1

ENB375: Structural Engineering 2

• 4 hours/week (3hours lectures and 1 hour (tutorial) and 12 Credit Points

• Pre-requisites: ENB270• Lecturing and Tutoring Staff

– Mahen Mahendran– S. Kesawan, V. Jatheeshan - PhD students – steel

structures– Dr S. Gunalan, Dr P. Keerthan

ENB375: Structural Engineering 2

Rationale and TopicsRationale and Topics• Behaviour and Limit states design of

STEEL STRUCTURES (mainly elements such as ties, columns and beams)

• Structural mechanics theories and applications

ENB375: Structural Engineering 2

Teaching approachg pp• Lectures - 3hrs/week• Tutorials – 1 hr/week• Formative Assessments - 2• Summative Assessments

– Problem Solving Tasks – 25%– Analysis and Design Projects – 25%– Final examination – 50%

ENB375: Structural Engineering 2

To pass this subject, you needTo pass this subject, you need

• a grade of 4 in yourSummative Assessments.

2

How do We do it?• We have excellent resource materials – see

Blackboard• I must do my best, and will!• Will you do the same?• Set specific goals based on your strengths

and eaknessesand weaknesses• Attendance; Regular work; Use all the

available resources (including us); Give us regular feedback and communicate always

ENB375: Structural Engineering 2

• ContentsContents• Recommended Books and Notes• Australian Steel Institute (ASI)

Student Membership–www.steel.org.au

STEEL

• One of the very efficient civilyengineering materials; it enablessustainable construction

• Steel has high strength andstiffness with low self-weight, goodd tilit ll d f tductility, allows easy and fasterfabrication and erection and isrelatively cheap.

3-D Steel Structures

Made of• One-dimensional elements

– Ties, Columns, Beams and Beam-columns, Torsion members

• Two-dimensional elements– Frames, Plates,

• Connections – Welds, Bolts, Screws, Pins, Rivets, Clinches

• Design based on independent behaviour

3

Engineering Design ProcessStructural engineer must have a good

understanding of the following:understanding of the following:• Structural Design Principles• Behaviour of Structures• Structural Analysis• Detail Design• Fire and corrosion protection required• Properties of Construction Materials and• Methods of Construction

Engineering Design Process

• Investigationg• Conceptual Design – Core activity• Preliminary Design – Core activity• Final Design – Core activity• DocumentationDocumentation• Tendering• Construction

Engineering Design Structural Engineer must work with• Architects• Service engineers• ContractorsTo produce the best solution (constructable,

adequate strength, serviceable, functional, aesthetics, environment, economical)

Design Process

• Understanding the problem and client’sg prequirements

• Trial solution• Improving the trial solution• Determining the member sizes and detailsg• Design task 30-40% overall cost

4

Design Methods

• Computation using Standardsand Codes of Practice–Most Common

• Testing of Prototypes

This Unit• Deals with the analysis and design tasks

of individual members and simpleof individual members and simplestructures

• Includes evaluation of relevant loads,appropriate idealisation of structures(both members and connections), use ofappropriate methods of analyses andappropriate methods of analyses, anddesign of members (safe and cost-efficient)

Building Code of Australia (BCA)

• Designers are usually required to deisgni d ith t t t / l tin accordance with statutory/regulatoryguidelines. In Australia, designs aretypically regulated by the BCA

• Designs are said to have been “Deemed toComply” with the BCA if they are inComply with the BCA if they are inaccordance with the relevant AustralianStandards.

Standards and Codes of Practice• Minimum Criteria for structural adequacy

t bl f d ffi i t d ito enable safe and efficient design• Loads/Actions, analysis methods, limits, etc

– Building Code of Australia– AS1170 Parts 1 to 4 (Action codes)– AS4100 AS4600 AS2327 (Design Codes)– AS4100, AS4600, AS2327 (Design Codes)– AS3678, AS3679, AS1163, AS1554,

AS1252, AS1111 (Material quality codes)

5

Standard Steel Sections

• Hot-rolled Sections: UB, UC, PFC/TFC,, , ,EA/UA

• Standard Welded Sections in the form of3-plate I-sections: WB, WC

• Cold-formed structural steel hollowsections: CHS, SHS, RHS

Standard Steel Sections

Structural Steel Sections

Tensile Stress-Strain CurvesGrade 400

750 750(a) (b)

?Grade 350

Grade 250

600

450

300

150

Stre

ss M

pa

S tre

ss M

pa

300

150

600

450

yielding strain hardening

350

250

?

150

0 0.1 0.2 0.3

Strain

150

0

0.002(0.2%)

0.006 0.012

Strain

Steels are alloys of iron with small quantities of carbon, manganese, chromium, vanadium, copper,….: CARBON EQUIVALENT

Important Terms

• Elastic limit• Elastic range• Young’s modulus of elasticity (E) *• Yield stress fy (or 0.2% proof stress) ***• Ultimate tensile strength fu *

St i h d i d f t (D tilit )• Strain hardening and fracture (Ductility)

*** Most important & Used in all design actions

6

Yield Stress

• Is it the same in compression ?• What happens in shear?

