Welded Connections
Welded Connections
In welded connections, different elements are
connected by heating their surfaces to a plastic
or fluid state. There may or may not be pressure,
and there may or may not be filler material. Arc
welding is the general term for many processes
that uses electrical energy in the form of an
electric arc to generate the heat necessary for
welding.
Types of Welding
Notwithstanding the availability of both gas and arc welding, welded connections in
steel structures are ordinarily done by arc welding.
To obtain satisfactory connections, additional metal is used for joining different
elements.
In electric arc welding, the additional material is a metallic rod, which is used as the
electrode. In this type of welding, the electric arc are produced between the elements
being welded and the electrode heats the elements and the electrode to the melting
point.
This transformation of electrical energy into thermal energy and the resulting high
temperature(up to 5500 degree Celsius) causes metallic electrode to melt off into the
joint.
Small droplets of the molten metallic electrode are in fact driven onward to the joint.
Types of Welding
Molten steel must be protected from the surrounding air; otherwise, gases
contained in the molten steel can combine chemically with oxygen and nitrogen
in the air.
This chemical reaction leaves small pockets of gases in the weld after it has
cooled down, making it porous.
The resulting weld will be brittle with very little resistance to corrosion.
To prevent this undesirable brittleness of the weld, two types of arc welding are
commonly used.
One is called Shielded Metal Arc Welding (SMAW) and other is Submerged (or
hidden) Arc Welding (SAW).
Shielded Metal Arc Welding (SMAW)
In SMAW, the weld is protected by using an
electrode covered with a layer of mineral
compounds.
Melting of this layer during the welding produces
an inert gas encompassing the weld area.
This inert gas shields the weld by preventing the
molten metal from having contact with the
surrounding air.
The protecting layer of the electrode leaves a slag
after the mold has cooled down.
The slag can be removed by peening and
brushing.
Submerged Arc Welding (SAW)
In SAW process, the arc is not visible
because the surface of the weld and
the electric arc are covered by a
blanket of granular fusible material to
protect it from the surrounding air.
In this method, a bare metal
electrode is used as a filler material.
Compared with SMAW, SAW welds
provide deeper penetration.
Also, SAW welds show good ductility
and corrosion resistance and high
impact strength.
Gas Material Arc Welding (GMAW)
In this process the electrode is a continuous wire
that is fed from a coil through the electrode
holder.
The shielding is entirely from an externally
supplied gas or gas mixture. The GMAW process
using Carbon dioxide shielding is good for the
lower carbon low-allow steels which are usually
used in buildings and bridges.
Flux Cored Arc Welding (FCAW)
This process is similar to GMAW, except that the
continuous fed filler metal electrode is tubular
and contains flux material within its core.
The core material provides the same functions
as does the coating in SMAW or the granular flux
in SAW.
This process is useful procedure for field welding
in severe cold weather conditions as well as to
speed up high rise construction.
The electrode material is specified under various specifications and
is given in the table. The designations such as E60XX or E80XX
indicates 60 ksi (415 MPa) and 80ksi (550 MPa), respectively for
tensile strength 𝐹𝑢 . The E denotes electrode. The X’s represent
numbers indicating the usage of the electrode
Advantages of Welding
In welded connections, in general fewer pieces are used.
This will up the detailing and fabrication process.
In welded connections, gusset and splice plates may be
eliminated. Bolts or rivets are not needed either. Thus, the
total weight of a welded steel structure is somewhat less
than that of the corresponding bolted structure.
Connecting unusual members (such as pipes) is easier by
welding than by bolting.
Welding provides truly rigid joint and continuous structure.
One possible drawback of welding is the
need for careful execution and
supervision.
For this reason, welding is sometimes done
in the shop and bolting in the field.
Inother words, shop-welding is
complemented by the bolting in the field.
Welded Connections
Welded Connections
Welded Connections
Fillet Welds
Fillet welds can be either equal-leg or unequal-leg, as shown in the
Figure. The intersection point of the original faces of the steel
elements being connected is called the root of the weld. The
surface of the weld should have a slight convexity.
In computation of the strength of the weld, theoretical flat surface is
used. The normal distance from the root to the theoretical face of
the weld is called the throat of the weld.
Fillet Welds
Welded Connections
Welded Connections
𝑃 = 𝑓𝑣_𝑎𝑙𝑙𝑜𝑤 𝐴𝑒 𝑜𝑟 𝑅𝑛 = 𝐴𝑒 𝐹𝑛𝑤
𝐴𝑒 = 𝑡𝑒 𝐿𝑒
𝑡𝑒 = 0.707𝑡𝑤
𝐴𝑒 = 0.707𝑡𝑤 𝐿𝑒
Example
𝑡𝑤 = 8 𝑚𝑚 𝐹𝑢𝑤 = 485 𝑀𝑃𝑎 P = 400 kN 𝑓𝑣_𝑎𝑙𝑙𝑜𝑤 = 0.6𝐹𝑢𝑤 = 291 𝑀𝑃𝑎
𝑡𝑒 = 0.707𝑡𝑤 400 kN = 𝑓𝑣𝑎𝑙𝑙𝑜𝑤 (0.707𝑡𝑤 2𝐿𝑒 )
𝐴𝑒 = 𝑡𝑒 2𝐿𝑒 =0.707𝑡𝑤 2𝐿𝑒
𝑃 = 𝑓𝑣_𝑎𝑙𝑙𝑜𝑤 𝐴𝑒 400 kN = 291 MPa (0.707)(8)(2)(𝐿𝑒 )
400000 𝑁
𝑃 = 𝑓𝑣𝑎𝑙𝑙𝑜𝑤 (0.707𝑡𝑤 2𝐿𝑒 ) 𝐿𝑒 = 291 𝑀𝑃𝑎 (0.707)(8)(2)
𝐿𝑒 = 122 mm
Round up when designing!!
