Structural Design of A Reinforced Box Culvert
Structural Design of A Reinforced Box Culvert
Structural Design of A Reinforced Box Culvert
BY
JANUARY, 2016
TABLE OF CONTENTS
1.0.0. INTRODUCTON2
1.1.0. BACKGROUND STUDY...2
1.2.0. SCOPE OF STUDY.5
1.3.0. AIM..5
1.4.0. OBJECTIVES..6
1.5.0. JUSTIFICATION.6
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3.0.0. METHODOLOGY.29
3.1.0. LOAD CASES FOR DESIGN...30
3.2.0. LOADING..30
3.3.0. LOADING CALCULATIONS..30
3.4.0. MOMENT CALCULATION.31
3.5.0. BENDING MOMENT ANALYSIS AND DIAGRAM31
3.6.0. REINFORCEMENT AND DETAILING..31
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LIST OF FIGURES
FIGURE 1....STREAM CROSSING CULVERT
FIGURE 2.DIFFERENT SHAPES OF CULVERT
FIGURE 3.A BOTTOM LESS ARCH CULVERT THAT ALLOWS FOR FISH
PASSAGE
FIGURE 4.DIAGRAM SHOWING VARIOUS INLET CONTROL METHODS
FIGURE 5DIAGRAM SHOWING VARIOUS OUTLET CONTROL METHOD
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1.0
INTRODUCTION
1.1
BACKGROUND STUDY:
Current in-stream design projects are moving away from the use of hard structures, such as
gabions, bank revetment, and culverts, and are increasingly employing a more natural,
biotechnical engineering approach. While stream restoration and bank stabilization efforts
may be prohibitive in terms of cost in the short run, ample evidence suggests that natural
stream channel stability is achieved by allowing the river to develop a stable dimension,
pattern, and profile such that, over time, channel features are maintained and the stream
system neither aggrades nor degrades(Rosgen 1996). Environmentally sensitive design
guidelines have recently been developed by several agencies, combining modern hydraulic
criteria and economical construction and maintenance costs, with consideration of natural
stream channel integrity, flood prevention, and habitat issues.
Culverts have the potential to destabilize streams if capacity and stream morphology are not
considered jointly, resulting in increased sediment supply and erosion, flooding, habitat loss,
and property damage. By artificially narrowing a channel, structures and hardscape methods
often have the unintended consequences of creating erosional eddies up and downstream of
structures, or creating a down-cutting response in order to make up for the lost cross-sectional
area (California Regional Water Quality Control Board, 2003). However, design alternatives
and construction guidelines exist that increase the effective transport of varying flow events
through culverts and under bridges, for use in situations where creating or modifying instream structures is necessary.
The Maryland State Highway Administration (SHA) has created new design procedures that
limit the impacts of constructing culverts and bridges in streams (Kosicki, Davis 2000).
These guidelines have shifted from traditionally focusing solely on the relationship between
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the stream and the highway structure for major flood events, to adopting a design process that
maintains the consistency of dimension, pattern, and profile of the stream with particular
attention given to maintaining bankfull width and width/depth ratio(Kosicki, Davis 2000).
This agency has also experienced successful results in using additional floodplain culverts in
order to relieve the hydraulic load on the main channel culvert so as to limit downstream
scour and erosion (Kosicki, Davis 2000). By emphasizing stream geomorphology in their
structural design process, the SHA predicts a reduction in future maintenance problems and
flood hazards.
However, the use of large bores or multiple culverts is not a solution in itself; without
applying a geomorphic approach, oversized culverts or auxiliary cells can become sediment
traps that clog one or more culvert barrels. If the stream passage is larger than the bankfull
width, a stream will ultimately change to reestablish bankfull flow conditions (Kosicki 2003).
An approach incorporating the Rosgens Stream Classification system (Rosgen, 1996), as
well as conventional hydraulic design tools such as HY-8 and HECRAS, has facilitated the
SHAs permit approval process while creatively addressing common problems such as scour,
degradation, head-cutting, and lateral movement (Kosicki 2003, Similarly, the Maryland
Department of the Environment, Water Management Administration, has created a set of
guidelines for the waterway construction process (Marylands Waterway Construction
Guidelines, 2000). These efforts are also in response to a growing need for the stabilization,
modification, or rehabilitation of streams due to the effects of urbanization or previous
channel construction. Their design recommendations incorporate a consideration of the
Rosgen Stream Classification, as well as an understanding of the root causes of the channel
instability. In some situations, various culvert designs are suggested which can facilitate the
flow of flood waters across a floodplain, and promote the conditions for improved fish
passage.
