tower. At 30 mph the wind load on a 24 ft by 12 ft face is 2.3 psf, which is 664 lb of push, and a tower that free-standing fails at 30 mph is tied by 4 ft vertical and 18 ft horizontal.
How the math works
- The 4 to 1 ratio (OSHA 1926.451(d)(12)): a scaffold must not be taller than 4 times the minimum base dimension unless it is restrained. Outriggers extend the base and support a 3 to 1 tower in the OSHA outrigger provision, and a tower tied to a permanent structure at 4 ft vertical and 18 ft horizontal (Cal/OSHA 4520(b)) is the common 8 to 1 configuration. The ratio is measured to the least base dimension, horizontal.
- Wind pressure (ASCE 7-16 velocity pressure): q = 0.00256 x V squared in pounds per square foot, so 25 mph is 1.6 psf, 30 mph is 2.3 psf, 40 mph is 4.1 psf, 50 mph is 6.4 psf, and 70 mph is 12.5 psf. A scaffold is not a solid wall, so ASCE 7 §29.5 applies a solidity factor to the gross face: about 0.4 for open frames, 0.6 with the planks and guardrails on, and 1.0 with a tarp, a debris net, or a sheeted face, and ASCE 7 §29.5.2 sets a 25 mph minimum design wind speed for a scaffold.
- Overturning: the wind force (pressure x projected area) acts at mid-height, so the overturning moment is force x height / 2, and the resisting moment is the dead weight x half the base width. Wind stability for a scaffold is commonly checked at a factor of 3, and OSHA 1926.451(d)(1) requires a scaffold capable of supporting its own weight plus 4 times the maximum intended load.
- Ties (OSHA 1926.451(d)(12), Cal/OSHA 4520(b)): ties within 2 ft of the top, at intervals not over 10 ft vertically and 18 ft horizontally, with the 4 ft vertical by 18 ft horizontal schedule on a tied tower, and a tie at the ends. Anchors are rated, installed in material that can carry the load, and not into a veneer, a block wall with no bond beam, or a gutter bracket.
- Moving and wind (OSHA 1926.451(d)(12)): the tower is moved by rolling at the base with the decks cleared, the casters unlocked and the brakes set after the move, the tower plumb, and no moving in a wind over 25 mph. No climbing on the frames to gain height, and a 1 in 12 slope on a rolling tower is the practical limit.
How tall can a scaffold tower be without ties?
Four times the least base dimension: a 5 ft base gets a 20 ft tower, and a 6 ft base gets 24 ft. That is the OSHA 1926.451(d)(12) free-standing limit. Outriggers extend the base to support a 3 to 1 tower, and a tied tower at 4 ft vertical by 18 ft horizontal is the 8 to 1 configuration most towers actually run.
How often do you tie a scaffold tower?
Within 2 ft of the top, then at intervals not over 10 ft vertically and 18 ft horizontally, with a tie at the ends, and the Cal/OSHA 4520(b) schedule is 4 ft vertical by 18 ft horizontal. Tie into a structure that can take the pull: a bond beam, a column, a wall with rated anchors, not a gutter, a downspout, or a brick veneer.
What wind speed stops scaffold work?
Over 25 mph is the practical stop for a mobile tower: do not move it, and the tower with a sail (tarp, net, insulation board) starts to load. The 0.00256 x V squared formula says a 40 mph day is 4.1 psf, which on a 24 ft by 12 ft face is 1,180 lb of push, and that is the tower on the point of tipping.
Do scaffold outriggers count as ties?
They count as base restraint, not as building ties. Outriggers extend the effective base width, which is how a 5 ft bay becomes a 9 ft base for a 3 to 1 tower, and they must be pinned at the frame level, deployed on the ground the tower will stand on, and checked with the casters. A tower with outriggers and no building ties is a wide-base tower, not a tied tower.