Hong Kong Institute of Vocational Education (Tsing Yi) s1

HONG KONG INSTITUTE OF VOCATIONAL EDUCATION (TSING YI)

Department of Construction

2011/12 Sessional Examination (Autumn Semester)

Course Name (Code) : Higher Diploma in Civil Engineering (51301F)

Year of Study : L4 (51301F)

Mode of Study : FT

Unit : Temporary Works (CBE3033)

Date : 9 January 2011

Time : 1:30 p.m. – 4:30 p.m.

Time Allowed : THREE (3) Hours

Instructions to Candidates:

1. Answer ANY FOUR (4) questions

2. All questions carry EQUAL marks.

3. This question paper has THIRTEEN (13) pages.

4. This question paper contains SIX (6) questions.

Examination Data:

Density of Concrete 25 kN/m3 unless otherwise stated

Available from Invigilator: Graph papers

Q.1 Fig. Q1 shows a 12 m long and 9 m tall wall formwork designed for concrete placement in two separate lifts – the lower 5 m lift for 600 mm thick portion and the upper 4 m lift for 200 mm thick portion.

Anticipated site conditions (during concreting operation):

Concrete temperature 6° – 30°C

Unit weight of concrete 25 kN/m3

Rate of concrete delivery 4 trucks each of 4.5 m3 per hour.

(i) Given the following formula in computing concrete pressure acting on formwork, determine and plot the design concrete pressure distribution in 1-m intervals for the whole wall form.

P in kPa or Dh

(9 marks)

(Ans. Upper Lift Pmax=100 kPa;

Lower Lift Pmax=95.23 kPa).

(ii) Given the following formwork design and material information:

Unit weight of concrete 25 kN/m3

Deflection not to exceed 1/360 of span

Formwork Materials

/ Permissible Moment of Resistance / Permissible Shear Load / Stiffness
EI
Plywood Sheeting / 0.420 kN-m /m / 6.86 kN /m / 2.50 kN-m2 /m
Horizontal
Waling (3 spans) / 1.50 kN-m / 6.90 kN / 89.14 kN-m2
Vertical Soldier (3 spans) / 5.11 kN-m / 31.21 kN / 102.58 kN-m2
Tie Rods / Not Applicable

Use the beam formulas on Page 8 to determine the maximum spacing of different formwork components to withstand the maximum design concrete pressures on both upper and lower wall form panels (assuming them uniformly acting)

(16 marks)

(Ans. Waling at 100 mm; Soldiers at 1125 mm; Ties at 450 mm c/c)

Q.2 Fig.Q2 shows a typical panel of soffit formwork measuring 1800x6300 mm in plan designed for the construction of a 350 mm thick concrete slab at the top.

Use the beam formulas on Page 8 to check the adequacy of the given spacing of the formwork components. Assume the following design and formwork material information will be used.

Design Information

Unit weight of concrete 25 kN/m3

Construction operation load 2.0 kN/m2

Deflection not to exceed 1/360 of span

Formwork Material Information

Formwork Materials

/ Permissible Moment of Resistance / Permissible Shear Load / Stiffness
EI
Plywood Sheeting / 0.420 kN-m /m / 3.86 kN /m / 2.50 kN-m2 /m
Bearers each 1.800 metres long / 1.00 kN-m / 6.80 kN / 79.14 kN-m2
Joists each 6.300 metres long / 5.39 kN-m / 30.21 kN / 100.58 kN-m2

(25 marks)

[Ans. Bearers & Joists Spacing OK;

Prop Spacing reduced to 1260 mm c/c (4 spans)]

Q.3 Fig. Q3 shows the elevation layout of a typical transverse row of an elevated working platform structure measured 7.5 m (W) and 7.0 m (H). 1.1 m high edge boards will be provided at both free edges of the working platform level. The scaffold frame is assembled from mild steel tubing and fittings all complying with BS1139. The longitudinal row spacing is 1.35 m centre to centre.

Given the following configuration of the scaffold structure and design loading information:

Design Loading
Vertical Operation Load / 2.0 kPa
Lateral Point Loads / 2, 3 and 4 kN/m
(longitudinally at the levels shown in Fig. Q3)
Self weight of Platform Structure & Scaffolding / 1.0 kPa
Design wind speed / 25 m/s
Scaffold Structure Configuration
Lift Height / 1.4 m
Bay Length / l 1.5 m c/c in transverse direction
l 1.35 m c/c in longitudinal direction
Solidity Ratio of scaffolding exposed to transverse wind: / 4%

(a) Determine the total overturning moment acting on the whole platform structure. Assume that the dynamic wind pressure q (in kPa) for wind speed Vd (in m/s) is given by and pressure coefficient Cf = 1.3 and 2.0 for circular scaffolding tubing and the edge board respectively.

(5 marks)

(Ans. O/T Moment = 64.59 kN-m per row)

(b) Determine the total leg load in kN in each individual verticals (standards) of the scaffold under the combined vertical and lateral loading condition. Using Table Q3 to check the axial loading capacity of the verticals against lateral buckling. Identify if there are any verticals under tension. Assume scaffolding tubes of “USED” condition will be used.

(13 marks)

(Ans. Max Leg Load=9.77 kN – No Buckling;

One Outermost Leg in Tension)

(c) Assume swivel couplers each of 6.25 kN in capacity will be used to fix diagonal braces. Determine the minimum number of diagonal braces required for each row of scaffold structure to resist the total design horizontal load. Draw the layout of the diagonal braces provided.

