Temporary Works Calculator

Under BS 5975, a Temporary Works Coordinator (TWC) must be appointed for all but the simplest temporary works. They ensure designs are checked, installations are inspected, and loading is monitored. On smaller sites, the site manager often fulfils this role with appropriate training.

Safe working load depends heavily on extension. Per BS EN 1065 manufacturer data (Speedy Hire / Acrow): a Size 1 prop carries about 34 kN closed (≈1.6m) but only 12 kN at full extension (≈3.12m); a Size 2 runs 39 kN down to 11 kN; a Size 3 runs 35 kN down to 7 kN. Capacity can fall around 65% from closed to fully extended, so always read the de-rated SWL for your actual prop height from the prop plate or the manufacturer table — do not assume the closed-length figure.

For vertical (wall/column) formwork, BS 5975:2019 clause 17.4.2 gives a conservative maximum lateral pressure of 25 × the depth of pour (kN/m²) — i.e. full hydrostatic head at a concrete weight of about 25 kN/m³. The full head applies to fast pours and self-compacting concrete. A slower pour lets the lower concrete stiffen before the top is placed, capping the pressure below hydrostatic; CIRIA Report 108 gives the less conservative method that accounts for pour rate, temperature and mix. This figure is the lateral thrust on wall formwork — it is not the vertical load on slab-soffit props, which carry the slab self-weight (25 kN/m³ × thickness) plus the BS 5975 construction imposed load (0.75 kN/m² access, 1.5 kN/m² in work or storage areas).

BS 5975 does not set fixed kN or height triggers — it grades each design by a check category. Category 0 (standard/proprietary solutions, e.g. props to a manufacturer table) and Category 1 (simple works) can be checked by another competent person in the same firm; Category 2 (more complex) needs an independent checker; Category 3 (complex, unusual, or high-consequence) needs an independent third-party check, often with full recalculation. As a site rule of thumb, anything beyond standard propping or low staging — heavy or tall falsework, shoring next to excavations, large pours, or anywhere failure could seriously harm people — should be designed and checked by a temporary works engineer. The Temporary Works Coordinator decides the category.

Temporary works must be inspected: before loading/use, daily when loaded, after any impact or adverse weather, and before each concrete pour. Formal weekly inspections should be recorded. Props must be checked for plumb, pin security, and base plate bearing.