Fundamentals
Loads and load combinations explained (ULS and SLS)
Dead, live, wind and seismic loads, and how codes combine them into ULS and SLS load combinations. Worked floor example and the combinations for six design codes.
Before a structure can be analysed, you have to decide what loads it carries and how those loads can act together. Design codes give both: the size of each load, and a set of load combinations with factors that make the design safe. This tutorial explains the load types, how Struxt3D generates them, and the combinations it uses for each code family.
The load types
| Load | Symbol | What it is | In Struxt3D |
|---|---|---|---|
| Dead (permanent) | D, G | Self-weight of the structure, plus finishes, screeds, ceilings and services that are always there. | Case Dead: self-weight of members, slabs and walls from their sections and density, plus Finishes and services on floors, roofs and decks. |
| Live (imposed) | L, Q | People, furniture, stored goods, maintenance on roofs, crowds on footbridges. They come and go. | Case Live: the Floor use load on floors, the code's roof load on roofs, the footbridge load on decks. |
| Wind | W | Pressure and suction on the faces of the building, drag on open frames. | Cases Wind +X, Wind −X, Wind +Y, Wind −Y. See wind loads. |
| Earthquake | E | Inertia forces when the ground shakes. | Cases Seismic X and Seismic Y by response spectrum. See seismic design. |
| Earth / fluid pressure | H | Soil or water pushing on walls. | Case Earth, added by the retaining wall template. |
Live loads by floor use
Each code gives a characteristic imposed load for each kind of floor. Struxt3D offers four floor uses; the values (kN/m²) it takes from each code are:
| Use | Eurocode | ASCE 7 | KDS | NBCC | AS/NZS | NSCP |
|---|---|---|---|---|---|---|
| Office | 3.0 | 2.4 | 2.5 | 2.4 | 3.0 | 2.4 |
| Residential | 2.0 | 1.92 | 2.0 | 1.9 | 1.5 | 1.9 |
| Retail | 4.0 | 4.79 | 4.0 | 4.8 | 4.0 | 4.8 |
| Storage | 7.5 | 6.0 | 6.0 | 6.0 | 5.0 | 6.0 |
| Roof | 0.4 | 0.96 | 1.0 | 1.0 | 0.25 | 1.0 |
These are typical defaults. National annexes, local amendments and special uses (plant rooms, car parks, assembly) can need other values.
Why loads are factored
Loads are uncertain, and so are material strengths. Limit state design handles this with two kinds of check:
- Ultimate limit state (ULS) — the structure must not collapse. Loads are multiplied by load factors greater than one (1.2 to 1.6 for variable loads), and materials are weakened by resistance factors. The structure must carry the factored load.
- Serviceability limit state (SLS) — the structure must work in normal use: deflections, cracking, vibration. Loads are taken at their characteristic (unfactored) values.
Not every load reaches its maximum at the same time. A full office live load during a design storm is unlikely, so codes reduce the accompanying loads in a combination (EN 1990 calls these ψ factors). Some combinations use a low dead load factor (0.9 or 1.0) because weight can help: it holds a structure down against wind uplift and overturning.
Worked example: an office floor
A 200 mm concrete slab, 1.5 kN/m² finishes and services, office use, Eurocode:
With ASCE 7 (L = 2.4 kN/m²): 1.2 × 6.4 + 1.6 × 2.4 = 11.5 kN/m², and 1.4D = 9.0 kN/m², so 1.2D + 1.6L governs. Codes differ in factors and in resistance factors, so the final member sizes come out close.
The combinations Struxt3D uses
Struxt3D builds the strength combinations from the code you choose in the materials step, for the load cases the model has. W stands for each wind direction in turn (four combinations for each W rule); E for the earthquake, applied as ±(EX + 0.3EY) and ±(0.3EX + EY) when both directions are on. Every code also gets D + L (service) for deflections.
| Code | Strength combinations | Source |
|---|---|---|
| Eurocode | 1.35D + 1.5L · 1.35D + 1.5L + 0.9W · 1.35D + 1.05L + 1.5W · 1.0D + 1.5W · D + 0.3L + E | EN 1990 eq. 6.10, ψ0 = 0.7 (imposed), 0.6 (wind), ψ2 = 0.3; seismic eq. 6.12b |
| AISC / ACI | 1.4D · 1.2D + 1.6L · 1.2D + 1.0W + L · 0.9D + 1.0W · 1.2D + 1.0E + L · 0.9D + 1.0E | ASCE 7-22 §2.3.1 (strength-level wind and seismic) |
| KDS | 1.4D · 1.2D + 1.6L · 1.2D + 1.0L + 1.3W · 0.9D + 1.3W · 1.2D + 1.0L + 1.0E · 0.9D + 1.0E | KDS 41 10 15 |
| CSA | 1.4D · 1.25D + 1.5L · 1.25D + 1.4W + 0.5L · 0.9D + 1.4W · D + 0.5L + E | NBCC 2020 Table 4.1.3.2.-A |
| AS/NZS | 1.35D · 1.2D + 1.5L · 1.2D + W + 0.4L · 0.9D + W · D + 0.3L + E | AS/NZS 1170.0 §4.2.2, ψc = 0.4, ψE = 0.3 |
| NSCP 2015 | 1.4D · 1.2D + 1.6L · 1.2D + 1.0W + f1L · 0.9D + 1.0W · 1.2D + 1.0E + f1L · 0.9D + 1.0E | NSCP 2015 203.3.1; f1 = 1.0 for live loads over 4.8 kPa, else 0.5 |
Earth pressure (H) takes the dead load's factor, except in NSCP where it has its own (1.6, and 1.4 with 1.4D). Combinations with wind or earthquake only appear when that load is switched on.
The wind speed you enter matters for which factor is right. Codes that use a 3-second gust at strength level (ASCE 7, NSCP, AS/NZS) put 1.0 on W; codes that use a mean or basic speed (Eurocode, KDS, NBCC) use 1.3 to 1.5. See wind loads.
The envelope
With five to twenty combinations, you want the worst value of each force at each point — the envelope. In the results, Envelope (tagged ENV) shows the largest and smallest values over all strength combinations; design checks look at every combination and report the one that governs. Displacements show the service combination instead (tagged SLS).
Try it in Struxt3D
- Choose a template with floors, such as Multi-storey frame, and go to What is it made of?. Pick the Design code — this sets the combinations.
- In How is it held and loaded?, choose the Floor use. Each button shows the live load in kN/m² from your code.
- Set Finishes and services (kN/m²), switch Wind and Earthquake on or off. The note at the bottom says how many strength combinations the code gives, plus D + L for deflections.
- Use Show on the model to see each load case drawn on the 3D model.
- Press Run analysis. In the results, the Load case list groups Load cases (LC1, LC2…), Combinations (C1, C2…) and the envelope.
Summary
- Dead loads are permanent, live loads vary; wind and earthquake act sideways.
- ULS combinations factor loads up for strength; SLS uses characteristic loads for deflection.
- The low dead load factor (0.9) matters for uplift and overturning.
- The envelope is the worst of all strength combinations at each point.