Steel Structures Design for SSC JE Civil: IS 800 Rules, Bolted and Welded Connections, Tension and Compression Members, Beams

Design of steel structures is another important part of the SSC JE Civil paper, with questions on connections, member types, slenderness and shape factors. This guide on steel structures design for SSC JE covers the IS 800:2007 provisions, bolted and welded connections, the design of tension and compression members, effective lengths and the plastic analysis basics, with solved examples, practice questions and FAQs.

Basics

• The code for the design of steel structures is IS 800:2007 (limit state method); the older working stress version was IS 800:1984.
• Structural steel Fe 410 (E 250) has a yield stress f_y = 250 N/mm² and an ultimate tensile strength f_u = 410 N/mm².
• Modulus of elasticity of steel E = 2 × 10⁵ N/mm²; Poisson’s ratio 0.3; density 7850 kg/m³.
• Partial safety factors for materials: γ_m0 = 1.10 (yielding), γ_m1 = 1.25 (ultimate stress), γ_mb = 1.25 (bolts) and γ_mw = 1.25 (shop welds).
• Common rolled sections: ISMB, ISLB, ISJB (beams), ISMC (channels), ISA (angles), ISHB (columns).

Bolted Connections

• Diameter of the hole d₀ = d + 1.0 mm (for 12–14 mm bolts), d + 1.5 mm (16–24 mm) and d + 2.0 mm (above 24 mm).
• Minimum pitch = 2.5 d; maximum pitch = 32 t or 300 mm for tension members, whichever is less (t = thickness of the thinner plate).
• Minimum edge distance = 1.5 d₀ (rolled, machine flame-cut edges) or 1.7 d₀ (sheared or hand flame-cut edges); maximum = 12 t ε.
• Failure modes of a bolted joint: shearing of the bolt, bearing of the plate, tearing of the plate (net section), block shear and shearing of the plate at the edge.
• Efficiency of a joint = (strength of the joint / strength of the solid plate) × 100.

Welded Connections

Tension Members

• The design strength is the least of: yielding of gross section T_dg = A_g f_y/γ_m0; rupture of the net section T_dn = 0.9 A_n f_u/γ_m1; and block shear.
• The net area of a plate with bolt holes in a chain A_n = (b − n d₀ + Σ p²/4g) t (Cochrane’s formula for staggered holes).
• The maximum slenderness ratio of tension members is 400 in general, and 350 for members subjected to reversal of stress due to wind or earthquake loads.
• An angle connected by one leg has a shear lag effect and its strength is reduced.

Compression Members

• Slenderness ratio λ = KL / r, where r is the least radius of gyration; it should not exceed 180 for members carrying dead and live loads.
• The Euler buckling load P_cr = π²EI / (KL)².
• Built-up columns use lacing or battens; the lacing bars are inclined at 40° to 70° to the axis.
• The base plate and the slab base distribute the column load on the foundation.

Flexural Members (Beams) and Plastic Analysis

• A beam may be laterally supported (full compression flange restraint) or unsupported (lateral torsional buckling).
• The bending strength is M_d = β_b Z_p f_y / γ_m0 (plastic section) for laterally supported beams.
• The shear strength V_d = A_v f_yw / (√3 γ_m0).
• Deflection limit for a simply supported beam under live load: span/300 (with brittle finishes) and span/240 in other cases.
• Shape factor S = Z_p / Z_e = plastic modulus / elastic modulus.

A plastic hinge forms when the section reaches the plastic moment M_p = f_y Z_p. A simply supported beam becomes a mechanism after one hinge; a fixed beam needs three hinges.

Roof Trusses

• Common types: Howe, Pratt, Fink, fan and north-light trusses; the truss is made of angles or tubes.
• Spacing of trusses: generally 1/3 to 1/5 of the span; purlins are placed over the truss nodes.
• Loads: dead load, live load and wind load (IS 875 Part 3).
• Pitch of a roof truss is the ratio of rise to span; for the A.C. sheet roofs the pitch is about 1/5 or more.

Solved Examples

Example 1. A column of 3 m length is hinged at both ends and its least radius of gyration is 30 mm. Find the slenderness ratio.
λ = 3000/30 = 100.

Example 2. Find the effective length of a column of 4 m, with one end fixed and the other free.
2.0 × 4 = 8 m.

Example 3. Find the throat thickness of a 6 mm fillet weld.
0.7 × 6 = 4.2 mm.

Example 4. What is the minimum pitch for a 20 mm bolt?
2.5 × 20 = 50 mm.

Quick Tips

• Memorise the effective length factors 1.0, 0.8, 0.65 and 2.0.
• Remember that the throat thickness is 0.7 times the weld size.
• Remember the minimum pitch of 2.5d.
• Learn the shape factors of the common sections.

Quick Revision Points

• IS 800:2007 uses the limit state method.
• γ_m0 = 1.10 and γ_m1 = 1.25.
• Slenderness ratio = KL/r.
• Effective length for both ends fixed is 0.65L.
• The shape factor of a rectangle is 1.5.

Practice Questions With Answers

1. What is the shape factor of a rectangular section?
   Answer: 1.5
2. What is the effective length of a column fixed at both ends?
   Answer: 0.65 L
3. What is the throat thickness of a fillet weld of size s?
   Answer: 0.7 s
4. What is the minimum pitch of bolts?
   Answer: 2.5 d
5. What is the maximum slenderness ratio of a compression member (dead and live loads)?
   Answer: 180
6. What is the yield stress of Fe 410 (E 250) steel?
   Answer: 250 N/mm²
7. Which type of bolt works on friction?
   Answer: HSFG bolt
8. What is the shape factor of a circular section?
   Answer: 1.7
9. What is the partial safety factor for yielding?
   Answer: 1.10
10. Which code is used for steel design?
    Answer: IS 800:2007

How to Practise This Chapter

Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Civil Engineering notes page, read the previous topic Reinforced Concrete (RCC) Design and the next topic Irrigation Engineering and Hydrology.

Frequently Asked Questions

Which code is used for the design of steel structures?

IS 800:2007.

What are HSFG bolts?

High-strength friction grip bolts that transfer load by friction.

What is a fillet weld?

A weld of triangular cross-section joining two surfaces at right angles.

What is slenderness ratio?

The ratio of effective length to the least radius of gyration.

What is the effective length of a cantilever column?

Twice its length.

What is shape factor?

The ratio of plastic modulus to elastic modulus of a section.

What is the pitch of bolts?

The centre-to-centre distance of bolts in a line.

What is block shear?

A failure in which a block of material tears out along the bolt line.

What is a plastic hinge?

A section at which the plastic moment has been reached and rotation occurs at a constant moment.

Is this topic asked in SSC JE?

Yes. Steel structures are asked in SSC JE Civil.

What is a lacing?

Diagonal bars connecting the main members of a built-up column.

What is the yield stress of Fe 410 steel?

250 N/mm².