👷🏻‍♀️Intro to Civil Engineering Unit 7 – Structural Analysis & Design Fundamentals

Key Concepts and Terminology

• Structural analysis involves examining the effects of loads on physical structures and their components
• Structural design focuses on creating structures that can safely resist the loads they are subjected to
• Loads refer to the forces applied to a structure, which can be classified as dead loads (permanent), live loads (temporary), or environmental loads (wind, snow, earthquakes)
• Stress is the internal force per unit area within a material, resulting from applied loads
• Strain represents the deformation or change in shape of a material due to applied stress
• Hooke's Law describes the linear relationship between stress and strain in elastic materials, where stress is directly proportional to strain
• Elastic limit is the maximum stress a material can withstand without permanent deformation
  • Beyond the elastic limit, materials enter the plastic region, where permanent deformation occurs
• Yield strength is the stress at which a material begins to deform plastically

Forces and Loads in Structures

• Dead loads are permanent, constant forces acting on a structure, such as the weight of the structure itself and fixed equipment
• Live loads are temporary, variable forces acting on a structure, such as occupants, furniture, and vehicles
• Environmental loads include forces from natural phenomena, such as wind, snow, and earthquakes
  • Wind loads are horizontal forces that vary with wind speed, direction, and the shape of the structure
  • Snow loads are vertical forces that depend on the local climate and the geometry of the structure's roof
  • Seismic loads are dynamic forces induced by ground motion during earthquakes
• Impact loads are sudden, short-duration forces resulting from collisions or explosions
• Combination loads consider the simultaneous occurrence of different load types, as prescribed by building codes
• Load paths describe how forces are transmitted through a structure's components to the foundation
• Tributary area is the area of a structure that contributes load to a specific structural element (beams, columns)

Types of Structural Systems

• Beam-and-column systems consist of horizontal beams supported by vertical columns, forming a skeleton-like framework (steel or reinforced concrete frames)
• Truss systems are composed of triangular arrangements of straight members connected at joints, efficiently transferring loads through axial forces (bridges, roofs)
• Arch structures transfer loads to supports primarily through compressive forces, allowing for long spans (vaulted ceilings, bridges)
• Shell structures are thin, curved surfaces that resist loads through a combination of bending and membrane forces (domes, tunnels)
• Suspension systems support loads through tension cables or rods, anchored at their ends (suspension bridges, cable-stayed roofs)
• Shear wall systems are vertical planar elements that resist lateral forces through in-plane shear and bending (reinforced concrete or masonry walls)
• Moment-resisting frames are beam-and-column systems designed to resist lateral loads through rigid connections that transfer bending moments
• Braced frames are beam-and-column systems with diagonal bracing members that resist lateral loads through axial forces

Material Properties and Behavior

• Strength is a material's ability to resist stress without failure, which can be measured in tension (ultimate tensile strength), compression (compressive strength), or shear (shear strength)
• Stiffness is a material's resistance to deformation under load, quantified by its modulus of elasticity (Young's modulus)
• Ductility is a material's ability to undergo significant plastic deformation before failure, allowing for redistribution of stresses and warning signs of impending failure
  • Ductile materials, such as steel, exhibit large deformations before failure
  • Brittle materials, such as concrete or glass, fail suddenly with little plastic deformation
• Toughness is a material's ability to absorb energy before fracture, represented by the area under the stress-strain curve
• Fatigue is the weakening of a material caused by repeated loading and unloading cycles, leading to eventual failure at stresses below the ultimate strength
• Creep is the gradual deformation of a material over time under constant load, particularly significant in materials like concrete and wood
• Thermal expansion is the change in a material's dimensions due to temperature changes, which can induce additional stresses in restrained structures
• Durability is a material's ability to resist deterioration due to environmental factors (corrosion, weathering, chemical attack)

