In steel structures, a connection is the joint where two or more structural members are fastened together. Connections are as important as the members themselves because they ensure safe transfer of forces, structural integrity, and stability of the structure.
1. Types of Connections in Steel Structures
A. Based on Method of Joining
1. Riveted Connections (Old structures): Good ductility, reliable but labour intensive.
2. Bolted Connections:
(a) Ordinary Black Bolt: Transfer load by bearing/shear. Simple but slippage may occur.
(b) HSFG Bolt: Load transferred by friction. No slip, good fatigue resistance.
3. Welded Connections: High strength, neat appearance, no drilling. Needs skilled labour.
B. Based on Nature of Forces Transmitted
1. Shear Connections (Simple): Transfer shear only (e.g., Beam-beam).
2. Moment-Resisting (Rigid): Transfer shear + moment + axial (e.g., Welded joints).
3. Semi-Rigid: Transfer partial moment.
C. Based on Structural Action: Pinned (Trusses) vs Rigid (Portal frames).
D. Based on Location: Beam-Beam, Beam-Column, Column-Column, Column Base.
2. Importance of Connections
Safe Load Transfer, Structural Stability, Economy, Ease of Construction, Ductility, Durability.
The primary duty of a designer is to ensure the safety, stability, and serviceability of the structure throughout its intended life, while achieving economy and compliance with relevant codes and standards.
1. Ensures Safety: Prevents collapse, resists all loads (dead, live, wind, seismic).
2. Provides Serviceability: Limits deflection, vibration, cracking.
3. Ensures Stability: Against overturning, sliding, buckling.
4. Achieves Economy: Efficient use of materials.
5. Complies with Codes: Follows IS 456, IS 800, etc.
6. Ensures Constructability and Durability.
One-Line: The primary duty of a designer is to design a structure that is safe, stable, serviceable, economical, and in accordance with relevant codes throughout its design life.
A structural system is an arrangement of interconnected structural elements designed to resist, support, and safely transfer loads from the structure to the ground. It ensures loads are received, transferred through a load path, and safely delivered to the foundation.
The required length of weld is determined by equating the factored force to be transferred with the design strength of the weld per unit length.
1. Lap Joint: Plates overlap. Load transferred by shear/bearing. Simple but eccentric loading.
2. Butt Joint: Plates end-to-end with cover plates. Double cover is preferred (no eccentricity).
3. Single Shear Joint: Bolt shears on one plane (Lap joint).
4. Double Shear Joint: Bolt shears on two planes (Double cover butt joint).
5. Bearing Type: Load transfer by bearing (slip allowed).
6. Friction Type (HSFG): Load transfer by friction (no slip).
Mechanical properties define behavior under load:
1. Yield Strength: Indicates stress where permanent deformation begins. Basis for design.
2. Ultimate Tensile Strength: Max stress before failure. Used for bolts/welds.
3. Modulus of Elasticity (E): Measure of stiffness. Governs deflection.
4. Ductility: Deformation before failure. Warning sign.
5. Toughness: Energy absorption (impact loads).
6. Hardness: Resistance to wear.
7. Poisson’s Ratio: Ratio of lateral to longitudinal strain (0.3).
8. Fatigue Strength: Resistance to repeated loading.
9. Creep Resistance: Deformation over time (high temp).
Limit State Design (LSD)
LSD is designed to satisfy:
1. Limit State of Strength (Safety): Prevents collapse (Yielding, buckling, fracture).
2. Limit State of Serviceability (Usability): Prevents excessive deflection, vibration, cracking.
It uses Partial Safety Factors for loads (increase load) and materials (reduce strength). It is realistic, rational, and economical.
A factor applied separately to loads and material strengths in Limit State Method to account for uncertainties.
- For Loads (γf): DL=1.5, LL=1.5, DL+LL+WL=1.2.
- For Materials (γm): Yielding=1.10, Ultimate=1.26.
Used only in Limit State Method, not Working Stress Method.
Used to determine critical load for long, slender columns where failure is by buckling, not crushing.
Formula: Pcr = π²EI / Le²
Assumptions: Long/slender, Hooke's law applies, perfectly axial load, no initial imperfections.
Effective Lengths: Hinged-Hinged (L), Fixed-Fixed (0.5L), Fixed-Hinged (0.7L), Fixed-Free (2L).
1. Determine Factored Axial Load (Pu).
2. Choose Single (40-70°) or Double Lacing.
3. Transverse Shear (V) = 2.5% of Axial Load.
4. Force in Bar (F): V/sin(θ) or V/2sin(θ).
5. Select Section (Le/r < 145).
6. Check Strength (Pd > F).
7. Design Connections (Bolts/Welds).
8. Spacing ≤ 0.7 × L.
9. Check Overall Slenderness (Increase by 5%).
1. Calculate Pu.
2. Select trial section.
3. Determine Le and buckling class.
4. Calculate slenderness ratio (increase 5% for laced).
5. Find fcd.
6. Check Pd = A × fcd > Pu.
7. Design connections and detail.
1. Ideal pin joints (no moment).
2. Loads act only at joints.
3. Members straight/prismatic.
4. Axial force only.
5. Self-weight neglected/lumped.
6. Small deformations.
1. Method of Joints (Equilibrium of joints).
2. Method of Sections (Equilibrium of cut section).
3. Method of Tension Coefficients.
4. Graphical Method.
5. Matrix Method.
1. Calculate support reactions.
2. Select joint with max 2 unknowns.
3. Assume tension.
4. Apply equilibrium equations.
5. Solve and move to next joint.
6. Continue until all forces determined.
Top Chord (Rafter), Bottom Chord (Tie), Web Members, King/Queen Post, Struts, Purlins, Bracing, Gusset Plates, Cleat Angles.
1. Laterally Supported: Compression flange restrained. High capacity.
2. Laterally Unsupported: Compression flange free. Prone to lateral-torsional buckling.
1. Tensile Failure of Connection.
2. Shear Failure of Connection.
3. Bearing Failure.
4. Block Shear Failure.
5. Tear/Rupture of Member.
6. Local Buckling.
7. Weld Failure.
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