How Suspension Bridges Work: Structural Engineering Explained

Structural loads are classified as dead loads (cargas muertas) or live loads (cargas vivas). Dead loads are constant weights that do not change over time, such as the weight of the structure itself, walls, and permanent fixtures. Live loads are variable weights that can change in magnitude or location over time, such as people, furniture, and movable equipment. Furniture is classified as a live load because it can be moved around. Dead loads are calculated from the unit weights of construction materials using tables with material unit weights.

Dead loads (cargas muertas) are permanent loads that do not vary in magnitude or position, including self-weight of structural elements and non-structural components. Live loads (cargas vivas) are variable loads due to occupancy and use, including people, furniture, and movable equipment. Live loads are more random and difficult to predict, requiring statistical approaches. The distinction is important because dead loads are always present while live loads can vary significantly. Engineers must consider both types for safe structural design.

Gravity loads acting on structures include dead load and live load. Dead load is the load that does not move throughout the life of the structure, such as the self-weight of the structure. Live load is the load that can move from one place to another throughout the life of the structure. Live load also includes impact loads (such as trains on railway bridges). The value of live load depends on the purpose of the building: residential buildings typically have 2-3 kN/m² (200-300 kg/m²), while commercial buildings have higher values due to more people and equipment.

Dead loads are permanent gravity loads of constant magnitude and fixed position, calculated as material density multiplied by volume, encompassing all construction materials including walls, floors, roofs, ceilings, partitions, finishes, and fixed equipment. Live loads are variable loads of changing magnitude and position caused by building use, including occupants, furniture, and storage. Environmental loads like wind and earthquake are classified separately despite not being dead loads. For large influence areas exceeding 40 square meters, live loads can be reduced using Equation 205-3, with reductions limited to 50% for single-level loading members and 40% for others. Minimum design loads for common materials are referenced from ASCE standards and the National Structural Code of the Philippines.

Dead load (dead load) refers to permanent, static loads on a structure such as the weight of the structure itself, walls, floors, and fixed equipment. Live load (live load) refers to variable, dynamic loads such as people, furniture, movable equipment, and environmental loads like wind and snow. Live load values vary based on the building's occupancy type (residential, commercial, industrial). Understanding these load types is fundamental to structural design as they determine the total loads that structural elements must resist.

This section covers three fundamental mechanical properties of materials. Strength is the ability to resist external loading, with three types: tensile (pulling), compressive (pushing), and torsional (twisting). Elasticity is the property allowing materials to return to their original shape after load removal. Stiffness is the resistance to deformation under load, related to elastic modulus. These properties form the foundation for understanding material behavior under stress.

Mechanical properties of materials are structure-sensitive characteristics dependent on crystal structure, bonding forces, and imperfections. Key properties include: (1) Elasticity - ability to regain original shape after deformation; (2) Proportional Limit - maximum stress maintaining uniform strain-rate; (3) Elastic Limit - maximum stress without permanent set; (4) Yield Point - onset of permanent deformation; (5) Strength - ability to resist external forces without breakdown, measured as ultimate strength before destruction. These properties determine material suitability for industrial applications in tools, machines, and structures.

This section covers three fundamental mechanical properties of materials. Elasticity is the ability to regain original shape after load removal, demonstrated through rubber bands and steel within elastic limits. Plasticity is the ability to retain permanent deformation after load removal, shown through polythene bags. Strength is the ability to resist external forces, with four types: tensile (pulling), compressive (crushing), shear (sliding), and torsional (twisting). These properties form the foundation for understanding material behavior under different loading conditions.

Key mechanical properties include: (1) Elasticity - tendency to regain shape after unloading; (2) Plasticity - permanent deformation capability; (3) Tensile strength - corresponds to ultimate stress, higher ultimate stress means higher tensile strength; (4) Ductility - ability to be drawn into wires, directly related to plastic deformation range; (5) Hardness - corresponds to yield strength, higher yield strength means higher hardness; (6) Toughness - allows plastic deformation, higher plastic deformation range means higher toughness; (7) Stiffness - resistance to deflection or deformation; (8) Malleability - ability to convert into sheets; (9) Creep - strain under constant loading.

Tensile strength is the first and most important mechanical property, defined as the maximum stress a material can withstand while being stretched or pulled before breaking. It is expressed as maximum load per unit area. Elasticity is the ability of a material to return to its original shape and size after the removal of an applied force. These properties are fundamental for understanding material behavior under different loading conditions and are essential for material selection in engineering design.

Nearly all bridges use three basic designs: beam, arch, and suspension. Beam bridges rest on supports at each end, with triangles added for strength (truss bridges). They can be lengthened with piers or trestles. Arch bridges use wedge-shaped stones or materials, with the key stone at the top, and can be lengthened by adding more arches. Suspension bridges use cables stretched over tall towers, with thousands of steel wires in each cable. The Tacoma Narrows Bridge collapse taught designers to eliminate solid surfaces that catch crosswinds.

This segment covers six major bridge types: (1) Beam bridge - simplest and oldest type, supported by abutments at each end, made from timber, iron, steel, or concrete, carries pedestrians and vehicles; (2) Truss bridge - load-bearing superstructure composed of beams, carries various loads, features triangles which are excellent for holding loads; (3) Arch bridge - semicircular shape that holds the load, one of the oldest and most popular types, made from stone, concrete, timber, or iron; (4) Cantilever bridge - projects horizontally into space supported on only one end, carries pedestrians, automobiles, and rail; (5) Suspension bridge - uses tension to hold up the deck, supported by cables, towers, and supports, modern versions use iron and steel cables; (6) Drawbridge (bascule bridge) - movable bridge that allows ships to pass underneath using a counterweight system.

There are four main types of bridges: beam, truss, suspension, and arch. Beam bridges are the simplest with flat designs. Truss bridges use a crisscross lattice of steel for strength. Suspension bridges hang from cables like the Golden Gate Bridge. Arch bridges have been around since Roman times and are strong, elegant, and efficient at handling weight because the pressure pushes down and out, meaning the arch itself carries the weight.

This section covers fundamental bridge types based on structural principles. Beam bridges use simple horizontal members spanning two supports but cannot span great distances due to self-weight limitations. Truss bridges overcome this by using assemblies of smaller elements experiencing only axial forces (compression/tension), enabling greater spans. Trestle bridges feature repetitive short spans with frequent supports. Arch bridges transfer loads using pure compression through curved elements, requiring temporary construction supports and strong abutments to resist horizontal thrust. These basic types form the foundation for understanding more complex bridge designs.

Arch bridges are structural systems where all forces are transmitted as compressive (druck) forces rather than tensile forces, making them ideal for brittle materials like stone, concrete, or cast iron; they come in three main forms—half-circle arches with minimal horizontal thrust requiring lighter abutments, segmental arches that generate significant horizontal forces needing robust foundations especially in rocky terrain, and multi-centered arches with multiple radii that allow uniform stone formats and mutual thrust support between segments—all constructed using temporary formwork (gerüst) that supports the structure until it achieves full structural effectiveness.