Bridge guidelines for Selection of Bridge components

Types of bridge

Slab, girder with deck slab, Arch type, Suspension, Cable stayed, Truss type

Arch bridge 

Hydraulic lift bridges

Truss bridge

Cable stayed

An ideal submersible bridge should necessarily have following characteristics:

i. Open foundations keyed into rock.

ii. Firm and defined banks.

iii. Both banks at almost same level.

iv. Straight Nallah / river.

v. O.F.L. 1.0 m below bank level.

vi. Height of bridge above bed level up to 6m.

vii. Spans up to 10m.

viii. Solid slab superstructure.

SPAN ARRANGEMENT 

    1) 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. 

    2) Such span arrangement generally calls for solid slab superstructures, which impart stability to the bridge during floods. 

    3) Longer spans call for girder and slab type arrangements, which are not desirable for submersible bridges, since they offer more obstruction to flow.

FOUNDATION FOR SUBMERSIBLE BRIDGES

1) An ideal situation would be to rest foundations on rock. 

2) The founding stratum being non-scourable poses no problem of stability, durability and maintenance.

3) 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.

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. 

KERBS AND RAILINGS

Kerbs should be such that it offers less resistance to flow of water. The height of kerb should be limited to minimum required say 250 mm above slab. Discontinuous kerbs are desirable and railing should be collapsible or removable type otherwise it will obstruct the flow of water.

WEARING COAT

 

For submersible bridges concrete wearing coat is preferred to bituminous wearing coat because under long submerged condition more damages are expected in case of bituminous wearing coat. Concrete wearing coat is, however, not desirable for longer span as it develops cracks due to deflection in superstructure. In such cases bituminous wearing coat is desirable though it needs continuous maintenance.

High level submersible bridge

A bridge designed to be overtopped during floods of extraordinary flood (year 2005)

Selection type of Foundation

 

FOUNDATION TYPES 

Generally two types of foundations are adopted for bridge structures. 

(i) Shallow foundations - Open foundations

- Raft foundations

(ii) Deep foundations - Pile foundations

- Well foundations

Selection of Foundation :_

Type of foundation	Span to Height Ratio	Remarks
Raft	1 to 1.25	Hard rock not available upto economic depth
Open	1.25 to 1.5	Hard rock available upto economic depth
Pile	1.25 to 1.75	Suitable for larger spans, Hard rock not available upto economic depth
Well	1.5 to 2	Suitable for larger spans, Hard rock not available upto economic depth

Open foundation:-

Open foundations are preferred over any other type. These are to be provided when good-founding strata is available at shallow depth and there is not much problem of dewatering. R.C.C. footings are preferred over P.C.C. footing in case of RCC piers.

 

Well:-

The shape of well can be, Single Circular, Double D-Type,Dumbell Type, Twin Circular

Some important points to be noted regarding well foundations are as follows –

a. If the external diameter of single circular wells exceeds 12 m relevant provisions of clause 708.1.2 of IRC: 78-2000 shall apply.

b. The steining thickness of well shall not be less than 500 mm and shall satisfy the following

relationship

h = kd√L

where h = minimum thickness of steining in m 

d = external diameter of circular well in m

L = depth of wells in m below top of well cap or LWL whichever is more

K = constant(for wells in cement concrete 0.03,brick masonry 0.05 and twin D wells

0.39(For details refer to clause 708.2.3 of IRC: 78-2000). 

c. In case of PCC wells the concrete shall not be learner than M-15. In case of conditions of severe exposure, steining shall not be leaner than M-20. The horizontal annular section of well steining shall be checked for ovalisation moments taking account of side earth pressure.

d. M.S. cutting edge shall not be less than 40 kg/m to facilitate sinking through all types of strata. In case of well curb the internal angle should be kept at about 300 to 370.Well curb shall not be leaner than RCC M-25.

e. The bottom plug provided should be such that the top is kept not lower than 300 mm in the centre above the top of the curb sump to be provided below the level of cutting edge. Well filling above the bottom plug shall be done generally with sand. Top plug of 300 mm in M-15 shall be provided over filling.

RAFT

TYPES OF RAFT FOUNDATIONS

The raft foundation in vogue can be broadly classified in three categories.

- R.C.C. solid slab raft.

- R.C.C. Channel raft.

- Raft for box bridges.

 

a)      To be avoided for major wide streams like Godavari, Tapi, Krishna, Penganga etc. It is suitable for low height bridges with soft or sandy strata.

b)      Not to be provided where sand mining and consequent lowering of bed is anticipated.

c)      The top of raft shall be placed 0.60 meter below the LBL

d)      Provision shall be made to tie the cutoff walls in current direction if they are exposed in due course due to sand lifting. Cross cutoff wall shall invariable be provided in each/alternate spans.

