(SEM VI) THEORY EXAMINATION 2022-23 DESIGN OF CONCRETE STRUCTURES

DESIGN OF CONCRETE STRUCTURES – KCE-601

Section-wise Important Questions & Ready Answers

SECTION A

(Attempt all questions – 2 marks each)

(a) Lever Arm and Development Length

The lever arm is the perpendicular distance between the line of action of compressive force in concrete and tensile force in steel in a reinforced concrete section. It governs the moment of resistance of the section.
Development length is the minimum length of reinforcement required to develop full tensile stress in steel through bond with concrete, ensuring safe load transfer.

(b) Under-Reinforced vs Over-Reinforced Section

An under-reinforced section fails by yielding of steel before crushing of concrete, providing sufficient warning and ductility.
An over-reinforced section fails suddenly by crushing of concrete without warning and is therefore not permitted in limit state design.

(c) Need for Shear Reinforcement in Beams

Shear reinforcement is provided to resist diagonal tension cracks, increase shear strength, improve ductility, and prevent sudden brittle shear failure of beams.

(d) Types of Bond in RCC

Bond in RCC is classified into adhesive bond, frictional bond, and mechanical bond (due to deformed bars). These ensure composite action between steel and concrete.

(e) Functions of Longitudinal Reinforcement

Longitudinal reinforcement resists tensile stresses, controls cracking, provides flexural strength, and improves ductility of reinforced concrete members.

(f) Factors Affecting Short-Term Deflection

Short-term deflection depends on span, loading, modulus of elasticity of concrete, moment of inertia of section, reinforcement percentage, and boundary conditions.

(g) Assumption for Limit State of Collapse in Compression

At the limit state of collapse in compression, concrete strain is assumed as 0.0035, and stress distribution follows the parabolic-rectangular stress block as per IS 456:2000.

(h) Classification of Columns

Columns are classified based on slenderness (short and long columns), loading (axially loaded, eccentrically loaded), shape (rectangular, circular), and reinforcement (tied or spiral).

(i) Modes of Failure of Retaining Wall

Retaining walls may fail due to overturning, sliding, bearing capacity failure, and structural failure of stem or base slab.

(j) Types of Footings

Footings are classified as isolated footing, combined footing, strap footing, raft footing, and pile footing depending on soil condition and loading.

SECTION B

(Attempt any three – 10 marks each)

2(a) Design Philosophies of Concrete Structures

Concrete structures have been designed using three philosophies: Working Stress Method, Ultimate Load Method, and Limit State Method.
The working stress method ensures safety under service loads but ignores ultimate strength. The ultimate load method considers collapse loads but neglects serviceability. The Limit State Method, adopted in IS 456:2000, ensures safety against collapse and serviceability, making it the most rational and widely used approach.

2(b) Beam Subjected to Bending, Shear and Torsion (Numerical)

For the given beam (300×600 mm) subjected to factored bending moment, shear force, and torsion, equivalent bending moment and shear force are calculated as per IS 456. Tensile reinforcement is designed for combined action, followed by design of shear reinforcement using torsional shear equations.
(Step-wise design with formulas and final detailing is expected in exam.)

2(c) Design Procedure of Two-Way Slab by LSM

The design involves determination of effective span, slab type based on aspect ratio, calculation of design moments using IS 456 coefficients, checking depth for deflection, design of main and distribution reinforcement, and verification for shear and serviceability.

2(d) Design of Column (Numerical)

The column is classified as short based on slenderness ratio. Axial load capacity is calculated using interaction equations. Longitudinal reinforcement is provided between 0.8% and 6% of gross area, along with lateral ties as per IS 456 requirements.

2(e) Types of Retaining Walls

Common retaining walls include gravity walls, cantilever retaining walls, counterfort retaining walls, and buttressed walls. Selection depends on height of retained earth, soil condition, and economy.
(Neat sketches are expected in exam.)

SECTION C

(Attempt any one – 10 marks)

3(a) Design of Rectangular Beam for Bending

Given bending moment and width, effective depth is calculated. The section is checked as under-reinforced. Required tensile steel is obtained using limit state equations. Adequacy of depth is verified and reinforcement detailing is done as per IS 456.

3(b) Design of Simply Supported Beam with UDL

Effective span is determined, factored load calculated, bending moment and shear force evaluated. Depth is checked for deflection control, reinforcement designed for bending, and shear reinforcement provided near supports.