(4 marks)

(Ans. 4 Diagonals per row needed)

(d) Determine the safety factor against overturning for the working platform structure under the given wind load only when it is standing free from any operations. Calculate the minimum total kentledge load in KN per row required if the safety factor calculated is less than 1.2.

(3 marks)

(Ans. FS = 1.80 > 1.2; No kentledge load is req’d)

Hint: Formulas for calculating scaffolding leg loads induced by overturning moment M are given as follows:

Q.4 (a) Fig. Q4(a) shows an irregular row of falsework assembled from steel tubes and fitting (to BS1139) is to be erected over a shaky ground surface. Considering the most critical effective length of individual verticals A to I, use Table Q3 to make the best estimate of the maximum allowable leg load of the falsework system.

(8 marks)

(Ans. 11.90 kN)

(b) Prepare separate sketches for raking shoring fabricated from timber and steel standard tubing and fittings to support an endangered masonry wall.

(8 marks)

(9 marks)

Q.5 (a) A 25m x 25m x 10m deep RC pumping station is to be built below ground close to an existing river. To facilitate the construction of internal structural elements as well as the installation of electrical and mechanical equipments, a strutting free space inside a cofferdam is required throughout its excavation stage.

Suggest by preparing layout arrangement and construction detail sketches of a cofferdam suitable for the proposed pumping station construction.

(8 marks)

(b) Distinguish between standard solutions and design solutions in the context of temporary works design options.

(8 marks)

(c) A steel bracket required three anchor bolts when it is mounted on an external wall of a building under the Building Ordinance of HKSAR Government. Show with illustrations the related theory of mechanics behind the ordinance.

(9 marks )

Q.6 (a) Fig. Q6(a) shows a proposed elevated working platform on a sloping ground, 2 bays x 4 lifts high, erected for 1-2 workers to carry out repairing work for a floor ceiling. Review carefully the adequacy of the safety feature provision of this temporary works and suggest an improved layout with new labeled features.

(5 marks)

(b) A propped cantilever steel sheet piling cofferdam is designed to resist a 6.5 m (H) high vertical cut as detailed in Fig. Q6(b) with raking struts at 45° to the horizontal spaced at 3.2 m centre to centre. Given the following soil properties and loading information :

Active earth pressure coefficient Ka 0.25

Passive earth pressure coefficient Kp 2.50

Unit weight of soil 18 kN/m3

Surcharge Load 15 kN/m2

Use the free earth support method to :

(i) determine and draw the net earth pressure diagram for the cofferdam wall.

(4 marks)

(ii) determine the depth of point of zero net earth pressure (Z);

(2 marks)

(Ans. 0.81 m)

(iii) calculate the axial load in each raking strut; and

(8 marks)

(Ans. 330 kN)

(iv) determine the minimum depth of embedment (D) that the pile has to be driven if the factor of safety for moment equilibrium is designed to be not less than 2.0. (Hint start D=X+Z = 2.80 m for the first iteration.)

(6 marks)

(Ans. 3.18 m)

- End of Questions-

Continuous Beam for more than 3 spans w

Max. Bending Moment= 0.107wL2

Max. Shear = 0.607wL L L L L L L

Max. Deflection =

3-span Continuous Beam w

Max. Bending Moment= 0.100wL2

Max. Shear = 0.600wL L L L

Max. Deflection =

Max. Support Reaction = 1.1 wL

2-span Continuous Beam

Max. Bending Moment= 0.125wL2

Max. Shear = 0.625wL L L

Max. Deflection =

Max. Support Reaction = 1.25 wL

TABLE Q3

Maximum Permissible Compressive Loads in Steel Scaffolds

(which are manufactured in accordance with BS 1139: Section 1.1:1990 with a yield stress of 225 N/mm2)

Effective Length (mm) / Permissible Axial Compressive Load (kN)
As “New” Tubes / As “Used” Tubes
250 / 76.4 / 70.0
500 / 74.5 / 63.3
750 / 70.7 / 60.1
1000 / 64.3 / 54.7
1250 / 55.3 / 47.0
1500 / 45.3 / 38.5
1750 / 36.4 / 30.9
2000 / 29.3 / 24.9
2250 / 23.9 / 20.3
2500 / 19.8 / 16.8
2750 / 16.6 / 14.1
3000 / 14.1 / 11.9
3250 / 12.1 / 10.3
3500 / 10.5 / 8.9
3750 / 9.2 / 7.8
4000 / 8.1 / 6.9
4250 / 7.2 / 6.1
4500 / 6.4 / 5.5
4750 / 5.8 / 4.9
5000 / 5.2 / 4.4
5250 / 4.7 / 4.0
5500 / 4.3 / 3.7
5750 / 4.0 / 3.4
6000 / 3.6 / 3.1
8000 / 1.3 / 1.1

200 200 200

4 m

9 m

5 m

600

SECTIONAL ELEVATION OF WALL FORM

Fig. Q1 (Not To Scale)

Joist

Bearer Plywood

Spacing at Sheeting

400 mm c/c

1575 1575 1575 1575

Prop

Spacing

Joist

Bearer

900 900

ELEVATION A-A

(All dimensions are in mm unless otherwise stated)

Fig. Q2

Elevated Working Platform

1.1 m

2 kN/m

1.4 m /

1.4 m /

1.4 m /

1.4 m /

1.4 m /

5 Bays each 1.5 m = 7.5 m

Fig Q3 (Not To Scale)

Vertical A B C D E F G H I

Ground Profile

Fig Q4(a) (Not To Scale)

Surcharge 15 kPa

1 m

Raking Strut

Waling

6.5 m

45°

Steel Sheet Piling X

Fig. Q6(b)

- End of Paper -

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