 

Raft foundation is, however, not recommended when

- Spans more that 10m raft being uneconomical.

- Bridge foundation that can not be inspected during its service life.

- Serious problem of dewatering due to large in flow of water/standing water.

- Where open foundations are feasible.

 

In other cases of small span bridges on weak soils, the raft foundations may be a most practicable solution.

The size of the stones to be used is of prime importance. It is the function of velocity of water in the stream, lest the apron gets washed away and endangers the raft foundation. Stone size and the thickness of apron need be carefully designed. If the stones of required size are not available economically, then concrete blocks, concrete or wire crates may be provided. The details of wire crates are available in IRC : 89, Appendix 1.

While deciding the size of stone, the velocity of flow at the bed level should be considered. If the maximum velocity which occurs at certain depth below the surface of the flow is considered, the size of the stone works out to be too large and is unnecessary. The minimum weight of stone to be used in apron shall not be less than 40 kg. It is desirable to provide bigger stones/concrete blocks near to the piers on D/s as the phenomenon of scour is predominant at this location.

We find that when the velocity at bed level exceeds 4.0m/sec. the weight of stone required for apron is quite large. It may be difficult to get such large stones from quarries etc. We may, therefore think of concrete blocks or concrete stone blocks/crators.

 

Selection Type of Substructure

Type designs available would provide sufficient information about the dimensions of the P.C.C. piers and abutments up to a height of 10m. These type designs available are for non-seismic zones only. For heights more than these R.C.C. pier of suitable dimensions will have to be considered.

Selection Type of Superstructure

Henceforth PWD will adopt standard  spans for bridges  from 5m  to 40m in multiple of 5m only. E.g. 5m, 10m , 15m , 20m , 25m ,30m, 35m,  40m, 45m and so on.

For selection of SUPERSTRUCTURE

a) For span up to 15 m span, solid slab, portal frame, Arch Portal, box cell and composite construction shall be allowed.

b)   For span between 10m to 20m

In case of submersible bridges, solid slab, spine slab  shall be provided for span of lengths 5m, 10m, 15 m and 20m

 

In case of high-level bridges, RCC girder, portal frame, arch bridges shall be provided

c)    For spans more than 20 m.

1 RCC girder ( precast/cast-in-situ) up to 25 m span

2 PSC girder ( precast/cast-in-situ)30m to  40 m span

3 PSC box girder for spans more than 40 m.

4 Steel girders with concrete deck slab for spans more than 30 to 45m m

d) Continuous superstructure shall not be allowed except two span continuous.  Superstructure constructed by balanced cantilever method may be allowed owing to difficult site conditions like deep back water, deep valley etc. However such cases shall be limited to sites where launching/lifting of precast girders is not possible.

e) For ease in execution and uniformity, the transverse spacing of girders shall generally be fixed to 2.5 m  irrespective of width of bridge, footpath etc.

f)  Specific arrangements shall be provided in the end portion of the slab to carry the OFC or water pipe lines. (Otherwise they carry it on top of curb creating many problems). Services as far as possible shall be taken over pier cap in suitable pipe arrangement. In case of PSC superstructure, services shall not be allowed by drilling hooks or J bolts from girders.

g)   Following types of superstructure needs to be completely banned.

1. Superstructure with articulation joint

2. Balanced cantilever in service

              3. Superstructure with central hinge

 

SR No	Description of super structure	Submersible Type Span length.	Width of bridge	High level  Type Span length.	Width of bridge	Remark
1	RCC solid slab	5m, 10m, 15m	7.5m and 11.55m	5m, 10m, 15m	7.5m and 11.00m
2	RCC spine slab	15m, 20m	7.5m and 11.55m	------	------
3	RCC box cell	3m, 7m, 8m	7.5m and 11.55m	3m, 7m, 8m	-------
4	RCC T beam & slab. Cast- in-situ	10m, 20m, 25m	7.5m and 11.55m	10m, 20m, 25m	7.5m and 11.00m
5	Precast girder and slab	--------	--------	10m, 20m, 25m	7.5m and 11.00m
6	Composite girder and slab.	----------	-------	30m, 35m, 40m,	7.5m and 11.00m	RCC slab and steel plate girder
7	Bridge cum Bhandhara	5m, 10m	7.5m and 11.55m
8	Arch Bridges	-------	--------	5m, 10m	7.5m and 11.00m

 

For submersible bridges, spans up to 20m will be preferred. Solid slab spans ,spine slab spans and T  girder shall  be proposed. Spans longer than 20 m shall not be provided as it attracts heavy current pressure on it and liable to be washed away. 

