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Page 1

BRIDGE DESIGN 1

GUIDELINES FOR BRIDGE DESIGN

INDEX

FROM THE DESK OF SEDC(BR) ....................................................................... 2

FROM THE AUTHORS ........................................................................................ 3

FOREWORD .......................................................................................................... 4

PREFACE ............................................................................................................... 5

FEW WORDS ........................................................................................................ 6

CONTENTS ............................................................................................................ 7

CHAPTER - 1 : INTRODUCTION .........................................................................11

CHAPTER - 2 : ESTIMATION OF DESIGN DISCHARGE SCOUR DEPTH,
LINEAR WATERWAY AND AFFLUX.................................................................... 25

CHAPTER - 3 : COMPONENTS OF BRIDGE STRUCTURE ............................. 45

CHAPTER - 4 : SUBMERSIBLE BRIDGES ....................................................... 101

CHAPTER - 5 : INNOVATIVE STRUCTURES AND BRIDGE ASTHETICS ..... 107

CHAPTER - 6 : PREPARATION OF BRIDGE PROJECT ................................. 121

CHAPTER - 7 : PRESTREESING HIGH PERFORMANCE CONCRETE
ANTICORROSIVE TREATMENT ...................................................................... 129

ANNEXURES...................................................................................................... 137

Page 2

GUIDELINES FOR2

The Engineers, who join this organisation, have to study various codes and
books for considerable time to bring themselves up to a desired level of
knowledge, as many of them are not in touch with the requisite literature. They
do not get proper references at the required moment. They do not precisely know
the practices followed in the State P.W.D. as regards the design of bridges and
buildings. It was, therefore, considered necessary to prepare guidelines, which
will help the engineers to know our practices and also know some useful
references for further study.

An effort has been made to discuss almost all aspects relating to the bridge
design, which are necessary for preparing ‘Bridge Project’ and further preparing
the detailed designs. One practical example is also enclosed as annexure to
understand the process better. It may please be remembered that these
guidelines are useful for the beginners in Bridge Design. The provisions are only
guiding principles and hence the designer should also study Text Books, Codes,
and Specifications etc. for preparing the Designs.

The first edition of these guidelines was published in the year 1997. The same
are now being revised as per revisions in IRC codes and prelevant practices in
State P.W.D.

Some new chapters on submersible bridges, innovative structures, bridge
asthetics, and high performance concrete have been added. Also some
additional information on hydraulics including Unit Hydrograph Method.Raft
foundations and some informative sketches have been incorporated. Hence, it is
felt that the second edition of these guidelines will be useful not only to the new
entrants in the Designs Circle but also serves as guidelines to the Field
Engineers of the Department.

AUGUST 2007 Superintending Engineer
Designs Circle(Bridges)

Konkan Bhavan, Navi Mumbai

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FROM THE DESK OF SEDC(BR)

Page 104

GUIDELINES FOR104

4.2.5 FIXING LENGTH OF THE BRIDGE AND NALLAH TRAINING

Linear waterway requirement in case of high-level bridges is computed using relation W = CvQ given in
IRC-5 General features of Design. The same formula can be used for linear waterway of submersible
bridge but in this case Q shall be the designed discharge at OFL.

In case of defined channel where the flood is confined within the banks, fixing the length of bridge is easy
since it may be almost same as that for High Level Bridge. Balancing of the obstructed area in the cross
section of the channel by possible nallah training helps in deciding the length of submersible bridge. The
percentage obstruction while flood level at RTL should be kept minimum while deciding the length of the
submersible bridge.

4.3 HYDRAULIC DESIGN OF SUBMERSIBLE BRIDGES

4.3.1 DISCHARGE THROUGH SUBMERSIBLE BRIDGES

The bridges designed to pass about 25 percent to 30 percent of Inglis discharge are found to be quite
efficient submersible bridges. The flood level at which the discharge is 30 percent of Inglis discharge
decides the level of the bridge. It may, however , be noted that this can not be considered as the formula for
fixing the bridge height, though it can be considered as governing factor for deciding the height of the
bridge. The concept of OFL may be used for deciding the design flood level for submersible bridges. OFL
is the flood level that occurs in the stream in normal monsoon. It is interesting to note that OFL generally
tallies with 30 percent Inglis discharge in Maharashtra State. Though OFL is considered as design flood
level, it is useful only for deciding the road top level and length of the bridge. Design calculations (hydraulic
or structural) are, however, based on HFL and all intermediate critical conditions.

4.3.2 SCOUR DEPTH CONSIDERATION

The scour depth should, be worked out based on HFL corresponding to Inglis floods or Modified Inglis
floods applicable to the area and also for critical condition like flood at bridge top.

4.3.3 OBSTRUCTION TO FLOW OF WATER

The bridge structure and its approaches obstruct the free flow of water in the stream. It is not practicable to
design a bridge to allow 100 percent discharge. Some obstruction is, therefore, required to be allowed in
the stream due to the structure as a practical approach to bridge planning.. In case of submersible bridges
the obstructions could be limited to 20 percent at designed flood level (with 30 percent Inglis discharge).
However, the obstruction at HFL should also be restricted to 20 percent.

The obstruction to flow of water causes afflux on upstream of the bridge site. This increases the area under
submergence on up stream. It should be verified that such submergence does not adversely affect the
nearby habitats and agricultural land etc.

Page 105

BRIDGE DESIGN 105

4.3.4 HYDRAULIC FORCES

Submersible bridges are designed for hydraulic forces depending upon the applicability of Inglis or
modified Inglis formula. The stresses in the structure need to be checked for at least 3 flood levels (i) flood
at OFL, a condition that will occur frequently, (ii) flood at RTL, this condition, will give maximum obstruction
to flood water, (iii) flood at HFL.

