Structural Engineering

Outline Introduction to Structural Engineering Design Process Forces in Structures Structural Systems Materials Definitions of Important Structural Properties Triangles UNITS (Dimensional Analysis)

Structural Engineering What does a Structural Engineer do? A Structural Engineer designs the structural systems and structural elements in buildings, bridges, stadiums, tunnels, and other civil engineering works (bones) Design: process of determining location, material, and size of structural elements to resist forces acting in a structure

Design Process

Engineering Design Process Identify the problem (challenge) Explore alternative solutions Research past experience Brainstorm Preliminary design of most promising solutions Analyze and design one or more viable solutions Testing and evaluation of solution Experimental testing (prototype) or field tests Peer evaluation Build solution using available resources (materials, equipment, labor, cost)

Design Process in Structural Engineering Select material for construction Determine appropriate structural system for a particular case. Justify (tell me why) you used these particular structural systems. Determine forces acting on a structure Calculate size of members and connections to avoid failure (collapse) or excessive deformation

Forces in Structures

Forces Acting in Structures Force induced by gravity (F=ma) Dead Loads (permanent): self-weight of structure and attachments Mass Vs. Weight Compression, Tension, bending, torsion

Forces Acting in Structures Vertical: Gravity Lateral: Wind, Earthquake

Forces in Structural Elements 100 lb Compression 100 lb Tension

Forces in Structural Elements 100 lb Bending Torsion

Structural Systems

Typical Structural Systems Arch

Typical Structural Systems Truss C T Forces in Truss Members

Typical Structural Systems Frame

Typical Structural Systems Flat Plate

Typical Structural Systems Folded Plate

Typical Structural Systems Shells

Providing Stability for Lateral Loads Racking Failure of Pinned Frame Braced Frame Infilled Frame Rigid Joints

Materials Used in Civil Engineering Metals Cast Iron Steel Aluminum Concrete Wood Fiber-Reinforced Plastics

Engineering Properties of Materials Steel Maximum stress: 40,000 – 120,000 lb/in2 Maximum strain: 0.2 – 0.4 Modulus of elasticity: 29,000,000 lb/in2 Concrete Maximum stress: 4,000 – 12,000 lb/in2 Maximum strain: 0.004 Modulus of elasticity: 3,600,000 – 6,200,000 lb/in2 Wood Values depend on wood grade. Below are some samples Tension stress: 1300 lb/in2 Compression stress: 1500 lb/in2 Modulus of elasticity: 1,600,000 lb/in2

Concrete Components Sand (Fine Aggregate) Gravel (Coarse Aggregate) Cement (Binder) Water Air

Fiber-Reinforced Composites Composite Laminate Polyester Polymer Matrix Epoxy Vinylester Glass Functions of matrix: Force transfer to fibers Compressive strength Chemical protection Fiber Materials Aramid (Kevlar) Carbon Function of fibers: Provide stiffness Tensile strength

Properties of Materials (Why are they used)

Definition of Stress T Example (English Units): T = 1,000 lb (1 kip) Section X T Example (English Units): T = 1,000 lb (1 kip) A = 10 in2. Stress = 1,000/10 = 100 lb/in2 Example (SI Units): 1 lb = 4.448 N (Newton) 1 in = 25.4 mm T = 1,000 lb x 4.448 N/lb = 4448 N A = 10 in2 x (25.4 mm)2 = 6450 mm2 (1 in)2 Stress = 4448/6450 = 0.69 N/mm2 (MPa) Section X Stress = Force/Area T

Definition of Strain T Strain = DL / Lo DL Example: Lo = 10 in. DL = 0.12 in. Strain = 0.12 / 10 = 0.012 in./in. Strain is dimensionless!! (same in English or SI units)

Engineering Properties of Structural Elements Strength Ability to withstand a given stress without failure Depends on type of material and type of force (tension or compression) Tensile Failure Compressive Failure

Engineering Properties of Structural Elements Stiffness (Rigidity) Property related to deformation Stiffer structural elements deform less under the same applied load Stiffness depends on type of material (E), structural shape, and structural configuration Two main types Axial stiffness Bending stiffness

Axial Stiffness T DL Stiffness = T / DL Example: T = 100 lb Lo Stiffness = T / DL Example: T = 100 lb DL = 0.12 in. Stiffness = 100 lb / 0.12 in. = 833 lb/in.

Bending Stiffness Displacement Force Stiffness = Force / Displacement Example: Force = 1,000 lb Displacement = 0.5 in. Stiffness = 1,000 lb / 0.5 in. = 2,000 lb/in.

Stiffness of Different Structural Shapes Stiffest Stiffer

Types of Structural Elements – Bars and Cables Bars can carry either tension or compression Cables can only carry tension

Types of Structural Elements – Beams Loads Tension Compression

Triangles

Formulas SOH, CAH, TOA c2 = a2 + b2 H O A