Steel Bridges Seminar

VISVESVARAYA TECHNOLOGICAL UNIVERSITY, BELGAUM

STEEL BRIDGES A Seminar Report

By Aamodh.K (USN: 4JC09EC001) & Sahas.S (USN:4JC09EC077)

II Sem. B.E.: Electronics and Communication

As a part of CV220: Elements of Civil Engineering and Engineering Mechanics

SRI JAYACHAMARAJENDRA COLLEGE OF ENGINEERING, MYSORE – 570 006 (Autonomous) 2009-2010

INTRODUCTION

Bridges are great symbols of mankind’s conquest of space. The sight of the Golden Gate Bridge in the Pacific Ocean, or the Rajiv Gandhi Sea-Link (BandraWorli) in Mumbai, fills one’s heart with wonder and admiration for the art of their  builders. They are the enduring expressions of mankind’s mankind’s determination to remove all  barriers in its pursuit of a better and freer world. wo rld. Their design and building schemes are conceived in dream-like visions. But vision and determination are not enough. All the physical forces of nature and gravity must be understood with mathematical  precision and such forces have to be resisted by manipulating manipulating the right materials in in the right pattern. This requires both the inspiration of an artist and the skill of artisan . There are about 1,20,000 bridges of all types and spans in India and about 50% of these bridges are more than 100 years old. Though more than 1000 bridges are rebuilt / rehabilitated every year, the backlog is enormous. Old bridges are ar e facing following following types of prob lems: 1. Aging and fatigue consideration 2. Increased loading standards for axle load 3. Increased longitudinal loads 4. Rebuilding meter gauge bridges for broad gauge work.

Most of these problems can be overcome by using steel as the core structural material. Steel is suitable for most span ranges, but particularly for longer spans. Right from Rabindra setu to Bandra-Worli sea link, structural steel has been the natural solution for long span bridges. Howrah Bridge, also also known as Rabindra Rabindra Setu, is to be looked at as an early classical steel bridge in India. This engineering marvel is still serving the nation, deriding all the myths that people have about steel. The sweeping 5.6 km Bandra worli sea link is the latest completed steel marvel in India. The length of steel cables used in the sea link is equivalent to the earth's circumference. circumference. This makes it all the more important importa nt to know the advantages that steel  possesses over concrete which has made bridges like Rabindra Setu outlast other concrete bridges.

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ADVANTAGES OF STEEL IN BRIDGE CONSTRUCTION

Steel is a very versatile material having many advantages over the other material. First cost, life-cycle costs, and environmental effects favor steel as the material of choice for bridges. New materials, new design concepts, and a better u nderstanding of the trade-off between structural reliability and life-cycle costs make the next millennium an exciting time for steel technology. In evaluating the trade-off between structural reliability and life-cycle costs, engineers need to keep in mind the potential advantages steel structures can offer. These include but are not limited to the following:

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Reduced dead loads.

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More economic foundations.

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Shorter execution time.

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Lighter weight than concrete for superstructures of comparable spans, reducing the inertia effects induced by seismic events.

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Ductility and toughness of material to allow absorption of loading well above design values without catastrophic failures.

When constructed in insurgency affected areas like North-East and J&K and in high seismicity areas where damage to the bridges is more likely, steel bridges provides easier and faster options for rehabilitation. More over, structural redundancies can be easily inbuilt in steel bridges.

In addition to the various points cited above, structural steel as the basic bridge construction material involves several other advantages, which have also played an important part in this shift of an engineer’s ideology from concrete bridge construction to steel bridge construction and have been discussed later.

