HONG KONG INSTITUTE OF VOCATIONAL EDUCATION (TSING YI)
Department of Construction
2010/11 Sessional Examination (Autumn Semester)
Course Name (Code) : Higher Diploma in Civil Engineering (51301F)
Higher Diploma in Civil Engineering (51301S)
Year of Study : L4 (51301F)
2 (51301S)
Mode of Study : FT
Unit : Temporary Works (CBE3033/CBE4407S)
Date : 21 January 2010
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 TWELVE (12) 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 A 6 m tall vertical double skin wall form is designed to resist a uniform wet concrete pressure of 70 kPa. Assume the following design and formwork material information will be used.
Design Information
Unit weight of concrete 25 kN/m3
Concrete Pressure 70 kPa
Deflection not to exceed 1/360 of span
Minimum Tie Rod Spacing 800 mm
Formwork Material Information
Formwork Materials
/ Permissible Moment of Resistance / Permissible Shear Load / StiffnessEI
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 / Not Applicable / Not Applicable
Use the beam formulas on Page 8 to determine :
(i) maximum spacing of different formwork components to withstand the design concrete pressure; and
(23 marks)
Ans. Waling at 150 mm, Soldier at 800mm, Tie Rods at 900 mm c/c
(ii) maximum tension in the tie rod.
(2 marks)
Ans. 50.4 kN
Q.2 (a) Fig. Q2(a) shows a 9 m tall column formwork designed for concrete placement in a single lift from its top. It has a zigzag profile on one vertical face ABCD while all the other three faces are truly vertical. Anticipated site conditions during concreting operation will be as follows:
Concrete temperature 4° – 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 wall form.
Hint: in kPa, or
(15 marks)
Ans.
Pmax / Hydrostatic D*h / Design Pressure / Elev. above bottomkN/m2 / kN/m2 / kN/m2 / m
148.63 / 0.00 / 0.00 / 9.00
144.58 / 25.00 / 25.00 / 8.00
141.57 / 50.00 / 50.00 / 7.00
139.22 / 75.00 / 75.00 / 6.00
137.32 / 100.00 / 100.00 / 5.00
148.63 / 100.00 / 100.00 / 5.00
145.29 / 125.00 / 125.00 / 4.00
142.68 / 150.00 / 142.68 / 3.00
140.56 / 175.00 / 140.56 / 2.00
138.81 / 200.00 / 138.81 / 1.00
137.32 / 225.00 / 137.32 / 0.00
(ii) Determine the resultant concrete thrust acting on the zigzag face.
(5 marks)
Ans. 2702.78 kN
(b) Explain the duties of a main contractor who designs and erects temporary works for construction uses under common law.
(5 marks)
Q.3 (a) Fig. Q3(a) shows the plan dimensions of a building basement site which will be bound by a cofferdam with steel sheet piling driven along its perimeter. A working platform 12 m by 18 m will be provided for construction plants access.
(i) Draw a sketch to show a feasible internal strutting and working platform arrangement if all strut spacing should not exceed 3 metres centre to centre.
(5 marks)
(ii) Draw a sketch to show the sectional elevation Section A-A if there are two levels of propping inside the cofferdam.
(5 marks)
(b) A free standing masonry façade of 9 m in height, as part of an old masonry building structure is required to be shored and kept stable from overturning. Prior to the start of demolition works of its adjacent structural elements of the building structure, a raking shore and access scaffold assembled from steel tube and fittings will be erected. Draw sketches to show the arrangement of major shoring components.
(5 marks)
(c) Prepare a general method statement for installation of a steel sheet piling cofferdam around a bridge pier site in a body of river water. Draw illustrated diagrams to show different construction processes.
(10 marks)
Q.4 Fig. Q4 shows the elevation layout of a typical transverse row of an elevated working platform structure measured 6.5 m (H) and 7.0 m (W). 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 LoadingOperation load / 2.0 kPa
Self weight of Platform Structure & Scaffolding / 1.0 kPa
Design wind speed / 30 m/s
Scaffold Structure Configuration
Lift Height / 1.3 m
Bay Length / l 1.4 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 wind 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.28kNm
(b) Determine the total leg load in kN in each individual verticals (standards) of the scaffold under the combined vertical and horizontal loading condition. Using the Table Q4 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.7.38 kN, No buckling; One leg under 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 designed wind load. Draw the layout of the diagonal braces provided.
