The use of lumber for formwork and falsework must be in accordance with the standard CSA O86 – Engineering design in wood. The Canadian Wood Council (CWC) Wood Design Manual should be used as a reference guide. (CSA S269.1 Clause 6.8.1)
Modification factors
The CSA S269.1, Clause 6.8.3, specifies the following modification factors for wood elements use for formwork and falsework.
| Service condition: Normal service condition is dry. | \(K_S=1\) |
| Load duration factor: For columns, normal duration. For all other member and connection, short duration. | \(K_D=1\) \(K_D=1.15\) |
Further more, the CSA O86 specifies the following modification factors, depending on the assembly of the panel and the sizes of the lumbers.
For joisted panels and stud wall
CSA O86, clause 6.4.4, System factor, \(K_H\), for visually graded lumber
| Case 1: Applies to bending, shear and axial properties if the system consist of more than three parallel members spaced 610mm apart maximum. For example, 4×6 joists supporting a concrete slab sheathed with a lightly nailed plywood. | \(K_H=1.1\) |
| Case 2: Applies to bending and shear properties if the sheathing of the system described by case 1 is nailed to the joists with 2in long nails min, spaced 150mm max along the edge of the sheathing panels and spaced 300mm in the field of the panels. | \(K_H=1.4\) |
Size factor for visually graded lumber
| Lumber | \(K_{Zb}\) and \(K_{Zv}\) |
| 2×4 | 1.7 |
| 2×6 | 1.4 |
| 2×8 | 1.2 |
| 2×10 | 1.1 |
| 2×12 | 1.0 |
| 4×4 | 1.7 |
| 4×6 | 1.5 |
| 4×8 | 1.3 |
Bearing – Compression perpendicular to grain, \(K_{Zcp}\)
From CSA O86, Clause 6.5.6.4
For width over depth ratio equal or less than 1 \(K_{Zcp}=1.00\)
For width over depth ratio equal or more than 2 \(K_{Zcp}=1.15\)
Interpolation is allowed for intermediate ratios
| Orientation | Lumber | Nominal \(\frac wd\) | Actual \(\frac wd\) | \(K_{Zcp}\) |
| On edge | 2×4 | 2.0 | 2.3 | 1.15 |
| On edge | 2×6 | 3.0 | 3.7 | 1.15 |
| On edge | 2×8 | 4.0 | 4.8 | 1.15 |
| On edge | 2×10 | 5.0 | 6.2 | 1.15 |
| On edge | 2×12 | 6.0 | 7.5 | 1.15 |
| On edge | 4×4 | 1.0 | 1.0 | 1.00 |
| On edge | 4×6 | 1.5 | 1.6 | 1.075 |
| On edge | 4×8 | 2.0 | 2.1 | 1.15 |
| Flat | 1×6 | 0.2 | 0.1 | 1.00 |
| Flat | 2×4 | 0.5 | 0.4 | 1.00 |
| Flat | 2×6 | 0.3 | 0.3 | 1.00 |
Axial – Compression parallel to grain, \(K_{Zc}\)
Clause 6.5.5.2.4
Treated wood
| Treated lumber are rarely or never used for temporary construction such as formwork and falsework. | \(K_T=1\) |
Slenderness ratio
Clause 6.8.4.2 for bracing member maximum allowable slenderness ratio is 50.