design of concrete structures 2010 level 4 semester 1

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UNIVERSITY OF MORATUWAFaculty of EngineeringDepartment of Civil EngineeringB.Sc. Engineering DegreeLevel 4, Semester 1 ExaminationCE 3060: Design of Concrete Structures

Time Allowed: Three (03) Hours January 2010

INSTRUCTIONS TO CANDIDATES1. This is a closed book examination.2. This paper contains four questions on 8 pages.3. Answer three questions.4. This examination accounts for 60% of the module assessment. The total

maximum marks attainable is 60.5. The marks allocated to each question or part ofa question are shown in square

brackets.6. Unless otherwise stated the dimensions in all figures are given in millimetres.7. Candidates may use their own unmarked copies ofBS 8110 and BS 8007.8. Assume reasonable values for any data not given in or with the examination

paper. Clearly state such assumptions made on the script.9. If you have any doubt as to the interpretation of the wording ofa question,

make your own decision, but clearly state it on the script.10. All examinations are conducted under the rules &regulations ofthe

University.

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Question 1An open water storage tank of capacity 100 m3 is to be constructed for a factory forfire fighting purpose. The tank is to be constructed above ground and the selecteddimensions are shown in Figure Q 1.

1 -- ---1 O m - - - - 12 O ; ; m i [ : " 2 1 , - - - - m - L - + l

I2 0 0 m m Cross sectionFigure Ql

The following information is also available. Grade of concrete to be used for the tank - C35A Coefficient of thermal expansion of concrete - lOX10-6 fO C Fall in temp. between hydration peak and ambient (Tl) - 30C Fall in temp. due to seasonal variations (T2) - 10C Reinforcement - Grade 460 (Type 2 deformed bars)

You may assume any missing information but state your assumptions clearly.(a) Assuming the thickness of the wall as 200 mm, calculate moments and forces of

the long wall under serviceability and ultimate limit states. [4 marks](b) Calculate the amount of reinforcement required for the long wall to resist

(i) serviceability and ultimate bending moment due to water load. Usethe limiting steel stress method to calculate reinforcement requirementunder serviceability condition.(ii) thermal and shrinkage movement in immature concrete.

Consider 12 mm bars for both main and distribution reinforcement. [8 marks](c) Provide a suitable arrangement of reinforcement for the long wall. [2 marks](d) Check for deflection of the wall. [3 marks](e) Check for shear at the base of the wall. [3 marks](f) Sketch a suitable detail at the wall-floor slab joint. [2 marks](g) Calculate the crack width of the section for the bending moment calculated in (a)

and the reinforcement arrangement provided in (c). [8 marks]In calculating the crack width you may use the following formula.Neutral axis depth x of a section subjected to flexural action is given by thefollowing formula. Take Modular ratio a. = 15.

xl d =ap [ J I + : p - I ]

(h) Comment on calculation of reinforcement requirement for flexural action based onlimiting steel stress method and limiting crack width method. [2 marks](i) How do you prevent leakage at construction joints in water retaining structures?. [2 marks]

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Quest ion 2Figure Q2.1 shows elevation and plan views of a square elevated water tank ofcapacity 500 rrr', The tank is supported on a framed structure as shown in FigureQ2.1. All columns are connected by bracings at two levels. The selected crosssectional dimensions of the structural elements of the supporting structure are asfollows.All braces - 250 mm x 250 mmAll columns - 300 mm x 3.1)0mm(a) What are the simplified methods of analysis available to analyze the supportingframe structure to obtain bending moments and shear forces in braces andcolumns due to vertical loads. Do not carry out any numerical calculations.[4 marks](b) Briefly explain how you would analyse the supporting frame structure for windload. Do not carry out any numerical calculations. Limit your answer to one page.[ 5 marks](c) Figure Q2.2 shows bending moment and redistributed bending moment of abracing for two load cases. Comment on the redistributed bending moments.[ 6 marks](d) The internal column "A" from foundation to first bracing level is subjected todesign ultimate axial load of 1000 kN and end design ultimate moments of 30kNm and 70 kNm at bottom and top of the column respectively, causing doublecurvature due to wind load. The foundation of this column is a normal pad footingdesigned to resist moment. The height of the column from the top of thefoundation to the centre of the bracing is 6 m.

