LOADS, LOAD COMBINATIONS AND BRIDGE MODEL DIAGRAMS WITH PROCEDURE DIAGRAMS OF STRAIGHT “I” CONRETE

GIRGER

CALCULATION SAMPLE OF “I” CONCRETE GIRDER MODEL BRIDGE

DEAD LOAD :

Dead Load of Bridge includes the following category of loads:

Selfweight of Structure eg, deck Main Girder, cross Girder or Shell element such as box girger 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 IRC2000

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 Pedestrain walkways P = P’-{40L-300)/9}=4.36 KN/m2

Where P’ = 500 kg/m2 as per Cl 209.4, L = Clear Span of bridge = 20m

The Vehicluar live loads has to determine governing values of the following global effects:

positive longitudinal moments within the span negative longitudinal moments at internal supports greatest longitudinal moments at changes of girder cross-section

maximum shears at supports maximum shears at changes of web resistance maximum reactions critical combinations of moment and shear (usually at supports) maximum torsions (usually most critical for box sections) maximum moments, shears and torsions on cross girders, cross bracing and slabs

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 straight “I” concrete girder sample project, we have to take out the maximum positive Mxy among plate no: 1111, 1121,1131,1 141,5047, 5057, 5398 and 5388 which has corresponding maximum positive bending moment stress under selfweight of structure. The case identified under this vehicular live load case will be the one which yeilds maximum poistive bending stress Mxy

and will be taken as Ist LL case.For determining the critical negative longitudinal moments within span (for continiuous bridge) for straight “I” concrete girder sample project, we have to take out the maximum negative Mxy among similar plate nos. which has corresponding maximum negative bending moment stress under selfweight of structure. The case identified under this vehicular live load case will be the one which yeilds maximum negative bending stress Mxy and will be taken as IInd LL case.For determining the critical Shear stress within span for straight “I” concrete girder sample project, we have to take out the maximum Shear stress SQx and SQy among plate no1116, 1126, 1136, 1146, 5052, 5062, 5403, 5393, 1307,1317, 1327, 1337, 5083, 5043, 5414 and 5404 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 yeilds maximum Shear Stress SQx and SQy and will be taken as IIIrd LL case.For determining the maximum support reaction within span for straight “I” concrete girder sample project, we have to take out the maximum support reaction among the intermediate supporting pier whose supporting nodes are: 72, 84, 642, 645 and 966 which is at intermediate support of piers . The case identified under this vehicular live load case will be the one which yeilds maximum Support reaction Fx, Fy and Fz and will be taken as IVth LL case.

All of theses cases of live load will be trerated seperately with the IS combination for obtaining the corresponding critical results.

WIND LOAD:

For Basic wind speed acting on bridge superstructure has been given on Table 4 of page 28 of IRC 2000For 20m height of Bridge superstructure above water level basic wind speed = 136 Km/hr = 37.77 m/sec and corresponding pressure = 119 Kg.m2 = 1.167 KN/m2.For 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

References for American Society of Civil Engineers (ASCE 7-05 & 02)

Basic wind speed:

Basic wind speed in U.S. is shown in Figure 6.1. The basic wind speed is basic on statistical data collected from airport across U.S.  The wind speeds correspond to 3-second gust speeds at 33 ft above ground for exposure C category.  The wind speeds represent 50-year return period.

Exposure to wind:

Change from ASCE 7-98 to 02 & 05. Exposure A is deleted. A distinction was made surface roughness categories and exposure categories.  Interpolation between exposure categories is permitted by rational analysis.  Wind is to be evaluated in the worst case of the two 45-degree sectors either side of the wind direction.

Surface roughness categories:

Surface roughness B: Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.

Surface roughness C: Open terrain with scattered obstructions having heights generally less than 30 ft. including flat open country, grasslands, and all water surfaces in hurricane-prone regions.

Surface roughness D: Flat, unobstructed areas and water surfaces outside hurricane-prone regions including smooth mud flats, salt flats, and unbroken ice.

  Exposure categories:

Exposure B: Surface roughness B in upwind direction for 2630 ft or 10 times the height of the buildings.  For buildings with mean roof height 30 ft or less, the upwind distance is reduced to 1500 ft.

Exposure C: apply to all cases not in B or D. Exposure D: Surface roughness D in upwind direction for 500 ft or 10 times the height

of the buildings.  Expose D shall extend inland from the shoreline for a distance of 600 ft or 10 times the height of the building.

