Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech IV Year ADVANCED STRUCTURAL DESIGN Previous Question Paper (February/March - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Note: Use of IS 456, IS 3370, IS 800, IRC 6, IRC 112 and Steel Tables is allowed.
Main
Design a cantilever retaining wall to support a bank of earth 6 m above basement level. Consider the backfill surface to be inclined at 18° with the horizontal. The angle of repose of soil is 30°. The unit weight of the soil is 18 kN/m3. The coefficient of friction between the soil and concrete is 0.5. Bearing capacity of the soil is 250 kN/m2. Use M 30 grade concrete and Fe 500 grade steel. Sketch the reinforcement details.
Answer
To design the cantilever retaining wall, we first determine the active earth pressure coefficient (Ka) for the inclined backfill using the formula:
With φ = 30° and β = 18°, Ka ≈ 0.36. The total active earth pressure (Pa) acting at H/3 from the base is calculated as Pa = 0.5 × γ × H² × Ka, where H = 6 m + base thickness. For a preliminary base thickness of 0.6 m, H = 6.6 m, resulting in Pa ≈ 142.5 kN/m.
The design steps are as follows:
- Stability Analysis: Check for overturning (Factor of Safety > 1.5) and sliding (Factor of Safety > 1.5). The resisting moment is provided by the self-weight of the wall and the weight of the soil wedge on the heel slab.
- Stem Design: Calculate the maximum bending moment at the base of the stem (Mu = 1.5 × Pa × H/3). Design the section for Mu using M30 concrete and Fe500 steel, ensuring the depth is sufficient to avoid compression reinforcement.
- Base Slab Design: Determine the pressure distribution under the base. Design the heel slab for downward pressure (soil weight minus upward soil pressure) and the toe slab for upward soil pressure.
- Reinforcement: Provide main vertical reinforcement on the back face of the stem, curtailed as the moment decreases. Provide distribution steel horizontally. Ensure proper development length at the junction of the stem and base.
Reinforcement detailing requires the main bars to be anchored into the base slab with a U-hook or sufficient development length. The toe slab reinforcement is placed at the bottom, while the heel slab reinforcement is placed at the top to resist the hogging moment caused by the backfill weight.
Design a Counterfort retaining wall to support an earth fill of 7 m above the ground level. The depth of foundation is 2 m below ground level. The SBC of soil is 200 kN/m2. The angle of repose of soil is 35°. The unit weight of the soil is 19 kN/m3. The coefficient of friction between the soil and concrete is 0.55. Spacing of counterforts = 3 m c/c. Use M 25 grade concrete and Fe 500 grade steel. Sketch the reinforcement details.
Answer
The design of a counterfort retaining wall involves determining the dimensions of the stem, heel slab, toe slab, and counterforts, followed by reinforcement detailing.
1. Design Parameters: Total height H = 7 m + 2 m = 9 m. Angle of repose φ = 35°. Coefficient of active earth pressure Ka = (1 - sin 35°) / (1 + sin 35°) = 0.271. Unit weight of soil γ = 19 kN/m3.
2. Stability Analysis: Calculate the total horizontal pressure Pa = 0.5 * Ka * γ * H2 = 0.5 * 0.271 * 19 * 81 = 208.2 kN per meter run. The base width is typically taken as 0.6 to 0.7 times the total height; assume B = 6 m. Check for sliding (factor of safety > 1.5) and overturning (factor of safety > 1.5) using the weight of the concrete and the soil wedge.
3. Stem Design: The stem acts as a continuous slab spanning horizontally between counterforts. The design moment at any depth h is M = (Ka * γ * h * L2) / 12, where L is the spacing of counterforts (3 m). Calculate reinforcement for the maximum moment at the base.
4. Counterfort Design: The counterfort acts as a T-beam or L-beam cantilever. It resists the total horizontal pressure from the stem. The main reinforcement consists of vertical bars anchored into the base slab to resist the tension force generated by the earth pressure.
5. Heel and Toe Slab: The heel slab is designed as a continuous slab spanning between counterforts, carrying the weight of the soil above it. The toe slab is designed as a cantilever slab subjected to upward soil pressure.
6. Reinforcement Detailing: Provide vertical reinforcement in the stem on the earth side, horizontal reinforcement in the stem, and main tension steel in the counterforts. Ensure proper development length and curtailment of bars as per IS 456:2000 standards. Sketch the layout showing the stem, base slab, and counterforts with spacing of main and distribution steel clearly marked.
Design the exterior panel of a flat-slab floor system for a proposed office building 30 m × 30 m divided into panels of 5 m × 5 m. Loading = 6 kN/m2. Use M 25 concrete and Fe 500 grade steel. Use 450 mm × 450 mm size columns. Assume the height of the storey as 3.5 m.
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An RC grid roof slab is to be designed for a live load of 3 kN/m2, for a conference hall of 12 m × 14 m. The spacing of ribs in the mutually perpendicular directions is 2 m c/c. Use M 25 concrete and Fe 500 grade steel. Analyse the grid floor and design the reinforcement in the Rib and slab. Sketch the reinforcement details.
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Design a circular water tank of capacity 600 kilo liters resting on ground and having a flexible base condition. Use M 30 grade concrete and Fe 500 grade steel. Bearing capacity of the soil at the site is 300 kN/m2. Design the tank as per IS 3370. Sketch the reinforcement details.
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Design a circular water tank of 15 m diameter and 3.5 m height. Free board is 300 mm. The tank rests on firm ground. The walls are fixed at base and free at top. Use M 35 grade concrete and Fe 500 grade steel. Bearing capacity of the soil at the site is 350 kN/m2. Design the tank as per IS 3370. Sketch the reinforcement details.
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Design a reinforced concrete (RC) slab culvert for a state highway to suit the following data: Carriage way = Two Lane (7.5 m wide). Use M30 concrete and Fe 500 grade steel. Kerbs = 600 mm wide, Clear span = 7.3 m, Wearing coat = 75 mm. Width of bearing = 0.4 m. Loading = IRC Class AA. Design the RC deck slab and sketch the reinforcement details.
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Design a simply supported Gantry girder to carry an electric overhead travelling crane, for the following data: Span of the gantry girder = 7.5 m; Span of crane girder = 18 m; Crane capacity = 250 kN; Self weight of crane girder excluding trolley = 200 kN; Self weight of trolley = 60 kN; Minimum hook approach = 1.2 m; Distance between wheels = 3.5 m; Self weight of rails = 0.3 kN/m. Design the Gantry girder and draw a sketch showing its cross section details along with welding details.
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