Introduction to Structural Engineering

Tony Freidman

Background

• Graduate of University of Missouri – Rolla– B.S. in Civil Engineering– B.S. in Architectural Engineering– Research in Architectural specialties– Research on V-T-M diagram development for

reinforced concrete column design

• Currently enrolled as a Ph.D. student at Washington University – St. Louis– Research on MR Damper performance– Research on Structural Health Monitoring

• http://www.youtube.com/watch?v=uKeENdyIluI&feature=related

• http://www.youtube.com/watch?v=AsCBK-fRNRk&feature=related

• http://www.youtube.com/watch?v=YCfgfccpHpc&feature=related

• http://em-ntserver.unl.edu/Mechanics-Pages/Group1/sld001.htm

Structural Engineering is used so that the events in the preceding

videos never take place.

“Engineers shall hold paramount the safety, health and welfare of the public and shall strive to comply with the principles of sustainable

development in the performance of their professional duties. “

- 1st Fundamental Engineering Canon

Structural Engineering Overview

• What is a Structural Engineer?

• What do they do?

• What do they design?

Structural Engineering Overview

• What is a Structural Engineer?

• What do they do?

• What do they design?

What is a Structural Engineer?

• Engineer– Mathematics of design

• Architect/Artist– Vision– Aesthetics of design

• Mediator– Liason between parties

on a project• Salesman

– Must sell your idea, yourself

Structural Engineering Overview

• What is a Structural Engineer?

• What do Structural Engineers do?

• What do they design?

What do SE’s do?

• Designer– Consultant

• Take a design, and fit a structural system to that• Expert witnesses in lawsuits

– Inspector• Fieldwork, Job site inspections• Oversee the materials (concrete, steel, etc.)• Inspect the building – pre- and post-construction

– Demolitions• Building deconstruction• Structural Retro-fits

Structural Engineering Overview

• What is a Structural Engineer?

• What do Structural Engineers do?

• What do they design?

SE’s design/analyze Structures

• What is a structure?– A system designed to resist or support

loading and dissipate energy• Building Structures

– Houses– Skyscrapers– Anything designed for continuous human occupation

• Non-building Structures– Bridges– Tunnels– Dams

Forces

• Influence on an object that causes a change in a physical quantity

• Considered “vectors” – magnitude and direction• Static Force

– Unchanging with time• Walls• Floors

• Dynamic Force– Changing with time

• People• Furniture

Forces• Axial Forces

– Acting along one axis, directly on a point or surface

• Momential (Bending) Force– Acting along an axis, at a certain distance from a

point, causes a folding motion– M = F*d

F

Forces

• Tensile Force– Pulling on an object – stretching it– Steel shows “necking” when too much tensile

force is applied

• Compressive Force– Pushing on an object – collapsing it– Concrete crushes when too much

compressive force is applied

Forces

• Strain– Tensile-related property– Deformation / Length

• Stress– Compression-related

property– Force / Area

• Compare using stress-strain graph

What constitutes loading?

• Loading is a force being enacted on the structure– Many sources of load

• Gravity/Weight• Wind• Snow• Earthquake• Man-made

– Two Types of Structural Loading• Dead Loads – static, ever-present (i.e. Walls, Floors, etc)• Live Loads – dynamic, changing (i.e. People, Desk, etc)

What should we build our structures out of??

• Common Structural Materials– Timber– Masonry– Concrete– Steel– Composites

How do we judge the materials?

• Common Material Properties– Strength – Tensile/Compressive– Density– Hardness– Ductility / Brittleness– Elasticity– Toughness

Strength

• Ability of a material to withstand loading– Tensile strength – ability of a material to withstand a

pulling force• Steel is good at this, but concrete performs very poorly.• http://www.youtube.com/watch?v=YdqvGGFIbfc

– Compressive strength – ability of a material to withstand a pushing force

• Wood, concrete, steel, and masonry perform well• http://www.youtube.com/watch?

v=WC6AgX2N1Go&feature=related• http://www.youtube.com/watch?

v=i5qwvtEqC5o&feature=related

Density

• Mass per unit volume of a material– Units – mass/vol - kg/m3 or lb-m/ft3

– Typically, materials with a high density are very strong and offer great protection.