  Note: This also allow the building to be designed in different exposure in different direction.  A building located at edge of city or sea shore may be designed for exposure category in one direction and other exposure in other direction.

  Important factor, I for wind load

Building category Non-Hurricane prone regions and Hurricane

prone regions with V = 80-100 mph and Alaska

Hurricane prone regions with

V > 100 mph

I    (Low hazard) 0.87 0.77II   (all others) 1 1III  (substantial hazard) 1.15 1.15IV  (essential facility) 1.15 1.15

Building classifications

Building enclosure and openings and protection:

Open building: A building having each wall at least 80% open.Partially enclosed building: building that meet the following conditions;

1. Ao > 1.10Aoi

2. Ao > 4 ft2 or 0.1 Ag and Aoi/Agi  0.2Ao = total area of openings in a wall the received positive pressureAg = the gross area of wall in the same wall of Ao.Aoi = the sum of area in the building envelop not including Ao.Agi = the sum of gross area of building envelop not including Ag.  Examples of partially enclosed buildings: Airplane hangars.  One side of the building has door opening that larger than other 3 sides. Enclosed buildings: buildings not qualified for open or partially enclosed.

EARTHQUAKE:

A per IS1893-2002, we have mentioned some of the table for seismic zone, response reduction factor, Importance factor Type of Soil, damping factor for reference.

Importance Factor I = 2.0 (For Bridge)Response Reduction Factor R = 5 (SMRS Structure)Seismic Zone Factor (V) = 0.36 For most Severe Seismic ZoneType of Structure = 1 for RCC Structure

Rock/Soil Site Factor (SS) = 3 for Soft soilDamping Ratio = 5% = 0.05Depth of Foundation for Abutment pier etc = 3m (Assumed initially)

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 rollingloads in state of quasi-static load .The ternm Imact factor has been introduced in the Code so as to fullfill 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)Span ≥ 30m from Fig 5 of page 23 Impact Factor

(I.F.) = 22%

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) = Braking Load = 25% of Vehicle Axle Load acting 1.2m above deck surface

LOAD COMBINATION:

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

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

critical combinations of moment and shear (usually at supports) maximum torsions (usually most critical for box sections) maximum moments, shears and torsions on cross girders, cross bracing and slabs

In addition it is necessary to determine governing values of load moments, shears and torsions on the concrete slab or orthotropic deck.The Load Combination for bridges as per American Association of State Highway and Transportation Officials –Load Reduction Factored Design (AASHTO-LRFD) Article 3.4 based on strength Limit state as follows:

η[1.25DC + 1.5DW + 1.75 ((LL+IM) + CE + BR)+ 0.5TU] ---- Strength I η[1.25DC + 1.5DW + 1.4 (WS) + 0.5TU] ---- Strength II η[1.25DC + 1.5DW + 1.35 ((LL+IM) + CE + BR)+0.4WS + WL + 0.5 TU] ------

Strength V η[DC + DW + (LL+IM) + CE + 0.3WS + WL + 1.0TU] ---- Service load I η[DC + DW + 1.3((LL+IM) + CE) + 1.0TU] ---- Service load II

For Constructibility limit state as follows: η[1.25DC + 1.5C + 1.25Wc] ---- Constructibility load I

where η = Load modifier as described in article 1.3.2 of AASHTO-LRFD η ≥ 1.05 for non-ductile component and connection

η = for Conventional design, details comply with those specifiactionη ≥ for Component and Connection which additional ductility-enhancing measure have been specified by thise required specificationη = 1.00 for all other limit stateDC = dead load : component & attachmentsDW = dead load : wearing surfaces & utilitiesLL = vehicular Live LoadsIM = vehicular dynamic allowanceCE = Vehicular Centrifugal forcesWS = wind Load on StructureWL – Wind Load on Live loadTU = Uniform temperatureBR = vehicular Braking ForceD = Dead loadsC = Construction Loads

And Wc = Wind Loads for construction condition for an assumed critical direction magnitude of wind may be less than that used for final design.

