Welded Connection LSM

 Welding is a process to join two pieces of metal by causing fusion between them. The metal pieces to be connected are brought closer and the metal is melted by means of electric arc or oxyacetylene flame along with a filler material added to the joint to form a pool of molten material that cools to form a joint which is usually stronger than the base material.

 Types of Welded Connection

1. Butt Weld

2. Fillet Weld

3. Slot Weld and Plug Weld

Butt weld: 

In butt welding, two metal pieces are connected which are nearly parallel to each other and don't overlap.

Types of Butt Welds

                                Source: Autodesk Advance Steel Forum

Specification

•  The Size of the Butt weld shall be specified by the effective throat thickness. In case of full penetration butt weld, it shall be taken as the thickness of thinner part joined.
• In case of incomplete penetration, the effective throat thickness shall be taken as the minimum thickness of the weld metal common to the parts joined excluding reinforcement. In case of absence of actual data, it may be taken as 5/8th of the thickness of the thinner material.
• The effective length of butt weld shall be taken as the length of full-size weld.
• The minimum length of butt weld shall be four times the size of the weld.
• If intermittent butt welding is used, it shall have an effective length of not less than four times the weld size and space between the two welds shall not be more than 16 times the thickness of the thinner member joined.

Design Strength of Butt weld

The design strength of the butt weld in tension or compression is given by,

                              ${{T}_{dw}}=\frac{{{f}_{y}}{{L}_{w}}{{t}_{e}}}{{{\gamma }_{mw}}}$

${{f}_{y}}=$ Smaller of yield stress of the weld (${{f}_{yw}}$) and the parent metal (${{f}_{y}}$)
${{L}_{w}}=$ Effective length of weld
${{t}_{e}}=$ Effective throat thickness of the weld
${{\gamma }_{mw}}=$ Partial safety factor
        $=1.25$ for shop welding
        $=1.5$ for site welding

The design strength of the butt weld in shear is given by,

                              ${{V}_{dw}}=\frac{{{f}_{yw1}}{{L}_{w}}{{t}_{e}}}{{{\gamma }_{mw}}}$

${{f}_{yw1}}=$ Smaller of shear stress of weld $\frac{{{f}_{yw}}}{\sqrt{3}}$ and parent metal $\frac{{{f}_{y}}}{\sqrt{3}}$

Fillet Weld

In Fillet welding, two metal pieces are connected at an angle. It is also known as Tee joint or corner joint.

                                                                 Source: Wikipedia

Specification

1. Size of the Fillet Weld
     (a) The size of normal fillet weld shall be taken as the minimum weld leg size.
     (b) For deep penetration welds with penetration not less than 2.4 mm size of weld is minimum leg size + 2.4 mm
     (c) For fillet welds made by semi-automatic or automatic processes with deep penetration more than 2.4 mm, if purchaser and contractor agree.
                                       S = minimum leg size + Actual penetration 

2.  The minimum size of a fillet weld is specified as 3 mm.
     To avoid the risk of cracking in the absence of preheating the minimum size is:
     For less than 10 mm thick plate - 3 mm
     For 10 to 20 mm thick plate       - 5 mm
     For 20 to 32 mm thick plate       - 6 mm
     For 32 to 50 mm thick plate       - 8 mm

3. Effective Throat thickness
It shall not be less than 3 mm and shall not generally exceed 0.7t [Kt]* (or t under special circumstances) where t is the thickness of the thinner plate at the elements being welded.

* Value of K for different angles between Fusion Faces

Angle between Fusion Faces	60 - 90	91 - 100	101 - 106	107 - 113	114 - 120
Constant, K	0.70	0.65	0.60	0.55	0.55

4. Effective Length
It is the length of the weld for which specified size and throat thickness exist.
In drawing only effective length is shown.
Welding length = Effective length + twice the size of the weld
Effective length > 4 times the size of the weld

5. Lap Joint
Minimum lap joint = 4 times the thickness of the thinner plate or 400 mm, whichever is more.

6. Intermittent welds
Length > 4 times the size of the weld or 40 mm whichever is more
Minimum clear spacing = 12 t for compression joints and 16 t for tension joints

Intermittent welds shall not be used in positions subjects to dynamic repetitive and alternating stresses.