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CLASSIFICATION OF STEEL BRIDGES Steel bridges are classified according to 

the type of traffic carried

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the type of main structural system

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the position of the carriage way relative to the main structural system

Classification Based On Type Of Traffic Carried

Bridges are classified as

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 Highway Or Ro ad Brid ges

Bridges mainly for transport of highway or heavyloaded vehicles. Ex:Mahatma Gandhi Setu (Patna-Hajipur,across Ganges, Bihâr)

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 Railway Or Rail Bridges

Bridges mainly for rails to cross across a river or large pits. Ex: Pamban Bridge (Rameshwaram)

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 Road - Cum - Rail Bridges

These bridges offer way to both rail and small (somecases include heavy) loaded vehicles. Ex: Vivekananda setu (Howrah – Kolkata) (fig 1.0)

Fig 1.0 Vivekananda setu

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*Note:  Important terms ap pearing in th e study of steel bridges:

Forces acting on steel bridge(or any bridge): (i) Dead lo ad  :

This term refers to the weight of the bridge itself. Like any other

structure, a bridge has a tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is constructed (i.e., the wood, concrete, steel, or aluminum)

(ii) Live load   :

This term refers to traffic that moves across the bridge as well as

normal environmental factors such as changes in temperature, precipitation , and winds. (iii) Dynamic load:  The third factor refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind  and earthquakes. All three factors must be taken into consideration in the design of a bridge. Parts of the steel bridge:  Abutment - Heavy supporting structures usually attached to bedrock and supporting

 bridge piers.  Bedrock  - Portion of Earth's mantle made of solid rock on which permanent

structures can be built. Piers - Vertical columns, usually made of reinforced concrete or some other strong

material, on which bridges rest. Suspenders - Ropes or steel wires from which the roadway of a bridge is suspended. Truss - A structure that consists of a number of triangles joined to each other.

Truss

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Classification Based On The Main Structural System

Many different types of structural systems are used in bridges depending upon the span, carriageway width and types of traffic. Classification, according to make up of main load carrying system, is as follows:

(i) Girder bridges A girder is a support beam used in construction. Girders often have I beam cross section for strength, but may also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure, which supports smaller beams. A girder is commonly used many times in the building of bridges.

Fig 2.0 Girder bridge

A girder bridge, in general, is a bridge built of girders  placed on bridge abutments and foundation piers. In turn, a bridge deck is built on top of the girders in order to carry traffic. Girders combine strength with economy of materials and can therefore be relatively light. There are several different subtypes of girder bridges: (a) Rolled steel girder bridge: This type of girder bridge is made up of I beams that are rolled into the shape of a steel mill. These are useful for spans between 10 meters and 29.5 meters. Rolled steel girders are practically available with a web height of up to one meter.

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(b) Plate girder bridge: It is made out of mostly flat steel sections that are later welded or otherwise fabricated into an I beam shape. In some cases plate girders may be formed in a Z shape rather than I shape. One distinguished advantage of plate girders over rolled steel girders is that the web of plate girder can be taller than that of a rolled steel girder, providing greater strength than rolled steel girders. The thickness of the top and bottom flanges of a  plate girder does not have to be constant; the thickness can be changed (typically at a field splice) to save on material costs. Stiffeners are occasionally welded between the compression flange and the web to increase the strength of the girder.

They can be

used for spans between 10 meters and 100 meters. Cross section of a typical plate girder is shown in figure 2.1(b.)

Fig.2.1 (b) Plate girder bridge section

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(c) Box Girder Bridge: Box girder bridges are built from steel girders in a rectangular box shape instead of an I beam shape. A typical box girder has two webs and two flanges. Cross section of a typical box girder is as shown in figure 2.1(c). They can be used for continuous spans up to 250 meters.

Box girders hold an important advantage over plate and rolled steel girders. A plate girder or rolled steel girder is simple to design and build, but works well only for straight spans. However, if the bridge needs to be curved, the beams are subject to twisting forces. This can be altered by building several shorter straight spans with a curved bridge deck or by using box girders. The added second web in a box girder adds stability and increases resistance to twisting forces. Though box girders po ssess several advantages over their counterparts, building these steel box girders is more difficult, because the welding of inner corners between the flanges webs has to be done either by a robot or a human, depending on who can fit inside. Ex: Bridge of Niteroi, Rio de Janerio, Brazil.

