Qwestrum Engineering360 · Civil Engineering · Steel Structures
Bolted and Welded Connections
For bolts compute the least of shear and bearing strength per bolt and divide the load to get the number required; for fillet welds use the throat thickness 0.707a and the weld design strength to find the length.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.
Key formulas & points
Skim these first — then read the full notes below.
- Minimum pitch: 2.5d; edge distance per IS 800 Table 19
- Eccentric bolt group: resolve force to centroid + moment
- Fillet weld size limited by thinner plate thickness
Topic details
Introduction
Connections transfer forces between members and are often the weakest link, so IS 800 requires each mode to be checked. A bolt in a bearing-type connection can fail in shear across its shank or in bearing/tearing of the connected plate, and the bolt value is the smaller of these.
Scope in B.Tech and GATE syllabus
Weld design centres on the fillet weld, whose strength acts on the throat area (0.707 times the leg size). The weld design strength per unit length times the effective length gives the capacity, and end returns are added for continuity.
Why this topic matters in practice
Eccentric connections, where the load does not pass through the bolt-group centroid, generate an additional moment; the design resolves the direct force and the moment-induced force vectorially at the most heavily loaded bolt or weld point.
Key relations & formulas
Formulas (Indian textbook notation)
(√3 γ_mb)
(fillet weld throat s = 0.707 a)
Notation and sign conventions
Relation 1 —
Formulas (Indian textbook notation)
Write this relation with symbols exactly as in Design of Steel Structures — SK Duggal before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
(√3 γ_mb)
Write this relation with symbols exactly as in Design of Steel Structures — SK Duggal before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
(fillet weld throat s = 0.707 a)
Write this relation with symbols exactly as in Design of Steel Structures — SK Duggal before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Fundamentals and definitions
In a bearing-type bolted connection, load is transferred by the bolt bearing against the plate and shearing across its cross-section. Shear capacity depends on the bolt’s net tensile area at threads and the number of shear planes; bearing capacity depends on plate thickness, bolt diameter, ultimate strength and edge/pitch distances through the factor k_b.
Governing relations in practice
Pitch and edge-distance limits prevent the plate from tearing between holes or splitting at the edge; the minimum pitch of 2.5d ensures bolts do not interfere and the plate retains enough net area.
Design and analysis considerations
Fillet welds resist load on their throat, the plane of minimum area at 45° across the weld. The design strength uses f_u/√3 with the weld material factor γ_mw = 1.25 (shop) or 1.5 (site), reflecting weld quality control differences.
Advanced theory and extensions
Eccentric loading superimposes a torsional moment on the direct shear in a bolt group; each bolt then carries a direct component plus a moment component proportional to its distance from the centroid, and the resultant on the critical bolt must not exceed its capacity.
Assumptions and validity limits
State assumptions explicitly before using any relation for bolted and welded connections — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Steel Structures viva and GATE descriptive questions, listing valid assumptions often earns separate marks.
Step-by-step problem approach
1. Read the question and list given data with SI units (common in Steel Structures papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to bolted and welded connections.
4. Use equation 1:
5. Use equation 2:
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to bolted and welded connections.
4. Use equation 1:
.
5. Use equation 2:
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
Applications & exam relevance
Bolted and Welded Connections appears in industrial sheds and high-rise frames. In Indian civil curricula this topic is tested because it connects theory to design of steel members and connections.
GATE and semester exams often combine bolted and welded connections with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use bolted and welded connections?" — answer with a lab, mini-project, or plant visit example if possible.
Common mistakes in exams
• Taking the bolt shear value without checking the often-lower bearing/tearing value.
• Using the leg size instead of the throat (0.707a) for fillet weld area.
• Ignoring the additional moment in eccentric bolt or weld groups.
• Violating minimum pitch or edge-distance requirements.
• Using the leg size instead of the throat (0.707a) for fillet weld area.
• Ignoring the additional moment in eccentric bolt or weld groups.
• Violating minimum pitch or edge-distance requirements.
Quick revision checklist
Before attempting bolted and welded connections problems, confirm you can:
1. Minimum pitch: 2.5d; edge distance per IS 800 Table 19
2. Eccentric bolt group: resolve force to centroid + moment
3. Fillet weld size limited by thinner plate thickness
2. Eccentric bolt group: resolve force to centroid + moment
3. Fillet weld size limited by thinner plate thickness
Revise the solved examples in Design of Steel Structures — SK Duggal and one previous-year GATE or university paper for this unit.
Worked examples
Try the problem first — open the solution when you are ready to check.
Shear capacity of a single bolt
Problem
A 20 mm diameter bolt of grade 4.6 (f_ub = 400 MPa) acts in single shear through its threaded portion (net area A_nb = 245 mm²). Take γ_mb = 1.25. Find the design shear strength of one bolt.
Solution
Design shear strength V_dsb = n A_nb f_ub / (√3 γ_mb) with n = 1 shear plane = 1 × 245 × 400 / (√3 × 1.25) = 98 000 / 2.165 = 45 264 N ≈ 45.3 kN. The bearing capacity would be checked separately, and the lower of the two governs the load carried per bolt.
Conceptual check — Bolted and Welded Connections
Problem
In a Steel Structures semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of bolted and welded connections." What should a complete answer include?
Exams & GATE
Duggal — combined shear and tension in bolt group is frequent exam type.
📖 Standard books (India)
Design of Steel Structures — SK Duggal
Read: Syllabus unit
IS 800 steel design for Indian practice
Explore related topics
See real civil engineering careers
After exams and interviews, see how engineers actually built careers — milestones and decisions from people in the field.