(SEM VI) THEORY EXAMINATION 2021-22 DESIGN OF CONCRETE STRUCTURES

DESIGN OF CONCRETE STRUCTURES (KCE601)

Section-wise Detailed Answers – As per University Exam Pattern

SECTION A

(Attempt all questions – descriptive answers)

Q1(a) What are the disadvantages of R.C.C. structures?

Reinforced Cement Concrete structures, though widely used, have certain disadvantages. One major drawback is their heavy self-weight, which increases the load on foundations. RCC construction also requires skilled labor, proper supervision, and quality control, without which structural performance may be compromised. Another disadvantage is the time required for construction, as concrete needs proper curing before gaining full strength. RCC structures are also susceptible to cracks due to shrinkage, temperature variations, and improper detailing. Additionally, once constructed, alterations or modifications in RCC structures are difficult and expensive.

Q1(b) Data required for design mix concrete

Design mix concrete requires detailed information about materials and conditions. This includes the grade of concrete required, type and grade of cement, properties of aggregates such as size, shape, and specific gravity, desired workability, method of compaction, exposure conditions, and quality control level. Environmental conditions and durability requirements as per IS 456:2000 are also essential inputs. These parameters help in achieving the required strength and durability economically.

Q1(c) Crack pattern in simply supported beams

In a simply supported beam, cracks develop primarily due to bending and shear forces. Flexural cracks appear near the mid-span at the bottom face of the beam where tensile stress is maximum. These cracks are vertical in nature. Near the supports, diagonal cracks develop due to combined action of shear and bending. These diagonal cracks originate at the tension face and propagate towards the compression zone. The crack pattern helps engineers understand stress distribution and structural behavior.

Q1(d) Where bond stress is developed in steel bar and concrete?

Bond stress develops at the interface between the steel reinforcement and surrounding concrete. This stress ensures that both materials act together as a composite unit. When a reinforced concrete member is subjected to load, tensile forces are transferred from concrete to steel through bond stress. Proper bond prevents slippage of steel bars and ensures effective stress transfer, which is essential for structural safety.

Q1(e) Reinforcement in one-way slab

In a one-way slab, the main reinforcement is provided in the direction of the shorter span because bending occurs mainly in that direction. Distribution reinforcement is placed perpendicular to the main reinforcement to control cracking and distribute load uniformly. The main bars are placed at the bottom of the slab in tension zone, while distribution bars are placed above them. Proper spacing and anchorage are essential for effective load transfer.

Q1(f) Define landing and riser

A landing is a horizontal platform provided between two flights of stairs or at the top and bottom of a staircase. It allows rest and safe movement. A riser is the vertical portion of a step that connects two successive treads. The riser height determines the comfort and safety of stair usage and must conform to standard building codes.

Q1(g) Axially loaded column

An axially loaded column is a compression member subjected primarily to a load acting along its longitudinal axis without significant bending. In practice, perfect axial loading is rare, but columns designed as axially loaded assume minimal eccentricity. Such columns experience uniform compressive stress across the cross-section and are commonly used in low-rise structures.

Q1(h) Why columns are designed for minimum eccentricity?

In reality, it is impossible to apply load perfectly along the centroid of a column due to construction tolerances and load misalignment. Even small eccentricities can induce bending moments. Therefore, IS 456:2000 mandates that all columns be designed for minimum eccentricity to ensure safety against unexpected bending and instability.

Q1(i) Purpose of foundation in a structure

The foundation transfers the load of the structure safely to the underlying soil. It distributes the load over a larger area to prevent excessive settlement. Foundations also provide stability against sliding, overturning, and uplift forces. Additionally, they protect the structure from differential settlement and ensure long-term structural integrity.

Q1(j) Cases where retaining walls are constructed

Retaining walls are constructed when there is a difference in ground levels that must be maintained. They are commonly used in hill roads, basements, bridge abutments, waterfront structures, and embankments. Retaining walls prevent soil movement and ensure stability of adjacent structures.

SECTION B

(Attempt any three – explained in detail)

Q2(c) Design steps of one-way slab

Design of a one-way slab begins with identifying the effective span based on clear span and support conditions. The type of slab action is confirmed by checking span ratio. Loads including self-weight, live load, and floor finish are calculated. Factored bending moment is computed using IS code coefficients. Effective depth is determined based on bending moment and material grades. Area of main reinforcement is calculated and bar spacing is finalized. Shear check and deflection check are then performed to ensure safety and serviceability. Finally, detailing of reinforcement is carried out as per code provisions.

Q2(d) Classification of columns and why RCC columns are preferred

Columns are classified based on material as masonry, steel, timber, and RCC columns. RCC columns are preferred because they offer high strength, durability, fire resistance, and economy. They can be cast into various shapes and sizes, making them suitable for modern construction. RCC columns also have better load-carrying capacity compared to plain concrete columns due to reinforcement.

Q2(e) Design of strip footing for brick masonry wall

In designing a strip footing, the total load on the wall including self-weight is first calculated. The required width of footing is obtained by dividing the load by safe bearing capacity of soil. Thickness of footing is then determined based on bending moment and shear considerations. Reinforcement is designed to resist bending stresses. Proper cover and detailing are ensured for durability. M20 concrete provides adequate strength and durability for such footings.

SECTION C

(Attempt any one – detailed solution)

Q3(b) Design steps of a simply supported RCC beam

Design of a simply supported RCC beam starts with determining effective span based on support conditions. Loads including dead load, live load, and self-weight are calculated. Factored bending moment and shear force are obtained using load factors. Effective depth is selected based on bending moment capacity. Area of tension reinforcement is computed using limit state method. Shear design is carried out to determine stirrup spacing. Checks for deflection, cracking, and development length are performed as per IS 456:2000. Finally, reinforcement detailing is prepared ensuring adequate cover and anchorage.

Q4(a) Design procedure of RCC beam subjected to equivalent shear and bending moment

When a beam is subjected to torsion along with bending and shear, equivalent bending moment and equivalent shear force are calculated. These equivalent values are used to design reinforcement. Longitudinal reinforcement is provided to resist combined bending and torsion, while transverse reinforcement resists shear and torsional stresses. Design ensures safety against combined effects and satisfies codal provisions.

Q5(a) Design of simply supported roof slab

Design begins by identifying slab type based on span ratio. Loads including self-weight, live load, and finishes are calculated. Effective depth is determined using bending moment equations. Area of main and distribution reinforcement is calculated. Spacing of bars is finalized as per code limits. Deflection check is carried out using span-to-depth ratio. Proper detailing ensures durability and serviceability.

Q6(a) Design of short RCC column

The effective length of column is determined based on end conditions. Slenderness ratio is checked to confirm short column behavior. Axial load capacity is calculated using IS code formula. Area of longitudinal reinforcement is computed and distributed evenly. Lateral ties are designed to provide confinement. Reinforcement detailing ensures adequate cover and spacing.

Q7(a) Deflected shape of cantilever retaining wall

Under earth pressure, the stem bends like a vertical cantilever fixed at the base, with maximum bending at the base. Heel slab bends upward due to soil pressure acting upward, while toe slab bends downward due to soil pressure acting downward. Understanding deflected shape helps in proper reinforcement placement and safe design.