← Design of Structures (Steel, Concrete & Masonry) · AAI Manager (Civil)

Chapter 1 of 10

Structural Steel — Connections

In the AAI Manager (Civil) syllabus under Design of Structures (Steel, Concrete & Masonry) · 2 parts

Bolted & Riveted Connections · Welded Connections

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (17 sections)

Part 1 of 2

Bolted & Riveted Connections

Last reviewed 16 Sept 2026 · 6 min read

Connections in steelwork

Steel members are joined by rivets, bolts or welds. Rivets have largely given way to high-strength bolts and welding, but riveted joints still exist in older bridges and are covered in exams.

Types of bolts

Bolt	Designation	Behaviour
Black (unfinished) bolts	Property class 4.6 ( $f_{u b}$ = 400 N/mm²)	Bearing type; used in light structures
Turned and fitted bolts	Close-tolerance holes	Bearing type, less slip
High-strength bolts	Classes 8.8 ( $f_{u b}$ = 800), 10.9 ( $f_{u b}$ = 1000)	Bearing type or friction type
High-strength friction-grip (HSFG) bolts	Pre-tensioned 8.8/10.9 bolts	Load carried by friction between plates; no slip at service load

Class 4.6 means $f_{u b}$ = 4 × 100 = 400 N/mm² and $f_{y b}$ = 0.6 × 400 = 240 N/mm².

Types of joints

Lap joint — plates overlap; bolts in single shear; eccentricity causes bending.
Butt joint — plates butt with cover plates on one side (single shear) or both sides (double shear).

Modes of failure

Shear failure of the bolt (across the shank or threads).
Bearing failure — crushing of the plate (or bolt) at the hole.
Tearing (rupture) of the plate across the net section.
Shear-out / splitting of the plate at the end (insufficient end distance).
Tension failure of the bolt (in tension connections).
Block shear of the connected part.

Detailing rules (IS 800:2007)

Code ProvisionIS 800 — holes, spacing and edge distance

Hole clearance over bolt diameter: 1 mm for bolts up to 14 mm; 2 mm for 16–24 mm; 3 mm for bolts over 24 mm.
Minimum pitch: $2.5 d$ ( $d$ = nominal bolt diameter).
Maximum pitch: $32 t$ or 300 mm, whichever is less ( $t$ = thinner connected plate). In tension members: $16 t$ or 200 mm; in compression: $12 t$ or 200 mm.
Minimum edge and end distance: $1.7 \times$ hole diameter for sheared or hand-flame-cut edges; $1.5 \times$ hole diameter for rolled, machine-flame-cut, sawn or planed edges.
Maximum edge distance: $12 tε$ ( $ε = 250/ f_{y}$ ), 40 mm + 4t for exposed edges.
Tacking fasteners in built-up members at a maximum spacing of about 32t or 300 mm.

Bearing-type bolts in shear

FormulaDesign strength of one bolt

Shear capacity:

V_{d s b} = \frac{V _{n s b}}{γ _{mb}} = \frac{f _{u b}}{3 γ _{mb}} (n_{n} A_{nb} + n_{s} A_{s b})

$n_{n}$ , $n_{s}$ = number of shear planes through threads and through the shank; $A_{nb}$ = net tensile area of bolt (≈ $0.78 \times π d^{2} /4$ ); $A_{s b}$ = shank area; $γ_{mb}$ = 1.25.

Bearing capacity:

V_{d p b} = \frac{2.5 k _{b} d t f _{u}}{γ _{mb}}, k_{b} = least of (\frac{e}{3 d _{0}}, \frac{p}{3 d _{0}} - 0.25, \frac{f _{u b}}{f _{u}}, 1.0)

$t$ = total thickness of connected plates experiencing bearing in one direction; $d_{0}$ = hole diameter; $e$ = end distance; $p$ = pitch.

Bolt value = the smaller of $V_{d s b}$ and $V_{d p b}$ .

Reduction factors

Long joints: if the joint length $l_{j}$ exceeds $15 d$ , multiply shear capacity by $β_{l j} = 1.075 - \frac{l _{j}}{200 d}$ , with $0.75 \leq β_{l j} \leq 1.0$ .
Large grip: if total grip $l_{g}$ exceeds $5 d$ , $β_{l g} = \frac{8 d}{3 d + l _{g}} \leq β_{l j}$ (grip not to exceed $8 d$ ).
Packing plates thicker than 6 mm: $β_{p k} = 1 - 0.0125 t_{p k}$ .

Strength of the plate and efficiency

Plate rupture across the net section: $T_{d n} = \frac{0.9 A _{n} f _{u}}{γ _{m 1}}$ ; yielding of gross section $T_{d g} = \frac{A _{g} f _{y}}{γ _{m 0}}$ .