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culvert. The purpose of this project is to design and analyse a reinforced box culvert. A culvert
is a covered channel of relatively short length designed to pass water through an embankment
(e.g. highway, railroad, and dam). The design requires a hydrological study of the upstream
catchment to estimate the maximum (design) discharge and the risks of exceptional
(emergency) floods. The sizes of the culvert are based on hydraulic, structural and geotechnical
considerations. Indeed, the culvert height and width affect the size and cost of the embankment.
The culvert impact on the environment must also be taken into account, e.g. flooding of the
upstream plain. The design process is a system approach. The system must be identified, as
well as the design objectives and constraints. A detailed analysis of it must be conducted and
questions should be asked at the end if the final design meets the objectives. The culvert design
begins with the report from a survey and hydraulic design reports, this report is used in
conjunction with existing roadway plan to then accurately specify the culvert length, design fill
and other items relating to the completed culvert plan.
1.2
SCOPE OF STUDY:
This project is limited to the structural design and analysis of a reinforced box culvert, no
attempt will be made to discuss the hydrological aspect of the design, but hydrological
parameters will be discussed in the literature review section.
1.3
AIM:
The aim of this project is to analyse and design a reinforced box culvert according to AASHTO
LRFD Bridge Design Specifications, and in accordance with the British Standard (B.S) code.
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1.4
1.5
OBJECTIVES
Determine the total load acting on the various parts of the culvert
Analyse the culvert and come up with bending moment and shear force diagram.
JUSTIFICATION:
Failure of culverts occur for various reasons, this includes maintenance, environmental and
installation related failures. But the major type of failure related to culverts are road collapses,
if the failure is sudden and catastrophic it can lead to loss of life. The dominant reason for
collapse of culverts is poor or inadequate design and analysis of the culvert. The purpose of
this project cannot be over emphasised as accidents due to failure of culverts can be lead to
loss of life and properties. In the hydrological analysis of culverts taking into account factors
like head flow, discharge, etc. are highly important in the effectual design of a culvert as any
error in the hydrological design can cause damage to the environment, Undersized culverts can
cause problems for oceanic life and also affect the quality of water available in that area via
erosion. Poorly designed culverts tend to become packed with sand and other unwanted rubble
during periods of medium to high rainfall which can lead to flooding of the road way above
the culvert. Therefore it is crucial for a culvert to be sufficiently designed both structurally and
hydrologic ally according to standards to withstand any unexpected environmental trials
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INLET STRUCTURE: An arrangement of wing walls and apron that smoothens the
hydraulic transition from open channel to culvert flow and increases maximum capacity. It
may also be the mounting point for a trash rack.
OUTLET STRUCTURE: An arrangement of apron, wing walls and sometimes energy
absorption structure at the end of a culvert. (The pacific stream keepers federation, Al
jonsson 2001)
PIPING: This refers to water flowing along the outside of a culvert. This can lead to erosion
and failure. (The pacific stream keepers federation, Al jonsson 2001)
SLOPE: It is the measurement of the change in elevation with distance. (The pacific stream
keepers federation, Al jonsson 2001)
TRASH RACK: It is a metal grate placed at the upstream end of a culvert to prevent woody
debris, rocks etc. from entering the culvert. . (The pacific stream keepers federation, Al
jonsson 2001)
BOX CULVERT: It is a culvert of rectangular cross section, commonly of precast concrete.
(The pacific stream keepers federation, Al jonsson 2001)
BEDDING: It refers to the fine gravel or crushed rock placed around culverts to evenly
distribute load. (The pacific stream keepers federation, Al jonsson 2001)
CRITICAL DEPTH: Critical depth can best be illustrated as the depth of water at the
culvert outlet under outlet control at which water flows are not influenced by backwater
forces. Critical depth is the depth at which specific energy of a given flow rate is at a
minimum. For a given discharge and cross-section geometry, there is only one critical depth.
(Iowa storm water management manual, 2009).
