Understanding SLS Load Combinations in Eurocode Design: Characteristic, Frequent and Quasi-Permanent Loads Explained Part 3 of 3
- MTS DNC ENERGY CONSULTANTS LIMITED
- Jul 13
- 5 min read
🧮 Worked Example 1: Office Floor SLS Load Combinations
To understand why different SLS combinations are required, consider a typical office floor.
The floor is designed for:
Permanent Actions
Self-weight of structure, finishes and fixed services:
Gk = 5.0 kN/m²
Variable Actions
Office imposed load:
Qk,office = 3.0 kN/m²
Movable partitions:
Qk,partition = 1.0 kN/m²
For an office area (Category B), typical values are:
Factor | Value |
ψ₀ | 0.7 |
ψ₁ | 0.5 |
ψ₂ | 0.3 |
🔴 Characteristic Combination – Rare Loading
The characteristic combination represents a rare but realistic situation where the leading variable action reaches its maximum expected value.
The formula is:
Ed = Gk + Qk,1 + ψ₀Qk,2
Assuming office occupancy is the leading variable action:
Permanent load:
= 5.0 kN/m²
Office load:
= 3.0 kN/m²
Partition load:
= 1.0 kN/m²
Calculation:
Ed = 5.0 + 3.0 + (0.7 × 1.0)
Ed = 8.7 kN/m²
What does this represent?
This represents a situation such as:
A fully occupied office
Higher-than-normal furniture loading
A temporary busy period
It is unlikely to occur frequently but is possible during the life of the building.
This type of combination may be used when checking:
Potential damage
Structural and non-structural elements
Short-term effects
🟡 Frequent Combination – Normal Repeated Use
The frequent combination represents a loading condition that occurs regularly.
The formula is:
Ed = Gk + ψ₁Qk,1 + ψ₂Qk,2
Calculation:
Permanent load:
5.0 kN/m²
Office load:
0.5 × 3.0
Partition load:
0.3 × 1.0
Therefore:
Ed = 5.0 + (0.5 × 3.0) + (0.3 × 1.0)
Ed = 6.8 kN/m²
What does this represent?
This represents the normal working condition of the office.
For example:
Normal employee occupancy
Typical furniture arrangement
Regular daily operation
This combination is more representative of what occupants experience most of the time.
It is commonly relevant for:
Human comfort
Vibration assessment
Operation of sensitive equipment
🟢 Quasi-Permanent Combination – Long-Term Loading
The quasi-permanent combination represents loads that are expected to remain present for long periods.
The formula is:
Ed = Gk + ψ₂Qk,1 + ψ₂Qk,2
Calculation:
Ed = 5.0 + (0.3 × 3.0) + (0.3 × 1.0)
Ed = 6.2 kN/m²
What does this represent?
This represents the average long-term loading condition.
Examples:
Average occupancy
Typical furniture loads
Long-term effects on the structure
It is commonly used for:
Long-term deflection
Creep calculations
Appearance of the structure
📊 Summary of Office Example
Combination | Purpose | Load Applied | Result |
Characteristic | Rare maximum service condition | Full leading variable load | 8.7 kN/m² |
Frequent | Regular service condition | Reduced variable loads | 6.8 kN/m² |
Quasi-permanent | Long-term average condition | Further reduced loads | 6.2 kN/m² |
🏠 Worked Example 2: Roof Loading With Snow and Maintenance Loads
Consider a flat roof designed for:
Permanent Actions
Roof structure, insulation and finishes:
Gk = 2.5 kN/m²
Variable Actions
Snow load:
Qk,snow = 0.75 kN/m²
Wind load:
Qk,wind = -1.0 kN/m²
"Wind may act either as pressure or suction depending on the roof geometry and the design check being considered. For roof uplift checks, wind suction is treated as an opposing action, while for gravity load checks wind pressure may increase the applied load."
Roof maintenance load:
Qk,maintenance = 0.40 kN/m²
The roof is located below 1000m altitude.
Typical snow combination factors:
Snow factor | Value |
ψ₀ | 0.5 for snow and 0.6 for other |
ψ₁ | 0.2 |
ψ₂ | 0.0 |
For roof maintenance loading, the applicable value depends on the roof category and National Annex.
🔴 Characteristic Combination – Snow Governing
The characteristic combination assumes snow is the leading variable action. Formula:
Ed = Gk + Qk,snow + ψ₀Qk,wind + ψ₀Qk,maintenance
Calculation:
= 2.5 + 0.75 + (0.6 × 1.0) + (0.6 × 0.40)
= 2.5 + 0.75 + 0.84
Ed = 4.09 kN/m²
or with wind as the leading variable action. Formula:
Ed = Gk + Qk,wind + ψ₀Qk,snow + ψ₀Qk,maintenance
Calculation:
= 2.5 + 1 + (0.5 × 0.75) + (0.6 × 0.40)
= 2.5 + 1 + 0.375 + 0.24
Ed = 4.115 kN/m²
What Does This Represent?
