Construction Techniques: Foundations, Masonry, Openings, Frames, Roofs, Fire Safety, Floors, Modular Coordination and Prefabrication

Practice questions (20)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

1. Which description fits English bond in brickwork?

   1. Stretchers only in every course, with each course shifted by half a brick against the one below it
   2. Headers and stretchers alternating within every course, with a closer beside the quoin
   3. Alternate courses of headers and stretchers, with a queen closer beside the quoin header
   4. Bricks laid on edge so that a cavity is left inside the wall
   Show answer

   Answer: C — Alternate courses of headers and stretchers, with a queen closer beside the quoin header

   English bond alternates whole courses of headers and stretchers, and a queen closer beside the quoin header sets the lap so that vertical joints do not line up. Flemish bond is the tempting alternative, but it alternates headers and stretchers within each course. Stretcher-only courses are stretcher bond, and edge-laid bricks enclosing a cavity are rat-trap bond.

2. An edge column cannot project beyond the property line. Its footing is joined by a beam to the footing of an interior column, and the beam balances the eccentric load. This arrangement is called a

   1. combined rectangular footing, a single slab under both columns
   2. raft foundation under the whole building plan
   3. stepped pad footing centred exactly under the edge column
   4. strap (cantilever) footing
   Show answer

   Answer: D — strap (cantilever) footing

   In a strap footing two separate bases are tied by a strap beam that carries the moment from the eccentric edge load to the interior footing. A combined footing is the tempting alternative, but it is one common slab under both columns rather than two bases linked by a beam. A pad centred under the edge column would need to cross the boundary.

3. A one-brick-thick (200 mm nominal) wall of modular bricks is 6 m long and 3 m high and has one door opening of 1 m × 2.1 m. Taking 500 bricks per m³ of masonry (nominal size), the number of bricks required is

   1. 1,800
   2. 1,590
   3. 2,066
   4. 1,431
   Show answer

   Answer: B — 1,590

   Net face = 6 × 3 − 1 × 2.1 = 15.9 m²; volume = 15.9 × 0.2 = 3.18 m³; bricks = 3.18 × 500 = 1,590. The figure 1,800 is the tempting one because it ignores the door. The figure 2,066 comes from dividing by the actual brick volume (about 650 per m³), which counts mortar space as brick.

4. Which formwork system rises continuously at a controlled rate while concrete is being placed, and so suits silos, chimneys and lift cores?

   1. Table (flying) form lifted whole by crane to the next floor
   2. Climbing form lifted in stages after each pour has hardened
   3. Tunnel form casting room-sized cells with walls and slab together
   4. Slip form
   Show answer

   Answer: D — Slip form

   A slip form creeps upward without stopping as the concrete in it sets, giving a joint-free shaft. The climbing form is the near miss: it is raised in stages between pours, so it leaves construction joints. Table and tunnel forms serve slabs and repetitive cells.

5. Under vertical load on a king post truss, the king post acts as

   1. a tension member hanging the tie beam from the apex
   2. a compression strut carrying the ridge load down to the tie beam
   3. a bending member spanning between the two purlins at the ridge
   4. a zero-force member that is only there for lateral stability
   Show answer

   Answer: A — a tension member hanging the tie beam from the apex

   The principal rafters and the struts are in compression and push on the tie beam, which is in tension; the king post hangs the middle of the tie beam from the apex, so it is a tension member. Calling it a compression strut is the tempting error because it looks like a column.

6. Structural steel does not burn, yet an unprotected steel frame needs fire protection. The main reason is that

   1. steel melts at the temperatures reached in ordinary building fires, and the frame then runs away
   2. steel burns slowly once heated and adds its own heat to the fire load of the contents
   3. steel loses much of its strength at a few hundred degrees, so loaded members can fail early
   4. steel conducts heat so well that it ignites the timber and furniture next to it
   Show answer

   Answer: C — steel loses much of its strength at a few hundred degrees, so loaded members can fail early

   At roughly 550 to 600 °C steel keeps only about half its yield strength, which a building fire can reach in minutes, so a loaded unprotected member can fail. It melts only at far higher temperatures, which is the tempting wrong answer, and it adds no fire load because it is non-combustible.

7. A modular brick has a nominal (modular) size of 200 × 100 × 100 mm and is laid with a 10 mm mortar joint. Its work (manufacturing) size is

   1. 210 × 110 × 110 mm
   2. 190 × 90 × 90 mm
   3. 200 × 100 × 100 mm
   4. 180 × 80 × 80 mm
   Show answer

   Answer: B — 190 × 90 × 90 mm

   Work size = modular size − joint = 200 − 10 = 190 mm, and likewise 90 and 90 mm. Adding the joint (210 mm) reverses the relation, and leaving the size unchanged ignores the joint that completes the module.