(1) Show that the above column is to be designed as a slender column. [3 marks](2) Determine the additional moment due to the slenderness effect. [3 marks](3) Calculate the main reinforcement requirement for the above column. You mayassume a reinforcement arrangement which gives d/h =0.85. You may usethe k values given in the design chart for columns in calculating additionalmoment due to slenderness effect. Use grade 30 concrete and grade 460 steel.[13Marks]

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T31m DDDi4mDO 4V-- - - V. - c V~Vm

uL IL:-.J

250 X 250 mm300 X 300mm

G

ElevationNot to scale

Figure Q2.1Plan

600

Load Case I

370150 150

100550 ~...-/ / '" Redistributed

595 moment

Figure Q2.2595

Load Case 2

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Question 3For a multi-storey building, it is proposed to use prestressed precast concrete beamsand slab panels to reduce construction time. The precast floor system consists of 125mm thick precast concrete slab panels spanning onto pretensioned concrete beams,which are at 2.5 m spacing. The pretensioned beams have a simply supported span of5.0 m. The details of the floor system are shown in Figure Q.3. The pretensionedbeams are designed as class 2 members with Grade 40 concrete. The concrete strengthat transfer is 30N/mm2. The imposed load on the floor is 3.0 kN/m2. The load on theslab due to finishes and partitions can be taken as 1.0 kN/m2. The density of concreteis 24 kN/m 3

(a) Using BS 8110: Part 1: 1985 find the allowable stresses, from first principlesshow that a 250 mm x 400 mm rectangular section is adequate for thepretensioned beams. Assume that the total losses are 25% and the loss ratio a.is 0.80. [13 Marks]

12!i,4 01 500 I2

Figure Q.3(b) If 9.3 mm diameter standard strands of ipli = 1770N/mm2 (cross sectional area

= 52mm2) are to be used, determine a suitable effective prestressing force andeccentricity by drawing a Magnel diagram, to satisfy SLS of cracking at thecritical section. [12 marks](c) Determine the eccentricity limits for mid span and support sections for theprestressing force selected in (b) above.(i) Select a suitable tendon arrangement for the mid span section.(ii) Is it necessary to debond some tendons close to the supports? [9 marks]

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Question 4Part A(a) Briefly explain the effect of using prestressing tendons with limited straincapacity on the ultimate moment capacity of a beam. [5 marks](b) A rectangular prestressed concrete beam of breadth 200mm and depth 550mm isprestressed with two tendons, each of area 300mm2, one 50 mm above the soffitand the other 150mm above the soffit. The initial prestress in each tendon is800N/mm2. At failure the concrete and the compression zone is assumed to becarrying a uniform stress of 30 N/mm2. Concrete has a Young's Modulus of 32kN/mm2. Perform two iterations of a calculation to determine the beam's ultimatemoment capacity in sagging bending, taking due account of the initial elasticdeformation of the concrete due to prestress. The tendon stresses can be measuredfrom Figure Q. 4A with sufficient accuracy. [12 marks]

(f(MPa)1600 -..- -- ~-.""!.----1200 .

.006 .012 sFigureQAA

PartBAn exterior column (A) and an interior column (B) of a building are to be supportedon a rectangular footing as shown in Figure Q.4B. The two columns carry thefollowing loads.

Dead load Imposed load(kN) (kN)ColumnA 750 500ColumnB 1000 750

400 x 400 mm ~---B

P roperty Iine _ III

. - . - . - . - . - . ~ - . -i .~- 4.8m -l Figure Q AB7


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(a) Assuming the thickness of the footing as 600 mm, calculate the length andwidth of the footing to obtain uniform bearing pressure under the base.Take the allowable bearing capacity of soil as 250 kN/m2 and density ofconcrete as 24 kN/m3 [6 marks](b) Provide a suitable arrangement of reinforcement in transverse direction.[ 5 marks](c) Assuming the same arrangement of reinforcement calculated in (b) forlongitudinal direction, check for shear. [ 6 marks]

Use grade 30 concrete and grade 460 steel.

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Design data for slnglyreinforced sections:SI units 118odular ratio

1 0 0 ',,(N/mm2) 2 5 05 0 2 0 0

design of concrete structures 2010 level 4 semester 1

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