# RCC & Prestressed Concrete : Miscellaneous Notes

By Deepanshu Rastogi|Updated : September 2nd, 2021

STAIRCASE

Reinforced concrete stairs are an important component of a building and often the only means of providing occurs between the various floors of building.

Technical terms in Stairs

1. Tread: The horizontal upper portion of a step
2. Riser: It is the vertical distance between two consecutive treads and riser is the vertical portion of step.
3. Winder: The radiating or angular tapering step.
4. Landing: The horizontal slab provided between two flights.
5. Nosing: The outer projecting edge of a tread.
6. Flight: This consists of a series of steps provided between landing.

Classification of Stairs

1. Straight Stairs: This consists of steps leading in the same direction. It is provided in long in long narrow stairs case.
2. Dog-legged Stair: In the type, the succeeding flights rise in opposite directions
1. Effective Span

Clause 33 of IS 456 gives the ruler for calculating the effective spans.

a. When the stairs span longitudinally and are supported at the top and bottom by beams, the effective span is the distance between the respective centres of beam.

1. When the stairs span longitudinally with the landing slab also spanning in the same direction as the stairs, the effective span is the centre to centre distance between the supporting beam or walls.

1. When the stairs span longitudinally and are supported by landing an top and bottom, which span in transverse direction, the effective span is the total going of the stair plus the half width of the landing on each end or 1m whichever is smaller.

 x y Span in m < 1m < 1m G + X + y < 1m ≥ 1m G + X + 1 ≥ 1m < 1m G + y + 1 ≥ 1m ≥ 1m G + 1 + 1

4. In the case of stair spanning transversely (horizontally in the transverse direction) the effective width of stair is taken as effective span.

(a)   self wt. of the stair slab

(b)  self wt. of step slab-type stairs, [it is taken as 25 kN/m3 × Average thickness of step].

(c)   self wt. of finish

RETAINING WALLS

INTRODUCTION

Retaining walls are usually built to hold back soil mass. However, retaining walls can also be constructed for aesthetic landscaping purposes.

CLASSIFICATION OF RETAINING WALLS

EARTH PRESSURE (P)

• Earth pressure is the pressure exerted by the retaining material on the retaining wall. This pressure tends to deflect the wall outward.
• Types of earth pressure :
• Active earth pressure or earth pressure (Pa) and
• Passive earth pressure (Pp).
• Active earth pressure tends to deflect the wall away from the backfill and opposite for passive earth pressure.

ANALYSIS FOR DRY BACK FILLS

Maximum pressure at any height, p = kaγh

Total pressure at any height from top,

pa = 1/2[kaγh]h = [kaγh2]/2

Bending moment at any height

M = paxh/3 = [kaγh3]/6

∴ Total pressure, Pa = [kaγH2]/2

∴ Total Bending moment at bottom,

M = [kaγH3]/6

Where, ka = Coefficient of active earth pressure

= (1 – sinϕ)/(1 + sinϕ) = tan2ϕ

= 1/kp, coefficient of passive earth pressure

• ϕ = Angle of internal friction or angle of repose
• γ = Unit weigh or density of backfill

BACKFILL WITH SLOPING SURFACE

pa = ka γH at the bottom and is parallel to inclined surface of backfill

• Where θ = Angle of surcharge

∴ Total pressure at bottom

= Pa = ka γH2/2

DESIGN OF MASONRY STRUCTURES

Masonry in general is defined as any structural assemblage of masonry units like stones, bricks, blocks etc. with a binding material which is known as mortar. The walls of the masonry building and the building itself are designed to be stable, strong and durable enough to withstand the most severe combination of loads called as design load. Masonry in general is defined as any structural assemblage of masonry units like stones, bricks, blocks etc. with a binding material which is known as mortar. The walls of the masonry building and the building itself are designed to be stable, strong and durable enough to withstand the most severe combination of loads called as design load.

TERMINOLOGIES

• Bed Block: A block bedded on a wall, a column or a pier in order to disperse a concentrated load on a masonry element.
• Cavity wall: It is the wall consisting of two leaves with each leaf separated by a cavity and ties together with metal ties or the bonding units in order to ensure that the two leaves act as one unit. The space between the two leaves is either left free as a continuous cavity or filled with non-load bearing insulating or water proofing material.
• Curtain wall: It is a non load bearing wall subjected to lateral loads only. It may be laterally supported by vertical or horizontal structural members.
• Effective height: It is the height of wall or column which is required for computing the slenderness ratio.
• Grout: It the mixture of cement (or any other binding material). sand and water with a pourable consistency for filling the voids.
• Leaf: Inner or outer section of a cavity wall is called as leaf.

Maximum slenderness ratio for reinforced load bearing wall

 End Condition Ratio of span to effective depth Simply supported 35 Continuous 45 Spanning in two directions 45 Cantilever 18

MASONRY REINFORCEMENT

Table: Reinforcement specification in load bearing construction

 Tensile Strength MS Bars confirming to IS 432 (Part I) 140 MPa for diameter  20 mm130 MPa for diameter > 20 mm HYSD Bars (IS 1786) 230 MPa Compressive Strength Size and spacing of reinforcement 130 MPa The maximum size of reinforcement used in masonry shall be 25 mm diameter bars and minimum size shall not be less than 5 mm.

In structural design of masonry structures, the most commonly adopted design philosophy is the allowable stress design method. As per this design method, the structure shall be designed for the following load combinations:

1. DL + IL
2. OL + IL + WL (or EQL)
3. DL + WL
4. 0.9DL + EQL

In the design load combination wherein wind load (or earthquake load) is being considered, there the permissible stresses may be increased by 33.33%.

1. 0.75 (DL + IL + WL (or EQL))
2. 0.75 (DL + WL)
3. 0.75 (0.9DL + EQL)

EFFECTIVE HEIGHT OF WALLS

It is the height of waif or column which is required for computing the slenderness ratio.

Table: Effective height of wall

 S.No. Condition of Supports Effective Height 1. Lateral as well as rotational restraint (that is, full restraint) at top and bottom. For example, when the floor/roof spans on the walls so that reaction to load of floor/roof is provided by the walls, or when an RCC floor/roof has bearing on the wall (minimum 9 cm), irrespective of the direction of the span (foundation footings of a wall give lateral as well as rotational restraint). 0.75 H 2. Lateral as well as rotational restraint (that is, full restraint) alone end and only lateral restraint (that is, partial restraint) at the other. For example, RCC floor/roof at one end spanning or adequately bearing on the wall and timber floor/roof not spanning on wall, but adequately anchored to it, on the other end. 0.85 H 3. Lateral restraint, without rotational restraint (that is, partial restraint) on both ends. For example, timber floor/roof, not spanning on the wall but adequately anchored to it on both ends the wall, that is, top and bottom. 1.00 H 4. Lateral restraint as well as rotational restraint (that is, full restraint) at bottom but have no restraint at the top. For example, parapet walls with RCC roof having adequate bearing on the lower wall, or a compound wall with proper foundation on the soil. 1.50 H

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