Ties and wind
Tie count and tie pattern check (area per tie)
- where
- façade area of the scaffold (m²)
- scaffold length (m)
- scaffold height (m)
- ties required
- maximum façade area per tie (m²)
- area per tie in your pattern (m²)
- horizontal tie spacing (m)
- vertical tie spacing (m)
- ties in your pattern
- pattern check (1 or less passes)
Explain this equation
Works out how many ties a scaffold face needs and checks that your tie pattern keeps within the maximum area each tie may carry.
Wind pushes and sucks on the scaffold face, and each tie carries the share from the area around it. The TG20 compliance sheet gives the maximum area per tie for the configuration, sheeting and site. The face area divided by that, rounded up, is the least number of ties. Your pattern's spacing across times spacing up is the area each tie actually serves; if that is no more than the allowed area the pattern passes.
Tie load is proportional to the area each tie supports.
Sense check: 210 m² at 32 m² per tie needs at least 7 ties; a 4.2 × 4 m pattern gives 16.8 m² per tie (13 ties), comfortably within the limit.
Watch out: sheeting and netting increase the wind load a lot and reduce the area allowed per tie. Ties must also be tested.
Checks a tie pattern against the maximum façade area per tie and estimates how many ties and anchors to order.
Enter your values to see the result.
Good to know
Take the allowed area per tie (or the tie pattern itself) from the TG20:21 compliance sheet or eGuide for your height, load class, wind zone and cladding. The 32 m² default is the old BS 5973 figure for unsheeted tube and fitting scaffolds with removable ties and is only a placeholder: TG20:21 often needs closer ties, and sheeting or netting greatly increases tie demand. Example: ties at every second 2.1 m bay on every second 2 m lift give 4.2 × 4.0 = 16.8 m² per tie. NASC TG4 asks for anchors to be proof tested to 1.25 × the working load (7.6 kN for a 6.1 kN tie) on at least 5% of the anchors, minimum three, spread through the job; check the current TG4 edition.
Common mistake
Using an unsheeted tie pattern and then sheeting or netting the scaffold, which can more than double the wind load on each tie.
Limits
Area-based count only; it does not place ties or check tie capacity. Ends, corners, the top lift and loading bays need extra ties. Reveal ties should be kept to a minority of ties. 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
- NASC TG20:21 Good practice guidance for tube and fitting scaffolding (Operational Guide, Design Guide, User Guide and eGuide compliance sheets)
- BS 5973:1993 (withdrawn) tie frequencies for tube and fitting scaffolds, as quoted in IOSH forum discussion: unsheeted 32 m² per removable tie, 40 m² non-removable
- NASC TG4:19 Anchorage systems for scaffolding, 6.2 (proof test to 1.25 × working load, at least 5% of anchors and minimum 3; superseded by TG4:25)
- 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)
- HSE, Scaffolding: design, TG20 standard configurations, competence (CISRS) and inspection
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.