THEORY EXAMINATION (SEM–VI) 2016-17 DESIGN OF CONCRETE STRUCTURES II

DESIGN OF CONCRETE STRUCTURES – II (ECE601)

B.Tech (SEM VI) | Section-wise Solved Answers

SECTION – A

(10 × 2 = 20 marks)

(a) Principles of design of strap footing

Strap footing is designed so that the resultant load from columns passes through the centroid of the footing area. The strap beam transfers moment between footings and does not bear on soil.

(b) Advantages of flat plates over conventional slabs

Flat plates eliminate beams, reduce storey height, simplify formwork, allow flexible architectural layout, and provide faster construction.

(c) Factors affecting punching shear strength

Punching shear strength depends on column size, slab thickness, concrete grade, reinforcement ratio, and presence of column heads or drops.

(d) Situations favoring combined footing

Combined footings are preferred when columns are close, property lines restrict isolated footings, or soil bearing capacity is low.

(e) Reinforcement detail of combined footing

Combined footings have main reinforcement along the longer direction with distribution steel perpendicular to it. Rectangular and trapezoidal shapes are commonly used.

(f) Segmental retaining walls

Segmental retaining walls consist of modular concrete blocks stacked without mortar and rely on gravity and soil reinforcement for stability.

(g) Earth pressure theories

The two theories are Rankine’s theory and Coulomb’s theory, used to calculate active and passive earth pressures.

(h) Column head and drop

A column head is a widened portion at slab-column junction to reduce punching shear. A drop is a thickened slab portion around columns.

(i) Need of prestressing

Prestressing counteracts tensile stresses, controls cracks, allows longer spans, and improves durability and serviceability.

(j) Difference between RCC and PSC in flexure

RCC resists flexure after cracking, while PSC resists flexure mainly by induced compressive stresses that delay or prevent cracking.

SECTION – B

(Attempt any five – 10 marks each)

(a) Design of combined footing

The footing is designed by calculating total load, soil pressure, footing area, bending moments, shear forces, and providing reinforcement as per IS 456.

(b) Flat slab moment calculation

Moments are calculated using direct design method or equivalent frame method considering panel size, column spacing, and loading.

(c) Prestressed concrete

Merits: Higher load capacity, crack control, economy for long spans.
Demerits: Skilled labor required, higher initial cost.
Losses include elastic shortening, creep, shrinkage, relaxation, and friction losses.

(d) Design of rectangular water tank

Design includes wall thickness, base slab, roof slab, and reinforcement based on hydrostatic pressure and IS 3370 provisions.

(e) Design of wall footing

Footing width is determined from bearing pressure. Bending moment and shear are checked, and reinforcement is designed accordingly.

(f) Methods of prestressing

Prestressing is done by pre-tensioning and post-tensioning methods using tendons and anchorages.

(g) Design of flat slab with drops

Interior panel moments are calculated, slab thickness is checked for deflection, and reinforcement is provided in column and middle strips.

(h) Counterfort retaining wall

A counterfort wall uses triangular counterforts to reduce bending moments in the stem. It is economical for heights above 6 m.

SECTION – C

(Attempt any two – 15 marks each)

(3) Design of T-shaped cantilever retaining wall

Design includes calculation of earth pressure, stability checks for sliding, overturning, bearing pressure, and design of stem, heel, and toe reinforcement.

(4) Design of circular water tank

Hoop tension, bending moments, wall thickness, and reinforcement are calculated. Crack control is ensured as per permissible tensile stresses.

(5) Design of strap footing

Strap beam transfers moment between footings. Design involves soil pressure calculation, footing size, strap beam design, and reinforcement detailing.