Building Stability and Structural Design

The Stability Standards for Buildings

Building stability refers to the structural integrity of a building, ensuring it can safely withstand its own weight and the various loads applied to it throughout its lifespan. This involves understanding the structure's components, how it behaves under different forces, and the essential principles guiding its design and construction.

Decoding Eurocodes: EN 1993-1-1:2005

The Eurocodes are a set of harmonized technical rules for the structural design of construction works within the European Union. Understanding their notation is crucial:

• EN: Norme Européenne (European Standard)

• 1993: Refers to Eurocode 3, which covers the design of steel structures.

• 1-1: Denotes Part 1-1, which specifies general rules for steel structures.

• 2005: Indicates the year of publication of this specific version.

How a Building Structure Works

The structure of a building, essentially its skeleton or framework, is designed to support and safely transfer all loads to the ground. It is composed of two main parts:

1. Superstructure:
   Definition: The visible part of the building located above ground level.
   Components: Columns, beams, floor slabs, load-bearing walls, and roof.

2. Infrastructure:
   Definition: The part of the building located below ground level, responsible for transferring loads from the superstructure to the soil.
   Components: Footings (isolated, strip), rafts (radiers), and piles.

Actions (Loads) and Solicitations

• Actions (Loads): Forces or influences that act on the structure, tending to change its state of stress or deformation. These can include the self-weight of the structure, loads from people and materials, and environmental forces.

• Solicitations: The internal forces (e.g., moments, shear forces, normal forces) generated within structural elements in response to applied actions.

The Principle of Load Descent

The principle of load descent describes how loads are transferred downwards through a building's structure. Loads are progressively transmitted from the highest elements (e.g., roof) through floor slabs, beams, columns, and walls, eventually reaching the foundations and then the ground. This principle applies to both permanent loads and variable (dynamic) loads.

Requirements of a Structure

A building structure must meet several key requirements:

• Stability: Ability to maintain its equilibrium position under all anticipated loads without collapsing or overturning.

• Safety: Ensuring the protection of occupants and the public from harm.

• Resistance: Ability to withstand applied loads without failure.

• Rigidity: Ability to resist deformation under load, maintaining its shape and preventing excessive deflections.

• Economy: Cost-effective design and construction using appropriate materials and methods.

Classification of Structures by Material

Structures are often classified based on the primary material used for their load-bearing system:

• Reinforced Concrete (RC)

• Timber (Wood)

• Metallic (Steel)

Types of Structural Systems

The choice of a structural system is influenced by climatic, geographical, economic, cultural, and regulatory constraints.

1. Load-bearing Masonry (Economical):
   Description: All exterior walls and some interior walls are load-bearing, providing both support and weather protection.
   Materials: Natural stones, bricks, blocks.

2. Framed Structure (Ossature):
   Description: Load transmission is primarily through a system of columns and beams, with non-load-bearing infill walls.
   Materials: Wood, metal, reinforced concrete.

3. Mixed Systems: Combinations of materials, e.g., masonry + concrete; concrete + steel.

4. Traditional Systems: Earth (rammed earth, adobe), local wood.

Types of Loads Applied to a Building Structure

Loads are the forces that a building structure must resist. They are categorized based on their duration and variability.

A. Permanent Loads (D) or Dead Loads

These loads are constant in magnitude and position throughout the life of the building.

• Examples: Self-weight of the structure (foundations, columns, beams, slabs, walls, roof covering), fixed equipment, permanent partitions.

B. Variable Loads (Q) or Live Loads (Exploitation Loads)

These loads are non-permanent and can vary in intensity, direction, or duration during the building's lifespan.

• Calculation Formula: Typically expressed as Volumetric Weight (kN/m³) x Volume (m³) for distributed loads.

• Types:

  • Live Loads (kN/m²): Weight of people, furniture, and movable equipment.

  • Climatic Loads: Wind loads, snow loads.

  • Construction Loads: Temporary loads applied during the construction process.

  • Thermal Loads: Stresses induced by temperature changes.

C. Accidental Loads (FA)

These are exceptional, short-duration actions produced by rare phenomena, characterized by high intensity and often unpredictability.

• Examples: Earthquakes (seismic loads), explosions, fires, storms, tornados, impacts (e.g., vehicle collisions).