– However, a high density means that they are heavy and difficult to work with $$$$$

Hardness

• Ability of a material to resist permanent deformation under a sharp load– Relates to the elasticity of a material– Diamond is a very hard substance. If we built

a wall out of diamond, we could be sure that very few things would scratch it.

– However, Diamond is incredibly expensive and not as tough as other engineering metals. It wouldn’t stand up as well in impact loading versus other materials.

Ductility / Brittleness

• Ability of a material to deform without fracture– We want materials with high ductility, because

they will indicate structural failure without a sudden collapse.

– http://www.youtube.com/watch?v=BXpqW9B0eT4&feature=related – “Brittle failure”

Elasticity

• Ability of a material to deform and return to it’s original shape.– Important quantity

• Young’s Modulus• Ratio of stress to strain

– Stress = Force / Area (lbs./in2 or N/m2)– Strain = Deformation / Length (unitless)

• Generates a stress-strain graph• Related to the ductility of a material

Toughness

• Ability of a material to resist fracture when stressed (amount of energy absorbed per unit volume)– Units – J/m3 or Lb-f/ft3

– Area under the stress-strain curve, evaluated from 0 to the desired strain.

So, we know what properties are important in structural materials.

How do the common materials stack up against each other?

Timber

• Advantages– Cheap, renewable resource– Good in Tension – ~40 MPa

• Disadvantages– Susceptible to fire, nature– Not very hard– Not very strong– Limits on shape, size

Masonry

• Concrete blocks, clay bricks– Advantages

• Large compressive strength• Cheap• Good thermal properties – holds heat

well– Disadvantages

• Not a cohesive material. The strength could depend on the mortar, other factors

• Poor tensile strength, unless reinforced• Heavy material, requires skilled

laborers to use $$$$$• Height restriction• Susceptible to the weather

Concrete

• Combination of water, cement, small aggregate, and large aggregate.

• Advantages– Very versatile – can be modified with

admixtures for different effects– High compressive strength (4~7 ksi)– Fire resistant– Many diverse sizes and shapes - formwork

Concrete

• Disadvantages– Long curing time– Low tension

strength (~0.4 ksi)

– Fails in shear, unless reinforced

– Fairly heavy material to work with

Steel

• Advantages– High tensile and compressive strength (A36

Steel ~ 60 ksi)– Many varieties, depending on your need

• Carbon steel• Stainless steel• Galvanized steel

– Elastic material– Ductile material– Many shapes, sizes

Steel

• Disadvantages– Expensive – limited quantities / competition– Susceptible to fire, rust, impurities

Put them together and…

• Reinforced Concrete– Concrete with steel reinforcement

• Concrete handles compression• Steel takes the tension

– Can handle nearly 4 times the loading that concrete alone can handle

– More expensive material– http://www.youtube.com/watch?v

=dGbrp7Mfp2w

Composites

• Engineered compounds that have different physical or chemical properties– FRP – Fiber reinforced polymers– CFRP – Carbon-fiber reinforced polymers– Plastics– Categories of Glass– Categories of Wood

So, now we know what material will best suit our needs..

What should we build with it?

Structural Shapes

• Rectangle / Square

• Triangle– Interested in stability

• Truss

• Geodesic Dome

Shape Stability Exercise

• Split into teams of 5

• Build a triangle and square

• See which shape is the most stable– Can the unstable shapes be made stable?– How?

Rectangle

• Advantages– Proficient in

resisting vertical load.

• Disadvantages– No lateral

support

Triangle

• Advantages– Able to withstand

lateral & vertical loading

– Many triangular shapes available

• Disadvantage– Wide base = $$$$

Rectangle

• Advantages– Proficient in

resisting vertical load.

• Disadvantages– No lateral

(horizontal) load support

Need another bar for lateral support!

