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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2009/
Module Title: Prestressed Concrete
Module Code: CIVL 8013
School: School of Building & Civil Engineering
Programme Title: Bachelor of Engineering (Honours) in Structural Engineering
Programme Code: CSTRU_8_Y
External Examiner(s): Mr. J. O’Mahony, Dr. M. Richardson
Internal Examiner(s): Mr. Brian D. O’Rourke
Instructions: Three Question are to be attempted
Questions 1 and 2 are compulsory
Attempt either Question 3 or Question 4
Total 100 marks
Duration: 2 hours
Sitting: Winter 2009
Requirements for this examination: Mathematics Tables. Students may use their
Extracts to the British Standards PP 7312 and the Approved Design Aids booklet.
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
250mm 30mm strand inset
25mm strand inset
1 2 0 0 m m
L L
W -0.125 W
0.375 1.25 0.
0.070 (^) 0.
Moment = coefficient x W x L Reaction = coefficient x W W = total load on one span
Figure Q.1A: Coefficients for equal span continuous beams with uniform loading
Figure Q1 shows the cross-section of a 250 mm deep prestressed hollow-core floor slab. The slab is required to carry a characteristic imposed load at service of 5.0 kN/m^2 and a characteristic dead load of 1.5 kN/m^2. The maximum span of the slab must be determined for two cases:
(a) Slab only
(b) Slab plus 75mm in-situ composite screed
Consider only service mid-span stresses as critical
Figure Q. 1
Section data Concrete properties Design width = 1200 mm f ci = 35 N/mm^2 precast I (^) xx = 1195.2 x 10^6 mm^4 f cu = 50 N/mm^2 precast Z (^) bottom = -9521 x10^3 mm^3 f cu = 35 N/mm^2 screed Z (^) top = 9602 x 10^3 mm^3 E (^) f cu = 50 N/mm^2 (short-term) = 30 kN/mm^2 Cross-sectional area, A = 157.6 x 10^3 mm^2 E (^) f cu = 35 N/mm^2 (short-term) = 27 kN/mm^2 Unit weight of concrete = 24kN/m^3 Allowable stresses Transfer / construction Service f o min = -2.75 N/mm^2 f s min = -3.2 N/mm^2 f o max = 17.5 N/mm^2 f s max = 16.67 N/mm^2 Strand data 12.5 mm diameter, 8 @ 30mm inset from bottom Nominal cross-sectional area (A (^) ps ) = 93 mm^2 per strand Strand characteristic (failure) strength = 164 kN per strand Initial prestress force (jacking force) = 70% of breaking load Prestress losses = 20% (α = 0.8)
X X
Strand inset centre positions 20 strands maximum
T Beam Section
dimensions (mm)
Insitu 150mm concrete composite screed at service
Figure Q.
(a) Sketch clear annotated sketches of the stress and strain distributions of a prestressed section at the ultimate limit state of flexure in accordance with the BS 8110 equivalent rectangular stress block. (5 marks)
(b) Use a first principles approach to determine the ultimate moment of resistance of the beam section shown in Figure Q. 3 in accordance with BS 8110. (15 marks)
h = 700mm
b = 325mm
A (^) ps = 1900 mm^2
e prestress= -175 mm
P (^) transfer = 1200 kN
Eps = 195000 N/mm^2
Ec = 33000 N/mm^2
f cu = 50 N/mm^2
f pu = 1770 N/mm^2
175mm
N.A.
FIGURE Q. 3