Shear yielding

Steel Properties

Assumed the same for all steel grades• Modulus of elasticity E = 200,000 MPa• Shear modulus G = 80,000 MPa• Poisson’s ratio = 0.25• Density = 7850 kg/m3

• Coefficient of thermal expansion 11.7 x10-6/C.

• Properties at elevated temperatures??

Effect of Fire on Yield Strength

1

1.2Eqs. (1)-(4)

Eq (5)

G550

G300

fy,T/fy,2

0

0 4

0.6

0.8

1 Eq. (5)

0.42(G550)

0.6(G550)

0.95(G550)

1.2(G500)

Temp.(°C)0

0.2

0.4

0 100 200 300 400 500 600 700 800 900

0.4(G300)

0.6(G300)

1.0(G300)

Effect of Fire on Young’s Modulus

1

1.2Eqs. (6)-(8)

0.4

0.6

0.8

1 0.42(G550)

0.6(G550)

0.95(G550)

1.2(G500)

0.4(G300)

0

0.2

0 100 200 300 400 500 600 700 800 900

0.4(G300)

0.6(G300)

1.0(G300)

7

Material Quality StandardsAll structural steels are required to comply with

appropriate standards:appropriate standards:• Structural steel hot-rolled plates, floor plates

and slabs : AS3678• Structural steel hot-rolled bars and sections and

welded sections : AS3679 Parts 1 and 2S l l h ll i AS1163• Structural steel hollow sections: AS1163

• Structural and pressure vessel steel (quenchedand tempered plates): AS3597

Steel Grades• G250, G300, G350, G450 etc

Mi i ( t d) t th ti f f• Minimum (not measured) strength properties fy, fu

– Table 2.1 of AS4100– ASI Design Capacity Tables– Onesteel Product Data – www.onesteel.com

• fy varies with both grade and thickness whereas fud d l ddepends only on grade

• fy depends on chemical composition, method ofmanufacture and amount of working

Design CodesSteel structures code AS4100 specifies the

minimum requirements forminimum requirements for• Design, Fabrication, Erection and

Modification of Steelwork in structures(buildings, wharves and cranes); also toroadway, railway and pedestrian bridgestogether with our bridge codes

• Based on Limit States design Method.

Design CodesAS4100 excludes the following• Steel elements less than 3 mm thick• Steel elements with design yield stress fy

exceeding 450 MPa• Cold-formed members except the tubular

sections (RHS, SHS and CHS) complying withAS1163 – use AS 4600 – Cold-formed steel codeAS1163 use AS 4600 Cold formed steel code

• Composite steel-concrete members – useAS2327, the composite structures code

Unidentified steels – use fy = 170, fu = 300 MPa

8

Design Actions (Loads)• Dead Load G (Permanent action)

AS1170 1AS1170.1• Live Load Q (Imposed Action) AS1170.1• Wind Load / Action (Wu) AS1170.2• Earthquake Load/Action Feq AS1170.4• Temperature Induced Loads T• Temperature Induced Loads T• Construction Loads C• Other (Snow, …….)

Why do we use the term Actions now?

Load FactorsTo allow for “overload” possibility a Load

Factor is used with each load.• For G: 1.2, 0.9• For Q: 1.5, 0.0• For Wu: 1.0

Static structures??Dynamic loads (wind and earthquakeactions = Use Static load equivalents

Design Action Effects S*Factored loads are used to obtain design

action effects S* from the analysis ofaction effects S from the analysis ofstructural system, ie. beam, frame, etc.

• Axial Forces N*• Bending Moment M*• Shear Forces V*Shear Forces V• Deflections

Limit States Design MethodStructures must remain stable, safe and

serviceable under all design loads andbi ti d i th i d i lifcombinations during their design life

• Strength Limit State (what is it???)• Serviceability Limit State• Stability Limit State• Fatigue Limit State• Fire Limit State• Brittle Fracture Limit StateNot an allowable/permissible stress method(single factor used to allow for all unknowns)

9

Load Combinations for Strength Limit StateNominal loads x Load factors 1.35 G 1.2 G + 1.5 Q * 1.2 G + 1.5 l Q 1.2 G + Wu + c Q 0.9 G + Wu* * - common combinations

G + E + Q ( th k ti ) G + Eu + c Q (earthquake action) 1.2 G + Su + c Q (snow action)First 4 load combinations are for downward

load while the fifth one for upward load

Strength Limit State Design RequirementDesign Action Effect Ed*

Design Capacity Rd = R Design Capacity Rd Rwhere R is the nominal design capacity from

AS4100 and is a capacity reduction factordepending on the type of member and designaction effect (Table 3.4 in AS4100) to allow forunder-strength

Ed* - Tension (N*), compression (N*) and moment(M*) from structural analysis based on factoredloads to allow for over-loading

