Introduction to Design of Bridges: RCC Bridge Design Report

CE 3214
Sessional on Reinforced Concrete Structures
Report No: 01
Report Name: Introduction to Design of Bridges
Submitted By:
Name: Fuad Hasan
Roll: 1701003
Sec: A
Department of Civil Engineering,
KUET.
Bridges: RCC Bridge and its advantages:
A bridge can be defined as a structure to overcome physical obstacles, such as body of water, valley,
or road, without closing the way underneath. The purpose of the construction of bridges is providing
passage over the obstacle which by any other means is difficult or impossible to cross. Different types of
designs are found that each serve a particular purpose and can be applied to different situations. Designs
of bridges vary depends:
1. On the function of the bridge
2. The nature of the terrain where the bridge is constructed
3. The material used to make it
4. The funds available to build it.
Reinforced concrete bridges are mainly used as highway bridges due to certain properties which are the
followings:
1.
2.
3.
4.
5.
Durability
Rigidity
Economy
Comparatively easy with pleasing architectural appearance
Great resistance of fire and water.
Structural Component of Bridges:
Superstructure: It is composed of deck, girder, slab, guard rail etc.
: It consist of pier, abutment, cap, bearing etc.
mainly pile foundation and spread footings are used.
Types of Bridges:
Bridges are classified depending on the distribution of tension, compression, bending moment and shear.
However, followings are the different types of bridges:
1.
2.
3.
4.
5.
6.
7.
Beam bridge
Truss bridge
Cantilever bridge
Arch bridge
Arch bridge
Suspension bridge
Cable-stayed bridge.
Composite Bridges
-Concrete Composite Bridge
-stressed Concrete-Reinforced Concrete Composite Bridge
-in-place Composite Bridge
Composite steel concrete or pre-stressed-concrete are usually when the span length is from 40 to 90
ft. For these two case, the concrete deck is made to act integrally with supporting steel stringers by
means the device called shear connectors, welded to the top flange of the steel section and embedded
in the slab.
Pre-stressed-concrete bridges can be used for the characteristics of the composite section. The girders
are commonly precast and placed into the final location using crane .The deck slab is then cast in
place, bonded and tied to the precast sections by steel dowels.
Design consideration:
 Bridges with a very short span from about 10 to 25ft are designed as one –way slab bridge
which are economical.
 Bridges of span length of 25 to 50ft are also designed as one-way slab with longitudinal voids
along with fiber board tubes which decrease the dead load of the structure along with the
moment. As a result thickness decreases which is economical.
 If the span length is less than 6m or 30ft then structure is defined or called as culvert,
otherwise bridge.
Design specification:
 ACI-American Concrete Institute
 AASHTO-American Association of State Highway and Transportation Officials

-Load and Resistance Factor Design

-American Railway Engineering Association
Difference between ACI & AASTHO:
ACI: strength design method using factored load is preferable.
AATHO: service load and allowable stresses are preferable in this case.
Distinction between LRFD& Allowable Stress:
Allowable stress: Equation (1) corresponds to allowable stress. Left side of the equation
indicates design strength and right hand side denoted required strength.
LRFD: Equation (2) corresponds to LRFD. Left side of the equation indicates design strength
and right hand side denoted required strength
Load:
Live loads:
a. Truck loadings:The live loads specified by the American Association of State Highway Officials (Ref. 12.1)- whose
specification control the most of the American highway bridges consist of standard, idealized trucks or
live loads which are equivalent to a series of trucks. Two systems of loading are found:
 H loading(H20-44& H15-44)
 H-S loading(HS20-44& HS15-44)
For HS20-44:
 H indicates Highway.
 S indicates semitrailer
 20 indicates 20 tons.
 44 indicates 1944 edition.
H loading:
,
The H loading represents a two axle truck. The gross weight is divided between the front and rear axles.
Distribution of the load is like this, front wheels get 20% of the total weight and rear wheels get 80% of
the total weight. Each of these loads divides equally in the both side wheel according to the figure. For
slab design the center line of the wheel shall be assumed to be 1ft from the center of the curb. The
distance between front and rear wheel is 14ft. For standard H struck 10ft clearance lane with required.
