pengenalan-struktur-prategang
TRANSCRIPT
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ERWIN ROMMEL-LUKITO PRASETYO
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Perilaku Struktur
Balok Baja, Kayu atau Beton sbl. retak
Distribusi Tegangan
pada Potg. Lintang
di tengah bentang
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Perilaku Struktur
Balok Beton (retak)
Distribusi Tegangan
pada Potg. Lintang
di tengah bentang
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Perilaku Struktur
Balok Beton (retak)
Distribusi Tegangan
pada Potg. Lintang
di tengah bentang
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Perilaku Struktur
Balok Beton Bertulang
Baja Tulangan
Retak Retak Retak
Distribusi Tegangan
pada Potg. Lintang
di tengah bentang
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Perilaku Struktur
+ DsS
Distribusi Tegangan
pada Potg. Lintang
di tengah bentang
Distribusi Tegangan
pada Potg. Lintang
di tengah bentang
BETON PRATEGANG
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Konsep sistem
Prategang
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Prinsip beton prategang, gaya prategang berupa gayaaksila tekan diberikan pada bagian-bagian beton untukmengimbangi sebagian tegangan tarik yang timbul akibatbeban-beban yang bekerja.
Dalam bidang rekayasa jembatan, pengenalan beton prategang telah digunakan untuk mengatasi pembangunanjembatan beton bentang panjang. Biasanya jembatan tipeini disusun dari unit-unit pracetak kemudian disambungkandan dikencangkan dengan kabel prategang, ditempatkanpada posisi tumpuan jembatan.
Untuk jembatan pendek, pemakaian balok prategangsederhana telah terbukti ekonomis
Range penampang balok standar telah diberikan untukmenyederhanakan desain dan pelaksanaan konstruksijembatan.
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BOX GIRGER
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DOUBLE T-BEAM
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SILO STRUCUTRE
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EQUIPMENT AND
MATERIAL
PRESTRESSING
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ANCHORAGE & STRESSING JACK (1)
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ANCHORAGE & STRESSING JACK (2)
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STRESSING JACK (1)
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STRESSING JACK (2)
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STRESSING JACK (3)
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TENDON (1)
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TENDON (2)
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STRAND
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MTW
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METODE DAN SISTEM
PRATEGANG
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Pre-tensioning is used to describe a method of prestressing in which the tendons are tensioned before the concrete is placed, and the prestress is transferred to the concrete when a suitable cube strength is reached.
Post-tensioning is a method of prestressing in which the tendon is tensioned after the concrete has reached a suitable strength. The tendons are anchored against the hardened concrete immediately after prestressing.
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Stage 1 Stage 2 Stage 3 Stage 4
Tendons and
reinforcement are positioned in the
beam mould.
Tendons are stressed
to about 70% of their ultimate strength.
Concrete is cast into
the beam mould and allowed to cure to
the required initial strength.
When the concrete
has cured the stressing force is
released and the tendons anchor themselves in the
concrete.
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Stage 1 Stage 2 Stage 3 Stage 4
Cable ducts and
reinforcement are positioned in the beam
mould. The ducts are usually raised towards the neutral axis at the
ends to reduce the eccentricity of the
stressing force.
Concrete is cast
into the beam mould and allowed
to cure to the required initial strength.
Tendons are
threaded through the cable ducts and
tensioned to about 70% of their ultimate strength.
Wedges are inserted
into the end anchorages and the
tensioning force on the tendons is released. Grout is
then pumped into the ducts to protect
the tendons.
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*In contrast to reinforced concrete, the design of prestressed concrete members is initially based upon the flexural behaviour at working load conditions.
*The ultimate strength of all members in bending, shear and torsion is then checked, after the limit states of serviceability have been satisfied.
*The prime function of prestressing is to ensure that only limited tensile stresses occur in the concrete under all conditions within the working range of loads.
*To satisfy the limit state of cracking it is necessary to satisfy the stress limitations for the outermost fibres of a section.
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*In general the stress limitations adopted for bridges are identical to BS8110 : Part 1: Clause 4.1.3. When considering the serviceability limit state of cracking of prestressed concrete members, three classifications of structural members are given :
*Class 1 : No tensile stresses;
*Class 2 : Flexural tensile stresses, but no visible cracking;
*Class 3 : Flexural tensile stresses, but surface crack widths not exceeding a maximum value (0.1mm for members in aggressive environments and 0.2mm for all other members)
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The allowable compressive and tensile stresses for bonded Class 1 and Class 2 members at transfer and service load are provided by BS8110 and summarised as follows :
Transfer Condition Service Condition
Compression 0.50 fci 0.33fcu
Tension :
Class 1
Class 2: Pretensioned
Postensioned
1.0 N/mm2
0.45 fci
0.36 fci
0
0.45 fcu
0.36 fcu
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Stresses at transfer condition
Top fibre '
minfZ
M
Z
eP
A
P
t
i
t
i
c
i
Bottom fibre '
maxfZ
M
Z
eP
A
P
b
i
b
i
c
i
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Stresses at service condition
Top fibre maxfZ
M
Z
eP
A
P
t
s
t
i
c
i
Bottom fibre minfZ
M
Z
eP
A
P
b
s
b
i
c
i
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Re-arranging the above
inequalities by combining,
the expressions for Zt and
Zb can be obtained.
These two inequalities
may be used to estimate
the preliminary section for
design.
'
minmax ff
MMZ is
t
min
'
max ff
MMZ is
b
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eAZ
MfZP
ct
iti
'
min eAZ
MfZP
cb
ibi
'
max
eAZ
MfZP
ct
sti
max
eAZ
MfZP
cb
sbi
min