--BRACING--

Truss

• Combination of square and triangle

Truss

• Combination of square and triangle

Squares

Truss

• Combination of square and triangle

Triangles

Truss

• Combination of square and triangle– Both vertical and lateral support

Geodesic Dome

Domes

Domes

• Advantages– Very strong shape, gets strong as the dome

size increases– Perfect load distribution– No need for structural supports– Great aerodynamic performance

Structural Components

• Beams

• Girders

• Columns

• Floors

• Foundations

Column Girder

Beam

Load Path

• Floor

• Beams

• Girders

• Columns

• Foundation

• Soil/Bedrock

Foundations

• Support the building– Typically attached to columns

• Types– Shallow

• Spread footing – concrete strip/pad below the frost line• Slab-on-grade – concrete pad on the surface

– Deep• Drilled Shafts• Piles

Columns

• Carry the load from floors to the foundation– Never want the columns to fail COLLAPSE– Typically reinforced concrete or steel– Many sizes and shapes

Girders

• Attached column-to-column– Take the load

from the beams– Transfer it to the

columns– Generally shaped

as an I-Beam

Beams

• Attached between the girders– Take load from the

flooring system– Transfer it to the

girders– Generally solid

squares, I-beams

Flooring

• Composed of a subfloor and floor covering– Usually leave space for ductwork, wiring, etc.– Floor covering ranges from application to

application

Picture Credits• Geodesic Dome

– http://www.ecophotoexplorers.com/images/antarctica/Dome.jpg• Truss

– http://www.classictruss.com/art/wodd_truss_types_examples.gif• Truss 2

– http://www.wwta.ab.ca/images/pic%201.jpg• Truss 3

– http://www.garrettsbridges.com/images/pratt-truss-bridge.jpg• Stress-Strain

– http://www.mittalsteel.com/NR/rdonlyres/8E6DAA96-67F0-4C32-8702-FB02211647D9/0/InlandAutoApps_MildCS_ssGraph.gif

• Crushing Concrete– http://images.google.com/imgres?imgurl=http://cee.engr.ucdavis.edu/faculty/chai/

Research/Planar%2520Wall%2520Stability/Wallcolumn5.jpg&imgrefurl=http://cee.engr.ucdavis.edu/faculty/chai/Research/Planar%2520Wall%2520Stability/struct-walls.htm&h=432&w=293&sz=38&hl=en&start=10&sig2=u3TW2TnwJ_IRSpfaQhJuUQ&tbnid=7g-JN0VhU59wxM:&tbnh=126&tbnw=85&ei=qUTYR56oL6jmigGFuaiFAg&prev=/images%3Fq%3Dconcrete%2Bcrushing%2Bcompression%26gbv%3D2%26hl%3Den

• Pile Machine– http://upload.wikimedia.org/wikipedia/commons/5/54/DFmachine.JPG

• Pile Machine 2– http://upload.wikimedia.org/wikipedia/commons/8/8b/PileDriving.jpg

• Foundation Type– http://upload.wikimedia.org/wikipedia/en/6/66/Found-House-Apt.png

• Rebar Cage– http://i.pbase.com/u29/camera0bug/upload/17213579.P5070031.JPGco

py• Circular Columns

– http://web.mit.edu/istgroup/ist/images/research_images/microwave/figure1.gif

• Timber– http://www.certified-teak.com/images/timber_india.jpg– http://www.timberframeworks.com/images/File0001.jpg

• Steel– http://www.lakewoodconferences.com/direct/dbimage/50261363/

Stainless_Steel_Seamless_Tubes.jpg– http://ocw.mit.edu/NR/rdonlyres/Global/6/695E8EC6-5161-4B05-B6F1-

8BE09DC40F68/0/chp_steel_struct.jpg• Concrete

– http://www.lakewoodconferences.com/direct/dbimage/50261363/Stainless_Steel_Seamless_Tubes.jpg

– http://www.imagico.de/pov/pict/concrete.jpg• Masonry

– http://www.gomedia.us/arsenal/images/texturepacks/masonry-prev1.jpg– http://www.cca.org.nz/images/masonry.jpg

• Structural Engineer– http://vaengineer.com/images/mr__snell_0462.jpg

• Building– http://www.brasfieldgorrie.com/_images/portfolio/84/2.jpg

• Girder– http://www.popartuk.com/g/l/lgfp0432+men-on-a-girder-having-lunch-

new-york-city-collection-poster.jpg– http://oldcooperriverbridge.org/small/

oct_31_1927_pearman_resting_girder.jpg• Beam

– http://www.cedar-log-homes.com/log-homes-images2/beams-glulam-6.gif

• Flooring– http://www.domoteck.org/domoteck_com_images/

floor_cross_section.jpg