Bridge Project Planning and GAD Preparation

GAD (General Arrangement Drawing) Preparation for Bridge

yogendra borse

Bridge - Definition

A bridge is a structure having a total length of above 6 m for carrying traffic or other moving loads across a channel, depression, road or railway track or any other obstruction.

Types of Bridges

On the basis of Submergence

Generally, bridges constructed by State PWD are on river, creek, lakes etc. Hence on the basis of submergence of bridge under water, the river bridges / bridges on stream are divided in to following types:

A) High Level Bridge : A high level bridge is a bridge which carries the roadway above the Highest Flood Level (HFL) of a channel (including afflux) or the high tide level, with appropriate vertical clearance.

High level bridges are preferred on important roads such as expressways, National Highways. On some State Highways and other roads, depending upon the local  conditions High Level Bridges may be provided with certain conditions.

B)    Submersible Bridge:Submersible bridge is a bridge designed to be overtopped during floods.

Submersible bridges are economical due to reduction in height of piers and hence reduction in length of approaches. Design Circle has a practice to provide Submersible Bridge wherever possible considering the local conditions.

C)    High Level Submersible Bridge: This type of bridge is not defined in IRC Code but Design circle has a practice to provide such a type of bridge. In this type of bridge, vertical clearance kept minimum so that the Highest Flood Level barely touches the soffit. But this bridge is designed as Submersible bridge only.

On the basis of Length

A) Culvert-Culvert is a structure having a total length of upto 6 m between outer faces of walls, measured at right angles. Cross drainage structures with pipes will be termed as culvert, irrespective of length.

Culvert

Pipe Culvert

B)    Small bridge–it is a bridge having overall length of the bridge between the inner faces of dirt walls is upto30m and where individual span is not more than 10m.

C)    Minor bridge–It is a bridge having total length upto 60m

D)    Major bridge - It is a bridge having total length greater than 60m

E)    Long Span bridge- It is a bridge having span length (s) of 150 m or more.

On the basis of function- Based on functions, bridges are classified as under:

 River bridge / Bridge over stream

Viaduct- A viaduct is a bridge structure across dry ground/terrain.

Flyover - A bridge over another road for allowing traffic without interruption, with its approaches on both sides.

Grade separator - Grade separator is a bridge which facilitates movement of traffic by segregating ‘at-grade' conflicting movements to different levels.

Road Over Bridge (ROB) - Road Over Bridge is a bridge built over the Railway tracks.

Road Under Bridge (RUB) - Road under bridge is a structure built beneath the Railway tracks.

Foot Over Bridge (FOB) - A foot over bridge is a bridge over a road or other obstructions, for exclusive use of pedestrians.

Underpasses - Underpass is a structure allowing uninterrupted movement of traffic beneath a reference roadway. An underpass is classified as Cattle Underpass (CUP), Pedestrian Underpass (PUP) and Vehicular Underpass (VUP) depending on principal user.

Subways - A subway is usually meant for pedestrian use and comprises of structure below a road with stairs/ramps/escalator.

Overpass - Overpass is a bridge carrying another road over a reference road.

On the basis of Superstructure- Based on form and type of superstructures, bridges are classified as under:

Arch bridge

Solid slab bridge

Voided slab bridge

T-beam and slab bridge

Suspension bridge

Cable stayed bridge

Extra-dosed bridge

Box girder bridge

Single/Multi-cell Box bridge

Truss bridge

On the basis of System of Support for Superstructure - Bridges are classified on the basis of systems of support as under:

Simply supported

Continuous

Integral

Balanced cantilever

Cable supported

On the basis of Material - Based on material of structure, bridges are classified as under:

Masonry

Reinforced concrete

Pre-stressed concrete

Fiber reinforced concrete

Steel

Composite

Timber

On the basis of Construction technology - Based on technology for construction adopted, bridges are classified as under:

In-situ

Precast or prefabricated

Cantilever

Incremental launching

Span by span

Segmental

Some selected GADs are shown in Bridge GAD Drawings article, click and examine carefully to understand details.

Bridge Components

Decking, consisting of deck slab, girders, trusses, etc.

Bearings for the decking

Pedestals

Piers, Abutments and Returns

Foundations for the abutments and the piers

Expansion joints

Handrails, parapets and guard stones.

Approaches to the bridge to connect the bridge proper to the roads on either side;

River training works, like revetment for slopes for embankment at abutments, and aprons at river bed level.

