Structural loads and forces are the foundation of building design. They determine how a structure will behave under different conditions. Understanding these loads is crucial for engineers to create safe, stable buildings that can withstand various stresses.

This section dives into the types of loads structures face, from permanent dead loads to variable live loads and environmental forces. We'll explore how engineers calculate and analyze these forces to ensure buildings can handle whatever comes their way.

Types of structural loads

Permanent and variable loads

• Dead loads act as permanent, constant forces on a structure due to its own weight and fixed components
• Live loads create temporary, variable forces on a structure from occupancy, use, or environmental factors
• Environmental loads encompass wind, snow, and seismic forces, which vary dynamically in magnitude and direction
  • Wind loads push laterally on buildings, increasing with height
  • Snow loads add weight to roofs, varying by region and roof pitch
  • Seismic loads shake structures horizontally and vertically during earthquakes

Special load considerations

• Impact loads produce sudden, short-duration forces from collisions (vehicle impacts) or explosions
• Thermal loads induce forces through temperature changes, causing expansion or contraction of structural elements
  • Bridge expansion joints accommodate thermal movement
  • Concrete curing generates significant internal thermal stresses
• Soil pressure loads exert lateral forces on below-grade structures from surrounding soil and groundwater
  • Retaining walls must resist soil pressure to prevent overturning
  • Basement walls experience hydrostatic pressure in areas with high water tables

Force calculations from loads

Vector analysis and superposition

• Vector analysis determines the magnitude and direction of resultant forces from multiple applied loads
  • Forces are represented as arrows with length indicating magnitude
  • Vector addition combines individual force components
• Superposition principle allows combination of individual load effects to determine total force on a structure
  • Linear elastic behavior assumed for most structures
  • Effects of multiple loads can be analyzed separately then summed

Moment and free-body analysis

• Moment calculations determine rotational effects of forces acting on a structure
  • Moment = Force × Perpendicular distance from pivot point
  • Crucial for analyzing beams, columns, and connections
• Free-body diagrams visualize and analyze all forces acting on a structure or structural element
  • Isolate the element of interest
  • Show all external forces, including reactions
  • Ensure force and moment equilibrium

Load distribution techniques

• Tributary area concepts distribute loads to specific structural elements
  • Divide floor area to assign loads to supporting beams and columns
  • Roof loads allocated to trusses or rafters based on spacing
• Load path analysis traces transfer of forces through a structure from application point to foundation
  • Identify primary and secondary load-bearing elements
  • Ensure continuous load paths to avoid structural failures
  • Critical for understanding overall structural behavior

Wind and seismic load effects

Wind load analysis

• Wind loads create dynamic lateral forces varying with building height, shape, and surrounding terrain
  • Tall buildings experience higher wind pressures at upper levels
  • Aerodynamic shapes (rounded corners) can reduce wind loads
• Wind tunnel testing and computational fluid dynamics (CFD) simulations predict wind load effects on complex structures
  • Scale models tested in wind tunnels to measure pressures
  • CFD software simulates airflow patterns around buildings

Seismic load considerations

• Seismic loads result from ground accelerations during earthquakes, influenced by structure's mass, stiffness, and natural frequency
  • Heavier structures generally experience larger seismic forces
  • Stiffer structures tend to attract higher seismic loads
• Response spectrum analysis evaluates a structure's response to seismic excitation across different frequencies
  • Plots peak structural response vs. natural frequency
  • Used to determine design forces for various structural elements

Advanced seismic design techniques

• Base isolation systems separate the structure from ground motion using flexible bearings
  • Reduces acceleration transmitted to the superstructure
  • Commonly used for critical facilities (hospitals, data centers)
• Damping systems dissipate seismic energy, reducing structural response
  • Tuned mass dampers in tall buildings counteract sway
  • Viscous dampers act like shock absorbers in bridges
• Dynamic amplification factors account for increased effect of wind and seismic loads on flexible structures compared to static analysis
  • Tall buildings may experience 1.5-2 times the static equivalent force
  • Crucial for accurate design of long-span bridges and skyscrapers

Load combinations in design

Code-specified load combinations

• Load combinations represent realistic worst-case scenarios for structural design
  • Ensure structures can withstand multiple simultaneous loads
  • Account for low probability of all maximum loads occurring together
• Building codes and standards specify required load combinations to ensure structural safety and reliability
  • ASCE 7 provides widely used load combination equations
  • International Building Code (IBC) references these combinations

Load and resistance factor design

• Load factors applied to individual loads within combinations account for uncertainties in load estimation
  • Dead load factor typically 1.2-1.4 (less uncertainty)
  • Live load factor often 1.6 (more variable and uncertain)
• Strength design method (Load and Resistance Factor Design or LRFD) compares factored load combinations against factored structural capacities
  • Design strength must exceed required strength
  • Provides consistent reliability across different materials and load types

Advanced combination techniques

• Serviceability limit states use separate load combinations to ensure structures remain functional under normal use
  • Control deflections, vibrations, and cracking
  • Often use unfactored loads or lower load factors
• Probability-based load combination methods (First Order Second Moment approach) enable advanced reliability analysis in structural design
  • Consider statistical distributions of loads and resistances
  • Allow for more optimized designs in some cases
• Load envelopes determine critical load combinations governing design of specific structural elements or systems
  • Develop diagrams showing maximum forces at each point
  • Ensure all possible load scenarios are considered in design