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Lecture 14 – Welded Connections

Welding is a procedure that involves fusing two pieces of steel together by melting a sacrificial “flux” electrode to two pieces, thereby joining the pieces permanently together. They have some distinct advantages over bolted connections including:

• Welded joints are more rigid than bolted joints • Can directly connect pieces without the need for connection plates • Welds do not create holes in member (i.e., no need to check

fracture on net area) • Can join odd-shaped pieces together

Welds also have some disadvantages which may preclude their use, including:

• Welds are brittle, not ductile like bolted connections • Very labor intensive • Skilled labor required • Quality control is difficult to inspect • Potential fire hazard in areas of welding

Fillet Welds: The most common type of weld for structural steel connections is the “fillet” weld. This type of weld joins 2 pieces with flat faces at 900 angles. Some examples of fillet welds and their weld symbols are shown below:

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A closer inspection through the fillet weld itself is shown below to indicate some of the dimensions of a weld:

The most common type of fillet welding process is “Arc” welding, or sometime called “stick” welding. This process involves running an electric current through a sacrificial electrode creating an arc of extremely high temperature that fuses the steel pieces together. The electrode (stick) has a coating called a “flux” that, when subject to heat, produces a cloud acting as a barrier to impurities in the air entering the molten metal. A diagram of arc welding process is below:

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Some examples of other welds are shown below:

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Minimum & Maximum Size of Fillet Welds:

Below is a table relating the minimum size of fillet weld to the thickness of material to be welded per AISC Table J2.4 p. 16.1-96:

Material thickness of the

thicker part joined: Minimum size of fillet weld:

Up to ¼” inclusive 1/8” Over ¼” to ½” 3/16” Over ½” to ¾” ¼”

Over ¾” 5/16”

Maximum size of a fillet weld = See AISC p. 16.1-96 paragraph 2b = Thickness of thinner part up to ¼” thick

= Thickness – 1/16” over ¼” thick Design SHEAR Strength of Fillet Welds:

From AISC p. 16.1-98 Weld available strength = Rn LRFD Design Strength = φRn

= FwAw ASD Allowable strength = Ω

nR

where: φ = 0.75 for shear from AISC Table J2.5 p. 16.1-100 Ω = 2.00 for shear from AISC Table J2.5 p. 16.1-100 Fw = nominal strength of weld electrode, Table J2.5 = 0.60FEXX FEXX = weld electrode strength = 70 KSI for E70XX electrodes Aw = effective cross-sectional area of weld, in2 = cos(450) x (Weld Size) x (Weld Length)

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Example (LRFD) GIVEN: Two ¼” thick A36 steel plates fillet welded as shown below. Use E70XX weld electrodes. REQUIRED: Determine the maximum factored load, Pu, that can be applied based on shear strength of the welds.

Step 1 – Determine total length of fillet welds:

Total length = 2(4”) = 8”

Step 2 – Determine design shear strength of welds: Weld design strength = φRn

where: φ = 0.75 for shear from AISC Table J2.5 Rn = FwAw

Fw = nominal strength of weld electrode = 0.60FEXX FEXX = weld electrode strength, Table 8-3 p. 8-65 = 70 KSI for E70XX electrodes Aw = effective cross-sectional area of weld, in2 = cos(450) x (Weld Size) x (Weld Length)

Weld design strength = 0.75(0.60(70 KSI))(cos(450)(3/16”)(8”)) Weld design strength = Pu = 33.4 KIPS

4” 3/16

Pu