There are cases where  requirements of obligatory spans of odd length may be needed. eg  for spans of flyover on the junction of main roads. Obligatory spans for navigational  purpose which are 50m length or more, shall have  composite Superstructure  of Plate girder. In this case only deviation from standard spans will be permitted.

Selection of Railing/Crash barrier

The present practice is to use RCC Sanchi Type parapet for high-level bridges. The average speed of vehicles and volume of traffic  has increased over the years and so the  damages to railings/parapet. But it has been observed that , RCC railings/parapets are difficult to maintain, and  equally difficult to restore the   parapet damaged in accident or vehicle hitting etc. Hence it is recommended  that, RCC Sanchi type railings shall not be provided henceforth , instead RCC crash barriers shall be provided. The crash barrier will also ensure the safety of vehicles.

The height of crash barrier shall be 1.5m  and shall be designed for high containment.

In case of submersible bridge, MS angle and GI pipe railings are provided. But,  it has been observed that GI pipes of railing are not removed regularly before the onset of monsoon as anticipated in the Design of bridge. Non removal of pipes, causes additional  obstruction to the flow and increases the current pressure on superstructure endangering its safety. Hence the committee recommends not to use angle and pipe railings for submersible bridges. Instead,  “W” beam metal barrier shall be provided.

Selection of Type of Returns

The types of returns used are solid gravity, R.C.C. box, Tied back or fly back type etc.

1)      Riding return :- Commonly used where abutments have back batter, its basic function to adjust back batter  in such a way that vertical face of return wall is separated by expansion joint equally throughout height of the return wall.

2)      Gravity wall: - Resist earth pressure by gravity loads. Gravity walls require massive sections and therefore Masonry or cement concrete is used in such walls. Suitable upto height of 6 meters.

3)      Cantilever wall: - Reinforced concrete thin sections are used in the construction of cantilever, counterfort or buttress walls. Resist earth pressure through structural strength. Suitable up to height of 6 meters.

4)      Counterfort wall: - Counterfort walls are on opposite sides of backfill. Counterfort walls are provided to strengthen the main wall of the return. Suitable if height exceeds height of 6 meters.

5)      Buttress wall: - Buttress walls are on the same sides of backfill. Suitable if height exceeds height of 6 meters.

6)      Tied back wall or box return: - Suitable for high walls, suitable in case where walls on both sides are to be provided.

Grade of Concrete for each component as per IRC

 

Grade of Concrete:

It is recommended to use same/ Uniform grade of concrete for all components of bridge, except leveling course and annular filling. It is suggested to use M-40 grade of concrete for bridges in Konkan region (severe, very severe and extreme conditions) and M-30 for bridges in rest of Maharashtra. (Moderate condition) irrespective of PCC, RCC) except M-35 as a minimum grade  for Pre-stressed concrete.

 

Grade of steel:

For bridges in  Konkan region ( severe, very severe and extreme conditions),  corrosion resistant  rebar's i.e. CRS or HCR steel bars of TATA/ SAIL/ Jindal make only   shall be used.

Bearing:-

The part of the bridge structure which bears directly all the forces

from the structure above and transmits the same to the supporting structure. The same therefore, need be carefully decided. Span length plays important role in deciding the size of bearings and its pedestals and expansion gap, which also need be considered while deciding the top width of pier.

The selection of Bearings should be as follows :

Spans up to and including 10 m for solid slab superstructure :- Tar Paper

Span > 10 m and < 25m :- Neoprene

For larger spans :- POT/PTFE

Use of MSM bearing, spherical bearings shall be encouraged.

Cut roller, concrete/steel rollers, rocker bearings shall be banned. 

Through inspection of bearing twice a year must be carried out and record to that effect shall be maintained.

Replacement cycle of bearing must be strictly observed.

 

1)   Pure resting: - Tar paper Bearings, span up to 10m.

 

2)   Sliding: - A type of bearing where sliding movement is permitted, Applicable to small spans, but currently not recommended.

3)   Roller : - Steel rollers are used but maintenance issue.

4)   Rocker:- No sliding movement is permitted but which allows rotational

movement. Steel rocker, it has also maintenance issue.