Classification by Mode of Application

Loads are also distinguished by how they are applied to structural elements:

• Punctual Loads: Force concentrated at a single point (e.g., weight of a column on a footing). Unit: N.

• Linear Loads: Force distributed along a line or length (e.g., weight of a partitioning wall on a beam). Unit: N/m.

• Surface Loads: Force distributed over an entire area (e.g., weight of water in a reservoir, snow on a roof). Unit: N/m².

Foundations

The foundation is the critical part of a structure that rests on the bearing ground and transmits all permanent and live loads from the superstructure to the soil. Its role is to ensure stability and protect the entire construction.

Importance of Geotechnical Study

It is strongly recommended to conduct a soil study (geotechnical investigation) before designing and dimensioning foundations. This study optimizes the choice of foundation type, helps reduce costs, and may even recommend relocating the building to a more suitable area if necessary.

Factors Influencing Foundation Choice

• Soil type and properties (e.g., bearing capacity).

• Depth of good bearing soil.

• Ground slope (influences stability and design).

• Presence of groundwater.

• Loads to be transmitted from the structure.

• Hydrogeological conditions.

• Site characteristics (urban, rural, coastal, near railways).

• Available equipment and expertise of the construction company.

• Cost of foundations (an important but not decisive factor).

• Type of structure (e.g., bridge, residential building).

Types of Foundations

1. Shallow Foundations (Fondations Superficielles):
   Depth (P): Typically between 0.50 m and 3 m, used when good bearing soil is relatively close to the surface.
   Types:

   • Isolated Footings (Semelles Isolées): Support individual columns.

   • Strip Footings (Semelles Filantes): Support load-bearing walls or a row of aligned columns.

   • Raft Foundations (Radiers): A large plain concrete slab serving as a single foundation for the entire structure, used when the soil has low bearing capacity or the loads are significant.

2. Deep Foundations (Fondations Profondes):
   Depth (P): Generally m, used when poor quality soil extends to significant depths.
   Types: Piles.

3. Semi-Deep Foundations (Wells/Puits):
   Depth (P): Between 3 m and 8 m, used when good bearing soil is at a medium depth.

Principles for Foundation Stability

A building is in equilibrium under:

• Ascending Actions (S): Forces or pressures exerted by the soil acting upwards on the foundations.

• Vertical Actions (P): Downward loads from the structure (e.g., roof, frame, walls, floors, slabs, foundations).

To ensure stability, two main principles must be applied:

• Applying a safety coefficient to the soil's resistance (determined through geotechnical studies).

• Ensuring sufficient width of the foundation footing. The footing must be dimensioned to distribute loads over an adequate surface area.

These combined principles ensure that the construction can resist the forces it is subjected to, guaranteeing user safety and durability.

Construction Process for Foundations

1. Geotechnical Study: Essential initial step to understand soil conditions.

2. Building Layout (Implantation): Precisely marking the building's outline on the site.

3. Earthworks (Terrassement):

   • Excavating trenches for foundations.

   • Leveling the bottom of the excavations.

   • Placing a layer of blinding concrete (lean concrete) to stabilize the bottom of the trenches and prevent direct contact between reinforcement and soil.

4. Reinforcement Placement: Assembling and positioning of steel bars.

5. Formwork (Coffrage): Setting up temporary molds to shape the concrete.

6. Concreting (Bétonnage):

   • Using appropriate concrete (resistance, consistency), poured continuously to avoid weak points.

   • Vibrating the concrete to eliminate air pockets.

   • Applying curing (watering and protection) to prevent rapid drying.

Recommendation: Never pour concrete on unstable or wet soil. Verify levels and alignments; respect the concrete cover for reinforcements; use suitable concrete; and follow the directives of the civil engineer.

Soubassement (Abutment/Plinth)

The soubassement is an intermediate element located between the ground level and the building's ground floor slab. It has multiple functions:

1. Protective: Shields the construction from humidity (infiltrations) and impacts.

2. Structural (Load-bearing): Helps transmit loads to the foundation.

3. Aesthetic: Contributes to the visual appearance of the facade.

4. Stable Base: Provides a solid base for the erection of load-bearing walls or facades.

Materials for Soubassement

The choice of material depends on the function and environment:

• Reinforced Concrete (BA): Used for high loads and modern constructions.

