Understanding SLS Load Combinations in Eurocode Design: Characteristic, Frequent and Quasi-Permanent Loads Explained Part 1 of 3
- MTS DNC ENERGY CONSULTANTS LIMITED
- 5 days ago
- 5 min read
When engineers design a building structure, the objective is not only to ensure that the structure will not collapse, but also that it remains functional, comfortable and visually acceptable throughout its service life.
A building may be perfectly safe from a structural strength point of view but still experience problems such as:
Excessive deflection
Cracking
Excessive vibration
Water ponding on roofs
Poor user comfort
Damage to finishes
Problems with machinery operation
These issues are assessed using the Serviceability Limit State (SLS).
While the Ultimate Limit State (ULS) checks whether a structure has sufficient strength and stability against failure, the Serviceability Limit State (SLS) checks whether the structure performs satisfactorily during normal operation.
A key part of SLS design is understanding that variable loads do not normally act at their maximum value throughout the entire life of the building.
For this reason, EN 1990 – Eurocode: Basis of Structural and Geotechnical Design introduces different load combinations:
✅ Characteristic combination
✅ Frequent combination
✅ Quasi-permanent combination
Each combination represents a different probability of occurrence and is used for different types of structural checks.
🧱 ULS vs SLS – What Is the Difference?
Structural engineers generally consider two main limit states:
Ultimate Limit State (ULS)
ULS checks structural safety.
The question being answered is:
"Will the structure remain safe under extreme loading conditions?"
ULS considers situations such as:
Structural collapse
Loss of stability
Excessive stress
Failure of structural members
Examples:
Maximum snow load
Maximum occupancy load
Extreme wind loading
The aim of ULS is:
✅ Prevent collapse
✅ Protect life safety
✅ Ensure structural resistance
Serviceability Limit State (SLS)
SLS checks how the building behaves during normal use.
The question being answered is:
"Will the building continue to function properly and provide acceptable comfort during its lifetime?"
SLS checks include:
Deflection limits
Cracking limits
Vibrations
Long-term deformation
Appearance
Comfort of occupants
A floor may be strong enough to pass ULS but still fail SLS if it vibrates excessively or deflects too much.
⚖️ Understanding Variable Actions and Imposed Loads
One of the most common misunderstandings in structural design is the difference between:
Variable actions
Imposed loads
Although these terms are often used interchangeably, they are not exactly the same.
Variable Actions
A variable action is any load that changes in magnitude or location during the life of the structure.
Examples include:
Occupancy loads
Storage loads
Snow
Wind
Thermal actions
Vehicle loads
In Eurocode notation, variable actions are represented by:
Q
The characteristic value is:
Qk
where:
Q = variable action
k = characteristic value
Example:
Snow load:
Qk,snow = 0.75 kN/m²
Imposed Loads
An imposed load is a specific type of variable action associated with the use and occupancy of a building.
Examples:
People
Furniture
Movable equipment
Storage contents
Examples of imposed load categories include:
Category A – Domestic and residential areas
Category B – Office areas
Category C – Areas where people may congregate
Category D – Shopping areas
Category E – Storage areas
Therefore:
All imposed loads are variable actions, but not all variable actions are imposed loads.
For example:
A snow load is a variable action, but it is not an imposed load.
📐 Why Are Variable Loads Reduced in SLS Design?
In the previous article on ULS combinations, we explained that it is unlikely that all variable loads will reach their maximum value simultaneously.
The same principle applies to SLS design, but with an additional consideration:
The maximum characteristic load is not expected to occur very often during the lifetime of the building.
For example:
A typical office floor may have a design imposed load based on a high occupancy scenario.
However, throughout the life of the building:
The office may rarely reach maximum occupancy
Furniture layouts may change
Occupancy levels vary daily
Therefore, the structure normally experiences lower loads than the characteristic value.
Eurocode accounts for this by reducing variable actions using:
ψ combination factors
These factors represent different probabilities of occurrence.
🏗️ Understanding Characteristic, Frequent and Quasi-Permanent Loads
The three main SLS combinations represent different levels of loading frequency.
A simple way to understand them is:
🔴 Characteristic Loads – Rare Maximum Expected Loading
Characteristic loads represent:
The highest expected loads that may occur during the life of the building, but only rarely.
They are not the absolute maximum possible load, but a statistically determined value that has a low probability of being exceeded.
Examples:
A fully occupied office
A heavily loaded storage area
A significant but realistic snow event
Characteristic loads are used when checking:
Potential damage
Structural and non-structural elements
Conditions where a rare event could cause problems
The combination uses:
Permanent loads at full value
The leading variable load at full value
Other variable loads reduced using ψ₀
General expression:
Ed = Gk + Qk,1 + ψ₀,2Qk,2 + ψ₀,3Qk,3
🟡 Frequent Loads – Normal Heavy Usage
Frequent loads represent:
Loads lower than the characteristic value that occur relatively often during the life of the structure.
They represent more realistic repeated loading situations.
Examples:
A normally occupied office
Regular pedestrian movement
Typical equipment operation
Frequent combinations are used for checks such as:
User comfort
Vibration
Operation of sensitive equipment
Repeated loading effects
The combination uses:
Permanent loads at full value
Leading variable action reduced using ψ₁
Other variable actions reduced using ψ₂
General expression:
Ed = Gk + ψ₁,1Qk,1 + ψ₂,2Qk,2 + ψ₂,3Qk,3
🟢 Quasi-Permanent Loads – Long-Term Sustained Loading
Quasi-permanent loads represent:
Loads lower than frequent loads that are expected to be present for most of the building's life.
They represent long-term average loading conditions.
Examples:
Permanent furniture loads
Long-term occupancy effects
Sustained loading affecting creep
They are used for:
Long-term deflection
Creep calculations
Appearance of structures
Permanent deformation
The combination uses:
Permanent loads at full value
All variable actions reduced using ψ₂
General expression:
Ed = Gk + ψ₂,1Qk,1 + ψ₂,2Qk,2 + ψ₂,3Qk,3
📊 Summary of SLS Load Combinations
Combination | Load Level | Frequency | Typical Use |
Characteristic | Highest expected load | Rare | Damage checks, cracking, irreversible effects |
Frequent | Reduced service load | Occurs regularly | Comfort, vibration, machinery operation |
Quasi-permanent | Long-term average load | Present most of the time | Creep, long-term deflection, appearance |
💡 Simple Analogy
Imagine a restaurant.
Characteristic loading
A fully booked restaurant on the busiest night of the year.
This happens occasionally.
Frequent loading
A normal busy evening.
This happens regularly.
Quasi-permanent loading
The average number of customers present over a long period.
This represents the sustained condition.
The structure experiences all three situations during its lifetime.
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
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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.
