DOS Steel Bridge-1

Part 1: BRIDGE STRUCTURES

DESIGN OF STRUCTURES

STEEL DESIGN

Introduction

DefinitionA structure that allows people or vehicles to cross an obstacle such as a river, canal or railway, etc. is called a BRIDGE.

Function of a BridgeA bridge has to carry a service (which may be highway or railway traffic, a footpath, public utilities, etc.) over an obstacle (which may be another road or railway, a river, a valley, etc.) and to transfer the loads from the service to the foundations at ground level.

A bridge is a key element in a transportation system for three reasons:It likely controls the capacity of the system.It is the highest cost per Kilometer of the system.If the bridge fails, the system fails.

Structural Differences between a Building and a Bridge

1. Bridges are designed for heavy and concentrated moving loads whereas buildings are usually designed for static distributed loads.

2. The impact of moving loads is quite considerable as compared with residential and official buildings.

3. Fatigue may become a problem and hence may reduce the strength due to large number of loading cycles.

4. Greater part of the structure is exposed to atmosphere.

5. The controlling design specifications for bridges are provided by organizations different from those dealing with the building design. For example, AASHTO Specification may be employed for bridges in place of AISC Specification for steel buildings.

Classification of Bridges

According to materials of construction : reinforced concrete, prestressed concrete, steel, composite, timber etc.

According to form of superstructure : slab, beam, truss, arch, suspension, cable-stayed etc.

According to inter-span relation : simple, continuous, cantilever.

According to the position of the bridge floor relative to the superstructure : deck, through, half-through etc.

According to span : short, medium, long, right, skew, curved.

According to degree of redundancy : determinate, indeterminate

According to type of service and duration of use : permanent, temporary bridge, military

Classification of Bridges (According to span)

Classification of Bridges (According to materials of construction )

Slab on Steel Girder Bridge

Slab on Prestressed Concrete Girder Bridge

Classification of Bridges (According to form of superstructure)

Slab on Steel Girder Bridge

Steel sections may be hot-rolled shapes (for short-span bridge), Box section (medium span), or Plate Girder (medium span).

Box Girder Bridge is used for curved and longer span bridges. These bridges decrease the total depth requirement and can resist torsion to a large extent.


Truss Bridge


Cable Stayed Bridge

A typical cable stayed bridge is a continuous girder with one or more towers erected above piers in the middle of the span. From these towers, cables stretch down diagonally (usually to both sides) and support the girder.


Suspension Bridge

A typical suspension bridge is a continuous deck with one or more towers erected above piers in the middle of span. The deck maybe of truss or box girder.

Cables pass over the saddle which allows free sliding.

At both ends large anchors are placed to hold the ends of the cables.

Suspension bridge needs to have very strong main cable

Cables are anchored at the abutment so abutment has to be massive


Arch Bridge

Arches use a curved structure which provides a high resistance to bending forces

Arches are good choices for crossing valleys and rivers since the arch doesn't require piers in the center. Arches can be one of the most beautiful bridge types

Classification of Bridges (According to inter-span relation )

Classification of Bridges (According to the position of the bridge floor relative to the superstructure )

A Deck Bridge is a bridge built at or near the top level of the main supporting members of the superstructure, which hang below the deck and are not visible from the bridge.

A Typical Plate Girder Deck Bridge.

Arch Type Deck Bridge.


In case of Through Bridge, the carriageway issupported at the bottom of the main supportingmembers that are visible while traveling on thebridge.

A Typical Plate Through Bridge.

Steel Truss Through Bridge


If the roadway lies between the top and bottom chords, the bridge is said to be a half-through or pony bridge.

Deck + Through Type

Components of Slab on Girder Bridge

A Typical Slab on Girder Bridge

Components of Composite Box girder Bridge

A Typical Box Girder Bridge

Components of Orthotropic Deck Bridge

In case of Orthotropic Deck Bridges, an orthotropic deck consistingof longitudinal folded steel plate resting on cross girders, providedat a spacing of 3 to 5 m.

The cavities of the plate are filled with tar and gravel and topped bywearing surface.

Components of Truss Through Bridge

A Typical Truss Through Bridge

Components of Truss Through Bridge

Stringers: These are longitudinal bridge deck beams spanning between the transverse floor beams and placed parallel to the roadway

Floor Beams: Floor beams are the main girders of the bridge deck spanning between trusses or plate girders and running perpendicular to the roadway.

