BASIS OF DESIGN OF COMPOSITE STEEL BRIDGESRamy Gabr, M.Sc. , P.E.

BASIS OF DESIGN OF COMPOSITE STEEL BRIDGESParsons, Doha

The discussion presented herein is based on the bases of design of the AASHTO LRFD 2010 Section 6 – Steel structures.

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AGENDA

1. Introduction (Types, Pros and Cons)2. Limit State Design3. Section Properties4. Service Limit State5. Strength Limit State6. Fatigue and Fracture Limit State

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AGENDA

1. Introduction (Types, Pros and Cons)2. Limit State Design3. Section Properties4. Service Limit State5. Strength Limit State6. Fatigue and Fracture Limit State

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Introduction

Types of Steel Bridges1. Beam Bridges (15m – 100m)

a) Multi-girderb) Ladder deck

2. Box Girder Bridges (45m – 180m)3. Truss Bridges (40m – 500+m)4. Arch Bridges (30m – 500m)5. Cable-stayed Bridges (200m – 850+m)6. Suspension Bridges (>850m)

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Introduction

Advantages:• Pleasing aesthetics offering a

smooth cross section.• Structural and nonstructural

components are typically hidden.

• Better torsional resistance.• More economical (steel

weight) due to the increased bending strength offered by their wide bottom flanges.

Disadvantages:• Box girders should be no less

than 1.5m deep to allow access for maintenance and inspection.

• Higher fabrication and erection costs.

• Requires skilled workers during fabrication.

• Risks associated with working in enclosed spaces during maintenance.

Advantages and Disadvantages of Bridge TypesBox-Type Bridges

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Introduction

Advantages:• Simpler design• Lighter sections• Ease of fabrication and

erection• Ease of maintenance

Disadvantages:• Accumulation of debris and

acids on top of the flanges• Aesthetics is not preferred• Coating is more difficult than

box girder types

Advantages and Disadvantages of Bridge TypesBeam-Type Bridges

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AGENDA

1. Introduction (Types, Pros and Cons)2. Limit State Design3. Section Properties4. Service Limit State5. Strength Limit State6. Fatigue and Fracture Limit State

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Limit State Design

Limit State: a condition of a structure beyond which it no longer fulfills the relevant design criteria.

Specified limit states to achieve the objectives of constructability, safety and serviceability.

Other design provisions address inspectability, economy and aesthetics (Section 2.5). However, these issues are not part of the limit-state design philosophy.

Limit state in LRFD:

is the load or stress level is the load factor (load combinations) is the load modifier (ductility, redundancy, and importance)

are the nominal and factored resistance respectively is resistance factor

rn RRQ Q

rn RR &

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Limit State Design

Similarly, limit state in ASD:

is the load or stress level is the load modifier (ductility, redundancy, and importance)

are the nominal and factored resistance respectively is safety factor

Limit states in AASHTO LRFD – Section 6:• Service Limit State• Strength Limit State• Fatigue and Fracture Limit State• Extreme Event Limit State

rn RR

Q

Q

rn RR &

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Limit State Design

Limit states in AASHTO LRFD – Section 6:• Service Limit State

• The service limit states ensure the durability and serviceability of the bridge and its components under typical everyday (i.e. service) loads.

• The AASHTO LRFD includes four service limit state load combinations of which only two are applicable to steel bridges.

• Strength Limit State• Ensure strength and stability of the bridge and its

components under the statistically predicted maximum loads (i.e. factored) during the life span of the bridge.

• The strength limit states are not based upon durability or serviceability.

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Limit State Design

Limit states in AASHTO LRFD in Section 6:• Fatigue and Fracture Limit State

• Represents a more severe consequence of failure than the service limit states (brittle failure), but not necessarily as severe as the strength limit states (many passages trucks may cause a critically-sized fatigue crack while only one heavy truck can lead to strength limit state failure).

• The fatigue and fracture limit state is only applicable where the detail under consideration experiences a net applied tensile stress under FATIGUE load (S-N Curve).

• Using FATIGUE I (g=1.5, infinite life) and FATIGUE II (g=0.75, finite life) depends on comparing the expected ADTTSL against ADTTSL for 75-year equivalent to infinite life specified in AASHTO.

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Limit State Design

Limit states in AASHTO LRFD in Section 6:• Extreme Event Limit State

• Represents less frequent events such as earthquakes and vehicular collisions.

• These loads or events of such great magnitude that if designed for the levels of reliability or failure rates of the strength limit states would be economically prohibitive or unfeasible.

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AGENDA

1. Introduction (Types, Pros and Cons)2. Limit State Design3. Section Properties4. Service Limit State5. Strength Limit State6. Fatigue and Fracture Limit State

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Section Properties

Bridge deck sections are divided into:• Steel section (before composite action) (DC1 and CLL).

Applicable for bridge during erection and pouring of deck. Used to compute stresses for DL= OW+Concrete Deck

• Composite section beff: Table 4.6.2.6.4-1

STEEL SECTION

1

4

EFFECTIVE WIDTH

beff beff

a = 0.8w - 1.2ww

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Section Properties

Bridge deck sections are divided into :• Composite section:

• Short term(n): used with transient loads (LL+I)• Long term (3n) due to creep and shrinkage – used with

permanent loads (DC2 and DW) (6.10.1.1.1)beff

b/3n

EFFECTIVE SECTIONLong Term

Composite (3n=24)

beff

b/n

EFFECTIVE SECTIONShort Term

Composite (n=8)

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AGENDA

1. Introduction (Types, Pros and Cons)2. Limit State Design3. Section Properties4. Service Limit State5. Strength Limit State6. Fatigue and Fracture Limit State

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Service Limit State

Service limit state in AASHTO LRFD (Sections 6.10.4 and 6.11.4) includes:a) Elastic deformations: control of live load deflections

b) Permanent deformations: prevent permanent deflections due to expected severe traffic loadings that would harm rideability.