Fig.2.1 (c) Box girder bridge section

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 (d) Truss girders:

A truss is a structure composed of triangular units that are connected at joints called nodes. A truss bridge is a bridge consisting of such triangular units that are straight in shape. Truss bridges are suitable for span range of 30 meters to 375 meters. The cross sections of truss bridges is as shown in figure 2.1(d). Ex: Quebec Bridge (Canada) ,The Bridge over The River Kwai, Thailand.

Fig.2.1 (d) Some of the trusses used in steel bridges

One of the advantages of truss bridges is that they can be constructed with less material to give greater strength. Truss bridges are strong, since they are made of triangles, which are ridged. Another advantage about truss bridges is that they can be  built in a convenient area, then placed over where it has to bridge, although this is not always possible. That makes the truss bridge a perfect replacement to span over railways and roads. The disadvantage of a truss bridge is that it takes up more space and can sometimes become a distraction to drivers. Also, a lot of materials may be wasted if they are not designed properly i.e. there can be force members doing nothing for the structure.

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(ii)Cantilever bridge

A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1,500 feet (460 m), and can be more easily constructed at difficult crossings by virtue of using little or no falsework. Ex: Howrah Bridge, Forth Rail Bridge.

Fig2.1(e) Cantilever bridge

Building out from each end enables construction to be done with little disruption to navigation below. The span can be greater than that of a simple beam,  because a beam can be added to the cantilever arms. Cantilever bridges are very common over roads. Because the beam is resting simply on the arms, thermal expansion and ground movement are fairly simple to sustain. The supports can be simple piers, because there is no horizontal reaction. Cantilever arms are very rigid,  because of their depth

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Like beams, they maintain their shape by the opposition of large tensile and compressive forces, as well as shear, and are therefore relatively massive. Truss construction is used in the larger examples to reduce the weight.

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(ii) Rigid fra me bridges – In building construction field, ‘rigid frame’ is a steel skeleton frame in which the end connections of all members are rigid so that the angles they make with each other do not change. A bridge using such rigid frames is called a rigid frame bridge. In a standard girder bridge, the girder and the piers are separate structures. However, a rigid frame bridge is one in which the piers and girder are one solid structure. The cross section of a typical rigid frame bridge is as shown in figure 2.2 Rigid frame bridges are suitable in the span range of 25 m to 200 m.

Fig.2.2 Typical rigid frame bridge

(iii)  Arch bridges An arch bridge is a bridge with abutments at each end shaped as a curved arch. This curved structure provides high resistance to bending forces. Arches can only be used where the ground or foundation is solid and stable because unlike girder and truss bridges, both ends of an arch are fixed in the horizontal direction (i.e. no horizontal movement is allowed in the bearing). Thus when a load is placed on the  bridge (e.g. a car passes over it) horizontal forces occur in the bearings of the arch. Steel being the most elastic material adds extra efficiency to arch bridges. The cross sections of typical arch bridges is as shown in figure 2.3. Ex: Sydney Harbour bridge,Sydney.

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Fig.2.3 Typical arch bridges

(iv) Cable stayed bridges – 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. Steel cables are extremely strong but very flexible. Cables are very economical as they allow a slender and lighter structure, which is still able to span great distances. Cable stayed bridges are economical when the span is about 150 m to 700 m. Though only a few cables are strong enough to support the entire bridge, their flexibility makes them weak to a force we rarely consider: the wind For longer span cable-stayed bridges, careful studies must be made to guarantee the stability of the cables and the bridge in the wind. The lighter weight of the bridge, though a disadvantage in a heavy wind, is an advantage during an earthquake. However, should uneven settling of the foundations occur during an earthquake or over time, the cable-stayed bridge can suffer damage so care must be taken in planning the foundations. The modern yet simple appearance of the cablestayed bridge makes it an attractive and distinct landmark. The cross section of a typical cable stayed bridge is as shown in figure 2.4. Ex:Bandra-worli, Vidyasagar setu.