HS loading:
The HS loading represents a two –axle tractor along with a single-axle semitrailer. For this the
semitrailer axle load is added which is exactly the same amount equal to the rear axle. The allowable
length for semitrailer should be in between 14ft to 30ft.
b. Lane loading:
If the amount of stress is larger than the corresponding truck loadings, lane loading is used. Lane
loadings are used designed to approximate the effect of a series of trucks, govern the longer-span
bridges. Normal practice is when the span length is 60ft or more, lane loading is considered. In lane
loading concentrated load plus uniformly distributed load is taken in consideration.
c. Other roadway loading:
There is a possibility that the bridge may be required to carry electric railways, railroads freight cars,
military vehicles, or other extraordinary vehicle should be considered in design based on the location and
purpose of construction. The class of such traffic shall determine the loads to be used.
d. Sidewalk Loadings:
Sidewalk floors, stringers, and their support are usually designed for alive load at les 85psf of sidewalk
area.
Selection of loadings:
According to the AASTHO specification which state that for truck highways or other highway which
carry heavy truck traffic the minimum live load shall be the H15-S12 loading. It has become common
practice in the present that design bridge span on major highways for the H20-S16 loading, using the
lighter loadings only for structures on secondary roads, on which occurrence of heavy traffic will be
negligible.
Impact:
To allow vibration and the sudden application of load live-load stresses due to truck loading (or
equivalent lane loading) are increased. This increase is computed by the formula,
𝐼=
50
𝑙 + 125
Where, I = impact fraction of live-load stress
l = loaded length, ft.
The maximum impact allowance to be used shall be 30 percent.
Span length:
For simple span,
The span length shall be the distance center to center supports but shall not exceed clear
span plus thickness of slab.
S = c/c distance of supports ≤ clear span + thickness of slab (h)
The following effective span length shall be used in the calculation of the distribution of loads and
bending moments for slabs continuous over more than two supports:
Slabs monolithic with beam (without haunches):
S= clear span
Slab supported on steel stringers:
S= distance between edges of flanges plus one-half the stringer flange width
Slab supported on timber stringers:
S= clear span plus one-half the thickness of the stringer
Bending moment:
Bending moment per foot width of the slab shall be calculated according to the methods given under
cases 1 and 2. In both cases,
S= effective span length as defined under above heading, ft.
E= width of slab over which wheel load is distributed, ft.
P= load on one rear wheel of truck
P15= 12,000 lb for H15 loading
P20= 16,000 lb for H20 loading
Case 1: Main reinforcement perpendicular to traffic (spans 2 to 24ft inclusive)
The live load moment for simple spans shall be determined by the following formulas (impact not
included):
HS20 loading:
(S+2)*P20/32= moment, ft-lb per foot width of slab
HS15 loading:
(𝑆+2)*𝑃15/32= moment, ft-lb per foot width of slab
In slabs continuous over three or more supports, a continuity factor of 0.8 shall be applied to
the above formulas for both positive and negative moments.
P15 = 12,000 lb for H15 loading
P20 = 16,000 lb for H20 loading
Case 2: Main reinforcement parallel to traffic
Distribution of wheel loads E=4+0.06S; maximum 7.0ft.
Lane loads are distributed over a width of 2E.
Longitudinally reinforced slabs shall be designed for the appropriate HS loading.
For simple spans, the maximum live load moment (LLM) per foot width of slab, without
impact, is closely approximated by the following formulas:
HS20 loading:
Spans up to and including 50ft: LLM = 900S ft-lb
Spans 50 to 100ft: LLM = 1000(1.30S-20.0)) ft-lb
HS15 loading:
Use three-fourth of the values obtained from the formulas for HS20 loading.
Moments in continuous spans shall be determined by suitable analysis using the truck or
appropriate lane loading.
Edge beams (longitudinal):
Edge beams shall be provided for all slabs having main reinforcement parallel to traffic. The
beam may consist of a slab section additionally reinforced, a beam integral with and deeper
than the slab, or integral reinforced section of slab and curb. It shall be designed to resist a
live-load moment of 0.10PS, where P is the wheel load in pounds (P15 and P20) and S is the
span length in feet. The moment as stated for a freely supported span. It may be reduced 20
percent for continuous spans unless a greater reduction results from a more exact analysis.
