Welding of steel bridges

INTRODUCTION
Steel bridges are widely used around the world in different structural forms with different span
length, such as highway bridges, railway bridges, and footbridges. Steel is used for the
construction of bridges of sizes ranging from the very large to the very small. It is a versatile
and effective material that provides efficient and sustainable solutions. Steel has long been
recognized as the economic option for a range of bridges.
Early bridges were made of stone, wood and concrete. The arrival of the steam train in the
mid-18th century ushered in a new era in bridge design. A stronger material was needed as
bridges were required to carry heavier loads over longer spans. Iron was first used to bridge
the ‘Tees’ river in England in 1741. By the 1880s, steel had become a material of choice.
The main advantages of structural steel over other construction materials are its strength,
ductility, easy fabrication, and rapid construction. It has a higher strength to cost ratio in
tension and a slightly lower strength to cost ratio in compression when compared with
concrete. The stiffness to weight ratio of steel is much higher than that of concrete. Thus,
structural steel is an efficient and economic material in bridges.
Steel is a versatile and effective material that provides efficient and sustainable solutions for
bridge construction, particularly for long span bridges or bridges requiring enhanced seismic
performance.

Amongst bridge materials, steel has the highest and most favorable strength qualities,
and it is therefore suitable for the most daring bridges with the longest spans. Normal
building 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. A
special merit of steel is its ductility due to which it deforms considerably before it
breaks, because it begins to yield above a certain stress level.
Bridge steels have to perform in an outdoor environment with relatively large
temperature changes, are subjected to excessive cyclic live loading, and are often
exposed to corrosive environments.
The structural steel for steel bridges should be selected according to the required
material properties or the stress state where used, environmental conditions at the
construction site, corrosion protection method, construction method, etc. The physical
properties of structural steel such as strength, ductility, toughness, weldability, weather
resistance, chemical composition, shape, size, and surface characteristics are important
factors for designing and construction of steel bridges.
Structural steels for bridges are required to have fracture toughness and often
corrosion resistance that exceed general structural requirements.

Typical example of bridges made from steel
 Steel is an ideal material for bridges. It is an essential part
of modern bridges because it is strong, can flex without
fracturing and has a long life, even in the harshest
conditions. It can be used to build bridges of any length
because of its durability and ease of manufacture and
maintenance. New grades of steel increase the economic
advantages of steel, while ensuring that it meets the
increasing demands for high performance.
 Steel is a most versatile and effective material for bridge
construction, able to carry loads in tension, compression
and shear. Structural steelwork is used in the
superstructures of bridges from the smallest to the
greatest.
 There is a wide variety of structural forms available to the
designer but each essentially falls into one of four groups
namely:
(i) Beam bridges,
(ii) Arch bridges,
(iii) Cable stayed bridges and
(iv) Suspension bridges.

Materials Used in Steel bridges
 Types of steels used in bridges are:
1) CARBON STEEL
2) HIGH PERFORMANCE STEEL
3) HEAT TREATED CARBON STEEL
4) WEATHERING STEEL
5) STAINLESS STEEL
6) FIRE RESISTANT STEEL
 Depending on weather conditions, bridge length, and proposed use, each of
these options has different properties that make them the best material for
bridges. Based on these characteristics and more, bridge architects will choose
the best type of steel for the job.

STEEL USED IN BRIDGES
• Steel used for bridges may be grouped into the following three categories:
(i)Carbon Steel: This is the cheapest steel available for structural users where stiffness is more
important than the strength. Indian steels have yield stress values up to 250 N/mm2 and can be
easily welded. The steel conforming to IS: 2062 - 1969, the American ASTM A36, the British
grades 40 and Euronorm 25 grades 235 and 275 steels belong to this category.
(ii)High strength steels: They derive their higher strength and other required properties from the
addition of alloying elements. The steel conforming to IS 961:1975, British grade 50, American
ASTM A572 and Euronorm 155 grade 360 steels belong to this category. These steels need
special welding techniques for welding.
Another variety of steel in this category is produced with enhanced resistance to atmospheric
corrosion. These are called 'weathering' steels in Europe, in America they conform to ASTM A588
and have various trade names like ' cor-ten'.
(iii)Heat-treated carbon steels: These are steels with the highest strength. They derive their
enhanced strength from some form of heat-treatment after rolling namely normalization or
quenching and tempering. These steels can be welded with normal welding techniques.
• The physical properties of structural steel such as strength, ductility, brittle fracture,
weldability, weather resistance etc., are important factors for its use in bridge construction.
These properties depend on the alloying elements, the amount of carbon, cooling rate of the
steel and the mechanical deformation of the steel.