Elastomeric: - A bearing consisting of one or more internal layers of elastomeric boarded to internal steel laminates by the process of vulcanization. The bearing cateress for translation and / or rotation of the superstructure by elastic deformation.I.R.C.:83-1987, Electrometric Bearings. (Part-II), Used from 10m to 25m span

1)   POT-PTFE: - I.R.C.:83-2002, POT, POT-cum-PTFE, Pin and metallic guide bearings, (Part-III), used for larger spans more than 25m.

a.       Fixed POT Bearing :- A type of POT bearing which along with vertical load bears and transmits horizontal force in any director and allows rotation about any axis in horizontal plane without permitting any movement in horizontal plane.

b.      True sliding type POT-cum-PTFE Bearing: - A type of POT bearing which bears and transmit vertical load and allows movement in any direction in the horizontal plane and accommodates rotation about any axis in horizontal plane.

c.       Guided Sliding type POT cum PTFE Bearings: - A type of POT bearing which along with vertical load bears and transmits horizontal force in one direction only and allows movement perpendicular to that direction and allows rotation about any axis in horizontal plane.

d.      Free PTFE Sliding Assembly: - A type of PTFE sliding assembly, which along with vertical load bears and transmits horizontal force in one direction and allows movement perpendicular to that direction.

e.       Guided PTFE Sliding Assembly: - A type of PTFE  sliding assembly, which along with vertical load bears and transmits horizontal force in one direction and allows movement perpendicular to that direction.

1)    Metallic bearings: - A bearing consisting of a sliding assembly with restraint along a Bearing desired direction to bear and transmit horizontal force and capable of allowing movement in a direction and to the direction of horizontal force. Metallic Guide Bearings and capable of allowing rotation only about an axis perpendicular to the plane of sliding. Metallic Bearing cannot bear or transmit any vertical load. In structures with metallic bearings, no part of the bearing shall be at a height less than 500mm above affluxed design highest flood level. 

 

2)      Pin Bearing: - A bearing consisting of a metal pin provided within a metal cylinder to bear and transmit horizontal free along any direction in the horizontal plane and accommodating rotational movement about any axis. Pin bearing cannot bear or transmit any vertical load.

 

DOF of Bearings

Type of bearing	Translation Permitted		Rotation permitted			Loading Resisted
	Longitudinal	Transverse	Longitudinal	Transverse	Plan	Vertical	Longitudinal	Transverse
Roller
Single cylinderical	√	X	√	X	X	√	X	S
Multiple cylindrical	√	X	X	X	X	√	X	S
Non-cylindrical	√	X	√	X	X	√	X	S
Rocker
Linear	X	X	√	X	X	√	√	S
Point	X	X	√	√	√	√	√	√
Knuckle
Pin	X	X	√	X	X	√	√	S
Leaf	X	X	√	X	X	√	√	√
Cylindrical	X	S	√	X	X	√	√	S
Spherical	X	X	√	√	√	√	√	√
Plane Sliding	√	√	X	X	X	√	√	S
Elastomeric
Unreinforced	√	√	√	√	√	√	√	√
Laminated	√	√	√	√	√	√	√	√
Guided Pot
Longitudinal	√	X	√	S	X	X	X	√
Transverse	X	√	S	X	X	X	√	X

 

Application of Bearings

Type of Bearing	Movement Capacity in one dirction (mm)	Rotation in radians	Typical Application
			Straight	Curved	Steel	Concrete
Steel roller	100	0.19	√	-	√	-
Steel sliding	25	0.08	√	-	√	-
Pot	No limit	0.04	√	√	√	√
Disc	No limit	0.04	√	√	√	√
Spherical	No limit	-	√	√	√	√
Plain Elastomeric	10	Negligible	√	√	-	√
Laminated Elastomeric	60	0.025	√	√	-	√

 

RE wall function :-

To retain earth especially behind abutments of ROB.

Not suitable for river bridges

RE wall design:-

Horizontal loads are taken by Geogrids only, Facia Panels not designed to lateral loads. 

Expansion Joints:-

In the planning of bridges , efforts shall be made to keep number  expansion joints to bare minimum. For this purpose, deck continuity shall be extensively used for span of length more than 10 meter.

For spans 30 meter length, MS cover plate resting on MS angles with or without joint filler is to be used.

For spans more than 30m, expansion joints shall be provided as per IRC SP-69.

During the maintenance of BT wearing course or while laying of  new BT layer, it was noticed that gap between expansion joint gets filled with BT material and joint gets clogged exerting undue forces on superstructure. This shall be avoided by taking appropriate care by placing thermocol pads while laying of BT wearing course.  In certain cases expansion joint may need to close by temporarily welding the MS flat. Utmost precaution shall be taken to get smooth riding quality over the expansion joint.

 

Water Spouts

Waterspouts are required to drain out the rainwater from the deck surface quickly. The deck has camber or super elevation, which help rainwater get quickly towards kerbs. The waterspouts located near the kerb further disposes the water out. One water spout per 20 sq.m. of the deck area is considered adequate

Weep Holes

Adequate weep holes shall be provided in abutments, riding returns, solid returns and outer walls of box returns at not more than 2000 mm centre to centre horizontally and 1000mm centre to centre vertically, regularly staggered. The weep holes shall be provided up to the bed level.

For Hydraulics  following link can useful

https://yogipwd.blogspot.com/2022/02/bridge-project-planning-and-gad.html