Capacity Reduction Factor Allows for “understrength” due to

V i bilit f t i l t th• Variability of material strength• Accuracy of calculated design action

effects• Deterioration due to corrosion etc.• Quality of workmanshipQ y p• Extent of damage and loss of life

resulting from failure

Capacity Reduction Factor Does not Allow for “understrength” due to

• Human ErrorRelies on competent structural engineers

from QUTQuality assurance procedures in theQuality assurance procedures in the

design offices and construction sites

10

Capacity Reduction Factor Allows for “under-strength”

0 9 f b• = 0.9 for members• = 0.8 for bolted connections• = 0.6 for GP welds;

= 0 8 for SP welds0.8 for SP weldsR = Ultimate design capacity from

AS 4100

Serviceability Limit State• Deflection• Vibration• Bolt slip• Corrosion

• These are checked against acceptable limitvalues under appropriate loadvalues under appropriate loadcombinations. For example, calculated acceptable, and typically for beams,deflection limit is span/250.

Serviceability Limit StateShort-term effects Long-term effects• G• s Q• 1 Q• Ws• Es

h th h t t ( ) d l t ( )where the short term (s) and long term (1)factors are given in Table 4.1 of AS 1170.0.

Limit State Design• This does not mean elimination of

failurefailure.• It means the failure is unlikely to occur,

ie. the risk of failure during thestructure’s intended life time is very low,ie. Probability is 2 x 10-7 (compare withcar travel – 3.6 x 10-4, Swimming 2 x 10-5)

11

Fatigue Limit State• Load fluctuations (ex. Wind)• Fatigue cracks and fracture• Fatigue cracks and fracture• Structural failures at stresses well below

yield levels• Fatigue damage is repairable• Welded jointsWelded joints• Minimise stress concentrations• Fatigue life versus Stress range curves

Fire Limit State• Prevent premature collapse & fire spread• Passive approach (just protecting) versusPassive approach (just protecting) versus

Active approach• Open car parks – no fire protection is

needed• Fire load versus Fire performance should

be assessed and then protection providedbe assessed and then protection provided• Fire resistance level versus Period of

structural adequacy method

Brittle Fracture Limit State• Occurs suddenly in the tensile stress

regions at low temperatureregions at low temperature• Can occur at low stress levels (25% of fy)• Use steel with adequate notch toughness• Limit max steel thickness for a given steel

grade depending on the lowest serviceg p gtemperature (notch-ductile temperaturerange)

Brittle Fracture Limit StateCold water sinks Titanic or is

it really iceberg??y gUSS Ponaganaset broke into two

while at a dockside in Boston

: Reasons: cold day and tiny crack in welding

Below the Nil-ductility transition temperature, steel breaks in a brittle manner!!

12

Design Process• Step 1: Decide on the structural layout for the

building• Step 2: Determine the design loads and their

design load combinations with appropriate loadfactors

• Step 3: Structural analysis to determine thedesign action effects such as maximum bendingdesign action effects such as maximum bendingmoment M*, axial tension or compression forceN* and shear force V*

Design Process• Step 4: For the chosen member, determine the

design capacity = capacity reduction factor xnominal capacity R based on AS4100 rules

• Step 5: If Design Action Effect M* or N* or V * Design Capacity R, then design is ok.Otherwise choose another member size andrepeat the processrepeat the process

• Step 6: Produce design drawings: They shouldinclude the following: design data and details

Design Aids• Mahen’s Notes• AS4100 (1998) or HB2.2( )• AS1170 Parts 1 and 2 or HB2.2• ASI Design Capacity Tables www.steel.org.au• OneSteel Section Data Handbook• -www.onesteel.com/productsdb/products.asp

G l ’ T B k• Gorenc et al.’s Text Book• Bradford et al.’s Worked Examples Book

Summary• Design process• Design by calculation or testing• Design standards (loading, design and material

quality)• Various steel grades and sections (Grade 300,

350, 400--;UB/UC, WB/WC, PFC, EA, RHS----)• Important parameters: E f f• Important parameters: E, fy, fu

• Scope of AS4100 (t≥3mm, fy≤450MPa, not for cold-formed steel sections except RHS, SHS, CHS and composite sections)

13

Summary• Limit states design• Design loads/actions G, Q, Wu

• Factored load combinations to allow for overloading, 1.2G+1.5Q, 0.9G+ Wu

• Design actions S* (N*, V*, M*)• Design capacity φR (Nominal capacity = R)• Strength L S E ≤φR• Strength L.S. Ed ≤φR• Serviceability L.S• Stability L.S.• Have you got your books and notes?

Some Questions?• Why 1.2 for G and 1.5 for Q?• Why c <1 when all 3 loads act together?• Why 0 9G?• Why 0.9G?• Why Wu has no load factor?• What will be the load factor for Q in 2020?• If the building is not in the earthquake zone, do you

consider 1.2G+1.6Feq + cQ?• Why is φ different?• Why is φ different?• Which code do you use for the following cases? t=3

fy=400; t=2 fy=400; t=5 fy=550;• Can load factors be applied to design action effects

(M*) or only for actions?