outline introduction to structural engineering design process forces in structures structural...

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 Elements100

lb

Compression

100 lb

Tension

Forces in Structural Elements 100

lb

Bending

Torsion

Structural Systems

Typical Structural Systems

Arch

Typical Structural Systems

TrussC

T

CCT

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

• WoodValues 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

PolymerMatrix

Polyester

Epoxy

Vinylester

Fiber Materials

Glass

Aramid (Kevlar)

CarbonFunction of fibers:

•Provide stiffness•Tensile strength

Functions of matrix:

•Force transfer to fibers•Compressive strength•Chemical protection

Composite

Laminate

Properties of Materials

(Why are they used)

Definition of Stress

Section X

T

T

Section X

Stress = Force/Area

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 NA = 10 in2 x (25.4 mm)2 = 6450 mm2

(1 in)2

Stress = 4448/6450 = 0.69 N/mm2

(MPa)

Definition of Strain

L

T

T

Lo

Strain = L / Lo

Example:

Lo = 10 in.L = 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

L

T

T

Lo

Stiffness = T / L

Example:

T = 100 lbL = 0.12 in.

Stiffness = 100 lb / 0.12 in. = 833 lb/in.

Bending Stiffness

Stiffness = Force / Displacement

Example:

Force = 1,000 lbDisplacement = 0.5 in.

Stiffness = 1,000 lb / 0.5 in. = 2,000 lb/in.

Displacement

Force

Stiffness of Different Structural Shapes

Stiffest

StifferStiff

Types of Structural Elements – Bars and Cables

Bars can carry either tensionor compression Cables can only carry tension

Types of Structural Elements – Beams

Tension

Compression

Loads

Triangles

Formulas

• SOH, CAH, TOA

• c2 = a2 + b2

H

A

O