Basic Structural Analysis Methods

• Statics is the study of forces acting on a structure at rest, forming the basis for structural analysis
• Free body diagrams represent the forces and moments acting on a structural element, isolated from its surroundings
• Equilibrium equations are used to solve for unknown forces and reactions, based on the principle that the sum of forces and moments acting on a body in equilibrium must be zero
• Truss analysis methods, such as the method of joints and the method of sections, are used to determine the forces in truss members by applying equilibrium equations at each joint or section
• Beam analysis involves determining the internal forces (shear and bending moment) and deformations (slope and deflection) along a beam subjected to loads
  • Shear and moment diagrams graphically represent the variation of internal forces along a beam
• Moment distribution is an iterative method for analyzing indeterminate beams and frames, distributing fixed-end moments to adjacent members until equilibrium is achieved
• Influence lines graphically represent the variation of a specific response (reaction, shear, moment) at a point in a structure as a unit load moves along the structure
• Computer-based methods, such as the finite element method (FEM), discretize a structure into smaller elements and solve for the response using numerical techniques

Principles of Structural Design

• Limit state design considers the various conditions (limit states) at which a structure ceases to fulfill its intended function, such as ultimate limit states (collapse) and serviceability limit states (excessive deflection, vibration)
• Load and resistance factor design (LRFD) applies factors to loads and material strengths to account for uncertainties and ensure a target level of reliability
• Allowable stress design (ASD) ensures that the maximum stresses in structural members remain below a specified allowable stress, typically a fraction of the material's yield strength
• Capacity design promotes the formation of ductile failure mechanisms by ensuring that brittle failure modes have higher strength than ductile failure modes
• Redundancy is the provision of alternative load paths and multiple load-carrying components to prevent catastrophic failure in case of localized damage
• Ductility is incorporated into structural design to allow for energy dissipation and redistribution of forces during extreme events (earthquakes)
• Serviceability considerations ensure that a structure remains functional and comfortable for its intended use, limiting deflections, vibrations, and cracking
• Constructability is the ease and efficiency with which a structure can be built, considering factors such as material availability, transportation, and construction sequencing

Safety Factors and Building Codes

• Safety factors are used to account for uncertainties in loads, material properties, and construction quality, ensuring a margin of safety against failure
• Dead load factors are typically lower than live load factors, as dead loads are more predictable and have less variability
• Load combinations specify the various combinations of loads that a structure must be designed to resist, as prescribed by building codes
• Building codes are legal requirements that establish minimum standards for the design, construction, and maintenance of structures to ensure public safety and welfare
  • International Building Code (IBC) is a model code adopted by many jurisdictions in the United States
  • Eurocodes are a set of harmonized technical rules for the design of construction works in the European Union
• Occupancy categories in building codes define the level of importance and risk associated with different types of structures (residential, commercial, essential facilities)
• Wind and seismic provisions in building codes specify the design requirements for structures subjected to wind and earthquake loads, based on the structure's location, height, and occupancy
• Fire resistance requirements in building codes prescribe the minimum fire rating for structural elements and assemblies, based on the structure's occupancy and size
• Accessibility guidelines in building codes ensure that structures are designed to accommodate people with disabilities, specifying requirements for entrances, circulation spaces, and facilities

Real-World Applications and Case Studies

• Skyscrapers are tall, multi-story buildings that require advanced structural systems to resist gravity and lateral loads efficiently (Burj Khalifa, Shanghai Tower)
• Bridges are structures that span over obstacles, such as rivers or valleys, to provide passage for vehicles or pedestrians (Golden Gate Bridge, Sydney Harbour Bridge)
  • Suspension bridges are suitable for long spans and consist of a deck supported by vertical cables attached to main cables anchored at the ends
  • Truss bridges use a triangular arrangement of members to transfer loads efficiently and are commonly used for medium-span bridges
• Stadiums and arenas are large, open structures that require long-span roof systems and careful consideration of crowd loading and dynamics (Beijing National Stadium, Cowboys Stadium)
• Dams are massive structures built across rivers to create reservoirs for water storage, irrigation, and hydroelectric power generation (Hoover Dam, Three Gorges Dam)
• Tunnels are underground passages constructed for transportation, utilities, or storage purposes, requiring careful consideration of soil conditions and excavation methods (Channel Tunnel, Gotthard Base Tunnel)
• Earthquake-resistant design involves the use of ductile materials, damping devices, and base isolation systems to minimize damage during seismic events (Transamerica Pyramid, Taipei 101)
• Sustainable design incorporates principles of energy efficiency, resource conservation, and environmental responsibility into structural design (One Angel Square, Bank of America Tower)
• Forensic engineering involves the investigation of structural failures to determine the causes and prevent future occurrences (Tacoma Narrows Bridge collapse, I-35W Mississippi River bridge collapse)