    Step 1:- Conduct Survey on Site

PSD:-

PSD à Preliminary Survey Data

PSD word file link

Excel sheet for Hydraulics à Download Link 

PSD form as shown in “Guidelines of bridge Design”

        Survey Includes:-

                1) Levels Along Defined cross section

                2) Levels along Existing Cross section

                3) Levels Along River (To get idea of bed gradient, it may give approximate representation of Hydraulic Gradient)

Note:- For Tentative Planning in Office Levels can be obtained from GIS softwares, Chainages can be obtained from Latitude and Longitudes of Points using distance between those 2 points by Excel formula "=ACOS(COS(RADIANS(90-A2))*COS(RADIANS(90-A3))+SIN(RADIANS(90-A2))*SIN( RADIANS(90-A3) )*COS(RADIANS(B2-B3) ))*6371*1000"

                 4) Catchment Area Characteristics:- Toposheets, GIS softwares, BHUVAN website etc, I have demonstrated how Catchment area contour plan can be prepared in this link --> contour plan  .  Contour plan provides great help to study Catchment area characteristics. The detailed Steps for Calculating Catchment area and creating Catchment area plan to desired scale is written in Catchment area calculation from Bhuvan and QGIS software article.

                5) Getting tentative High Flood Level and Ordinary Flood Levels through local enquiry and Bank Marks left by river during floods.

                6) Existing Bridge information of Bridges constructed on Upstream and Downstream size (To get tentative idea regarding Span and Height and Flood characteristics of river flow after Obstruction of bridge)

                7) Geological features of River bed and banks

                8) River Characteristics :-

 Meandering, Straight, Silting, Scouring, Bringing wooden logs with flow, Backwater of reservoir etc.

Step 2:- Carry out Hydraulic Calculations:- 

                There are several methods by which Hydraulic calculations are carried out, though the calculations may seem complicated but the logic behind it is quite simple as demonstrated in video

calculations on excel sheet

Main aim of Hydraulic calculations is to found out

1) Linear Water Way :- Gives Idea regarding length of the bridge, Tentative Arrangement is assumed with specific locations of Abutment and Piers in such a way that It Gives minimum % Obstruction to flow hence minimum afflux.

2) % obstruction to flow after bridge construction :- here one thing is to note that Shifting bridge Location of shifts location of Abutments and Piers along the cross section which can change % obstruction significantly, Then I don't think it is good practice to carry out hydraulic calculations on Defined section which is most of the time completely different that actual Bridge location section. 

•         Obstruction includes bridge proper and approaches

•         Generally not more than 20% - 25% at HFL (high level bridges)

•         25% to 30% for submersible br.

•         HFL at RTL 40 % is permitted(submersible bridges)

•         For raft bridge, area above raft+0.3 may be taken.

3) Afflux :- It is calculated by Empirical formulas, and contributes in deciding soffit level of the bridge.

Afflux” Diff in water levels U/S and D/S of the structure measured at a location unaffected by high local flow velocities caused by the constriction

Strictly Afflux= max diff in elevation of the water surface at a location U/S of the structure with and without  the structure

Should generally not exceed 0.6m

In case of high level bridge by Molesworth formula

In case of submersible bridges by Broad Crested weir formula

Afflux at HFL by Molesworth Formula (high level bridge)

If afflux is contained within banks, higher higher afflux may be allowed.

Orifice formula

$$Q = C_o \sqrt{2g} \times LD_d \times \left(h + \frac{(1+e)u^2}{2g}\right)^{\frac{1}{2}}$$

e and Co are from graph Show PICs

h and u are unknown

2^nd eq. Q = W. (Dd+h) u

Applicable (Du-Dd) less than 0.25 times Dd

Scour depth

Scour estimation by observation, theoretical

Type of Bed material	weighted mean diameter of particles	Value of Silt factor
Fine silt	0.081	0.5
Fine silt	0.12	0.6
Fine silt	0.158	0.7
Medium silt	0.233	0.85
Standard silt	0.323	1
Medium sand	0.505	1.25
Course sand	0.725	1.5
Fine Bajri and Sand	0.988	1.75
Heavy Sand	1.29	2

 

For design of floor protection of raft or shallow foundation following scour values shall be adopted
In a straight reach	1.27	dsm
At a moderate bend	1.5	dsm
At a severe bend	1.75	dsm
At right angled bend	2	dsm

Scour by soundings during or immediately after flood

Guidelines of Bridge design book, written by Maharashtra Public Works Department provides guidelines to prepare a bridge GAD suitable in Western Ghats and Vidarbha region.

