Concrete structures miscellaneous
- A concrete beam of rectangular cross-section of size 120 mm (width) and 200 mm (depth) is prestressed by a straight tendon to an effective force of 150 kN at an eccentricity of 20 mm (below the centroidal axis in the depth direction). The stresses at the top and bottom fibres of the section are
-
View Hint View Answer Discuss in Forum
Due to direct stress,
stress = P = 150 × 103 = 6.25 N/mm2 A 120 × 200
Due to moment by eccentric loadstress = m .y I = - 150 × 103 × 20 × 100 1 × 120 × (200)2 12
= 3.75
stress at top = 6.25 – 3.75 = 2.5 N/mm2
stress at bottom = 6.25 + 3.75 = 10 N/mm2
(compression)Correct Option: A
Due to direct stress,
stress = P = 150 × 103 = 6.25 N/mm2 A 120 × 200
Due to moment by eccentric loadstress = m .y I = - 150 × 103 × 20 × 100 1 × 120 × (200)2 12
= 3.75
stress at top = 6.25 – 3.75 = 2.5 N/mm2
stress at bottom = 6.25 + 3.75 = 10 N/mm2
(compression)
- Un-factored maximum bending moments at a section of a reinforced concrete beam resulting from a frame analysis are 50, 80, 120 and 180 kNm under dead, live, wind and earthquake loads respectively. The design moment (kNm) as per IS: 456-2000 for the limit state of collapse (flexure) is
-
View Hint View Answer Discuss in Forum
As per IS: 456 - 2000 only earthquake or wind load need to be taken. Here higher is earthquake load (= 180 kNm). Hence it is considered for design.
∴ Factored Design moment = 1.2 (50 + 80 + 180) = 372 kNmCorrect Option: D
As per IS: 456 - 2000 only earthquake or wind load need to be taken. Here higher is earthquake load (= 180 kNm). Hence it is considered for design.
∴ Factored Design moment = 1.2 (50 + 80 + 180) = 372 kNm
- A reinforced concrete column contains longitudinal steel equal to 1 per cent of net cross-sectional area of the column. Assume modular ration as 10. The loads carried (using the elastic theory) by the longitudinal steel and the net area of concrete, are Ps and Pc respectively. The ration Ps/ Pc expressed as per cent is
-
View Hint View Answer Discuss in Forum
Strain in steel = strain in concrete.
⇒
Load on steel Ps = εs Es Ast
Load on concrete, Pc = εc Ec. AcoAst = 0.01 Ac Esc = 10(m) Ec ∴ Ps = εs Es × Ast = 0.1 Pc εc Ec. Aco Correct Option: D
Strain in steel = strain in concrete.
⇒
Load on steel Ps = εs Es Ast
Load on concrete, Pc = εc Ec. AcoAst = 0.01 Ac Esc = 10(m) Ec ∴ Ps = εs Es × Ast = 0.1 Pc εc Ec. Aco
- A pre-tensioned concrete member of section 200 mm × 250 mm contains tendons of area 500 mm2 at the centre of gravity of the section. The pre-stress in tendons is 1000 N/mm2. Assuming modular ratio as 10, the stress (N/mm2) in concrete is
-
View Hint View Answer Discuss in Forum
Pretension force, P = Ast Ps
= 1000 × 500
= 5000 kN
strain in concrete = strain in steelEcBc = 500 × 1000 = 10.10 N/mm2 200 × 250 - 500
Stress in steel = strain in concrete × Es= 10.10 × Es Ec
= 10 × 10.10
= 101.01 N/mm2
Final stress = 1000 – 101.01 = 899N/mm2Pc = 899 × 500 = 9.08 N/mm2 200 × 250 - 500 Correct Option: B
Pretension force, P = Ast Ps
= 1000 × 500
= 5000 kN
strain in concrete = strain in steelEcBc = 500 × 1000 = 10.10 N/mm2 200 × 250 - 500
Stress in steel = strain in concrete × Es= 10.10 × Es Ec
= 10 × 10.10
= 101.01 N/mm2
Final stress = 1000 – 101.01 = 899N/mm2Pc = 899 × 500 = 9.08 N/mm2 200 × 250 - 500
- The modulus of rupture of concrete in terms of its characteristics cube compressive strength (fck) in MPa according to IS 456: 2000 is
-
View Hint View Answer Discuss in Forum
0.7 √fck
Correct Option: D
0.7 √fck