(4 marks)
Ans. 2 Diagonal Braces required for each transverse row
(d) Determine the safety factor against overturning for the working platform structure under the given wind load when it is standing free from any operations. Calculate the minimum total kentledge load required if the safety factor calculated is less than 1.2.
(3 marks)
Ans. F.S. 1.18; Required Kentledge 0.15 kN per row
Hint: Formulas for calculating scaffolding leg loads induced by overturning moment M are given as follows:
Q.5 (a) Fig. Q5(a) shows a trench is being excavated adjacent to an existing concrete thrust block of a 45° horizontal bend of a 450 mm diameter water main. The highest design water pressure in the main is 200 kPa. Assume 2 struts will be installed at each side of the thrust block to transfer the thrust from the latter to the opposite side of the trench, determine the compressive force in each strut.
(8 marks)
Ans. 11.25 kN
(b) A 6 m high empty column formwork with a 4.5 m x 4.5 m square working platform at its top is erected to support the concrete placement for a concrete pier structure as shown in Fig Q5(b). The upper 1.5 m part of the pier form tapers from 4.5 m square at its top to 1.5 m square while the lower 4.5 m portion is 1.5 m square throughout. The pier form is provided with 4 raking struts each inclined at 45° at each vertical face and the total self-weight of the whole temporary structure is 10 kN. If it is designed to withstand a safety factor of 1.2 against overturning by a horizontal wind load of 30 kN acting at 4.5 m above ground level, determine the design axial load in each strut.
(8 marks))
Ans. 72.83 kN
(c) Prepare a safety check list for a mobile access tower assembled from mild steel tubes and fittings before put into use.
(9 marks)
Q.6 (a) A cofferdam is an important temporary structure in civil engineering construction. Any failure could lead to a disaster to both operatives and the general public in the vicinity of a cofferdam site. State possible causes of a cofferdam failure.
(5 marks)
(b) A propped cantilever steel sheet piling cofferdam is designed to resist a 6.0 m (H) high vertical cut as detailed in Fig. Q6(b). Given the following soil properties and loading information :
Active earth pressure coefficient Ka 0.25
Passive earth pressure coefficient Kp 2.30
Unit weight of soil 18 kN/m3
Surcharge Load 10 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. 0.80 m
(iii) Calculate the prop load if the props are spaced to leave a working clearance of 3.0 m on plan for equipment passing between the ground level and the bottom of excavation;
(7 marks)
Ans. 174.51 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).
(7 marks)
Ans. 3.04 m
- End of Paper -
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 =
Single-span Simply Supported Beam (Symmetrically loaded)
Max. Bending Moment= L
w
Max. Shear = 0.5wL
X
Max. Deflection @@
Where w = Uniformly Distributed Load (kN per metre run)
X = Span
L = Loaded Length
EI = Bending Stiffness of Beam
TABLE Q4
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
3 m 3 m 3 m
A
4 m
B
C
9 m
5 m
D
6 m
SECTIONAL ELEVATION SIDE ELEVATION
Fig. Q2(a) (Not To Scale)
A
12 m 24 m
12 m Steel Sheet Piling Cofferdam Site
Base Level 7.5 m below ground
18 m
12 m
Fig. Q3(a) A
``
Elevated Working Platform
1.1 m
1.3 m
1.3 m
1.3 m
1.3 m
1.3 m
5 Bays each 1.4 m = 7.0 m
Fig Q4
2 Struts at each side
of conc. block 45° Horizontal Bend
Trench under construction
PLAN OF TRENCHING WORK
Existing Conc.
Thrust Block
Centreline of the Existing 450 mm f Pipeline
Fig. Q5(a)
4.5m x 4.5m Square 1.5 m 1.5 m 1.5 m
Working Platform
Wind Load
1.5 m
30 kN
Conc. Pier Form
Tapered from W (10 kN) 1.5 m
4.5m x 4.5 m to
1.5 m x 1.5 m square
4.5 m
One Raking Strut
to each face
3 m
Ground
45°
3 m 1.5 m 3 m
Fig. Q5(b)
Surcharge 10 kPa
Prop 1 m
Waling
6 m
Z
D
Steel Sheet Piling X
Fig. Q6(b)
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