Link to Download the book --> Guidelines of Bridge Design

Link to Download Bridge Bluebook Vol -2 --> > https://drive.google.com/file/d/1Z2nF4N19mDw7RA-rf0orWSGnjeBZvrqw/view?usp=sharing

4) Various Levels :- 

VARIOUS LEVELs

O.H.F.L : Observed highest flood level ever  recorded ( 50 years/100 years)

H.F.L. (Inglis): Flood level giving Manning’s discharge = Inglis Discharge

H.F.L.(M.I.): Flood level giving Manning’s discharge = modified Inglis Discharge

O.F.L.: Ordinary flood level. This is level of flood when cleared , will not give  interruptions, more then permissible to traffic during floods

Perm. Interruptions

National Highways:  No interruptions.

SH and MDR : 6 times a year and for a period not exceeding 12 hr at a time.

ODR: 6 times a year and for a period not exceeding 24 hr at a time

VR: 6 times a year and for a period not exceeding 72 hr at a time

Estimation of flood discharge

IRC-5 Clauses

Cl.103.1 Qd shall be based on max flood discharge of 50 years return cycle.

100 years for imp bridges (MORTH)

Methods total 4

1) Q observed as per records

2) Rainfall and other characteristics

3) Area velocity method

4) Hydrograph methods

a) Empirical formula

b) Recognized method

More than one method adopted and Qmax selected

Freak flood Q or exceptional flood of high intensity , failure of dam need not be considered

Normal peak Q or spillway Q

Modified Rational Formula

$${Q_{50}}={0.278C{I_{50}}A}$$

Where, 

C = Runoff coefficient

A= Catchment area in Sq.Km

I₅₀ = 50 year rainfall intensity mm/hr = R_(50)tc/t_c

$$C = X {(R.F.)}^{0.2}$$

R = 50 year 24 hrs point rainfall (cm) from figure given in the report for the country

    F : Areal reduction factor 

    X : 0.249 to 0.498 depends on soil type and location 

t_c = Time of Concentration in hours

It is the time taken by water to travel from most distant point on the periphery of catchment to the point of interest.

$${t_c}=\left[{{L^3}\over H}\right]^{0.375}$$

L is Length of longest stream ( in kms) from source to bridge site

H = Height of farthest point above the point of interest along the river ( in meter) i.e. height of farthest point from bed level

Runoff coefficient
Depends upon nature of soil, soil cover and location of catchment :
S. No.	Description of the Catchment	Value of X
1	Sandy Soil/Sandy loam/Arid areas	0.249
2	Alluvium/silt loam/coastal areas	0.332
3	Red soil/clayey loam/cultivated plains/tall crops/wooded areas	0.415
4	Black cotton clayey soil/lightly covered/plain & barren	0.456
5	Hilly soil/plateau and barren	0.498

3.1 Dickens formula (1885) :- Used in Vidarbha Region or for largge Catcchment areas

$$Q = {C\times {A^{3\over 4}}}$$

Q => (ft³/sec), A-sq. mile

C  = 800 to 1000 for rainfall 25’’ to 50”

    =  1000 to 1400 for MP, adjacent Vidharbha

    = 1400 to 1600 in Western ghats.

3.2 Ryve Formula

Sometime later modified Dickens formula for south India (Madras Presidency)

Inglis C.C.(PWD) 1930 Bombay state

Inglis formula : 

$$Q={{C\times A}\over {\sqrt{A+4}}}$$

Q in  (ft³/sec), A in sq. mile

For smaller catchment areas Inglis formula gives significantly higher values so the coefficient needs some modification and accordingly Value of "C" is modified and is given in below table

SR No	Catchment Area in sq.miles	Value of C
1	2	4600
2	3	4800
3	4	5000
4	5	5300
5	6	5550
6	7	5700
7	8	5850
8	9	6000
9	10	6100
10	11	6200
11	12	6300
12	13	6400
13	14	6500
14	15	6600
15	16	6700
16	17	6775
17	18	6850
18	19	6925
19	20	7000

Following formula can be used in excel to interpolate corresponding 'C' value

=INDEX(C:C,MATCH(x,B:B,1))+((x-INDEX(B:B,MATCH(x,B:B,1)))/(INDEX(B:B,MATCH(x,B:B,1)+1)-INDEX(B:B,MATCH(x,B:B,1))))*(INDEX(C:C,MATCH(x,B:B,1)+1)-INDEX(C:C,MATCH(x,B:B,1)))

Note:- All above formulas gives Discharge in Cusec, it must be converted in Cumec by dividing it by 35.41 when using Metric System.