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i. culvert clearances
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ARCH CULVERT: Just as the name implies, arch culverts are culverts with the barrel
shape of an arch, they could be of two types (1) Full arch culverts; which have a bottom
and hence when placed on a river, do not allow the passage of natural marine life (2) arch
culverts without bottom, they only consist of the top arch and so they allow the flow of
aquatic life through the culvert.
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SPRUNG ARCH CULVERT: It is simply the combination of an arch culvert and a box
culvert, they are rarely used.
NOTE: The shape of a culvert may differ from one place to another, as the culvert type and
shape is based on a number of design factors e.g. road embankment height, requirements for
hydraulic performance, environmental impact, limitation on upstream water surface
elevation. (Wikipedia.com)
First of all, a bridge is a structure built across a physical obstruction like a river,
mountain etc. usually for the transportation of humans and goods, while a culvert is
simply a passage built to allow the flow of water through a barrier or obstruction.
A bridge basically uses a system of columns (piers) and beams to transfer load from the
main deck of the bridge to the foundation and down to the earth while a culvert does not
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make use of any beam whatsoever, it consists of a top slab, a bottom slab and side walls
which can be designed as retaining walls.
If the size (height) of the structure in question is greater than 20ft (>20ft) the structure is
a bridge but if less it can be classified as a culvert. (iamcivilengineer.com, 2015).
Most bridges do not have a floor i.e. they are not joined at the foot of the piers, while
culverts have floors(bottom slab).(iamcivilengineer.com,2015)
2.5.0
WHY CULVERTS?
Harvesting or other agriculture based tasks can do a lot of damage to stream habitat and affect
the water quality. Workers who need to move vehicles and equipment across streams must
consider how they can do so and still protect the natural stream and aquatic life. For this
reason a culvert is best suited to tackle the situation.
Culverts as hydraulic structures have a number of advantages which are outlined below
Prevent Erosion
Prevent flooding
Another major advantage/reason why a culvert should be used is the ease of construction and
installation, culverts could either be cast-in-situ or precast, but for economic reasons, a
precast culvert is advised. Culverts are also very portable and are usually readily available
locally. Operators can install and remove them quickly.
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A culvert may break the continuity of water in a stream if its outflow is lifted above the
water level downstream of the culvert.
The water velocity in a culvert may be higher than in the natural stream because the
culvert is straight and constricts the stream into a narrower channel. Also, if the culvert
contains little or no substrate (e.g. gravel, rocks, or cobbles), then the smoother bottom
and sides will offer less resistance to the flowing water.
A culvert may break the continuity of the streams substrate. It may have less, if any,
substrate along its stream bottom and, presumably, the ground underneath the culvert
would be compacted as a result of construction.
Culverts channelize the stream and do not allow it to migrate laterally across its
floodplain. This channelization may cause increased erosion and sedimentation.
Culverts serve as an entry point of pollutants (e.g., salt, silt, or soot) that accumulate from
water that runs off of roads into roadside ditches.
Culverts may change the temperature of the stream water. If the area around the culvert
and road receives more energy from the sun because the tree canopy was removed, water
temperatures may be elevated. However, if the stream is slow relative to the length of the
culvert (i.e., if the stream in the culvert is very shallow, slow-moving, and has to travel
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According to the centre for environmental excellence by AASHTO the following methods
may be applied to limit the negative environmental effects of culverts:
Culvert shape: A different culvert shape (e.g., ellipse, culvert arch, or box culvert) may
be chosen to achieve fish passage requirements.
Invert level: The invert level at an inlet or outlet is very important for managing flow
effects at contractions (inlets), expansions (outlets), and flow regime in a culvert barrel.
Invert levels affect habitat upstream and downstream of culverts. Lowering the invert
may be necessary to allow the placement of natural substrate on the culvert bottom. Care
should be taken to ensure a stable channel upstream and downstream of the culvert
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because erosion due to increased flow velocities can progress in both directions and
create barriers to fish passage.