• Stone: Suitable for low heights.

• Solid Brick: Common in traditional constructions.

• Concrete Blocks: Also used in traditional constructions.

For waterproofing and insulation of the soubassement wall, bituminous coatings, protective membranes, or a peripheral drain are often used.

Types of Soubassement

1. Hérisson (Fill Slab):
   Description: The least expensive option, particularly for flat terrains, where granular fill is compacted for a solid base.

2. Vide Sanitaire (Crawl Space)

:
Description: A raised space (typically 40 to 80 cm) between the ground and the ground floor slab. It avoids extensive excavation but is more costly than a hérisson.

Sous-sol (Basement):
Description: The most expensive option, requiring significant excavation. It allows for additional rooms (garage, utility room, cellar). Requires an optimal drainage system, especially in flood-prone areas.

Floor Slabs (Planchers)

A floor slab is a flat, horizontal structural element that separates the levels of a building. While usually horizontal, it can be sloped or stepped.

Components of a Floor Slab

A floor slab consists of three main parts:

1. Load-bearing Part (Partie Portante):
   Function: Supports the weight of the floor itself, live loads, and transmits them to the supporting structure (beams, walls).
   Materials: Wood, reinforced concrete, steel, or composite materials.
2. Floor Covering (Revêtement):
   Function: Constitutes the finished floor surface, resting on the load-bearing structure.
   Materials: Tiles (ceramic), wooden parquet, various paving, polished concrete, epoxy resin.
3. Ceiling (Plafond):
   Function: Installed beneath the load-bearing element, contributing to aesthetics, and sometimes providing acoustic and thermal insulation. It also limits water penetration.
   Materials: Plaster coating, prefabricated plasterboards, or other materials.

These three combined parts ensure the slab's strength, functionality, stability, and aesthetic appeal.

Functions of a Floor Slab

• Resistance and Stability: Capable of bearing loads and maintaining structural integrity.
• Support and Load Transmission: Distributes loads to the supporting elements.
• Durability: Withstands wear and tear over time.
• Acoustic and Thermal Insulation: Limits noise and heat transfer between levels.
• Fire Resistance: Acts as a barrier against the spread of fire.

Types of Floor Slabs by Construction Method and Materials

1. Hollow-Core Slab (Plancher à Corps Creux):
   Description: A precast floor system.
   Composition:
   • Poutrelle (Joist/Beam): Reinforced concrete or prestressed concrete, supporting the floor's weight.
   • Entrevous (Filler Block): Lost formwork (non-removable).
   • Compression Slab: Minimum 4 cm concrete layer poured over the filler blocks.
   • Metallic Reinforcement: For structural integrity.
2. Reinforced Concrete Slabs (Planchers en Béton Armé):
   Classified by manufacturing and construction methods: cast-in-place, semi-prefabricated, and prefabricated.
   • Solid Slabs (Dalles Pleines): A uniform reinforced concrete plate.
   • Flat Slabs with Drop Panels (Dalle Champignon): A reinforced concrete slab without beams, directly resting on columns with enlarged heads (drop panels).
     • Used for buildings requiring large spans and a flat ceiling.
     • Common in parking lots, industrial halls, and commercial centers.
   • Pre-Slab Floors (Planchers à Prédalle): A semi-prefabricated concrete floor where pre-slabs act as lost formwork and load-bearing elements, onto which a thin layer of concrete is poured on site.
     • Advantages: Reduced formwork and on-site concrete pouring, lighter weight.
     • Limitation: Spans are limited by the resistance of the pre-slabs.

Other Structural Elements of a Building

• Roof (Toiture)
• Swimming Pool (Piscine)
• Renovation and Various Works
• Chains (Chaînages): Vertical, horizontal, and sloped reinforcement bands for structural integrity.
• Walls (Murs): Soubassement, facade, shear walls (refend), gable.
• Columns (Poteaux)
• Beams (Poutres)
• Lintels (Linteaux): Beams over openings.
• Concrete
• Reinforced Concrete
• Slabs (Dalle)
• Paving (Dallage)
• Balcony
• Underground Parts: Earthworks, foundations, sewage systems.
• Finishing Works: Stairs, window sills, renderings (enduit).