Civilwar-Day 24-PWD-Limit state design

The moment of resistance of a balanced reinforced section is obtained by multiplying the lever arm with:

• Compressive force
• Tensile force
• Either compressive force or tensile force
• Compressive force or tensile force both

Answer explanation

In a balanced RC section, C = T; hence M_R = C·z = T·z. Either force with lever arm gives the same moment.

In an under-reinforced section of RCC beams, the depth of actual neutral axis is

• equal to the depth of critical neutral axis
• less than the depth of critical neutral axis
• more than the depth of critical neutral axis
• None of the above

Answer explanation

Under-reinforced: steel yields before concrete crushes → neutral axis lies shallower than the critical NA.

Partial safety factor for loads for the combination of Dead load + Live load ± Wind/Earthquake load is:

• 3
• 1
• 1.5
• 1.2

Answer explanation

For combined gravity + lateral loads: γf = 1.2 (IS 456/IS 1893 load combinations).

As per IS 456:2000, the maximum spacing of side face reinforcement bars is

• 300 mm
• 450 mm
• 250 mm
• 500 mm

Answer explanation

Clause 26.5.1.3: For deep sections, provide side face steel with spacing not exceeding 300 mm.

In the limit state of collapse, the partial safety factor for concrete (γm) is

• 1.15
• 1.5
• 2
• 1.67

Answer explanation

Material safety factor for concrete in collapse: γm = 1.50 (IS 456:2000).

For spans up to 10 m, the basic span-to-effective depth ratio for a cantilever beam is

• 7
• 20
• 23
• 26

Answer explanation

IS 456 basic ratios: Cantilever 7, Simply supported 20, Continuous 26.

The proof strain on the stress–strain curve is taken as

• 0.0035
• 0.003
• 0.002
• None of these

Answer explanation

Proof strain commonly refers to the 0.2% offset strain for steel → ε = 0.002.

The limiting value of neutral axis depth for Fe415 steel (Es = 2.1×10⁵ N/mm²) is

• 0.48 d
• 0.456 d
• 0.452 d
• 0.45 d

Answer explanation

From IS 456:2000, xu,max/d = 0.48 for Fe415.

What is the adoptable maximum spacing between vertical stirrups in an RCC beam of rectangular cross-section having an effective depth of 300 mm?

• 300 mm
• 275 mm
• 250 mm
• 225 mm

Answer explanation

As per IS 456, max spacing of vertical stirrups ≯ min(0.75d, 300 mm). With d = 300 mm → 0.75d = 225 mm.

A RC beam is subjected to: DL = 20 kN·m, LL = 30 kN·m, Seismic = 10 kN·m. The design bending moment for limit state of collapse is:

• 90 kN·m
• 75 kN·m
• 72 kN·m
• 45 kN·m

Answer explanation

Given: DL = 20 kN·m, LL = 30 kN·m, EQ = 10 kN·m

ULS load combinations (IS 456/IS 1893):

1) 1.5(DL + LL) = 1.5 × (20 + 30) = 1.5 × 50 = 75 kN·m

2) 1.5(DL ± EQ) = 1.5 × (20 ± 10) = 45 or 15 kN·m

3) 1.2(DL + LL ± EQ) = 1.2 × (20 + 30 ± 10) = 72 or 48 kN·m

4) 0.9DL ± 1.5EQ = 0.9×20 ± 1.5×10 = 18 ± 15 = 33 or 3 kN·m

Design bending moment (governing) = max = 75 kN·m