Limit state design is probabilistic and load factors are determined to ensure that the probability of exceedence of a particular set of design actions (i.e. characteristic actions times partial safety factors) is satisfactorily small. The probability of two different actions, e.g. limiting live load and limiting wind load, or Earthquake occurring simultaneously is clearly less than that for either individual occurrence. It is therefore desirable to achieve the same

reliability for different combinations of actions by adopting different partial safety factors on the individual components. The Following are two different combinations as mentioned in BS 5400 part 2:

1.05DC + 1.75DW + 1.50LLHA alone +1.30LLHA+HB (or LLHB alone) 1.05DC + 1.75DW + 1.4 WS + 1.10WL + 1.25LLHA alone +1.10LLHA+HB (or LLHB alone)

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

DL+ (LL+IL) DL+ (LL+IL) + BRL+X + WDL+Z DL+ (LL+IL) + BRL-X + WDL+Z DL+ (LL+IL) + BRL+X +WDL-Z DL+ (LL+IL) + BRL-X + WDL-Z DL+ (LL+IL) + BRL+X +EQ+X DL+ (LL+IL) + BRL-X + EQ+X DL+ (LL+IL) + BRL+X + EQ+Z DL+ (LL+IL) + BRL-X + EQ+Z

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 + BRL+X + WDL+Z DL+ VLL + BRL-X + WDL+Z DL+ VLL + BRL+X + WDL-Z DL+ VLL + BRL+X +WDL-Z DL+ VLL + BRL+X + EQ+X DL+ VLL + BRL-X + EQ+X DL+ VLL + BRL+X + EQ+Z DL + VLL + BRL-X + EQ+Z

“I” CONCRETE GIRDER:

After Completion of the geometry model, assigning properties and support conditions as mentioned in the bridge design summarised procedure document, the following model will be seen

Fig 1 Multi Span “I” concrete girder Bridge

After assigning wind definition, seismic definition and generating critical case vehicular loading the following screen will be seen.

This is the Earthquake defination tab for defining parameters as per IS1893-2002.

Fig 2 Multi Span “I” concrete girder Bridge Seismic Load Defination

Fig 3 Multi Span “I” concrete girder Bridge Wind Load Defination

Fig 4 Multi Span “I” concrete girder Bridge Vehicle load IRC 70R

Fig 5 Multi Span “I” concrete girder Bridge Vehicle load IRC 70R

This is the Wind defination tab for defining parameters as per ASCE-07-02 & 05.

Fig 6 Multi Span “I” concrete girder Bridge Vehicle load IRC 70RLoad Combination for Max moment

Fig 7 Multi Span “I” concrete girder Bridge Vehicle load IRC 70RBraking Load for Max moment

Fig 8 Multi Span “I” concrete girder Bridge Vehicle load IRC 70R

deflection for Max moment

Fig 9 Multi Span “I” concrete girder Bridge Wind load ASCE-07Along+Z

Fig 10 Multi Span “I” concrete girder Bridge Seismic load IS1893-2002Along+X

Fig 11 Multi Span “I” concrete girder Bridge Seismic load IS1893-2002Along+Z

After completion of analysis and design of superstructure from STAAD.pro, foundation design of pile cap ndpiles from Mpile, STAAD.foundation and STAAD etc the following shown DXF file in autoCAD will be seen.

For Simply Supported condition as well, we can follow the above mentioned similar procedure and obtain the model

geometry, loads and analysis as shown in figure below:

Some of the AutoCAD generated file format for the bridge component design

Vertical post Railings Main Supporting Girder beam

Pier Design for Support 2 Pier for Suppot 1

Pier for Support 2 Pile & Pile Cap Design Chart

Pile Cap2 for Support 2 Design Typical Pile Design

Pile cap3 for Support 2 Pile Cap Layout Design

Pile Cap1 for Support 1

*****************THIS IS THE END OF SUPERSTRUCTURE ANSLYSIS ****************

REFERENCES: ESDEP WG 15B STRUCTURAL SYSTEMS: BRIDGES Lecture 15B.2: Actions on Bridge Project No. NCHRP 12-52 AASHTO-LRFD DESIGN EXAMPLE HORIZONTALLY CURVED

STEEL “I” CONCRETE GIRDER BRIDGE FINAL REPORT Prepared for National Cooperative Highway Research Program Transportation Research Board National Research Council,

John M. Kulicki ,Wagdy G. Wassef, Christopher Smith, Kevin Johns, Modjeski and Masters, Inc; Harrisburg, Pennsylvania, October of Technology Madras

Indian Institute of Technology, Madras,7.4 Loads and Load combinations; Prof. S.R> Satish Kumar and Prof. A.R Santha Kumar

IS 1983-2002 part2 ASCE-07 & 02 WIND LOAD DEFINATION IRC6-2000