COMPLETE BRIDGE ANALYSIS AND DESIGN PROCEDURE

FIRST OF ALL, COMPLETE THE TYPE OF BRIDGE MODEL AS PER REQUIREMENT OF CODES, GEOLOGICAL CONDITION. THEN FOLLOW THE PROCEDURE FOR LOADINGS AND ANALYSIS AND DESIGN

DEAD LOAD :

Dead Load of Bridge includes the following category of loads:

Selfweight of Structure eg, deck Main Girder, cross Girder & deck itself etc. Wearing Coarse (Asphalt Concrete and its thickness to be overlaied as wearing Coat on

top of Concrete Deck normally taken as 100mm thickness)i.e. = 0.100 * 22 = 2.2 KN/m2. For Curved bridges in plan, it vary in thickness as transverse or cross slope i.e. superelevation is required).It should be calculated as per design vehcile, design speed and design radius of bridge.

All other loads; such as Crash barrier, sidewalk railings, electric poles etc should be put as loads on model as per clause 209.7 or it can be placed on model as a geometry with defined thickness and height of sidewalk railings, crash barrier as specified on IRC6-2000

LIVE LOAD :

Traffic loads on bridge decks are used to simulate the effects of vehicles and/or pedestrian loads. Some traffic loads represent the weight of real vehicles that can travel over the bridges; other values and distributions are chosen in such a way that they produce maximum internal forces in bridge structures similar to the ones produced by real vehicles.

Many highway bridges, in urban and non-urban areas, have sidewalks (footpaths) for pedestrian traffic and/or cycle tracks. On these areas a uniform distributed load is usually considered, Figure 10. Some codes indicate also that one wheel load applied on the sidewalks should be considered.

Parapets of footpaths and cycle tracks that are protected from highway traffic by an effective barrier are designed to resist horizontal distributed force applied at a height of 1m above the footway.

For Pedestrian walkways P = P'−(40 L−300 )

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The Vehicular live loads have to determine governing values of the following global effects: positive longitudinal moments within the span greatest longitudinal moments at changes of girder cross-section

maximum shears at supports maximum reactions

In addition it is necessary to determine governing values of load moments, shears and torsions on the concrete slab or orthotropic deck.For determining the critical positive longitudinal moments within span for bridge sample project, we have to take out the maximum positive Mxy among plate which has corresponding maximum positive bending moment stress under self weight of structure. The case identified under this vehicular live load case will be the one which yields maximum positive bending stress Mxy and will be taken as Ist LL case.For determining the critical Shear stress within span for bridge sample project, we have to take out the maximum Shear stress SQx and SQy among plate which is near to the support or within 2D distance from face of support where maximum shear can occur. The case identified under this vehicular live load case will be the one which yields maximum Shear Stress SQx

and SQy and will be taken as IInd LL case.For determining the maximum support reaction within span for bridge sample project, we have to take out the maximum support reaction among the intermediate supporting pier. The case identified under this vehicular live load case will be the one which yields maximum Support reaction Fx, Fy and Fz and will be taken as IIIrd LL case.

All of these cases of live load will be treated separately with the IS combination for obtaining the corresponding critical results.

IMPACT LOAD :

When moving of vehicle on the deck with design velocity the vehicle will imposed certain amount of magnified amount of load on the deck with respect to its axle load. In other term, we have to translate the dynamic effect of rolling loads in state of quasi-static load .The term Impact factor has been introduced in the Code so as to fulfill the dynamic impact of rolling load s on the deck.

As per IRC 2000 for IRC 70R class of loading (Cl 211.2) (b) (i)

Impact load should be used with impact factor on the Critical Live load cases individually.

WIND LOAD DEFINATION:

For Basic wind speed acting on bridge superstructure has been given on Table 4 of page 28 of IRC 2000For almost all bridges, placed over river at Valley between two ridges or hill, wind velocity is taken always perpendicular to direction of bridge and Category for Exposure condition is taken as Critical Condition of Exposure D; Building Category according to Essentiality and importance Factor of Structure as (IV) { See ASCE-07-02 & 05} for wind Load Calculation.Type of Building/Structure Enclosure = Enclosed Building {See ASCE-07-02 & 05} for wind Load Calculation

SEISMIC LOAD DEFINATION:

Follow code IS1893-2002 for seismic zone, response reduction factor, Importance factor Type of Soil, damping factor for reference.

BRAKING LOAD :

These forces (Figure 8) result from the traction or braking of vehicles and they are applied to the road surface, parallel to the traffic lanes.As per Cl 214.2 (a) & (b) of IRC6-2000 Braking Load acts 1.2m above deck surface.

TEMPERATURE AND SHRINKAGE EFFECT LOAD:

For Shrinkage and temperature loadsForce induced at each bearing due to strain developed by temperature and Shrinkage effect is as follows

Ft = KN at each bearing

Support

Where Cw = Width of carriageway of bridge le = eff. Span of bridge αt = Coefficient of linear expansion. = 1.17 x 10-7/C ∆T = assumed temperature difference Abp= Assumed Area of bearing plate bt = thickness of Bearing esk =Strain of elastomeric bearing due to shrinkage usually taken as 2 x 10-4 mm n = nos of bearings ƒr =Modulus of rupture for M20 = 0.24√fck where fck is grade of concrete, N/mm2

EARTH PRESSURE LOAD:

This Load has to be used for abutment or piers where the abutment or piers are at the extreme left end or right of bridge section. For detail load calculation and formulae see IRC6 -2000

CENTRIFUGAL LOAD:

For Curved section of bridge this load condition is used. For detail see IRC6- 2000

WATER CURRENT LOAD:

For mid pier of bridge this load condition is used. For detail see IRC6- 2000

BUOYANT FORCE:

For mid pier of bridge where the pier is submerged in water, this load condition is used. For detail see IRC6- 2000

LOAD COMBINATION:

The Analysis has to determine governing values as we have to identify the following global effects for corresponding vehicular live load cases:

positive longitudinal moments within the span greatest longitudinal moments at changes of girder cross-section maximum shears at supports maximum reactions

The load combination for bridge will as follows according to IS 1915-1961:

DL+ (LL+IL) DL+ (LL+IL) + Braking load+ Temperature & shrinkage Load + Earth pressure Load +

Centrifugal (if any) + Water Current Loads+ Buoyant Force + Seismic Loads DL+ (LL+IL) + Braking load+ Temperature & shrinkage Load + Earth pressure Load +

Centrifugal (if any) + Water Current Loads+ Buoyant Force + Wind Loads

The Vehicular Live load (dead load of vehicles in quasi-static condition) and Impact load (dynamic impact of vehicles) comprises the vehicles in dynamic state or we can simply represent Vehicular Live load i.e. VLL

DL+ VLL DL+ VLL + Braking load+ Temperature & shrinkage Load + Earth pressure Load +

Centrifugal (if any) + Water Current Loads+ Buoyant Force + Seismic Loads DL+ VLL + Braking load+ Temperature & shrinkage Load + Earth pressure Load +

Centrifugal (if any) + Water Current Loads+ Buoyant Force + Wind Loads

DESIGN:Procedure for Further design and Analysis

1. After running analysis, the model has to be designed. For this we’ll go to design TAB and select the steel sections if any and set the required parameters and select the appropriate code from drop down list , it may be IS 800, 801 or 802 and assign the CODE CHECK command.

2. For RCC also, we follow the same pattern.3. Now, after getting done for the maximum moment and shear support, we‘ll repeat for

maximum reaction at pier or abutment support conditions also.

4. From there, we design the piers or abutment walls.5. Now, after Completion of design of piers, we’ll get the support reaction of the

corresponding piers or abutment.

PILE OR MAT FOUNDATION:

1. First of all, we have to decide, whether, the foundation of pier is enough with open foundation, mat or piles to support the loads coming to pier due to span of bridge.This can be determined easily from the soil test report, geological site condition. For instance, if the substratum is hard, open foundation will be enough, just to anchor the hard rock soil with foundation to transfer loads. But, if it is too soft or soil strata, we have to go for either mat or mat with piles.

2. For Mat only if required, you just have to follow the mat design procedure as we’ve discussed before in building class.

3. But, if we need to go for piles as well, we’ve to go to Pile design software, named GEODELF MPILE software.

4. In MPILE, first of all, create the project name and its detail in project information TAB and save in required destination. Now, select the required model of pile analysis, remember, there are 6 nos of pile analysis model in MPILE software.

5. Now, create the soil types from the soil test report in Soil Layers. Care should be taken while creating the soil profile, all the soil parameters values should be consistent or should be in report or in the soil type or should match the nature of soil in that

geological location. For instance, value of γs of soil at bottom ant top layer, value of Cu

for cohesion soils at top and bottom layers, Φ (Phi) for cohesion less soils at top and bottom of layers, value of dz at 100%, and many more. For detail see, manual of MPILE foundation.

6. After successful creation of soil layers, now got to profiles and create the critical soil profile as given in soil test report bore log sheet.

7. After completion of bore log sheet profiles, now initially assumed the no of piles to be grouped for each pile head or combined mat for all piers or for the mat foundation. Assumed the cross section of piles circular, rectangular or square from piles types. Also specify the material types of piles and their material properties.

8. Specify the pile tip curves from tip curves TAB. It represent the end condition of pile either it is curve profile, blunt or bulged headed.

9. Now, Go to Location TAB of piles and give the grouping X, Y, Z coordinates, their profile name, pile tip curves, End bearing capacity of piles, for the calculation of end bearing capacity of any soil refer to manual MIPLE Foundation, skewness or angle of piles if any.

10.After completion of location of piles, goto loads TAB and copy all the reactions of the piers or abutment from the STAAD post processing table and paste it on excel sheet and copy it form Excel sheet and paste in these loads table.

11.Now, goto Run analysis of F5 for analysis of piles.12.After successful completion of analysis, we’ll get the P-∆ curve in X, Y, Z direction at

each boundary layer between the changes in soil profile of each pile. We’ll also get the load carrying capacity (Fx, Fy,Fz, Mx, My & Mz) of each individual pile in all the loading conditions of LOADS and COMBINATION mention in STAAD.

13.Copy all the load carrying capacity of piles in excel or notepad and find the critical load carrying capacity of piles. Also, get the critical P-∆ curve in X, Y, Z direction at each change in boundary layer between soil profiles.

14.Again goto STAAD model the piles with critical loads at top and spring support condition as we got in corresponding level in pile profiles. The value of spring constant, Kx, Ky or Kz as per required in nodes can be calculated from P-∆ curve in X, Y, Z direction given in MPILE result.

15.Analyze the file and get the pile design. For mate, we got the load carrying capacity of (critical) each pile. We’ll use those data and specify the location of piles in mat and specify the sub grade modulus of soil and spring constant of pile equal to average value of spring constant of the soil profile and analyze the mat foundation to check the pile support reaction whether or not the support reaction in each of the pile exceeds the load carrying capacity of pile. If it exceeds, increase the no of pile arrangement otherwise fine. Enjoy the foundation output.

*****************THIS IS THE END OF BRIDGE DESIGN ****************