Objectives of Bridge Design

Bridge design is comprised of Safety, Serviceability, Economy (Total life cycle cost), and Aesthetics.

To achieve the design goal the design process consists of collecting the data, creating a new form (Conceptual Design Stage), and finalizing the design for realization (Modeling, Dimensioning and Detailing Stage).

The minimum requirements of safety and serviceability are well provided by design specifications, which reflect the current state of bridge design technology. While Economy and Aesthetics are not specified in design code or standards.

Because the civil and structural engineer has also a cultural and social responsibility for bridge structures, it is absolutely necessary to create a beautiful structure.

There is an indissoluble connection between Aesthetics and Economy. So that, an optimal balance between these two objectives may require a design effort that is the true art of the engineer

Selection of type of bridge

Selection of type of bridge mainly depends on:Span lengthBridge lengthBeam spacingMaterial availableSite conditions (foundations, height, space constraints)Speed of constructionConstructabilityTechnology/ Equipment availableAestheticsCostAccess for maintenanceThe required clearancesErection possibilitiesFoundation choicesHydraulic characteristics of the stream, if one is involved.

Type of Superstructure Vs Span Lengths

Advantages of Steel Beam Bridges

1. Steel is a high quality, homogeneous and isotropic material that is perfectly elastic up to its yield point.

2. It has equal and high strengths in tension and compression.3. The material remains uncracked and exhibits appreciable ductility.4. Lesser construction time, compared with reinforced and prestressed

concrete bridges, reduces the overall cost.5. The basic skeleton of steel bridges may very easily be erected over various

gaps in natural surface.6. The design, erection and fabrication procedures for steel bridges are very

well established.7. Due to lesser self-weight of these bridges, the foundation cost is also

reduced.8. For their lesser depths, the steel bridges are preferred where underneath

clearance is important.9. Repair, rehabilitation and up gradation of steel bridges are usually easier

than concrete bridges.

AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS

Historical Development

The first US standard for bridges were published in 1931 (AASHO), the 17th edition of AASHTO Specifications in 2002

Working stress design (WSD), based on allowable stresses

In 1975-79 work on the new code, Ontario Ministry of Transportation, the 1st edition of the OHBDC in 1979

In 1986-87 feasibility study initiated by a group of bridge engineers

Work on the new code 1988-93

1st edition of AASHTO LRFD Code in 1994, the 2nd in 1998, 3rd in 2004– as an alternative document

By 2007, only AASHTO LRFD in the USA

Changes of LRFD from Standard Specifications

　Introduction of a new philosophy of safety

　 Identification of four limit states (strength, service, fatigue, extreme event)

　 Development of new load models (including new live load)

　 Development of new load and resistance factors

　 Revised techniques for the analysis and load distribution

　 Introduction of limit state-based provisions for foundation design

　 Introduction of isotropic deck design process

　 Commentary are now side-by-side with the standard

General design criteria in AASHTO LRFD Code

Probabilistic Design Philosophy

Probability and reliability based design.

Takes into account the statistical mean resistance and statistical mean load and dispersion of load and resistance measured by standard deviation.

The way to control the probability of failure to be within a reasonable limitis through the “Reliability Index (β)”　 This is better than the Factor of Safety

More β More Factor of safety

Resistance Factor　 Takes care of uncertainties in the resistance　 3 Main sources of uncertaintiesMaterial Property – uncertainty in the strength, chemical composition, defects.Fabrication – uncertainty in the dimensions and construction qualities.Analysis – many methods are approximate so there are a lot of uncertainties in predicting the resistance.

Resistance Factor

Load Multiplier

Iη =Importance factorThe owner may declare a bridge or any structural component and connection to be of operational importance.For strength and extreme event limit states

1.05 for bridge considered of operational importance e.g. the only bridge crossing the river1.00 for typical bridges0.95 for bridge considered non-important　

For all other limit states　 1.00 for all bridges

= Ductility factor (Brittle v.s. Ductile failure)The structural system shall be proportioned and detailed to ensure the development of significant and visible inelastic deformations at the strength and extreme event limit states before failure.For strength limit states　 1.05 for non-ductile components & connection which may fail in a brittle manner　 1.00 for conventional designs　 0.95 for components with enhanced ductility e.g. has additional stirrups for shear reinforcementsFor all other limit states　 1.00