WEATHERING STEEL
oTo protect steel from corrosion, some countries produce steels which by themselves can
resist corrosion. These steels are called as “weathering steels or Corten steels”.
oWeathering steels are high strength alloy weldable structural steels, which possess
excellent weathering resistance in many non-polluted atmospheric conditions.
oThey contain up to 3% of alloying elements such as chromium, copper, nickel,
phosphorous, etc.
oOn exposure to air, under suitable conditions, they form adherent protective oxide
coatings called ‘patina’, to inhibit further corrosion. This acts as a protective film, which
with time and appropriate conditions causes the corrosion rate to reduce until it is a
low terminal level. The corrosion rate is so low that bridges fabricated from unpainted
weathering steels can achieve a 120 year design life with only nominal maintenance.
oConventional coatings are, therefore, not usually necessary since the steel provides its
own protection.
oWeathering steels are 25% costlier than the mild steel, but in many cases the total cost
of the structure can be reduced if advantage is taken of the 30% higher yield strength
compared to mild steel.

STAINLESS STEEL
 Steel is now in general use for bridge construction but the use of stainless steel is
relatively recent, 10 years to 15 years. Initially used principally for its anti-corrosion
properties in safety components – guardrails, and handrails etc.,
 Stainless steel is now found in structural components, whether in the deck – in the form
of beams and welded plate sections, tie-rods – or in the suspension systems – in the
form of stays, cables and pylons.
 Stainless steels are also occasionally used to fabricate bearings and other parts for
bridges where high corrosion resistance is required. However, the relative high cost of
stainless steel has limited its use in primary bridge members.
 Stainless steels are subject to increased corrosion if they are placed in contact with
regular carbon steel. This requires the use of either stainless steel or galvanized
fasteners. In addition, special care is needed to avoid contact with or connections to
regular carbon steel components.
 Stainless steels used for bridges and footbridges belong primarily to two categories of
stainless namely austenitic and austeno-ferritics, also known as duplex, which combine
excellent corrosion resistance and elevated mechanical performance.

Fire resistant steel
Fire safety in steel structures could also be brought about by the use of certain types of
steel, which are called ‘Fire Resistant Steels (FRS)’.
These steels are basically thermomechanically treated (TMT) steels which perform
much better structurally under fire than the ordinary structural steels.
These steels have the ferrite – pearlite microstructure of ordinary structural steels but
the presence of Molybdenum and Chromium stabilize the microstructure even at
600⁰C.
The fire resistant steels exhibit a minimum of two thirds of its yield strength at room
temperature when subjected to a heating of about 600⁰C.
In view of this, there is an innate protection in the steel for fire hazards. Fire resistant
steels are weldable without pre-heating and are commercially available in the market as
joists, channels and angles.

STRUCTURAL STEEL SHAPES AND MECHANICAL PROPERTIES
For design of structures, long and flat products are commonly used.
The long products include: angles; channels; joists/beams; bars and rods; cold
twisted deformed (CTD) bars & thermo-mechanically treated (TMT) ribbed bars.
The flat products comprise: plates; hot rolled coils (HRC) or cold rolled coils
(CRC)/sheets in as annealed or galvanized condition.
The starting material for the finished products is as given below:
• Blooms in case of larger diameter/cross-section long products
• Billets in case of smaller diameter/cross-section long products
• Slabs for hot rolled coils/sheets
• Hot rolled coils in case of cold rolled coils/sheets
• Hot/Cold rolled coils/sheets for cold formed sections

Mechanical properties of some typical structural steels
 Table given below summarizes some of the important mechanical properties of
steel produced in India. The UTS mentioned in the table represents the
minimum guaranteed Ultimate Tensile Strength at which the corresponding steel
would fail.

WELDING JOINTS IN BRIDGES
 Welding is defined as “A localized coalescence of metals or nonmetals produced
by heating the materials to the welding temperature, with or without the
application of pressure, or by the application of pressure along and with or
without the use of filler material”.
 Welded connections are direct and efficient means of transferring forces from
one member to the adjacent member.
 Today, plates are joined primarily by welding. This involves the laying of molten
metal along joints; when cooled this metal has fused with the plates on each side
to form a joint.
 Arc welding is the most common type of welding used for structural steel,
although other methods are also used.