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Fig.2. 4 Layout of cable stayed bridges

(v) Suspension bridges – A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. The bridge d eck is suspended from cables stretched over the gap to be bridged, anchored to the ground at two ends and passing over tall towers erected at or near the two edges of the gap. Currently, the suspension bridge is best solution for long span bridges. Fig. 1.5 shows a typical suspension bridge. Ex:Akashi Kaikyo, Japan

Fig 2.5 Suspension bridges

Advantages over other bridges: The center span of the bridge can be made very long in proportion to the amount of materials required, allowing the bridge to economically span a very wide waterway. Also, it can be built high over water to allow the passage of very tall ships. Except for installation of the initial temporary cables, little or no access from below is required during construction, for example allowing a waterway to remain open while the bridge is built above

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Disadvantages over other bridges: Considerable stiffness or aerodynamic profiling may be required to prevent the  bridge deck vibrating under high winds. The relatively low deck stiffness compared to other (non-suspension) types of bridges makes it more difficult to carry heavy rail traffic where high concentrated live loads occur.

Classification Based On The Position Of Carriageway

The bridges may be of the "deck type", "through type" or "semi-through type". These are described below with respect to truss bridges:

(i) Deck type bridge The carriageway rests on the top of the main load carrying members. In the deck type  plate Girder Bridge, the roadway or railway is placed on the top flanges. In the deck type truss Girder Bridge, the roadway or railway is placed at the top chord level as shown in Fig. 3.1(a).

Fig.3.1 Typical deck, through and semi-through type truss bridges

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(ii) Through type bridge The Through Truss bridge consists of two side trusses connected across the top and bottom. Trains drive through the box formed by the members. Steel trusses were created from riveted iron plates and bars that create straight truss members. The stick-like members are connected together at the joints so that they form triangular and then rectangular shaped sections. This diagonal webbing effect gives the complete truss its strength to carry the heavy locomotives with a minimum amount of steel. The carriageway rests at the bottom level of the main load carrying members [Fig. 3.1(b)]. In the through type plate girder bridge, the roadway or railway is placed at the level of bottom flanges. In the through type truss girder bridge, the roadway or railway is placed at the bottom chord level. The bracing of the top flange or lateral support of the top chord under compression is also required.

(iii) Semi through type bridge The deck lies in between the top and the bottom of the main load carrying members. The bracing of the top flange or top chord under compression is not done and part of the load carrying system project above the floor level as shown in Fig. 3.1(c). The lateral restraint in the system is obtained usually by the U-frame action of the verticals and cross beam acting together.

The normal span range of different bridges is as shown below(fig 4.0)

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STEEL USED IN BRIDGES Steel used for building bridges and structures contains: (1) Iron (2) A small percentage of carbon and manganese (3) Impurities that cannot be fully removed from the ore, namely sulphur and Phosphorus (4) Some alloying elements that are added in very small quantities to improve the properties of the finished product, namely copper, silicon, nickel, Chromium, molybdenum, vanadium, columbium and zirconium. The strength of the steel increases as the carbon content increases, but some other properties like ductility and weldability decreases. Sulphur and phosphorus have undesirable effects and hence their maximum amount is co ntrolled. Steel used for building bridges may be grouped into the following three categories:

(1) Carbon Steels  – only manganese, and sometimes a trace of copper and silicon, are used as alloying elements. This is the cheapest steel available for structural uses where rigidity rather than strength is important. It comes with yield stress up to 275 N/mm2 and can be easily welded.

(2) High-Strength Steels – these cover steels of a wide variety with yield stress in the range of 300 to 390 N/mm2. They derive their higher strength and other required properties from the add ition of alloying elements.