This represents a realistic but rare situation:
Significant snow accumulation or wind positive pressure
Possible maintenance activity
🟡 Frequent Combination – Normal Roof Use
The frequent combination represents more regular loading.
Formula:
Ed = Gk + ψ₁Qk,snow + ψ₂Qk,wind + ψ₂Qk,maintenance
Calculation:
= 2.5 + (0.2 × 0.75) + (0.0 × 1) + (0.0 × 0.40)
= 2.5 + 0.15
Ed = 2.65 kN/m²
or
Ed = Gk + ψ₁Qk,wind + ψ₂Qk,snow + ψ₂Qk,maintenance
Calculation:
= 2.5 + (0.2 × 1) + (0.0 × 0.75) + (0.0 × 0.40)
= 2.5 + 0.2
Ed = 2.70 kN/m²
🟢 Quasi-Permanent Combination – Long-Term Effects
Formula:
Ed = Gk + ψ₂Qk,snow + ψ₂Qk,wind + ψ₂Qk,maintenance
Ed = 2.5 kN/m²
🧱 Why Are Roof Loads Treated Differently?
A roof is different from an occupied floor.
People do not normally occupy roofs continuously.
Roof maintenance activities are:
Occasional
Short duration
Dependent on weather conditions
Therefore, the probability of simultaneous maximum:
Snow loading
Wind loading
Maintenance loading
is extremely low.
Eurocode combinations reflect this.
🏗️ Which SLS Combination Should Be Used?
A common question is:
"Which SLS combination should an engineer use?"
The answer depends on the behaviour being checked.
The designer must select the combination appropriate for the limit being verified.
📐 Deflection Checks
Deflection is one of the most common SLS checks.
Two types are normally considered:
Immediate Deflection
Short-term deformation caused by applied loads.
Often checked using:
Characteristic combination
Frequent combination
depending on the material Eurocode and design requirement.
Long-Term Deflection
Long-term effects include:
Concrete creep
Shrinkage
Permanent deformation
The quasi-permanent combination is normally important because it represents sustained loading.
🏢 Vibration and User Comfort
People are sensitive to movement.
A floor may be structurally safe but uncomfortable if it vibrates excessively.
Examples:
Office floors
Sports halls
Footbridges
Laboratories
Frequent combinations are often relevant because they represent repeated normal use rather than rare maximum occupancy.
🧱 Cracking and Damage to Non-Structural Elements
Cracking can affect:
Concrete durability
Waterproofing
Finishes
Partitions
Façades
The appropriate SLS combination depends on:
Structural material
Crack limitation requirements
Applicable Eurocode
For reinforced concrete structures, crack control requirements are addressed in EN 1992.
⚙️ Machinery Operation
Some buildings contain sensitive equipment:
Examples:
Laboratories
Pharmaceutical facilities
Manufacturing buildings
Excessive movement can affect:
Equipment accuracy
Product quality
User operation
Frequent combinations are often relevant because they represent realistic operating conditions.
🌧️ Ponding and Roof Performance
Flat roofs require careful consideration of:
Deflection
Drainage
Water accumulation
Excessive deflection can create low points where water collects.
This can increase loading and create a progressive problem.
The relevant combination depends on the design situation and applicable standard.
References
EN 1990 – Eurocode: Basis of Structural and Geotechnical Design
EN 1991-1-1 – Actions on Structures: Densities, Self-weight and Imposed Loads
EN 1991-1-3 – Actions on Structures: Snow Loads
Relevant National Annex
Need Professional Help? Contact the Experts!
For Building Services Design – whether it’s HVAC, plumbing, or civil engineering – reach out to Nexus M&E Design for expert solutions tailored to your project needs.
If you require a technical assessment, BER rating, or assistance with SEAI grants, get in touch with the professionals at MTS DNC Energy Consultants for comprehensive guidance and support.
Disclaimer:
The content provided in this post is for informational purposes only and should not be construed as professional engineering, architectural, or surveying advice. While every effort is made to ensure the accuracy and reliability of the information presented, it is not a substitute for a thorough, site-specific inspection or the expertise of a qualified professional. For detailed guidance on foundation issues, structural integrity, or repairs, always consult with a licensed engineer, architect, or surveyor. The authors and publishers are not responsible for any damages or losses resulting from the use or reliance on this information.
