Understanding Eurocode Design Situations and Structural Actions

Posted on Mar 15, 2025 in Civil Engineering, Transportation, and Urban Services

Design Situations According to Eurocode Zero

Design situations are sets of physical conditions representing the real conditions occurring during a certain time interval for which the design will demonstrate that relevant limit states are not exceeded.

• Transient design situation: A design situation that is relevant during a period much shorter than the design working life of the structure and which has a high probability of occurrence. A transient design situation refers to temporary conditions of the structure, of use, or exposure, e.g., during construction or repair.
• Persistent design situation: A design situation that is relevant during a period of the same order as the design working life of the structure. Generally, it refers to conditions of normal use.
• Accidental design situation: A design situation involving exceptional conditions of the structure or its exposure, including fire, explosion, impact, or local failure.

Definition and Classification of Limit States

Limit states are states beyond which the structure no longer fulfills the relevant design criteria.

• Ultimate Limit States (ULS): States associated with collapse or with other similar forms of structural failure. They generally correspond to the maximum load-carrying resistance of a structure or structural member (Structural failure, Instability).
• Serviceability Limit States (SLS): States that correspond to conditions beyond which specified service requirements for a structure or structural member are no longer met (Vibrations, Cracking).

Effects of Actions on Structural Members

Actions on a structure, like forces and loads, create effects on the elements, causing internal stresses such as compression, tension, bending, shear, and torsion. These stresses can lead to deformations, like displacements or rotations. If they are too high, they can cause failures. Additionally, the loads must be properly distributed to maintain the structure’s equilibrium and stability. In short, the actions affect both the individual elements and the structure as a whole.

Classification of Actions on Buildings

• Action (F):
  • Set of forces (loads) applied to the structure (direct action).
  • Set of imposed deformations or accelerations caused, for example, by temperature changes.
• Effect of action (E): Effect of actions on structural members or on the whole structure.
• Permanent action (G): Action that is likely to act throughout a given reference period and for which the variation in magnitude with time is negligible, or for which the variation is always in the same direction (monotonic) until the action attains a certain limit value.
• Variable action (Q): Action for which the variation in magnitude with time is neither negligible nor monotonic.
• Accidental action (A): Action, usually of short duration but of significant magnitude, that is unlikely to occur on a given structure during the design working life.
• Seismic action (A_E): Action that arises due to earthquake ground motions.
• Geotechnical action: Action transmitted to the structure by the ground, fill, or groundwater.

Examples of Combination of Actions for ULS (STR/GEO)

• Persistent: Designing a residential building foundation under normal use conditions.
• Transient: Designing a temporary shoring system to support an excavation during construction.

Serviceability Criteria

• Floor stiffness
• Differential floor levels
• Storey sway
• Building sway
• Roof stiffness

Determinants of Load-Bearing Capacity

The load-bearing capacity of a structural element is determined by:

1. Material properties.
2. Size and shape of the element.
3. Support conditions.
4. Type and distribution of loads.
5. Safety factors and environmental conditions.

Flat Roof Section Components

This flat roof section consists of:

1. Waterproof membrane (black and white layer): Protects against water infiltration (e.g., EPDM, TPO, PVC, or bitumen).
2. Thermal insulation (yellow layer): Reduces heat loss (e.g., XPS, PIR, or mineral wool).
3. Vapor barrier (thin black-lined layer): Prevents moisture from entering the insulation.
4. Structural support (bottom gray layer): The base of the roof (e.g., concrete, steel, or wood).

Types of Foundations

Foundations are classified into:

1. Shallow Foundations:
   • Placed near the surface, suitable for stable soil. Types include strip, slab, pad, and raft foundations.
2. Deep Foundations:
   • Extend deep into the ground to reach stronger soil or rock. Types include piles and caissons.

The choice depends on soil conditions and load requirements.

Determining Design Values of Timber Strength

Timber strength design values are determined through testing, which measures the material’s performance under specific conditions. These tests assess properties like bending, compression, shear, and tension. The results are then adjusted with safety factors to account for variables like moisture content, defects, and variability in timber quality. The adjusted values are used in design codes to ensure safe and reliable performance.

Determining Final Deflection of Timber Beams

The components of deflection resulting from a combination of actions are shown in the Figure, where:

• w_c is the precamber (if applied);
• w_inst is the instantaneous deflection;
• w_creep is the creep deflection;
• w_fin is the final deflection;
• w_net,fin is the net final deflection.

Functions of Shear Walls

The main functions of shear walls are:

1. Resist lateral loads: Counteract wind and earthquake forces, transferring them to the foundation.
2. Increase rigidity: Reduce horizontal movements and deformations.
3. Prevent collapse: Absorb seismic energy and protect the structure.
4. Distribute loads: Help evenly distribute forces.
5. Provide additional support: Can also carry vertical loads.