(3) Heat-Treated Carbon Steels – these are the steels with the highest strength, and still retain all the other properties that are essential for building bridges. They derive their enhanced strength from some form of heat treatment after rolling, namely normalization or quenching-and-tempering.

(4) Weathering Steel –  this variety of steel is produced with enhanced resistance to atmospheric corrosion and these can be left unpainted in appropriate situations.

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DESIGN AND FLEXIBILITY An efficient bridge design balances the two fundamental rules of structural behavior and economy. Steel bridge concepts should maximize structural efficiency by reducing the amount of material and the number of components without compromising safety, serviceability, or constructibility of the structure. Simplicity and ease of fabrication are still paramount to cost effective steel bridge constructions. One of the benefits of a  properly conceived and executed bridge design is aesthetics. When structures have a clearly defined load path and members are correctly proportioned, they will be both cost-effective and aesthetically pleasing.

ECONOMIC ADVANTAGES OF STEEL High Strength To Weight Ratio

One of the biggest advantages of steel is weight savings, which means lower erection costs, since the bridge pieces can be handled with lighter equipment. High strength to weight ratio of steel minimizes substructure costs. In addition, it facilitates very shallow construction depths, which overcome problems with headroom and flood clearances, and minimises the length of approach ramps.

High Quality Material

Steel is a high quality material, which is readily available world wide in various certified grades, shapes and sizes. Prefabrication in controlled shop condition leads to high quality work at minimum cost. The quality control extends from the material itself and follows on through the processes of cutting, drilling, welding and fit-up. The total weight of steel constructions is a fraction of the total weight of concrete bridges. Also, steel has compressive and tensile strengths of 370 N/mm2, about ten times the compressive strength of a medium concrete and a hundred times its tensile strength. Therefore steel bridges can be u sed with long spans, even in earthquake-prone areas.

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Speed of Construction

The prefabrication of the components means that construction time on site in hostile environment is minimized. The speed of steel bridge construction reduces the durations of rail possessions and road closures, which minimises disruption to the  public using those networks. The lightweight nature of steel permits the erection of large components. Besides this, resource, such as water, aggregates etc may sometimes not be easily available at sites on this project, for the purpose of  production of concrete.

Versatility

Steel suits a wide range of construction methods and sequences. Installation may be  by cranes, launching, slide-in-techniques or transporters.

Steel gives the engineer

flexibility in terms of erection sequence and programme. Components can be sized to suit access restriction at site, and once erected the steel girders provide a platform for subsequent operations.

Recycling

Steel is the most environmentally friendly material used in bridge construction.

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 principal ingredient of the raw material for steel bridges is scrap steel. Steel is a ‘sustainable’ material. When a steel bridge reaches the end of its useful life, the girders can be cut into manageable sizes to facilitate demolition, and returned to steelworks for recycling. The increased emphasis of the green techniques for construction, steel is lot ‘Greener’ than concrete for bridges.

Repair & Rehabilitation

Steel bridges can readily be repaired after accidental damages. In case of damage to the bridge due to derailment/accident, damage due to a terrorist activity or damage due to natural causes such as earthquakes, floods etc. complete steel spans can be replaced without much delay which is not the case with PSC super structures. This aspect is very important in the case of Railways where longer disruption to rail traffic can not be afforded.

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Aesthetics

Steel has broad architectural possibilities. Steel bridges can be made to look light or heavy, and can be sculptured to any shape or form. The high surface quality of steel creates clean sharp lines and allows attention to detail. Modern fabrication methods have removed restrictions on curvature in both plan and elevation. The painting of steelwork introduces colour and contrast, and repainting can change or refresh the appearance of the bridge to appear as new.