Dη

Load Multiplier

　 = Redundant factorMultiple load path and continuous structures should be used. Main elements whose failure is expected to cause the collapse of the bridge shall be designated as failure-critical (non-redundant)　For strength limit states　 1.05 for non-redundant members e.g. a simple span bridges　 1.00 for conventional level of redundancy　 0.95 for exceptional level of redundancy e.g. multi-girder continuous beam bridge

　For all other limit states

　 1.00

Rη

Permanent LoadsDC = dead load of structural components and nonstructural attachmentsDW = dead load of wearing surface and utilitiesEL = accumulated locked-in force effects resulting from the construction processDD = downdragEH = horizontal earth pressure loadES = earth surcharge loadEV = vertical pressure from dead load of earth fill

LL = vehicular live load 　 CR = creep　 IM = vehicular dynamic load allowance FR = friction 　 PL = pedestrian live load SH = shrinkage 　

LS = live load surcharge BR = vehicular braking force　 TG = temperature gradient TU = uniform temperature　WA = water load and stream pressure CE = vehicular centrifugal force　 CT = vehicular collision force IC = ice load　 CV = vessel collision force EQ = earthquake　

WS = wind load on structure SE = settlement　WL = wind on live load

Transient Loads

Types of Loads

Limit States

There are four types of “limit states”

ULTIMATE LIMIT STATES –involving the strength and stability of the structure, both local and global

Strength I, II, III, IV

EXTREME EVENT LIMIT STATES - relates to the structural survival of a bridge during a major earthquake, flood, or collision

Extreme Event I, II

SERVICEABILITY LIMIT STATES – involving the usability of the structure including stress, deformation, and crack widths

Service I, II, III

FATIGUE LIMIT STATE - relates to restrictions on stress range to prevent crack growth as a result of repetitive loads during the design life of the bridge

　 All limit states are equally important (AASHTO LRFD 1.3.2.1)

Limit States

STRENGTH I: Basic load combination relating to the normal use of bridge.STRENGTH II: load combination for special vehicles specified by ownerSTRENGTH III: load combination where the bridge is subjected to high wind (> 90 km/h) and traffic is preventedSTRENGTH IV: load combination for long span bridges (>67 m span) which has large ratio of DC to LLSTRENGTH V: load combination where bridge and traffic on the bridge is subjected to wind velocity of 90 km/h

EXTREME EVENT I: load combination for structural survival under major earthquakeEXTREME EVENT II: load combination for structural survival under combination of events such as flood and vessel collision

SERVICE I: load combination for normal operation of the bridge and for checking compression in prestressed concreteSERVICE II: load combination for steel bridges to control yieldingSERVICE III: load combination relating to tension in prestressed concrete\ during service

FATIGUE: load combination for fatigue and fracture due to repetitive LL and IM

Load Combinations and Load factors

Load Combinations and Load factors

Examples of Load Combinations

1.25DC + 1.50DW + 1.75(LL+IM) (Strength I)　 1.25DC + 1.50DW + 1.4WS (Strength III)　 0.90DC + 0.65DW + 1.4WS (Strength III)　 1.50DC + 1.50DW (Strength IV)　 1.25DC + 1.50DW + 1.35(LL+IM) + 0.4(WS+WL) (Strength V)　 1.25DC + 1.50DW + 0.5(LL+IM) + 1.0EQ (Extreme I)　 0.90DC + 0.65DW + 0.5(LL+IM) + 1.0EQ (Extreme I)　 1.25DC + 1.50DW + 0.5(LL+IM) + 1.0 (CT or CV) (Extreme I)　 0.90DC + 0.65DW + 0.5(LL+IM) + 1.0 (CT or CV) (Extreme I)

For slabs and girders designs, we normally have only DC, DW and (LL+IM)

　 1.25DC + 1.50DW + 1.75(LL+IM) (Strength I)　 1.50DC + 1.50DW (Strength IV)　 1.00DC + 1.00DW + 1.00(LL+IM) (Service I)　 1.00DC + 1.00DW + 1.30(LL+IM) (Service II, Steel)　 1.00DC + 1.00DW + 0.80(LL+IM) (Service III, Prestressed)

Notes on Load Combinations

Note that the sections for maximum moment of dead load and live load are not the same!!!

