Structural Loads and Design Principles (AS/NZS 1170)
Structural design is about verifying safe load paths. This guide breaks down how engineers calculate loads, check spans, and ensure buildings perform well under permanent, imposed, and wind actions.
1. Actions: What a Building Must Resist
In simplified terms, we design for combinations of:
- Permanent (Dead) Actions: self-weight, fixed finishes, masonry, built-in joinery, plant permanently installed.
- Imposed (Live) Actions: people, furniture, movable partitions, storage loads; varies by building classification and use.
- Wind Actions: uplift and lateral forces; often the controlling action for roof tie-down and bracing in low-rise construction.
- Earthquake Actions: typically lower than wind for many low-rise buildings in Australia, but can govern in some regions and for irregular buildings.
- Other Actions: construction loads, ponding on flat roofs, temperature effects, shrinkage and creep in concrete, differential settlement.
Practical takeaway: The “worst case” is rarely a single action. It’s a combination of actions in a prescribed design scenario.
2. Strength vs Serviceability (Why “It’s Strong Enough” Isn’t Enough)
Two separate checks happen in most designs:
- Strength (Ultimate Limit State): does the member have enough capacity without failure?
- Serviceability (SLS): does it deflect, crack, or vibrate excessively in normal use?
Serviceability is where real-world client complaints live: bouncy floors, cracked plaster, doors that stick, tiles that debond, and ponding roofs.
3. Spans: What Controls Them
Span capability isn’t just about the beam size. Common span limiters include:
- Load intensity: A lightly loaded roof beam can span far longer than a heavily loaded floor beam.
- Member stiffness: Deflection depends heavily on stiffness, not just strength.
- Support conditions: Continuous beams behave very differently from simply supported beams.
- Point loads: Posts, concentrated loads (e.g., girder trusses), and stair loads can control the design.
- Vibration: Long-span lightweight floors can feel uncomfortable even if deflection is acceptable.
4. Load Paths: The One Concept That Explains 80% of Site Issues
A load path is the route that forces take to the ground. Failures (and most defects) happen where the load path is interrupted.
- Roof to wall: truss/rafter tie-down must connect into studs or posts designed to take uplift.
- Wall to floor: bearers/joists need correct bearing length, hangers, and blocking where required.
- Floor to foundation: posts must land on designed footings, not “near enough” soil or fill.
Expert tip: When something looks “overdone” on an engineer’s detail (straps, tie-down rods, extra studs), assume it’s part of a load path continuity requirement.
5. Common Site Mistakes (and Why They Matter)
- Substituting members without re-check: “Same depth” doesn’t mean same capacity or stiffness.
- Cutting webs or chords: Trusses and engineered joists are system-designed; cutting is not a minor change.
- Insufficient bearing: Short bearing causes crushing, rotation, and long-term settlement.
- Unplanned penetrations: Large services penetrations near supports can kill shear capacity.
6. Quick Field Checks (Builder-Friendly)
- Verify the member ID/grade matches the schedule (LVL type, steel section, rebar size).
- Confirm bearing lengths and packers are structural and continuous (not random shims).
- Check tie-down continuity from roof to footing (especially at corners and openings).
- Photograph before lining: bracing, straps, hold-downs, and connectors.