Design Strength of Fillet Weld

Design Stress of a fillet weld

                            ${{f}_{wd}}=\frac{{{f}_{wn}}}{{{\gamma}_{mw}}}$

                            ${{P}_{dw}}={{L}_{w}}{{t}_{t}}\frac{{{f}_{u}}}{\sqrt{3}{{\gamma }_{mw}}}$

${{P}_{dw}}=$ Design Strength of fillet weld
${{f}_{wn}}=$ nominal strength of fillet weld $=\sqrt{3}{{\gamma }_{mw}}$
${{L}_{w}}=$ Effective length of the weld
${{t}_{t}}=$ throat thickness $=KS$
$S=$ Size of the weld
${{f}_{u}}=$ smaller of ultimate strength of the weld and the parent material
${{\gamma }_{mw}}=$ Partial factor of Safety
        $=1.25$ for shop welding
        $=1.5$ for site welding

Slot Weld 

A welding technique in which a plate with a circular or elongated hole is kept with another to be joined and then welding is made along the periphery of the hole.

Plug Weld

A welding technique in which a plate with a circular or elongated hole is kept with another to be joined and then the entire hole is filled with filler material.

                                Source: pursuit engineering

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Bolted Connection LSM

A bolt is a metal pin with a head formed at one end and shank threaded at the other in order to receive a nut.

Types of bolts

1. Unfinished bolts

2. Finished bolts

3. High Strength Friction Grip (HSFG) bolt

Let's look at the important fomulas for designing a bolted connection based on IS 800:2007

Design Strength of Bolt,
${{V}_{dsb}}=\frac{{{V}_{nsb}}}{{{\gamma }_{mb}}} $

${{V}_{nsb}}=\frac{{{f}_{ub}}}{\sqrt{3}}({{\eta }_{n}}{{A}_{nb}}+{{\eta }_{s}}{{A}_{sb}})$

${{V}_{nsb}}=$ Nominal Shear Capacity of bolt
${{\gamma }_{mb}}=$ Partial factor of safety of materail bolt = 1.25
${{f}_{ub}}=$ Ultimate tensile strength of a bolt
${{\eta }_{n}}=$ Number of shear planes with threads intercepting the shear plane.
${{\eta }_{s}}=$ Number of shear planes without threads intervepting the shear plane.
${{A}_{sb}}=$ Nominal shank area of bolt $=\frac{\pi {{d}^{2}}}{4}$
${{A}_{nb}}=$ Net Shear area of the bolt at threads $=0.78\times \frac{\pi {{d}^{2}}}{4} $
$d=$ Nominal diameter of Bolt

Reduction Factor for Shear Capacity of Bolts

1. Reduction Factor for long joints

If the distance between the first and last bolt in the joint measured in the direction of load exceed 15 d, the shear capacity ${{V}_{db}}$ shall be reduced by the factor,

                             ${{\beta }_{{{l}_{j}}}}=1.075-0.005\frac{{{l}_{j}}}{d}$

                      Subjected to the limits $0.75\le {{\beta }_{lj}}\le 1.0$

2. Reduction Factor if Grip Length is Large

If the total thickness of the connected plate exceeds 5 times the diameter d of the bolts, the shear capacity ${{V}_{db}}$ shall be reduced by the factor,

               ${{\beta }_{{{l}_{g}}}}=\frac{8d}{3d+{{l}_{g}}}$

Subjected to conditions, maximum value $={{\beta }_{{{l}_{j}}}}$ i.e. ${{\beta }_{{{l}_{g}}}}<{{\beta }_{{{l}_{j}}}}$
In no case ${{l}_{g}}$ be greater than $8d$

3. Reduction Factor if Packing Plates are Used

If packing plates of thickness more than 6 mm are used them the shear capacity ${{V}_{db}}$ shall be reduced by the factor,