• It includes satisfaction of flange stresses not exceed: composite sections noncomposite sections

• These checks are to be made under the SERVICE II combination.

800span

LLall

yfhFR95.0

yfhFR80.0

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AGENDA

1. Introduction (Types, Pros and Cons)2. Limit State Design3. Section Properties4. Service Limit State5. Strength Limit State6. Fatigue and Fracture Limit State

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Strength Limit State

Strength limit state in AASHTO LRFD includes:a) Constructibility checks (Sections 6.10.3 and 6.11.3):

• Involves checking the steel cross section before composite action development (only DC1 and CLL).

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Strength Limit State

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Strength Limit State

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Strength Limit State

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Strength Limit State

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Strength Limit State

Strength limit state in AASHTO LRFD includes:a) Constructibility checks (Sections 6.10.3 and 6.11.3):

• Involves checking the steel cross section before composite action development (only DC1 and CLL).

• Flexural checks (Top Flange):o Vertical bending stress (fbu)o Lateral bending stress (fl)

• Wind loads• Horizontal component of web shear• Deck overhang loads• Amplification factor to account for second-order nonlinear effects (geometry)

o Yielding (Fy)o Local buckling and Lateral torsional buckling (Fnc)

o Web bend-buckling resistance (Fcrw)

ychflbu FRff

crwfbu Ff ncflbu Fff 3

1

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Strength Limit State

Strength limit state in AASHTO LRFD includes:• Flexural checks (Bottom Flange):

o Vertical bending stress (fbu)o Yielding (Fy)

is function of St. Venant torsional shear (1.0 in straight bridges, <1.0 in curved bridges)

• Shear checks (Web):o Usually shear during construction is not governing

yfhfbu FRf

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Strength Limit State

Strength limit state in AASHTO LRFD includes:b) Strength limit state checks (Sections 6.10.6 and 6.11.6):

• Flexural checks (positive moments)o Compact section

Where Mn is based on My & Mp

o Noncompact section, is based in yield and torsion

o Sections in horizontally curved steel girder bridges shall be considered as noncompact sections

• Flexural checks (negative moments)

o depends on the presence of longitudinal stiffeners and torsion, , depends on yield

nfxtlu MSfM 31

ncfbu Ff ntflbu Fff 31

ncflbu Fff 31

ntflbu Fff 31

ncF

ntF

nF

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Strength Limit State

Strength limit state in AASHTO LRFD includes:b) Strength limit state checks (Sections 6.10.6 and 6.11.6):

• Shear checks

o Unstiffened websis based on elastic shear yielding or shear

buckling resistanceo Stiffened webs

End panels: is based on shear yielding or shear buckling resistance taking into account stiffener spacingInterior panels: is based on shear yielding or shear buckling resistance with tension field action if satisfying its requirements

pnvu CVVV

nV

nV

nV

Interior panelEnd panel

Ddo 3 Ddo 5.1

D

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AGENDA

1. Introduction (Types, Pros and Cons)2. Limit State Design3. Section Properties4. Service Limit State5. Strength Limit State6. Fatigue and Fracture Limit State

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Fatigue and Fracture Limit State

Fatigue and fracture limit state in AASHTO LRFD (Sections 6.10.5 and 6.11.5) includes:a) Fatigue checks

• Load induced fatigue – compression and tensions flanges and shear studs (and any component produce net tensile stresses)

is the nominal fatigue resistance and it depends on the detail category of each component (A, B, B’, C, C’, D, E, E’)

• Distortion induced fatigue (detailing)o Distortion-induced fatigue occurs in the web near a

flange at a welded connection plate for a cross-frame where a rigid load path has not been provided to adequately transmit the force in the transverse member from the web to the flange.

nFf )()( nF )(

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Fatigue and Fracture Limit State

Fatigue and fracture limit state in AASHTO LRFD (Sections 6.10.5 and 6.11.5) includes:a) Fatigue checks

• Distortion induced fatigueo To control web buckling and elastic flexing of the web

(i.e. rigid load path), transverse connection to be at top and bottom flanges and the provision of “Special fatigue requirements for webs” shall be satisfy

• Special fatigue requirements for webs (Section 6.10.5.3)o Interior panels of webs with transverse stiffeners,

shall satisfy shear strength limit state under FATIGUE load combination (with twice the factored fatigue load considered). This involves checking maximum web shear-buckling instead of the stress ranges.

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Fatigue and Fracture Limit State

Fatigue and fracture limit state in AASHTO LRFD (Sections 6.10.5 and 6.11.5) includes:b) Fracture requirements (should be mentioned in the specs)

• All primary superstructure components and connections sustaining tensile force effects due to STRENGTH I load combination, require mandatory Charpy V-notch fracture toughness.

• Fracture-critical membersto be tested in conformancewith AASHTO T 243M(ASTM A 673M).

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Fatigue and Fracture Limit State

Thank YouQ & A