HONG KONG INSTITUTE OF VOCATIONAL EDUCATION (TSING YI)
Department of Construction
2007/08 Sessional Examination (Autumn Semester)
Course Name (Code) : Higher Diploma in Civil Engineering (51301)
Higher Diploma in Civil Engineering (51301A)
Year of Study:3 (51301)
2 (51301A)
Mode of Study: FT
Unit: Temporary Works (CSE4402)
Date: 17January 2008
Time: 1:30 pm –4:30 pm
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 TEN (10) pages.
4.This question paper contains SIX (6) questions.
Examination Data:
Density of Concrete 25 kN/m3
Available from Invigilator:Graph papers
Q.1Fig. Q1 shows a 7.4 m tall cantilever formwork designed for casting successive lifts of concrete 4 m each in height. The formwork has a steel truss strongback behind the plywood sheeting and is anchored by three rows of anchor bolts at 1.0 m vertically apart as shown in the figure. It weighs 20 kN per longitudinalmetre run. The horizontal spacing for each row of anchor bolts is 0.5 m center to center. Anticipated site conditions during concreting operation will be as follows:
Concrete temperature 5 – 32C
Unit weight of concrete 25 kN/m3
Rate of concrete deliveryEach 4-m lift will be completed in 8 hours.
(a)Given the following formula in computing concrete pressure acting on formwork, determine the design concrete pressure in kPa for the 4-m LIFT and draw the concrete pressure distribution diagram accordingly.
Hint: in kPa, or
(6 marks)
(b)Among the three horizontal rows of anchor bolts, it is assumed that the middle bolts are installed for additional safety only and will not offer any tension nor compressive forces under normal design condition. Calculate the tension ineach upper anchor bolt in kNfor the formwork having a safety factor of 1.2 against overturning about its lower rowof anchor bolts.
(12 marks)
(c)Calculate the tension in each anchor bolt in the middle row if all the anchor bolts of the uppermost row fail in pull-out mode.
(4 marks)
(d)State the general considerations in structural design of falsework
(3 marks)
Q.2Fig. Q2 shows the elevation layout of a typical transverse row of a birdcage scaffold structure measured 8.4 m (H) and 9.1 m (W). There are 12 rows of falsework framework in longitudinal direction each comprising of mild steel tubing and fittings all complying with BS1139. The falsework will be erected to support the construction of a concrete slab.
Given the following configuration of the falsework and design loading information:
Design LoadingConcrete slab thickness / 150 mm
Operation load / 2.0 kPa
Self weight of Formwork & Scaffolding / 1.0 kPa
Design wind speed / 40 m/s
Falsework Configuration
Lift Height / 1.4 m
Bay Length / 1.30 m c/c in transverse direction
1.35 m c/c in longitudinal direction
Solidity Ratio of scaffolding exposed to transverse wind: / 5%
(a)Determine the total wind moment acting on the falsework structure. Assume that the dynamic wind pressureq (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 form respectively.
(5 marks)
(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 Q2 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)
(c)Determine the minimum number of diagonal braces required to resist the horizontal wind load. Draw the layout of the diagonal braces provided for the falsework.
(4 marks)
(d)Determine the safety factor against overturning for the falsework when the soffit formwork is standing EMPTY but with operation load and calculate the kentledge required, if required,to meet the minimumsafety factor of 1.2
(3 marks)
Hint: Formulas for calculating leg loads induced by overturning moment M are given as follows:
Q.3A large panel of soffit formwork is designed for the construction of a 250 mm thick concrete deck slab.
Determine the maximum spacing of the formwork components given the following design and formwork material information are used.
Design Information
Unit weight of concrete - 25 kN/m3
Construction operation load – 2.0kN/m2
Deflection not to exceed 1/360 of span
Formwork Material Information
Formwork Materials
/ Permissible Moment of Resistance / Permissible Shear Load / StiffnessEI
Plywood Sheeting / 0.420 kN-m /m / 7.86 kN /m / 3.50 kN-m2 /m
Timber Bearers each 3 metres long / 1.00 kN-m / 6.80 kN / 79.14 kN-m2
Steel Joists supported by vertical props / 5.39 kN-m / 30.21 kN / 100.58 kN-m2
Hint:Assume all formwork components are continuous beams ofmultiple equal spans and the following formulas to be used.
Max. Bending Moment for a continuous beam under UDL
Max. Shear Force
Max. Deflection = 0.007 for plywood; and
= 0.004 for bearers and steel joists,
where ω= Uniformly Distributed Load (UDL) on span L (kN per metre run)
(25 marks)
Q.4A propped cantilever steel sheet piling cofferdam is designed to resist a 6 m (H) high vertical cut as detailed in Fig. Q4. Given the following soil properties and loading information :
Active earth pressure coefficient Ka0.30
Passive earth pressure coefficient Kp2.85
Unit weight of soil18 kN/m3
Surcharge Load 20 kN/m2
and the net earth pressure diagram shown in the figure, use the free earth support method to determine the:
(i)Depth of point of zero net earth pressure (Z);
(4 marks)
(ii)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;
(12 marks)
(iii)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 = 2.8 m for iterations).
(5 marks)
(iv)Suggest with illustration alternative design provision if there are existing underground utilities which obstruct the depth of piling to be driven as obtained in (iii)
(4 marks)
Q.5 (a)A very tall reinforced concrete core wall of constant section is proposed to be built well ahead of the construction of floor slab, beam and column elements of a high rise building for lateral wind loading resistance. A slip form construction method will subsequently be employed for the core wall construction. Explain with illustration the setting up of such a typical formwork system arrangement and discuss the advantages and limitations of this construction method.
(15 marks)
(b)Steel sheeting piling material is always used for cofferdam construction. Describe briefly the common causes of cofferdam failures and the appropriate design requirements in cofferdam design.
(10 marks)
Q.6(a)Trenching carried out on existing carriageways or adjacent to existing properties could be hazardous if an excavation contractor does not exercise due care before and during construction works. Evaluate possible hazards in general and suggest some preventive measures to minimize the associated risk.
(12 marks)
(b)Draw a sketch showing the layout of a mobile scaffold tower and prepare the necessary safety checklist before the temporary structure is put into use by construction workers.
(13 marks)
- End of Paper -
TABLE Q2
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
Concrete Level 0.2 m
Truss Strongback
Height of next lift 4 m
4 m
7.4 m
1 mAnchor Truss Strongback weighs
Bolt 20 kN per m longitudinally
1 m3 m
t
1 m
0.2 m
Concrete of Previous
Lift
0.75 m
Fig. Q 1
Edge Form for150 mmthick
Concrete Slab 150
Wind 6 lifts each
1.4 m high
Ground Level
7 bays each 1.3 m wide
Fig. Q2(Not to Scale)
20 kPa
Prop6.00 kPa 1 m
6 m
38.40 kPa
Z
DSteel Sheet Piling
X
Fig. Q4
(Net Lateral Earth Pressure Diagram)
(Not to Scale)
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