8. A uniform precast beam 10 m long is lifted at two points placed symmetrically, each at distance a from an end. The hogging moment at the lifting points is to equal the sagging moment at midspan. The distance a is

   1. 1.50 m
   2. 2.50 m
   3. 2.07 m
   4. 3.33 m
   Show answer

   Answer: C — 2.07 m

   With self-weight w, hogging = wa²/2 and sagging = wL²/8 − wLa/2. Equating gives 4a² + 4La − L² = 0, so a = 0.207 L = 2.07 m. A quarter-length pick, 2.50 m, is the tempting round figure, but it gives a larger hogging moment than sagging moment.

9. A workshop floor will carry wheeled loads and heavy abrasion. The most appropriate finish is

   1. polished marble slabs on a lime-mortar bed with fine joints
   2. a granolithic or metallic-hardened concrete topping
   3. parquet blocks of hardwood laid in mastic over the screed
   4. thin vinyl sheet laid over a smooth cement screed with welded joints
   Show answer

   Answer: B — a granolithic or metallic-hardened concrete topping

   Hard aggregate or metallic hardener in the topping resists abrasion and wheel loads. Marble is the tempting prestige finish, but it is soft, polishes slippery and chips under impact. Parquet and thin vinyl wear through in a workshop.

10. In the usual sequence of stripping formwork from a floor cast with ordinary Portland cement, which is removed first?

    1. The vertical side forms of beams, after about 16 to 24 hours
    2. The props under a beam of 5 m span, after about 7 days
    3. The props under a slab of 4 m span, after about 14 days
    4. The soffit form of a 6 m beam, with its props kept in place for the time being
    Show answer

    Answer: A — The vertical side forms of beams, after about 16 to 24 hours

    Side forms carry no load and can come off as soon as the concrete resists damage, about 16 to 24 hours. Props and soffits carry the self-weight and wait for days: slab props about 7 days for spans up to 4.5 m, beam props about 14 days for spans up to 6 m. The slab props are the tempting pick because slabs look lighter than beams, but they still wait days.

11. Stone masonry is to be raised with scaffolding. A double (mason's) scaffold is preferred to a single scaffold because

    1. its two rows of standards stand clear of the wall, so no putlog holes are needed
    2. it is hung from ropes at the top of the wall, so it needs no support from the ground below
    3. its putlogs rest on the new wall, so they can be pulled out later without any damage
    4. it has no ledgers or braces, so it is cheaper and quicker to assemble than a single scaffold
    Show answer

    Answer: A — its two rows of standards stand clear of the wall, so no putlog holes are needed

    In a double scaffold the two rows of standards carry their own ledgers, braces and transoms, so the stone wall is not pierced for putlogs. The single scaffold relies on putlogs resting in the wall, which suits brick, where a brick can be left out and filled later, but not irregular stone. The suspended scaffold is hung from ropes and is a different type.

12. Which of the following are requirements of good formwork?

    1. It must be rigid enough to keep its shape under the pressure of wet concrete
    2. Its joints must be tight enough to prevent loss of cement grout
    3. It should bond firmly to the concrete so that it remains in place permanently
    4. It should be removable without shock or damage to the young concrete
    Show answer

    Answer: A — It must be rigid enough to keep its shape under the pressure of wet concrete; B — Its joints must be tight enough to prevent loss of cement grout; D — It should be removable without shock or damage to the young concrete

    Rigidity, tight joints and damage-free stripping are the standard requirements, together with accuracy of size and reuse. A form that bonds to the concrete is a defect, because it tears the surface at stripping; release agents are used to prevent that. Only a permanent form is designed to stay, and it is a separate system.

13. Which of the following are sound fire-protection measures in principle?

    1. Wedging a fire door open so that people can move freely
    2. Filling the gaps around service pipes that pass through a fire wall with fire-stopping
    3. Giving every floor at least two independent escape routes
    4. Enclosing stairs and lift shafts in fire-resisting construction
    Show answer

    Answer: B — Filling the gaps around service pipes that pass through a fire wall with fire-stopping; C — Giving every floor at least two independent escape routes; D — Enclosing stairs and lift shafts in fire-resisting construction

    Fire-stopping keeps compartment walls intact where services pierce them; two independent routes keep an escape open if one is blocked; and enclosed shafts stop smoke and flame using stairs and lifts as chimneys. A fire door held open defeats compartmentation, because only a closed, self-closing door contains the fire.