Stream Characteristics
Minimum
Preferred Structure
Bridge
Ford or culverts
Table 1: Minimum preferred structure for fish passage (Goulburn broken catchment management authority)
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near support affects effective width, more the distance larger will be the effective width and
will reach highest when the load is at centre. The ratio of breadth (unsupported edges) and the
span also affects effective width. All factors mentioned above need to be taken into account
while obtaining the effective width. The IRC: 21-20006 Clause 305.16 gives an equation for
obtaining effective width for simply supported and continuous slab for different ratio of
overall width verses span for these two kinds of supports. The Code does not provide if one
of the support is continuous while other is simply supported. The Code is silent for other
types of supports such as fixed or partially fixed. Some designers use this formula and factors
for continuous slab is taken valid for partially restrained support in a situation like box
culvert. This does not appear to be in order. The reasons for this can be better realized by the
explanations given in sub para 3 above. Nevertheless, effective width need to be obtained in
box type structure also to evaluate affected area by moving load for considering these in the
design. The AASHTO9 for Standard Specifications for Highway Bridges 17th Edition 2002,
provides at para 16.6.4.3 under RCC Box that The width of top slab strip used for
distribution of concentrated wheel loads may be increased by twice the box height and used
for the distribution of loads to the bottom slab. This confirms what is mentioned in sub para
5 and is alright. However, any such dispersal for bottom slab different than top slab shall not
be practical when braking force effect is to be taken, which shall have to be for the same run
of the box structure as a whole. (B.N.Sinha & R.P. Sharma October December 2009).
2.11. 0
INLET CONTROL
If the culvert is operating on a steep slope it is likely that the entrance geometry will control
the headwater and the culvert will be on inlet control. Inlet control for culverts may occur as
unsubmerged or submerged. For the unsubmerged condition, the culvert invert slope is supercritical and the culvert acts like a weir. For the submerged condition, the culvert doesnt flow
full and acts like an orifice. (Robert Duane Nickols)
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The headwater is submerged and the outlet is submerged with the culvert flowing
full.
The culvert slope is sub-critical and the tail water depth is below the pipe critical
depth.(Robert Duane Nickols)
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silent as far as box is concerned. It will be in order to neglect effect of braking force on box
having large cushion. In such situation the braking effect will be absorbed by the cushion
itself and no force will be transmitted to the box beneath. Question will, however, arise up to
what cushion height no braking force need be taken. This height generally is taken to be 3 m.
Thus no braking force for cushion height of 3 m and more and full braking force for no
cushion, for intermediate heights of cushion the braking force can be interpolated. Braking
force by the moving loads on top slab of box having no cushion shall act on the box structure
and shall deform the box. The question is what length of box can be considered to share this
braking force. In another words what effective width of box shall be taken to obtain braking
force per unit run of box. One way is to take the effective width of box same as considered
for vertical effect of moving loads. (B.N.Sinha & R.P. Sharma October December 2009).
Moving loads create impact when these move over the deck slab (top slab). The impact
depends on the class and type of load. The IRC:6-2000 Code gives formula to obtain impact
factor for different kind of loads by which the live load is to be increased to account for
impact. The box without cushion where the top slab will be subjected to impact is required to
be designed for live loads including such impact loads. Any such impact is not supposed to
act on box with cushion. Hence no such impact factor shall be considered for box with
cushion. The impact by its very nature is not supposed to act at lower depth and no impact is
considered for the bottom slab of the box. It does not affect the vertical walls of the box and
not considered in the design. The IRC:6-200010, Code Clause 211.7 specifies that for
calculating pressure on the bearings and on the top surface of the bed blocks, full value of
appropriate impact percentage be allowed. But for design of pier, abutment below the level of
bed block, the appropriate impact percentage shall be multiplied by the factor given therein.
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Accordingly, the impact is to be reduced to 50% below bed block and zero at 3 m below,
proportionately reducing between this heights. Although these provisions are for bridges but
can be applied in case of box structure in absence of any specific provision in the Code for
box in this regard. The AASHTO9 at para 3.8.1.2 specifies that impact shall not be included
for culverts having 1m or more cover. This, however, will be on lower side compared to
considering zero impact for a cover (cushion) of 3 m. It is, therefore, suggested that
considering full impact on top slab without cushion and zero impact for 3m cushion and
interpolating impact load for intermediate height of cushion is on conservative side and can
be safely adopted. (B.N.Sinha & R.P. Sharma October December 2009).