Distribution Reinforcement:
It shall be placed in the bottoms of all slabs transverse to the main steel reinforcement which
provide for the lateral distribution of the concentrated live loads, except that reinforcement
will not be required for culverts or bridge slabs when depth of the fill over the slab exceeds
2ft. The required amount of distribution reinforcement shall be the percentage of main
reinforcement steel required for positive moment as by the following formulas:
For main reinforcement parallel to traffic:
Percentage =100/√𝑆 maximum =50 percent
For main reinforcement parallel to traffic:
Percentage = 220/√𝑆 maximum = 67 percent
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Construction Details:
Abutments:
Bridge abutments are required to transmit the load from the superstructure to the foundation
and act as retaining walls to hold back the earth fill behind them.
Bridge seats:
The bridge seats of abutment which support the fixed ends of concrete highway bridges are
quite frequently built as horizontal surfaces without parapets or back walls. The slab or the
beam of the deck rest directly on the bridge seat: in deck-girder bridges, transverse
diaphragm walls are constructed between the beams to hold back the earth above the bridge
seat. These diaphragms are thin wall, 6 or 8in. in thickness, with a nominal amount of
reinforcement.
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Brest walls:
There are two types of common breast walls used which are cantilever retaining wall of the
reinforced concrete and the type that act as a vertical slab supported horizontally by the deck
and foundation. Buttressed breast wall are rarely used in short span concrete bridges. Gravity
walls of plain concrete are sometimes used, especially where the height of the abutment is not
great.
Wing walls:
Gravity or cantilever-type wing walls are found to be used. It is wise to construct vertical
expansion joints at the junction of the wing wall and breast wall to avoid unsightly cracks
which are apt to form along these planes.
Fixed bearing:
It can safely be used at the both ends of the spans up to 40ft. The dowels which serve to tie
together the deck construction and the abutment are not designed to resist any appreciable
amount of bending moment and hence no joint rigidity can be assumed in the design of the
deck or abutment.
Expansion bearings:
At one end of the bridge an expansion bearing is provided for the span greater than 40ft. A
pair of steel plates are placed under each girder; the lower plate is anchored to the concrete in
abutment and upper plate is fastened to the other side of the girder. Frequently a thin layer of
graphite asbestos, zinc, or copper is placed between the plates to reduce the friction.
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For the shorter spans and light loadings, frequently the expansion device which is a layer of
tar paper inserted between the bridge seat and the deck. Obviously this is not desirable where
provision for expansion is of any importance, because of the questionable efficiency of the
construction.
In spans greater than 50ft, the deflection of the deck may rotate the ends of the girder so
much that the bearing pressure may become concentrated on a narrow strip along the front
edge of the bridge seat and may cause local damage if the bearing plate are used. In order to
counter-act this tendency, one plane and one curved plate should be used.
Deck joints:
The number of the expansion joints in a bridge deck should kept as low as possible. In singlespan bridges of the type under discussion, one such joint usually suffices; that joint is at one
end of the bridge.
Drainage:
Surface water on the bridge should be disposed as soon and as directly as possible. This is
accomplished by the crowning the roadway about 1/8 in. per ft. and pitching the gutters to
drain into inlets. In single span bridges built on a grade, no special provision for the
longitudinal drainage is ordinarily necessary. The water is carried by the transverse crown to
the curbs or gutters and thence to the low end of the bridge, where it can be deflected away
from the roadway.
If the bridge is horizontal longitudinally, a longitudinal camber can be built into bridge deck
by the elevation the middle portion of the span where forms are erected. This camber will
serve to carry the water in the gutter to either end to the span. A camber of about 1/10 in. per
ft. is satisfactory for concrete decks.
When necessary, special drain inlets can be constructed by the building cast-iron scuppers in
the gutters. The scuppers should be designed as to prevent the drain water from touching the
concrete. Drain inlets are generally necessary in multi-span bridges because of the
impracticability of carrying the surface water to the ends of the bridge.
References:
 en.wikipedia.org
 Winter G, O’Rourke C, Urquhart L & Nilson A (2017-2018), Design of Concrete
Structures (7th &10th edition).
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