In case of bridges situated on immediate downstream of a dam the possibility of sudden opening of the
floodgates shall also be considered in design. In such a situation stream carries extra discharge through
gates in addition to the overflow through weir/spillway. This condition warrants the bridge to be checked for
higher flood levels / discharge.

4.4 SPAN ARRANGEMENT

Height of submersible bridge from the bed level, in general, is about 5 m to 8 m. It is, therefore, desirable to
have spans up to 10 m. Such span arrangement generally calls for solid slab superstructures, which impart
stability to the bridge during floods. Longer spans call for girder and slab type arrangements, which are not
desirable for submersible bridges, since they offer more obstruction to flow.

4.5 FOUNDATION FOR SUBMERSIBLE BRIDGES

An ideal situation would be to rest foundations on rock. The founding stratum being non-scourable poses
no problem of stability, durability and maintenance.. Raft foundation may be a solution to the situation with
weak soil and less scour depth. Furthermore, raft foundation provides more stability to the structure as it
tries to bridge over the unevenness of the foundation.

4.6 STABILITY AGAINST OVERTURNING

It is necessary to check the superstructure against over-turning. Girder type bridges and box-girder types
are susceptible to such forces since they offer more obstruction to flow of water, as they are lighter as
compared to solid slab type superstructure. It is desirable to check all the types for stability against
overturning. It should be remembered that submerged weight of superstructure is almost 60 percent of its
original weight, thus reducing drastically the stabilizing forces. Girder and slab bridges should be
considered with entrapped air between the girders, which might exert upward pressure on the
superstructure reducing the stabilizing forces. The box type superstructure should be checked for stability
with the condition that the box is not filled with water . This situation may occur when the rise in water level
is sudden due to flash floods and also when the vents are choked and do not function efficiently. Single lane
submersible bridge with slab and girder system or box superstructure shall be avoided.

4.7 STOPPERS ON DOWN STREAM SIDE

During floods there is a possibility that the superstructure may slide due to its buoyant weight and the water
current forces. It is, therefore, necessary to provide downstream stoppers that will keep superstructure in
its position. Downstream stoppers are provided on pier cap.

Page 207

BRIDGE DESIGN 207

3.3 H.F.L. R.L. is reported as 98.71 m. However, the discharge by Manning’s formula fairly tallies
with that by Inglis discharge at H.F.L. R.L. 99.51 m. Hence R.L. 99.51 m is considered for the
design purpose.

3.4 The bridge is proposed to be designed as single lane high level bridge as requested by field
officers.

4. Foundations :

10.1 The Executive Engineer Alibag (P.W.) Division Alibag reported that exposed trap rock is in
the river bed hence open foundation is proposed. The foundation should be taken 2 m below
the G.L. for pier and at abutment location and anchored in rock for minimum 1.50 m depth.

5. Proposal :

10.1 It is proposed to construct single lane high level bridge in between ch. 90 m to 150 m. having
total length of bridge 60 m.

5.2 It is proposed to provide 6 span of 10.0 m c/c. With above proposal, the R.T.L. work out as
below.

(i) H.F.L. R.L. ..... 99.51 m

(ii) Afflux (Assumed) ..... 0.600 m

(iii) Vertical clearance ..... 1.200 m

(I.R.C.cl.106.2.1)

Soffit R.L. . ..... 101.31 m

(iv) Depth of superstructure ..... 0.660 m

(for 10 m span s.s. M 25 grade)

(v) Wearing Coat ..... 0.075 m

Road Top Level ..... 102.045 m

10.2 With the above proposal the percentage obstruction and afflux at H.F.L. works out to 19.70 %
& 0.52 m respectively.

5.4 The bridge is to be designed for single lane of I.R.C. Class A loading.

6. Sub Structure :

6.1 It is proposed to provide solid type of piers and abutment as per Designs Circle type
drawings.Piers and abutment are proposed in M 10.The top width of pier and abutment is
0.90 & 1.20 m respectively.The batter for piers is 1 in 25.

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Page 208

GUIDELINES FOR208

6.2 Solid returns in M 10 as per type plan are proposed for required length.

6.3 Surface reinforcement @ 5 kg/m2 is proposed for piers and abutments.

7. Superstructure :

7.1 Superstructure is proposed in M 25 as per Designs Circle’s type plan of solid slab of
10 m c/c.

7.2 Tar paper bearing are proposed below the solid slab.

7.3 R.C.C. parapet is proposed as per type design.

8 Standards :

8.1 The bridge is proposed to be single lane high level bridge with clear roadway 4.25 m without
footpath.

10.1 The bridge site lies in seismic zone III. However, the seismic design is not to be done as
length of bridge is 60 m.

9. Miscellaneous :

9.1 The wearing coat is proposed of 75 m (avg) thickness with 50 BM + 25 AC.

9.2 The bituminous pad expansion joints are proposed.

9.3 The provision of water spout, should be as per MOST type design with 150 mm diameter
pipes. Filling behind abutment & return shall be as per Appendix 6 of I.R.C.-78-1983.

10. Special Points :

10.1 The coefficient of rugosity for bed & banks are 0.03 & 0.035 respectively. This shall be
confirmed.

10.2 The angle of skew is assumed to be zero degree which shall be confirmed.

10.3 Approach on both (left & right) side should be suitably protected by providing pitching against
flood zone.

(D. P. Hadole) (S. B. Tamsekar)
Sub-Divisional Engineer, Executive Engineer,

Bridge Wing Unit-I, Bridge Wing Unit-I,
Designs Circle, Designs Circle,

Konkan Bhavan, Navi Mumbai. Konkan Bhavan, Navi Mumbai.

(K. S. Jandge)
Superintending Engineer (Bridges),

Designs Circle, Konkan Bhavan,
Navi Mumbai

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