Efficiency of joint = \frac{design strength of joint}{design strength of the solid plate} \times 100%

For a staggered bolt line, the net area uses $A_{n} = [b - n d_{0} + \sum \frac{p _{s}^{2}}{4 g}] t$ ( $p_{s}$ = staggered pitch, $g$ = gauge).

Part 2 of 2

Welded Connections

Last reviewed 16 Sept 2026 · 7 min read

Welding in steel structures

Welding joins metal parts by fusing them, usually with a filler metal, forming a continuous connection. Most structural welding is electric arc welding: shielded metal arc (SMAW, manual with coated electrodes), submerged arc (SAW, automatic, for long girder welds), gas metal arc (MIG/MAG) and flux-cored arc welding.

Advantages over bolting: no holes (full gross section effective); lighter joints (no cover plates or angles); rigid, continuous connections; airtight and watertight; neat appearance; easier alterations and additions.

Disadvantages: needs skilled welders and inspection; residual stresses and distortion from uneven heating; brittle fracture risk; defects not always visible; poor fatigue behaviour of some details; field welding needs good conditions.

Common weld defects

Incomplete penetration, lack of fusion, porosity (gas pockets), slag inclusion, undercut (groove melted into base metal), cracks (hot or cold), overlap. Inspection: visual, dye penetrant, magnetic particle, ultrasonic and radiographic tests.

Types of welds

Weld	Description	Typical use
Fillet weld	Triangular section in the corner of two surfaces at about right angles	Lap joints, T-joints, most connections
Butt (groove) weld	Fills a groove between plates in the same plane; single-V, double-V, U, J, bevel	Splicing plates, girder flanges, full-strength joints
Plug and slot welds	Weld in a hole or slot in one plate	Supplementing fillet welds, preventing buckling of wide plates
Spot/seam welds	Resistance welds	Light-gauge work

Types of joints: butt, lap, tee, corner and edge.

Fillet welds

The size $s$ of a fillet weld is the leg length (for equal legs). Failure is taken along the throat, the shortest dimension from the root to the face.

FormulaThroat thickness

t_{t} = K s

$K$ = 0.70 for faces at 90° (the usual case). For other angles between fusion faces: 60°–90° → 0.70; 91°–100° → 0.65; 101°–106° → 0.60; 107°–113° → 0.55; 114°–120° → 0.50.

Code ProvisionIS 800 — fillet weld size and length

Minimum size based on the thicker part joined:

Thicker part (mm)	Up to 10	10–20	20–32	32–50
Minimum size (mm)	3	5	6 (first run)	10 (8 for first run)

Maximum size: along a square edge, thickness minus 1.5 mm; along a rounded toe of a rolled section, three-quarters of the thickness at the toe.

Effective length: actual length minus twice the weld size (for end craters) unless end returns are provided; effective length not less than four times the size.

End returns: side fillet welds terminating at the end of a member should be returned round the corner for a distance not less than twice the size.

Lap length: not less than four times the thickness of the thinner part (or 40 mm).

Design strength of welds

FormulaIS 800 — design stress of a fillet weld

f_{w d} = \frac{f _{u}}{3 γ _{m w}}, γ_{m w} = 1.25 (shop), 1.50 (field)

Design capacity of a fillet weld $= f_{w d} \times t_{t} \times L_{w}$ .

$f_{u}$ = the smaller of the ultimate stresses of the weld metal and the parent metal.

For E250 steel ( $f_{u}$ = 410 N/mm²), shop weld: $f_{w d} = \frac{410}{1.732 \times 1.25} = 189.4$ N/mm².

Butt welds: designed like the parent metal — full-penetration butt welds are taken as strong as the plate (effective throat = thickness of the thinner part); for partial penetration, the effective throat is the depth of penetration (with limits).

Combined stresses in welds: for a fillet weld carrying normal stress $f_{a}$ and shear $q$ , the equivalent stress $f_{a}^{2} + 3 q^{2}$ must not exceed $\frac{f _{u}}{3 γ _{m w}}$ .

Welded connection of an angle (balancing welds)

An angle connected by one leg to a gusset carries its load along its centroidal axis, which is nearer the connected leg's back. To avoid eccentricity, the side welds are proportioned so that their resultant passes through the centroid:

P_{1} a = P_{2} b, P_{1} + P_{2} (+ P_{3}) = P

$P_{1}$ = force in the weld along the heel (nearer the centroid, carries more), $P_{2}$ = force at the toe, $a$ and $b$ = distances of the centroid from the heel and toe edges; $P_{3}$ = force in an end weld if provided.

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