(a) In a layout of B.G. Yard, an 8° curve branches off from a 4° main curve in an opposite direction. If speed is restricted to…

(a) Speed restriction on the main line at a contrary-flexure turnout

Method: Equilibrium cant formula e = G V² / (127 R) and negative superelevation at opposite curves.

For B.G., effective gauge G = 1750 mm = 175 cm.

Radius of branch curve: R_b = 1750 / 8 = 218.75 m. Radius of main curve: Rₘ = 1750 / 4 = 437.5 m.

Equilibrium cant on branch at V_b = 28.90 km/h: e_b = G V_b² / (127 R_b) = 175 × (28.90)² / (127 × 218.75) = 175 × 835.21 / 27781.25 = 146161.75 / 27781.25 = 5.2612 cm.

Since the branch curve is opposite to the main curve, the main-curve cant eₘ acts as negative superelevation on the branch. Thus cant deficiency on branch is: cd = e_b + eₘ 7.61 = 5.2612 + eₘ eₘ = 2.3488 cm.

On the main curve, the actual cant is eₘ = 2.3488 cm. With the same permissible cant deficiency 7.61 cm, the maximum equilibrium cant on the main curve is: eₘ_eq = eₘ + cd = 2.3488 + 7.61 = 9.9588 cm.

Therefore main-line speed: Vₘ = √(eₘ_eq × 127 × Rₘ / G) = √(9.9588 × 127 × 437.5 / 175) = √(9.9588 × 317.5) = √3161.93 = 56.23 km/h.

Speed restriction on the main line = 56.2 km/h, say 55 km/h (speeds are normally fixed in multiples of 5 km/h). The same result follows from the IRS safe-speed formula V = 0.27√((Ca + Cd)R) = 0.27√(99.6 × 437.5) ≈ 56 km/h.

(b)(i) Overburnt and underburnt bricks

Overburnt bricks: burnt at excessive temperature or for too long. They are dark/blackish, vitrified, hard, brittle, fused, distorted, low in porosity, high in strength but poor in shape, difficult to dress, and may contain internal cracks. Underburnt bricks: burnt at low temperature or for insufficient time. They are light/yellowish, soft, porous, high in water absorption, low in compressive strength, liable to efflorescence, weathering and volume change. These bricks are not recommended for permanent construction because overburnt bricks are brittle and non-uniform, break during handling, give poor bond and cannot be easily cut or dressed; underburnt bricks are weak, absorb water, cause dampness and efflorescence, and have low durability. They may be used only as road metal/aggregate (overburnt) or in temporary works (underburnt).

(b)(ii) Selection of building stone

General factors governing selection of stone: strength and durability; hardness, toughness and abrasion resistance; porosity and water absorption; density and specific gravity; weathering resistance against rain, frost, salt and chemicals; appearance, colour and texture; ease of quarrying, dressing and carving; grain size and uniformity; fire resistance; thermal expansion; workability; availability and cost; absence of soluble salts, cracks and flaws.

(I) Face work of a building: choose stone with pleasing and uniform colour/texture, ability to take polish or fine dressing, no staining minerals, low porosity to resist damp, sufficient strength, good weather resistance and economical availability. Suitable examples are granite, marble, compact sandstone and dense limestone. (II) Buildings facing the sea: marine atmosphere contains salt spray and wind-driven rain. Select dense, low-porosity, high-durability stones resistant to salt crystallisation, chemical attack and abrasion, such as granite, basalt, quartzite and compact sandstone. Avoid porous limestone, soft sandstone and soluble/efflorescent stones. Low water absorption, high strength and resistance to weathering are essential.

(c)(i) Parts of a masonry wall

Coping: sloping top cover on a parapet or boundary wall. It sheds rainwater and protects the top of masonry from percolation. Cornice: projecting moulded course at the top of a wall. It throws rainwater away from the wall and gives architectural finish. Lintel: horizontal member over a door/window opening. It supports the masonry above the opening and transfers the load to the jambs. Jamb: vertical sides of an opening in masonry. It supports the lintel or arch and provides fixing for door/window frames. Parapet wall: low wall at the edge of a roof, balcony or terrace. It provides safety against fall, hides roof projections and can act as a fire barrier.

(c)(ii) Parts of a sloping roof and the tie beam

Eaves: the lower edge of a sloping roof projecting beyond the external wall. It throws rainwater clear of the wall. Eaves board: a board fixed along the eaves at the feet of the common rafters. It closes the roof edge, supports the gutter and gives a finished appearance. Common rafters: inclined members placed at regular intervals from the ridge to the wall plate/eaves. They support the roof covering and transfer loads to the ridge, wall plates and tie beam. Tie beam: a horizontal member connecting opposite ends of the principal rafters or opposite walls at wall-plate level. Purpose of tie beam: it resists the outward thrust of the rafters, prevents the walls from spreading, keeps the roof frame triangular and stable, and also carries ceiling load where provided.