 There are a number of processes in which the weld can be formed namely:
1) SHIELDED METAL ARC WELDING(SMAW)
2) SUBMERGED ARC WELDING(SAW)
3) GAS METAL ARC WELDING (GMAW) AND FLUX-CORED ARC WELDING (FCAW)
4) ELECTROSLAG WELDING (ESW) AND ELECTROGAS WELDING (EGW)
5) SHEAR STUD WELDING
 Welding processes may be selected based upon the required welding position—
flat, horizontal, vertical or overhead.
 Vertical and overhead are often referred to as “out of position” and flat and
horizontal referred to as “downhand” or “in position”.

SHIELDED METAL ARC WELDING(SMAW)
• SMAW colloquially called “stick” welding, is used infrequently in most bridge shops due
to its lower productivity.
• However, because of its simplicity, it is used where access for equipment is limited, or
when transporting and positioning of equipment would be a major task.
• A prime example is tack welding—moving equipment around a structural component
may take more time than making the tack welds. Thus, SMAW is often used for tacking.
Portability makes SMAW useful in field applications, both in new construction and also
for field repairs.
• SMAW is characterized by versatility, simplicity, and portability. In the 1940s, 50s, and
60s, SMAW was commonly used for shop fabrication that could not be done with
submerged arc welding.
• The advent of gas metal arc welding and flux-cored arc welding, however, has displaced
much of the use of SMAW in the shop. Though SMAW is still sometimes used to
dependably deposit quality welds, it is slower and more costly than other methods of
welding.

SUBMERGED ARC WELDING(SAW)
• SAW, more familiarly known as “sub arc”, is the workhorse of the steel bridge fabrication industry,
well suited to full-penetration welds of large cross-sections and too long, mechanized welds.
• Given its common use in flange splicing, web splicing, web-to-flange welding and stiffener-to-
flange welding, SAW accounts for perhaps 90 percent of shop welding on steel bridges by volume.
it operates with larger-diameter electrodes, higher heat input, and higher deposition than other
arc welding processes, and so for many decades it has been the process of choice in bridge shops.
• Most SAW applications are mechanized. Long, uninterrupted, straight seams are ideal applications
for SAW.
• SAW is not suitable for vertical and overhead welding because the flux cannot be kept over the arc
in those positions; it falls away due to gravity. For shop fabrication, the work can be moved such to
facilitate a position suitable to SAW. However, field conditions prohibit such opportunities, and
thus restrict the suitability of SAW.
• SAW is generally the most popular welding process in a bridge fabrication shop because large
structural assemblies (like plate girders) with many long fillet welds or many thick full penetration
groove welds lend themselves to the high-deposition welds that SAW offers.

GAS METAL ARC WELDING (GMAW) AND FLUX-
CORED ARC WELDING (FCAW)
• These processes may be used semi-automatically, mechanized,
automatically, or robotically.
• Generally, these two processes are popular for welds that cannot be
readily made with SAW. This includes short welds, “out of position”
welds (vertical or overhead), tack welds, and robotic welds. Generally,
fabricators choose between GMAW and FCAW for such applications.
• Bridge fabricators typically turn to GMAW or FCAW for tack welds; for
shorter production welds, such as stiffener to flange welding; or for
welds that change direction, are difficult to access, or are out of
position (i.e., vertical or overhead).

GAS METAL ARC WELDING (GMAW)
• There are four common modes of metal transfer associated with GMAW: globular
transfer, spray transfer, pulsed spray transfer, and short circuiting transfer. The
first three are used for steel bridge fabrication, with spray and pulsed spray being
the most common.
• Use of GMAW in pulsed spray mode is particularly useful for out-of-position
welding.
• Short circuiting transfer is a very low heat input process that is ideal for thin
material but, because of this low heat input, is not reliable for achieving fusion in
thicker sections. Therefore, short circuiting is not permitted for use on bridges.
• Short circuit transfer is readily avoided through use of proper welding
procedures, and GMAW’s other modes of transfer are very beneficial in bridge
fabrication.

