Civil's Guide
Timber Span Tables
Max spans for domestic floor joists
| Cross section (mm x mm) | Imposed load 1.5 \(kN/m^2\) – Dead load 0.5 \(kN/m^2\) | ||||||
|---|---|---|---|---|---|---|---|
| Spaced at 400mm | Spaced at 450mm | Spaced at 600mm | |||||
| C16 | C24 | C16 | C24 | C16 | C24 | ||
| 47×95 | 1.77 | 2.05 | 1.68 | 1.94 | 1.47 | 1.70 | |
| 47×120 | 2.40 | 2.67 | 2.28 | 2.57 | 2.01 | 2.32 | |
| 47×145 | 2.89 | 3.22 | 2.78 | 3.10 | 2.52 | 2.81 | |
| 47×170 | 3.38 | 3.77 | 3.25 | 3.62 | 2.95 | 3.29 | |
Preliminary Sizes for Timber Roofs
| Structural form, Description and materials | Pitch | Span range (m) | Approximate proportions |
|---|---|---|---|
| Truss rafter, solid timber and punched metal plate fasteners | 15-40 | 3-15 | \(h\approx \frac{L}{75} to \approx \frac{L}{100}\) |
| Bolted timber truss with purlins and intermediate common rafters and ceiling ties | 15-45 | 5-20 | \(h\approx \frac{L}{60} to \approx \frac{L}{80}\) |
| Trussed girder, Warren type etc. Solid Timber, glulam, LVL | Less than 3 | 30-80 | \(h\approx \frac{L}{10} to \approx \frac{L}{15}\) |
Characteristic properties of Timber
| Stength properties \(N/mm^2\) | Class | C16 | C20 | C24 | C27 | D40 | D50 |
|---|---|---|---|---|---|---|---|
| Bending | \(f_{mk}\) | 16 | 20 | 24 | 27 | 40 | 50 |
| Tension Parallel | \(ft_{0k}\) | 8.5 | 11.5 | 14.5 | 16.5 | 24 | 30 |
| Tension Perpendicular | \(ft90_{mk}\) | 0.4 | 0.4 | 0.4 | 0.4 | 0.6 | 0.6 |
| Compression Parallel | \(f_{c0k}\) | 17 | 19 | 21 | 22 | 27 | 30 |
| Shear | \(f_{vk}\) | 3.2 | 3.6 | 4 | 4 | 4.2 | 4.5 |
| Stiffness Properties \(kN/mm^2\) | |||||||
| Mean modulus of elasticity parallel bending | \(E_{m,0,mean}\) | 8 | 9.5 | 11 | 11.5 | 13.0 | 14.0 |
| 5 percentile modulus of elasticity parallel bending | \(E_{m,0,k}\) | 5.4 | 6.4 | 7.4 | 7.7 | 10.9 | 11.8 |
| Mean modulus of eleasticity perpendicular | \(E_{m,90,mean}\) | 0.27 | 0.32 | 0.37 | 0.38 | 0.87 | 0.93 |
| Mean Shear modulus | \(G_{mean}\) | 0.5 | 0.59 | 0.69 | 0.72 | 0.81 | 0.88 |
| Density \(kg/m^3\) | |||||||
| 5 percentile density | \(\rho_{k}\) | 310 | 330 | 350 | 360 | 550 | 620 |
| Mean density | \(\rho_{mean}\) | 370 | 400 | 420 | 430 | 660 | 740 |
Span Tables
| Example of Use | Limit State | Instantaneous/Final | Recommended limits for beams spanning between 2 supports |
|---|---|---|---|
| Cracking of plasterboard, glass,ceramics etc. in roofs, ceilings or floorss | Irreversible | Final | \(w_{fin}\leq\frac{L}{250}\) where \(w_{fin} = deflection due to permanent + imposed loads + creep. |
| Apperance of roofs and ceilings with no attatched brittle finishes | Reversible | Final | \(w_{fin}\leq\frac{L}{150}\), where \(w_{fin}\) = deflection due to permanent + imposed loads + creep |
| In fire, at end of required period of fire resistance, where protecttion depends on attatched plasterboard, unless proved by test | Accidental | Instantaneous | \(w_{inst}\leq\frac{L}{150}\) where \(w_{inst}\) = instantaneous deflection due to permanent + imposed loads |