Timber combination sets and strength design cases
Load combinations are automatically divided into groups (called timber combination sets) in order to create strength design cases with appropriate load duration factors (or time effect factors if working to NDS LRFD) for use in the Tedds calculation.
Note: For timber beams, deflection design
cases can also be created. These are described elsewhere, see: Timber beam deflection checks - US and Timber beam deflection checks - EC.
Timber combination sets
Each timber combination set is created based on the load type occurring in the combinations according to the hierarchy defined in the table below.
| Timber combination set | Load type included (EC) | Load type included (US) |
|---|---|---|
| Seismic set | Seismic | Seismic |
| Wind set | Wind | Wind |
| Temperature set | Temperature | Temperature |
| Snow set | Snow | Snow |
|
Imposed set (EC only) / Live set (US only) |
Imposed, Imposed (Other), Roof Imposed | Live, Live (Other), Roof Live |
| Settlement set | Settlement | Settlement |
| Dead set | Dead, Slab Wet | Dead, Slab Wet |
Note:
- Dead load type includes ‘self weight’ and ‘slab dry’ load cases
- Snow load type includes all types of snow and ice loads; volumes, drifts, ice
- Seismic load type includes ELF and RSA
As load combinations are included in a set, only the remaining load combinations are considered in creating additional sets.
As an example, the following four load combinations:
- LC1: Dead
- LC2: Dead + Temperature
- LC3: Dead + Live
- LC4: Live + Wind
would result in the following four timber
combination sets:
- Wind set: LC4
- Temperature set: LC2
- Live set: LC3
- Dead set: LC1
The factor associated with load duration (Load duration factor - NDS ASD and EC, Time effect factor - NDS LRFD) is typically the same for all combinations in a timber combination set. If this is not the case the critical value will be used. i.e. the shortest time duration for the load duration factor (NDS ASD, and EC), or largest value for the time effect factor (NDS LRFD).
You can generate envelopes that correspond to the active timber combination sets so that the design forces can be easily reviewed. For details, see: Generate timber design envelopes.
Strength design cases
Timber combination sets relate directly to strength design cases in Tedds. The maximum member forces from all of the combinations in a timber combination set will create at least one design case in Tedds. When a member sees both tension and compression within a timber combination set, two design cases are created for those sets.
As an example, if a member had timber combination sets for Seismic, Wind, Live and Dead, and the member saw both tension and compression under seismic and wind sets, six design cases would be created as follows:
- Seismic: Tens
- Seismic: Comp
- Wind: Tens
- Wind: Comp
- Live
- Dead
Note: For glulam members designed to NDS, more design cases will be created when
either tension and compression or positive and negative moment forces are seen
across a member or group in a timber combination set.