1.1 Types of Structures
Structures are fundamental elements in civil engineering that support loads and transfer them safely to the ground. The types of structures covered in this section are:
BRIDGE STRUCTURE
Beams: Beams are horizontal members that primarily resist bending. They are typically used in buildings, bridges, and frameworks to support vertical loads.
Characteristics: Rectangular or I-shaped cross-sections; supports include simply supported, fixed, or cantilever.
Trusses: Trusses are frameworks composed of triangular units. Each member of a truss is assumed to carry only axial forces, either tension or compression.
Applications: Roof structures, bridges, and towers.
Types: Pratt truss, Warren truss, Howe truss, etc.
Frames: Frames consist of beams and columns rigidly connected together. They can resist both vertical and horizontal loads.
Types: Rigid frames (resist bending) and braced frames (use braces to resist lateral loads).
Arches and Cables: These structures are used to span long distances. Arches resist loads through compression, while cables resist loads through tension.
Examples: Arch bridges, suspension bridges, cable-stayed bridges.
Shells and Plates: These are thin structures that can support loads through their shape. They are used in modern buildings, roofs, and tanks.
Examples: Domes, cylindrical shells, folded plates.
1.2 Types of Loads
Loads are forces or other actions that result in stresses, deformations, or displacements in structures. This section covers:
Dead Loads (DL):
Permanent loads due to the weight of structural elements (e.g., beams, walls, floors).
Calculations involve determining the weight per unit area of materials.
Live Loads (LL):
Temporary or movable loads, such as the weight of occupants, furniture, or vehicles.
Governed by codes (e.g., ASCE 7) which specify standard load values for various types of structures (residential, commercial, bridges).
Wind Loads (WL):
Loads caused by wind pressure acting on the surfaces of a structure. The magnitude depends on factors like wind speed, building height, and shape.
Design involves calculating wind pressure using coefficients and gust factors as per relevant standards.
Snow Loads (SL):
Loads due to the accumulation of snow or ice, important in cold regions.
Calculated based on snow depth, density, and drifting effects, as per codes.
Seismic Loads (EL):
Loads due to earthquakes, causing dynamic shaking of the ground.
Determined using methods such as the Equivalent Lateral Force (ELF) method or Response Spectrum Analysis.
Thermal Loads:
Loads that result from temperature changes, causing expansion or contraction of materials.
Calculated using coefficients of thermal expansion and temperature differentials.
Impact Loads:
Sudden loads resulting from events like a vehicle collision or heavy equipment drop.
Design considerations involve dynamic impact factors.
Hydrostatic and Buoyant Loads:
Loads caused by fluid pressure acting on submerged structures or portions of structures.
Relevant for dams, retaining walls, and underwater structures.
Settlement Loads:
Differential settlement of foundations causing bending and cracking in superstructures.
Analyzed through soil-structure interaction studies.
1.3 Idealization of Structures
This section explains the simplifications made to analyze real-world structures:
Structural Models:
Simplifying complex structures into beams, frames, or trusses to make analysis feasible.
Assuming linear elastic behavior for materials.
Material Assumptions:
Homogeneous and isotropic materials (having uniform properties in all directions).
Support Conditions:
Idealizing supports as pinned (allowing rotation but no translation), fixed (restricting both rotation and translation), or roller (allowing horizontal movement).
1.4 Principles of Structural Design
This section introduces key concepts:
Safety and Serviceability:
Structures must be designed to ensure safety (resisting collapse) and serviceability (limiting deflections, vibrations, or cracking).
Load Combinations:
Engineers must consider various combinations of loads (e.g., DL + LL, DL + WL) as per codes to ensure comprehensive safety.
Load factors and combination rules provided by codes like ASCE 7 are explained.
Factors of Safety:
Discusses the importance of using safety factors in design to account for uncertainties in loads, materials, and construction.
Design Codes and Standards:
Emphasizes the need to adhere to building codes, such as the American Concrete Institute (ACI) standards, American Institute of Steel Construction (AISC) standards, or Eurocodes, to ensure safe and compliant design.
Conclusion
This chapter lays the groundwork for understanding the behavior of structures under different types of loads and introduces the fundamental concepts necessary for structural analysis and design. The subsequent chapters build upon this foundation, diving deeper into specific analysis methods, structural behavior, and design principles.
Learning Outcomes:
By the end of Chapter 1, readers should have a clear understanding of:
The various types of structural systems and their applications.
The types of loads that structures are designed to withstand.
The principles behind idealizing structures for analysis.
The safety and serviceability considerations in structural design.