Ties and wind
Sheeting or netting area and wind force
- where
- sheeted area (m²)
- length of sheeted face (m)
- height of sheeted face (m)
- sheeting or netting to order (m²)
- laps and fixing allowance (%)
- wind force on the sheeted face (kN)
- wind velocity pressure (kPa)
- force coefficient for sheeting or netting
- wind force per tie (kN)
- façade area per tie (m²)
Explain this equation
Works out the sheeting or debris netting to order and the wind force it puts on the scaffold and on each tie.
Area is length times height, plus an allowance for laps. Wind force equals the velocity pressure times a force coefficient (1.3 for sheeting or netting square to the wind) times the area. A tie carries the force on the area it serves, so the same calculation on the area per tie gives the tie load.
Wind force is pressure times a shape coefficient times area.
Sense check: 210 m² at 0.2 kPa and 1.3 gives about 55 kN on the face; with 16.8 m² per tie each tie takes about 4.4 kN.
Watch out: 0.2 kPa is a working wind. Storm wind at most sites is several times higher, and that governs tie design. Sheeted scaffolds need a design.
Works out the area of sheeting or debris netting and the wind push it puts on the scaffold and each tie.
Enter your values to see the result.
Good to know
Wind force = velocity pressure × force coefficient × area. BS EN 12811-1 Annex A gives a force coefficient of 1.3 for sheeting and netting with wind at right angles to the face (0.1 along the face for sheeting, 0.3 for netting). The 0.2 kPa default is the BS EN 12811-1 working (in-service) wind pressure; the storm design wind for your site from BS EN 1991-1-4 or the TG20 wind map is usually several times higher and is what governs ties. Use the area per tie from your tie pattern.
Common mistake
Adding sheeting to a scaffold designed unsheeted: the wind load and tie forces rise sharply, and TG20 needs a sheeted compliance sheet or a design.
Limits
Flat-face estimate. Ignores shielding, funnelling between buildings, corners and the top lift, where local pressures are higher. Netting coefficients depend on the product’s solidity: use manufacturer data where given. For planning and estimating only. This does not replace TG20:21 compliance (a TG20 compliance sheet or eGuide output) or a scaffold design by a competent person.
Sources
- BS EN 12811-1:2003 Temporary works equipment. Scaffolds. Performance requirements and general design (Table 3 load classes; 5.5 side protection; Annex A cladding force coefficients)
- NASC TG20:21 Good practice guidance for tube and fitting scaffolding (Operational Guide, Design Guide, User Guide and eGuide compliance sheets)
- Health and Safety Authority (Ireland), Code of Practice for Access and Working Scaffolds (sole boards ≥ 1000 cm², ties ≥ 6.1 kN, façade bracing at least every six bays, tower height to least base ≤ 3, toe-boards ≥ 150 mm)
Results are calculations from the published methods and figures shown, and depend on the data you enter. For planning and checking only. Scaffolds must be erected to NASC TG20 or a bespoke design by competent persons, and lifting operations planned under LOLER by a competent person. Never rely on these results instead of a design, a lift plan or manufacturer ratings.