2.15.0 CLEANING AND MAINTENANCE
One method to account for all culverts is to maintain an inventory of culverts and under-drains and
use a checklist from this inventory to account for culverts during inspections. Inspect culverts often,
especially in the spring and autumn, and after storm events, checking them for signs of corrosion,
joint separation, bottom sag, pipe blockage, piping, fill settling, cavitation of fill (sinkhole), sediment
buildup within the culvert, effectiveness of the present inlet/outlet inverts, etc. Check inlet and outlet
channels for signs of scour, degradation, agradation, debris, channel blockage, diversion of flow, bank
and other erosion, flooding, etc.
Practice preventive maintenance to avoid clogging of pipes and other situations which may damage
the culvert or diminish its design function. If a culvert is plugged with sediment, flush it from the
outlet end with a high pressure water hose. Take measures to reduce downstream sedimentation and
clean debris and sediment from the outlet ditch afterwards.
When replacing damaged culverts which handle the flow adequately, use the same size, shape, and
type of pipe. Changing any of these criteria may adversely affect the established stability of the ditch,
stream, and/or roadway.
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types, assess the channel stability to determine whether or not the channel is degrading or widening. If
the channels are unstable, widening, or degrading, a culvert system should not be used unless the
channel can first be stabilized.
2. For incised stream types F or G which have been stabilized, a single cell culvert which can convey
the design storm flow can be designed and constructed.
3. For stable stream types C or E in which debris jam potential is not significant, a multi-cell culvert
system should be constructed where practical. One cell is placed within the bank full channel which is
designed to carry the bank full flow. The invert of this barrel should be depressed according to
MGWC 4.5: Depressed Culverts. One to three cells are placed on either side of the floodplain to
convey the design storm flow with minimum constriction of the flow. All erosion and sediment
control devices, including dewatering basins, should be implemented as the first order of business
according to a plan approved by the WMA or local authority. (See the 1994 Maryland Standards and
Specifications for Soil Erosion and Sediment Control.)(Watershed Sciences, 2007)
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3.0.0
METHODOLOGY
According to (oyenuga. O. victor, 2001), a box culvert should be analysed as a rigid structure
with moments occurring at the corners. The Hardy Cross method of moment distribution is
best suited for the culvert analysis or the Kanis method of moment distribution.
3.1.0
Culvert empty: Full load on top of the slab, surcharge load and superimposed
surcharge load on earth fill.
Culvert full: Live load surcharge on top slab and no superimposed surcharge on
earth fill.
Culvert full: Live load surcharge on top slab and superimposed surcharge load
on earth fill.
3.2.0 LOADING
Top Slab: The load include, slab own weight, imposed load ad weight of earth fill. In
cases where the depth of the earth fill is greater than three times the width of the
culvert, the earth load can be assumed to be equal to earth loads of height three times
the culvert. When a point load such as wheel loads incident on a culvert without earth
fill, the dispersal should be based on tyre width. For a wheel load on a fill of height,h,
the load should be should be spread over an area of 4h 2 , that is 2h, by 2h. When h
equals or slightly(B.S. 5400 Part 2: 1978)
Walls: Loads on walls include own weight, effect of active pressure, effect of any
surcharge any pore water pressure. When the culvert is full, there will be water pressure
on the inside wall and wall should be designed to resist this pressure and assuming no
back fill. The walls need not be designed as tank walls. That is, no need to check for
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stresses in the steel as well as checking for crack widths, the walls should simply be
designed for flexural(bending, shear and axial pull).
Bottom slab: The top slab and all its imposed load, the walls and pressures on them
produce an upward pressure (reaction) from the ground and causes moment. The weight
of water in the culvert and weight of the bottom slab should be considered when
determining the maximum pressure on the ground but since they are borne by the
ground, directly, they do not generate moment.
Dead load:
Self-weight: thickness of slab cover x unit weight of concrete (KN/m 2 )
Earth load: height of road fill x unit weight of earth (KN/m 2 )
Live load :
Wheel load: wheel load x 2 (see B.S 5400 part 2:1978) (KN/m 2 )
The total load acted is then gotten from the addition of the live and dead load
F= (DL + LL) (KN/m 2 )
BOTTOM SLAB:
Load acting on bottom slab include load transferred from the top slab and the
upward pressure (reaction) from the walls which causes moment.
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wl 2
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