Ø  3.6  Manning's Method ( most used)

Compartmental method

$$V={1\over n}\times {R^{2\over 3}}\times {S^{1\over 2}}$$

n = Rugosity coefficients ( roughness of bed and bank)

R = A/P Hydraulic mean depth.

S = Hydraulic gradient.

Q = Discharge in m³/s

V = velocity in m/s

Surface	Perfect	Good	Fair	Bad
Clear, straight bank, no rift or deep pools	0.025	0.0275	0.03	0.033
Clear, winding, ineffective slopes	0.04	0.045	0.05	0.055
Very weedy stretches	0.075	0.1	0.125	0.15

 Show Chao’s book

Hard exposed rock 0.025

+0.005 if curved

rock outcrops = 0.030

Sand boulders pebbles = 0.030

Silt clay = 0.040

Vegetable growth = 0.045

Sluggish river reach = 0.055

Agricultural field 0.060

Deep vegetable growth = 0.07 to 0.100

Linear Waterway

Linear waterway at

            HFL /OFL=A/D

A=A2 +A1 (Q1/Q2) +A3(Q3/Q2)

D=HFL/OFL-lowest bed level

W=C(Q)^1/2

Depending upon the ht of substructure  choose spanning arrangement

VERTICAL CLEARANCE

Discharge (m3/s)	Vertical Clearance (mm)
Up to 0.3	150
0.3 to 3.0	450
3.0 to 30	600
30 to 300	900
300 to 3000	1200
Above 3000	1500

Fixing RTL

HFL/HTL/OFL     ……………………..

Vertical clearance  …………………..

Afflux                   ………………………...

                             Soffit level ……..

Depth of SS          ……………………….

Wearing coat thickness  ………………….

                           Road top level……..

   1) Flood Levels :- 

            For Submersible Bridges :- OFL

            For High Level Bridges :-   HFL

    2) Soffit :- 

            For Submersible Bridges :- OFL + Vertical Clearance + Afflux

            For High Level Bridges :-   HFL + Vertical Clearance + Afflux

    3) RTL :- 

            For Submersible Bridges :- Soffit + Depth of Super Structure    < HFL

            For High Level Bridges :-   Soffit + Depth of Super Structure    > HFL

For Bridges in Backwater Backwater HFL shall be substituted by FSL (Full Supply Level) of Reservoir, otherwise bridge may submerge.

Step 3:- Checking Stability of Bridge structure in river flow:- 

                As an Engineer it is our job to predict the sizes of the structure and prepare Estimates, the cost of the estimate shall not go beyond Administrative Approval, to do so the assumed sizes of Piers, Abutments and their Footings and Actual design sizes after design  shall not vary much. so the tentative Stability calculations are carried out. The required results obtained from Stability analysis are -

1) For Piers :-

    a) Stress in concrete at bottom of pier

    b) Stress in reinforcement at bottom of pier

2) For Footings :-

    a) FOS against sliding

    b) FOS against overturning

    c) Area Under Tension (AUT) < 20% for normal case and < 33% in seismic case

    d) Redistributed Pressure < SBC of founding strata

Anybody can calculate above values as per technical paper as per download link --> https://drive.google.com/file/d/19Rkb6-rnUu69CKh6jQK4ArF_SFqmWQBD/view?usp=sharing

In following video the quick way of stability calculations is demonstrated.

Step 4:- Drawing General Arrangement Drawing:- 

                As Drawings are essential for all civil engineering works, GAD is prepared showing all essential details. Manually it is time consuming process using LISP program the time can be reduced as shown in below video.

Some Essential things need to be Tabulated in GAD drawings as those are essential for estimate preparation.

1) Material Table :- To Know Material of each component and Grade of Material.

2) Hydraulic Parameters :- So that designer can design structure to those parameters

This method resulted in certificate shown below 

For other bridge component details read --> Click Here

Any Suggestions or corrections will be appreciated.