Bridge Design

• Spans: 
It depends upon the type of superstructure proposed.
Masonry arch : 3 to 15 m 
Slab bridges : Upto 9 m
Slab bridges : Upto 9 m
Girder and beams : 10 to 60 m
Truss bridges : 30 to 375 m with simply supported ends.
Suspension bridges : Over 500 m so for maximum span built in 1990 m 
Cable stayed bridges : 300 to 600 m

• Width of bridges:
It is based on traffic survey. It may be single lane or double lane with pedestrian platform on only one side or on both side.

• Single lane – 4.25m

• Double lane – 7.5m with kerb
• Multiple lane – 3.5m per lane

• Length of bridge:
It depends upon the waterway.

It’s the Distance between inner faces of two abutments.
[ L= (n X clear span) + (n -1)b ]

b= pier width
n= no of span
L= total bridge length

• Height of bridge:
It is 1.2 to 1.5 m above HFL.

• Piers: 
Types of piers generally used are:
     • Masonry piers
     • R.C.C. piers

• The forces acting on piers are:
Vertical load or inclined reaction from the superstructure
Water pressure
Static water pressure
Dynamic pressure due to flow of water
Impact due to cross currents
Tractive force
Wind pressure
Earthquake forces

• Foundations:
It may be spread foundation, pile foundation or well foundation.
The choice of foundation depends upon load expected and soil properties

• GLength:

In case of erodible soil, the depth of foundation is kept more than maximum scour depth, below the designed foundation depth. This depth below scour depth is called GRIP LENGTH.

• In road ways grip length > 1/3rd of max scour depth

• In railway bridges grip length > ½ of max scour depthrip 

• Handrail:
• It will eliminate the chance of user / vehicle falling down

• Imparts appearance to bridge
• Defines the width of bridge
• Vertical posts supporting handrails are provided at an interval of 2 m. They should be designed to resist a lateral horizontal force and vertical force each of 1.5 KN/m.

• Foothpath:

• Footpath provision on either side of bridge about 1.5m is must, it will carry about 108 persons/min

• Increase this width by 0.6 m per 54 persons /min

• Scouring:

• Erosion of river bed is called scouring action.

• Bridge scour is the removal of sediment such as sand and gravel from around bridge abutments or piers.

• If velocity of flow is more than critical velocity then scouring happens.

• Scoredepth is given by

[ d= 0.473(Q/f)0.33 ]

• f=1.76√(bed material size)

• Water Way:

Area through which water flows under the bridge superstructure is called waterway.
If the flow is unobstructed then its called Unobstructed Waterway- Natural waterway.
If the flow of water is obstructed then its called obstructed – linear water way.

• Abutment:

• Abutment is subjected to all forces that are acting on pier, but one additional force Earth

pressure also acts on abutment.

• Main difference between wing wall and abutment is that the live load is absent in case of wing wall.

• Abutment pier = every forth or fifth pier of arch bridge is made stronger and designed to serve all function of abutments, except earth filling.

• Afflux:

When natural water is obstructed by bridge piers or abutments, its area of flow reduces and hence

velocity increases which causes rise in head on upstream side. This rise of head on upstream side is called Afflux. calculated by

A) Merrimans formula-

Ha = [v^2/2g][(A/Ca)^2– (A/A1)]

B) Molesworth formula

Ha =[v^2/17.87 + 1/65.60][(a/A)2- 1]

• v- Natural velocity

• A- natural waterway area

• A1- enlarged area on up stream of bridge

• a- Artificial water way area

• C- Coefficient of discharge

In any case afflux should be less than 15cm.

• Maximum Flood Discharge:

 In design of bridges consideration is made that flood will occur once in 100 years , & that for

design of culvert consideration is made that flood will occur once in 20 years.

•  Measurement of discharge :

A) Direct measurement-

. Here cross section of area up to HFL is calculated and velocity of flowing water is measured by making use of surface floats.

. While measuring area of gorge at least 3 cross section are calculated and their average is reported.

. While measuring the velocity, surface float are used in case of small rivers and velocity rods

are used in case of large rivers.

B) Indirect measurements-

     • Rational Method

          . AIR/360 .............[area in H]

     • Emperical method-

         . Dickens Formula- Q = C.A3/4 (central & north India)

         . Ryves Formula- Q= CA2/3 (madras catchment)

         . Inglis Formula- Q=123A/(√(A+10.36)) (western catchment)

A-area of catchment in km2

C their respective constants

Q- flood discharge in m3/s

I- intensity of rainfall in mm/hour

R- runoff coefficient.