Durability

Steel bridges now have a proven life span extending to well over 100 years. In fact, old steel girders of vintage 1854 etc are also in use on branch lines. Steel has a  predictable life, as the structural elements are visible and accessible. Any signs of deterioration are readily apparent, without the need for extensive investigations. Corrosion is a problem requiring major maintenance. The potential durability of steel may be summarized in the following quote by a Mr. J.A.Waddell in 1921:

“The life of a steel bridge that is scientifically designed, honestly and carefully built, and not seriously overloaded, if properly maintained, is indefinitely long.”

HIGH PERFORMANCE STEEL

The “normal” steel, though has a lot of advantages over concrete, has several areas which can be improved upon. Over the last decade, taking the limitations of conventional steel into account a new grade of steel, known as ‘high performance steel’ has been developed.

All steels possess a combination of properties that determine how well steel  performs its intended function. Strength, weldability, toughness, ductility, corrosion resistance, and formability are all important to determine how well a steel performs. High-performance steel (HPS) can be defined as having an optimized balance of these properties to give maximum performance in bridge structures while remaining cost-effective.

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Properties Of High Performance Steel

Strength

The high strength of HPS comes from heat treatment and rolling processes during manufacture rather than from carbon content. HPS contains low levels of carbon, making it easy to weld under a variety of conditions. Weathering Capability:

Included in this development are HPS formulations with weathering capability that add savings in life cycle costs. These specials grades effectively resist weather and corrosion. Naturally occurring corrosion eventually forms a protective  barrier layer (patina) on the steel that greatly reduces further access to oxygen, moisture, and contaminants. Weldability:

High strength levels in steel are usually achieved by increasing the amount of carbon and other alloys. But high carbon levels make welding difficult, often leading to cracking during construction or in service. To avoid cracking, fabricators and erectors must perform carefully controlled techniques. These requirements increase fabrication and erection costs. HPS grades, on the other hand, have been developed to greatly improve weldability and to minimize need for pre- and post-heating. Fracture Toughness:

High Performance Steel has much higher fracture toughness than the conventional grades of steel used for bridge construction. HPS makes the transition from brittle to ductile at a much lower temperature than conventional grades. So HPS improves reliability by minimizing the chance of sudden brittle failure. This property provides more time for inspectors to detect and repair any fatigue cracks that might develop  before the structure becomes unsafe.

Fabrication:

Standard shop practices of girders of High Performance Steel may require some modification for drilling, reaming, and mill scale removal. For HPS 70W, drill bits and reamers will dull quickly unless the worked area is flooded with

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lubricants. To remove mill scale, abrasive blasting is the preferred method. Grinding mill scale from HPS grades has proved difficult.

CONCLUSION Bridges are the monuments of civil engineering profession. Bridges generally outlive their designers and provide a visual testimonial to the skill and ingenuity of their engineers and builders. Steel structures are poised for a dramatic resurgence, given the opportunities available with recent research and the development of HPS for innovative, cost-effective, and p leasing steel structures. It is concluding that the High Performance Steel is being used on highway and railway bridges successfully all over the world because of its inherent quality of better strength, resistance against fracture toughness, weldability and a very good resistance against weathering / corrosion. The weight of the structure is reduced tremendously reducing the cost of substru cture and foundations and over all reduced life cycle costs. Its introduction on Indian railways will be a very good decision for the upgradation of the present technology of design, fabrication and maintenance of steel bridges. It will not be long before much of today's concrete bridge infrastructure will have to be replaced, and properly designed steel bridges will all have their place.

REFERENCES Chatterjee, S., The Design of Modern Steel Bridges, first edition, BSP professional  books. McCormac, Jack., Structural steel design, fourth edition. Owens, G.W., Knowles, P.R., & Dowling, P.J. (1994): Steel Designers' Manual, Fifth edition, Blackwell Scientific Publications. en.wikipedia.org/wiki/Steel_Bridge www.steel-insdag.org/new/pdfs/chapter43.pdf www.steel.org/bridges/Myths_and_Realities.pdf www.fhwa.dot.gov/bridge www.reidsteel.com/steel_bridges.htm

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