FLUX-CORED ARC WELDING (FCAW)
• FCAW has been used in bridge fabrication for many decades, and is popular for shorter welds, such as
stiffener to flange fillet welds or small complete joint penetration groove welds that do not lend
themselves to mechanization.
• Within the category of flux-cored arc welding, there are two subcategories: gas-shielded FCAW (FCAW-
G) and self-shielded FCAW (FCAW-S).
• Different welding processes are often combined in a single joint for a variety of reasons. For example,
tack welding may be done with SMAW, and the rest of the joint may be filled with FCAW. Under most
circumstances, such intermixing of processes causes no difficulty. However, FCAW-S poses a specific
exception.
• Because of the relatively high amount of aluminum and magnesium present in FCAW-S, mixing other
processes, including FCAW-G, with FCAW-S in a single weld joint creates the potential for negative
interactions.
• Welding over a deposited FCAW-S weld can break apart the aluminum and magnesium compounds, and
the presence of these compounds in the subsequently deposited weld can have a negative impact on
the weld mechanical properties, and in particular the CVN toughness.
• For welding under field conditions where wind may disturb the gas shielding, FCAW-S is ideal.

ELECTROSLAG WELDING (ESW) AND
ELECTROGAS WELDING (EGW)
• Electroslag welding (ESW) and electrogas welding (EGW) are ideal for welding on
thicker materials, typically 1 inch thick or greater. Materials 12 inches thick and
greater have been welded with ESW using multiple electrodes. However, ESW is
not well suited for use on thinner materials because traditional processes are
more efficient.
• While use of EGW is not common in bridges, ESW is used for splicing bridge
flanges. Further, ESW is sometimes used in CJP weld T- and corner joints; in such
applications, ESW can be more efficient and also help minimize the welding
distortions which are more pronounced with multi-pass processes.
• Currently, the Bridge Welding Code does not allow for the use of ESW or EGW for
fracture critical members or high-performance steel and does not allow EGW for
welding quenched and tempered steels (which includes most grades of high-
performance steel) or joints in tension or stress reversal.
• Specifically for bridges, a variation of ESW known as “narrow gap” (ESW-NG), was
developed in the 1980s and 1990s .

SHEAR STUD WELDING
Shear studs are welded by arc stud welding (SW), which is “[a]n arc welding
process using an arc between a metal stud, or similar part, and the other
workpiece. The process is used without filler metal, with or without shielding gas
or flux, or with or without partial shielding from a ceramic or graphite ferrule
surrounding the stud, and with the application of pressure after the faying
surfaces are sufficiently heated”.
Shear stud welding is another welding process that is common in bridges but
unique compared to other processes. Arc stud welding is frequently referred to as
“stud welding” or “shear stud welding”, and is used to attach headed shear stud
connectors to beams.
NOTE:
Generally, the resistance welding (other than the unique case of ESW) is not used in
bridges; however, the longitudinal butt seam on structural tube and pipe is typically
welded with the electric resistance welding process.

WELDING OF STAINLESS STEEL
oStainless steels are generally available in austenitic, martensitic, and
ferritic microstructures as well as two-phase austenitic and ferritic
microstructures known as “duplex” stainless steel.
oAustenitic stainless steels are readily weldable, but there are some
key differences compared to welding carbon steel (i.e., non-stainless
steel)
oUsing special techniques, stainless steel can be welded to carbon or
low-alloy steels. Use of stainless steel at bearing locations is one
example. Such bearing applications typically consist of a thin sheet of
stainless steel attached to guide plates. A useful welding process for
this application is GTAW.

Preferred Weld Types in Bridges
The type of weld chosen for the connection has a significant effect on the
constructability of the design based on the effort required to make each
type of weld.
Generally, the relative constructability of the four most common types of
welds used in bridge fabrication is as follows, listed from highest
constructability to lowest:
1) Single-pass fillet welds
2) Multipass fillet welds (fillet welds sizes not greater than 1 inch)
3) Partial joint penetration (PJP) welds
4) Complete joint penetration (CJP) welds

For the bridge designer, two weld types are usual – a butt weld and a fillet weld.
The welds are formulated such that they have similar properties to the parent metal
being joined, so that the limiting yield and shear stresses are unaffected.
Achieving a good bridge weld, which is fatigue-resistant and of sufficient strength for
the life of the structure, depends on a number of factors:
• Use of the correct welding materials to attain proper weld metal strength, ductility,
and
toughness
• Sufficient cross-section and associated effective throat or weld size, as designed and
deposited
• Welding technique that results in good soundness (fusion as required and minimal
defects)
• Design details chosen and properly executed for good fatigue and fracture behavior
Good welds are a function of both design and production.

• In bridge practice, CJP weld single-
V and double-V butt joints and
single- and double-bevel T- and
corner joints are heavily used (see
figures 43 and 44). Joints with “U”
or “J” shaped preparations as
opposed to straight bevels are not
common; generally this is because
preparation of J and U bevels
requires machining, which is much
more time-consuming and
expensive.

Welding of steel bridges

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