14. Which statements about roofs are correct?

    1. A hip roof slopes on all four sides of a rectangular building
    2. A valley is the internal angle where two roof slopes meet
    3. A couple roof without a tie pushes the supporting walls outward
    4. In a mansard roof the upper slope on each side is steeper than the lower
    Show answer

    Answer: A — A hip roof slopes on all four sides of a rectangular building; B — A valley is the internal angle where two roof slopes meet; C — A couple roof without a tie pushes the supporting walls outward

    A hip roof has four slopes, a valley is an internal meeting of slopes, and without a tie the rafter feet spread and push the walls out. The mansard statement is reversed: the lower slope is the steep one and the upper slope is gentle, which is the tempting slip.

15. Which statements about foundations are correct?

    1. A raft foundation reduces differential settlement by tying the whole building to one slab
    2. Under-reamed piles suit expansive black cotton soil because their bulbs anchor below the zone of seasonal moisture change
    3. In Rankine's formula the minimum depth increases as the angle of repose increases
    4. A strap footing is used when an edge column's footing cannot be centred under the column
    Show answer

    Answer: A — A raft foundation reduces differential settlement by tying the whole building to one slab; B — Under-reamed piles suit expansive black cotton soil because their bulbs anchor below the zone of seasonal moisture change; D — A strap footing is used when an edge column's footing cannot be centred under the column

    A raft ties the whole plan together, under-reamed bulbs anchor against swelling and shrinkage, and a strap beam handles an eccentric edge footing. The Rankine statement is wrong: the ratio (1 − sin φ)/(1 + sin φ) falls as φ rises, so the minimum depth falls, which is the tempting reversal.

16. A foundation is to be taken down to Rankine's minimum depth D = (q/γ)[(1 − sin φ)/(1 + sin φ)]². With q = 200 kN/m², γ = 18 kN/m³ and φ = 30°, the depth in metres, correct to two decimal places, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 1.23

    sin 30° = 0.5, so (1 − 0.5)/(1 + 0.5) = 1/3 and its square is 1/9. q/γ = 200/18 = 11.111 m. D = 11.111/9 = 1.2346, which is 1.23 m to two decimal places. Check: 11.111 × 0.1111 = 1.2346.

17. A wall of modular brick is 10 m long, 3 m high and 0.2 m thick, with openings of 2.0 m² and 1.8 m². One m³ of this masonry holds 500 bricks of actual size 190 × 90 × 90 mm, and the rest is wet mortar. The wet mortar volume for the wall, in m³ correct to two decimal places, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 1.21

    Net area = 30 − (2.0 + 1.8) = 26.2 m²; volume = 26.2 × 0.2 = 5.24 m³. Bricks per m³ occupy 500 × 0.19 × 0.09 × 0.09 = 0.7695 m³, so mortar is 0.2305 m³ per m³. Mortar = 5.24 × 0.2305 = 1.2078, which is 1.21 m³. Check: 5.24 − 5.24 × 0.7695 = 5.24 − 4.0322 = 1.2078.

18. A room measures 6.0 m × 4.5 m and is laid out on a 3M planning grid (M = 100 mm). The number of 300 mm × 300 mm grid squares in the room is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 300

    3M = 300 mm. Along 6.0 m there are 6000/300 = 20 modules and along 4.5 m there are 4500/300 = 15 modules. Squares = 20 × 15 = 300. Check by area: 27 m² ÷ 0.09 m² = 300.

19. A segmental arch has a clear span (chord) of 10 m and a rise of 2 m. Using R = (c²/4 + h²)/(2h), the radius of its intrados, in metres to two decimal places, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 7.25

    c²/4 = 100/4 = 25 and h² = 4, so the numerator is 29; 2h = 4. R = 29/4 = 7.25 m. Check from geometry: half-chord 5 and R − h = 5.25 give 5² + 5.25² = 25 + 27.5625 = 52.5625 = 7.25². The check holds.

20. A symmetrical gable roof has a span of 12 m and a rise of 4.5 m, with no overhang. The length of each rafter, in metres, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 7.5

    Half-span = 6 m. Rafter = √(6² + 4.5²) = √(36 + 20.25) = √56.25 = 7.5 m. Check: a 3-4-5 triangle scaled by 1.5 gives 4.5-6-7.5.