Timber is a fantastic, sustainable building material, but it comes with a structural quirk: it creeps. Over time, under a sustained load, a timber beam will continue to deflect long after the initial load was applied. If you don't calculate this correctly according to Eurocode 5 (EN 1995-1-1), you will end up with sagging floors and cracked plasterboard.
The Dreaded $k_{def}$ Factor
The most common pitfall for junior engineers is failing to separate instantaneous deflection ($w_{inst}$) from final deflection ($w_{fin}$). In steel design, deflection is immediate. In timber design, you must apply the deformation factor, $k_{def}$, to account for moisture content and load duration.
For a permanent load ($G$), the final deflection is calculated as:
For variable loads ($Q$), the formula is slightly more complex, as it relies on the quasi-permanent combination factor ($\psi_2$):
Choosing the Right Service Class
The value of $k_{def}$ is entirely dependent on the Service Class (1, 2, or 3) and the material type (Solid timber, Glulam, LVL, etc.).
- Service Class 1: Heated indoor environments (e.g., internal floor joists). $k_{def}$ is typically around 0.6.
- Service Class 2: Covered exterior or unheated indoor (e.g., roof trusses). $k_{def}$ increases to 0.8.
- Service Class 3: Fully exposed to weather. $k_{def}$ jumps significantly, often over 2.0.
Using a Service Class 1 deformation factor for a Service Class 2 roof beam will result in a gross underestimation of the final roof sag. Always verify your environmental conditions before pulling values from the EC5 tables.
Total vs. Net Deflection
Finally, remember that EN 1995-1-1 requires you to check both $w_{inst}$ (to ensure the floor doesn't feel overly "bouncy" under live loads) and $w_{net,fin}$ (the total final deflection minus any pre-camber). Bypassing the instantaneous check is a fast track to unhappy clients.