Dead Load Moment : midspanLive Load Moment : some small distance away from midspanIf we add them together, we are conservative!

Critical moment for shear is d away from the support. We can calculate shear at this location for both dead load and live load If we know the height of the section.

We estimate the height from past experiences of similar projects.

If we don’t know, we calculate the shear at the support. This is conservative but may not be economical.

Typical Loads

Typical Loads on the bridge are:Dead Loads: DC/DWLive Loads of Vehicles: LLPedestrian Load: PLDynamic (Impact) Loads: IM

Dead Loads DC:

Dead load includes the self weight of:structural components such as girder, slabs, cross beams, etc…nonstructural components such as medians, railings, signs, etc…

But does not include the weight of wearing surface (asphalt)We can estimate dead load from its density

Dead Load of Wearing Surface: DW

It is the weight of the wearing surface (usually asphalt) and utilities (pipes, lighting, etc…)Different category is needed due to large variability of the weight compared with those of

structural components (DC).Asphalt surface may be thicker than designed and may get laid on top of old layer over and over.Density of asphalt paving material= 2250 kg/m3

Average Thickness of asphalt on bridge= 9 cm

Densities of Materials for Calculation of Loads

Design Lane

The design lane has a width equal to the lesser of 3600 mm or width of the traffic lane.

Roadway widths from 6000 to 7200 mm shall have two design lanes, each equal to one-half the roadway width.

The number of design lanes is taken as the integer part of the result when the clear roadway width in mm between curbs is divided by 3600.

Multiple Presence Factor

If the design lanes are more than one, reduction factor is applied on the live load force effect called Multiple Presence Factor denoted by m.

We have considered the effect of load placement in ONE lane

But bridges has more than one lane

It’s almost impossible to have maximum load effect on ALL lanes at the same time

The more lanes you have, the lesser chance that all will be loaded to maximum at the same time

Design Vehicular Live Load

AASHTO has 3 basic types of LL called the HL-93 loading (stands for Highway Loading, year 1993)1. Design truck2. Design tandem3. Uniform loads

(1) Design truckThe design truck is called HS-20 (stands for Highway Semi-Trailer with 20-kips weight on first two axles)Weight shown are for each one axle = 2 wheelsTotal Wt = 325 kN Distance between second and third axles may be varied to produce maximum effect.The design truck or tandem shall be placed transversely at 0.3m from the face of curb or railing for the design of bridge overhang and 0.6 m from edge of the design lane for the design of all other components.


(2) Design Tandem

The design tandem shall consist of a pair of 110 kN axles at a longitudinal spacing of 1.2m with the transverse center-to-center spacing of the wheels being 1.8 mm.

Lead to larger moment than the HS20 truck for simple-support spans less than about 13.4 m

(3) Uniform Lane Loading

Uniform load of 9.3 kN/m acting over a tributary width of 3 m. (i.e. the load is 3.1 kN/m2)

May be apply continuously or discontinuously over the length of the bridge to produce maximum effect


3 ways to add the design truck, design tandem, and uniform load together

1. Combination 1: one HS20 truck on top of a uniform lane load per design lane2. Combination 2: one Design Tandem on top of a uniform lane load per design

Lane3. Combination 3: (for negative moments at interior supports of continuous beams)

place two HS20 design truck, one on each adjacent span but not less than 15 m apart (measure from front axle of one truck to the rear axle of another truck), with uniform lane load. Use 90% of their effects as the design moment/ shear.

The loads in each case must be positioned such that they produce maximum effects (max M or max V)

The maximum effect of these 3 cases is used for the design

A pedestrian load of 3600 N/m2 is used on all sidewalks simultaneously with the vehicular design live load.

Separate bridges for pedestrian and bicycle traffic should be designed for a live load of 4100 N/m2.

Dynamic Load Allowance: IM

Sources of Dynamic EffectsHammering effect when wheels hit the discontinuities on the road surface such as

joints, cracks, and potholes.Dynamic response of the bridge due to vibrations induced by traffic

Actual calculation of dynamic effects is very difficult and involves a lot of unknowns.

To make life simpler, we account for the dynamic effect of moving vehicles by multiplying the static effect with a factor.

Add dynamic effect to the following loads:Design TruckDesign Tandem

But NOT to these loads:Pedestrian LoadDesign Lane Load

Documents

DOS Steel Bridge-1