               ${{\beta }_{pk}}=1-0.0125{{t}_{pk}}$

Thus Shear capacity of bolt

               ${{V}_{dsb}}=\frac{{{f}_{ub}}}{\sqrt{3}{{\gamma }_{mb}}}({{\eta }_{n}}{{A}_{nb}}+{{\eta }_{s}}{{A}_{sb}}){{\beta }_{{{l}_{j}}}}{{\beta }_{{{l}_{g}}}}{{\beta }_{pk}}$

Bearing Capacity of Bolts
               ${{V}_{dpb}}=\frac{{{V}_{npb}}}{{{\gamma }_{mb}}}$

               ${{V}_{npb}}=2.5{{K}_{b}}\times d\times t\times {{f}_{u}}$

${{V}_{dpb}}=$ Design bearing strength
${{V}_{npb}}=$ Nominal bearing srength
${{\gamma }_{mb}}=$ Partial factor of safety of material = 1.25
${{K}_{b}}=$ smaller of

$\frac{e}{3{{d}_{h}}},\left( \frac{p}{3{{d}_{h}}}-0.25 \right),\frac{{{f}_{ub}}}{{{f}_{u}}},1.0 $

$e=$ end distance
$p=$ pitch
${{d}_{h}}=$ diameter of hole
${{f}_{u}}=$ Utlimate tensile strength of plate
$t=$ Summation of the thickness of the connected plates experiencing bearing stress in the same direction.

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Riveted Connection

Connections

As Steel structure is an assembly of different steel members, they need to be connected to form a frame and to support and transfer the load to the foundation. 

Connections between different members of a steel framework not only facilitate the flow of forces and moments from one member to another but also allow the transfer of forces up to the foundation level.

There are 3 ways to connect these steel members:

1. Riveted Connections

2. Bolted Connections

3. Welded Connection

Riveted Connections

Riveting is a method of joining together pieces of metal by inserting ductile metal pins called rivets into holes of pieces to be connected and forming a head at the end of the rivet to prevent each metal piece from coming out.

Here are some formulas which  are used in the designing of riveted connections.

${{d}_{R}}=6.04\sqrt{t}$

${{P}_{s}}=n\times {{\tau }_{vf}}\frac{\pi }{4}{{d}^{2}}$ (Single Shear)

${{P}_{s}}=n\times 2\times {{\tau }_{vf}}\frac{\pi }{4}{{d}^{2}}$ (Double Shear)

${{P}_{t}}={{\sigma }_{at}}(p-d)t$

${{P}_{b}}=n\times {{\sigma }_{pf}}d\times t$

${{P}_{u}}={{\sigma }_{at}}d\times t$

${{d}_{R}}=$ Diameter of rivet by Unwin's formula
${{P}_{s}}=$ Shearing Strength of rivet
${{P}_{t}}=$ Strenth of plate per pitch length
${{P}_{b}}=$ Bearing Strength of Rivet
${{P}_{u}}=$ Strenth of soild plate per pitch length
$n=$ Number of rivet line
$p=$ Pitch
$d=$ diameter of hole
$t=$ thickness of the thinner plate

${{\tau }_{vf}}=$ Permissible Shearing strength of revit
${{\sigma }_{at}}=$ Permissible stress in plate in axial tension
${{\sigma }_{pf}}=$ Allowable Bearing Stress in rivet

Recommendations of IS 800:1984

Pitch

1. Minimum Pitch: $p>2.5{{d}_{R}}$
2. Maximum Pitch: $p< 16t$ or 200 mm (whichever is less) for tension members
$p< 12t$ or 200 mm (whichever is less) for compression members

In load is transfered through butting face in the compression members then, pitch shall not exceed 4.5 times the diameter of rivets for a distance from the abutting faces equal to 1.5 times the width of the member.

The gauge length shall not exceed (100mm + 4t) or 200 mm, whichever is less in compression or tensin members.

When rivets are staggered at equal intervals and the gauge does not exceed 75 mm, the distance specified in (2) maybe increased by 50 percent.

Edge and End Distance

1. $e> 1.7d$ (hand flamed out edges)
2. $e> 1.5d$ (machine flame cut, sawn and planed edges)
3. $e< 12t\varepsilon$, where $\varepsilon =\sqrt{\frac{250}{{{f}_{y}}}}$
4. $e< 40mm + 4t$ (where members are exposed to corrosive environment)

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