3544134000000000000000000000000000000.ppt

Bridge

1. Bridge Engineering (6)
Superstructure – Concrete Bridges
1.*Determination of concrete bridge superstructure:
1.1 the geometry of the bridge;
1.2 the span length; and
1.3 the method of construction
2.*Bridge types and suitability
2.1 Non pre-stressed concrete slab bridges
2.1.1 The simplest type in which the deck slab also serves a
s the principal load carrying element.
2.1.2 The solid, cored, or ribbed concrete slab is supported
directly on the substructure.
2.1.3 For spans below about 12m, a cast-in-place simple spa
ns or continuous span solid slab may be used; and for
spans over 12m, voided slabs become more economical.
2.1.4 Pre-cast slab decks usu. consist of a number of pre-cas
t voided slab sections placed side by side.
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2. Bridge Engineering (6)
Superstructure – Concrete Bridges
2.Bridge types and suitability
2.1 Non pre-stressed concrete slab bridges
2.1.5 The individual slab units are made to act integrally, eit
her by the use of longitudinal shear key or by casting a
concrete slab on top.
2.1.6 For simple spans of less than 15m, pre-cast slabs are u
su. used; for longer spans, box sections are preferable.
2.2 Non pre-stressed concrete girder bridges
2.2.1 This type consists of a deck slab supported by non pr
e-stressed concrete longitudinal girders.
2.2.2 The two most common forms are cast-in-place T-beam
s and box girders.
2.2.3 The spans may be simply supported or continuous.
__ simple spans are generally prismatic (of constant cro
ss-section);
__ continuous spans may be haunched;
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3. Bridge Engineering (6)
Superstructure – Concrete Bridges
__ diaphragms are normally provided at piers and
abutments;
__ intermediate diaphragms may be used to impro
ve load distribution particularly at mid-span.
2.2.4 The analysis should take into consideration the in
teraction of the axial, flexural and torsional stiffn
ess of the longitudinal girders and transverse com
ponents (deck slab, floor beams and/or diaphrag
ms)
2.3 Pre-stressed concrete slab bridges
2.3.1 For expressway interchanges:
____ because the inherent torsional stiffness of sla
bs bridges suitable for curved continuous structur
es, and structures with single column intermediat
e supports;
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4. Bridge Engineering (6)
Superstructure – Concrete Bridges
__ solid slabs for the short spans; round voided slab
s for the intermediate range; and rectangular void
ed slabs for the longer range.
2.3.2 For simple spans, but more commonly for contin
uous spans
2.3.3 Other issues in design and construction
__ For post-tensioned voided slabs, the section shoul
d be solid over the piers and the abutment bearin
gs.
__ At fixed piers the slabs are generally cast integral
with the piers.
__ At expansion piers sliding pot bearings are used.
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5. Bridge Engineering (6)
Superstructure – Concrete Bridges
__ Continuous multi-span decks used to reduce the n
umber of deck expansion joints, and thereby mini
mize maintenance problems.
__ Structures on horizontal curves or single column s
upports require particular attention to torsional s
tresses.
2.3.4 Pre-cast pre-tensioned concrete slabs, like pre-ca
st non pre-stressed concrete slabs are usually limit
ed to straight square spans and are not adaptable
to varied deck geometries;
2.3.5 Simple spans may be made partially continuous f
or live load in the same way as pre-cast beams or
box girders.
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6. Bridge Engineering (6)
Superstructure – Concrete Bridges
2.4 *Prestressed concrete girder bridges
2.4.1 In the short span range, pre-cast AASHTO beam
s with a composite cast-in-place non-pre-stressed
concrete slab or closed box girders with pre-cast t
op slabs are frequently used for simple spans;
2.4.2 In the medium span range, I-girders and closed b
ox girders with pre-cast top slabs are used for par
tially continuous spans;
2.4.3 In the medium to long span range, continuous pr
e-cast segmental trapezoidal open box girders wit
h the deck cast-in-place are common;
2.4.4 The longest spans are generally cast-in-place seg
mental box girders.
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7. Bridge Engineering (6)
Superstructure – Concrete Bridges
3. Design and Construction Techniques
__ For cast-in-place construction, the girders (T-bea
ms and box girders) and slab are formed together
and cast before the removal of formwork and sup
ports.
__ the usage of continuous spans, and thinner transv
erse post-tensioned slabs and a reduced number o
f longitudinal girders at a larger spacing;
__ the importance of temperature effects for box gir
ders due to the possibility of large differential tem
peratures between the top and bottom slabs;
__ the necessity of knowing exact construction loads
for cast-in-place segmental construction built by t
he balanced cantilever method, in order to calcula
te stresses and deformations at each stage.
 
 
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8. Bridge Engineering (6)
Superstructure – Concrete Bridges
__ the essentiality of knowing the creep characteristi
cs of the concrete for calculating deformations aft
er the addition of each segment, and for calculatin
g the redistribution of moments after completion
and final stressing;
__ the usage of intermediate diaphragms;
__ the same design and analysis items as given for re
inforced concrete girder bridges;
__ construction procedures for composite beams an
d slabs: erecting beams; placing forms for the sla
bs between beams; casting a reinforced concrete s
lab
__ The slab and beams act compositely for superimp
osed dead load and live load.

 
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9. Bridge Engineering (6)
Superstructure – Concrete Bridges
__ the advantage of reducing the number of deck joi
nts by using partially continuous pre-cast pre-stre
ssed beams for multi-span bridges;
__ the development of support moments due to super
imposed dead load, live load, differential tempera
ture, shrinkage and creep.
__ the achievement of continuity for superimposed d
ead load and for live load by casting diaphragms
at the time the deck concrete is placed.
__ the placement of longitudinal reinforced steel in t
he deck slab across the intermediate pier in order
to resist the tension from negative moments

at the
supports.
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10. Bridge Engineering (6)
Superstructure – Concrete Bridges
__ the connecting of the bottom flanges of adjacent beams
at the diaphragms to resist the tensile stress due to pos
itive moments generated by differential temperature, s
hrinkage and creep;
__ the application of segmental pre-cast pre-tensioned gir
ders to achieve continuous spans, beyond the range of
pre-cast girders;
__ the employment of single or multiple cell boxes with tr
ansverse segments post-tensioned together longitudina
lly;
__ the use of pre-cast segmental construction for medium
spans
__ the use of the balanced cantilever method for longer sp
ans where each segment is successively stressed after e
rection;
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11. Bridge Engineering (6)
Superstructure – Concrete Bridges
__ being aware of the time difference between segme
nt pre-casting and erecting against the deformatio
n of the structure;
__ the application of elastic analysis and beam theor
y in the design of segmental box girder structures.
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