Chapter – 7 Design Standards
7.16 Design Standards for Structures 1 General
This section deals with the standards to be adopted vis-à-vis for ROBs, flyovers, bridges, underpasses and culverts. It also provides for the type of materials and their specifications that would be adopted for the above structures, the loads and forces to be considered.
It is intended that the project road will accommodate 4-lane traffic (2-lane divided) at present and to be widened to 6 lanes at a later stage if required.
The design standards for bridges has been worked out on the basis of recommendations regarding loading and material strength characteristic contained in the current bridge design practices and are contained in the relevant IRC standards. The aspects regarding geometry and structural design of various components and settlement effects formed main considerations for design of bridges.
The design of bridges is based on various parameters and data such as design discharge of stream, HFL, scour level, characteristic of stream/river, sub-soil type, selection of site, etc. The selection of proper bridge site, computation of design discharge, bearing capacity and characteristic of soil are required to conceptualize a new bridge. The carriageway width, footpaths, crash barrier are provided as per MOSRT&H guidelines. Based on all these data, type of bridge, length of bridge, height of bridge, type of foundation whether shallow or deep is decided. Two or three alternatives of bridge superstructure and sub-structure are conceived and
the cost of each alternative worked out, the most economical alternative was selected. In case there is already an existing bridge and a new 2 lane bridge is proposed parallel to it, the spans of new bridge is kept same as that of existing bridge or larger span lengths in multiple of existing span is adopted so that pier and abutments of existing and new bridge are in line and no obstruction to flow of water takes place. The various data required for bridge design, method of computation of these data and parameters of bridge design are given below
7.16.2 Hydraulic and Hydrological Investigations
The objective of this investigation is to plan the structures so that the bridge / CD structure should pass safely the design discharge without disturbing the regime of river. The CD structure should not obstruct the flow of river and the length of bridge should be equal to regime width of the river as given by the formula for regime condition in IRC: 5. It is necessary to access correctly the discharge of river, HFL, scour depth, flood frequency, intensity of rainfall and average velocity of flow.
Discharge Computations
The design discharge for which the waterway of bridge is to be designed shall be the maximum flood discharge on record for a period of 100 years for major bridges and 50 years for minor bridges. In case where the discharges are not available it shall be calculated by various rational formula’s and methods given in literature.
The bridge must be able to pass the design flood reasonably. Design for extremely high flood is, however, not feasible for road structures. The consultant advises for minor Bridges and culverts 50 years return period and for major bridges flood 100 years return period is used.
The flood estimation methods for bridges are given below: - Maximum rainfall.
- Basin characteristics such as catchments area.
- River cross sections for area of flow at bridge site, at up stream and down stream section. - Longitudinal sections of the river through the bridge.
- Peak flood sequences.
- Two monthly maximum rainfall.
The following methods for design discharge are used for bridges.
- Empirical methods based on area and two months’ maximum rainfall. - Flood frequency method.
- Flood frequency index method. - Slope area method.
7.16.3 Cross-sectional Elements Width of Bridge
Structural width for bridges/flyovers/road over Rail Bridge:
It is proposed to make the highway a 4-lane highway. All of the bridges except two bridges satisfy the 2-lane requirement. At seven places the new 4 lane bridges have to be built. At other places only 2 lane bridges have to be built or not required due to bypass.
The cross sectional details of the bridge are as shown in figure 8.15. As shown in the figure the outer to outer of crash barrier or handrail and crash barrier is 10.25m. The structural width for all new bridges will be kept same and the entire formation width will be carried out on to the structure.
Any existing bridge of width less than 7.5m will be widened to 10.25m if possible. In case of new 4 lane bridges, there shall be two independent bridges with the overall deck width equal to 10.25m separated by the median width of 4.5m (inner to inner of crash barrier)
Median width
The width of median in the bridge portion shall be kept same as that of approaches . 7.16.4 Type of Super structure
When the length of the new bridges is less than 60m, the alignment of bridges is governed by alignment of the road. Considering small spans ranging from 9.0m to 25.0m (centre to centre of expansion gap) RCC T-beam and Slab type superstructure has been adopted here for overall economy, and easy and rapid construction. The following types of superstructures have been considered though in some cases RC Solid Slab type superstructure has been considered at end span to adjust total bridge length and linear waterway.
Type of Superstructure Span Length(c/c exp. Gap) i) RC Solid Slab Up to 10.0m
ii) RCC T–Beam & Slab 10.0 to 26.0 m iii) PSC I-girder 20.0 to 40.0m iv) Box girder 30 to 60.0m
The depth of superstructures has been decided based on structural considerations. Keeping in view the minimum vertical clearances above HFL, the road formation levels have been achieved. 7.16.5 Specification for Material
a) Concrete
The grade s of concrete will be either equal to or higher than those prescribed in IRC: 21-2000. i) Concrete Grades for various structural elements.
Grade of concrete in various structural elements shall be for moderate conditions of exposure. Superstructure
PSC Members M40
RCC T-Girder and Deck Slab M35
RCC Solid Slab M30
RCC Crash Barriers M40
Substructure
RCC substructures and foundations M35
Pedestals for bearings
Pot M40 Elastomeric M35
b) Steel
This shall conform to provisions given in IS: 1786, IS: 423 (Part I). i) Reinforcement Steel
This will be;
High yield strength deformed bars conforming to Fe 415/TMT. Mild steel shall be of grade Fe 240.
ii) Prestressing Steel
These should conform to IS: 6006.
System: 19K13 or 12T13 low relaxation multiple strands system
Cables: 19K13 or 12T13 low relaxation with strands of 12.7m nominal diameters. Sheathing: 90mm OD HDPE/ metal sheathing duct.
c) Bearings
i) POT cum PTFE Bearings
POT cum PTFE bearings shall be provided conforming to IRC provisions. d) Expansion Joints
Elastomeric strip seal type expansion joints shall be provided on all the bridges as per Clause No. 2607 of MORT&H specification for road and bridge works and interim specifications for expansion joints issued subsequently vide MORT&H’s letter no. RW/NH-34059/1/96-S&R dated 25.01.2001 and addendum thereto circulated vide letter of even no. 30.11.2001.
In case of bridges with smaller spans slab seal type expansion joint shall be provided. 7.16.6 Loads and Forces to be considered in Design
Vertical Loads a) Dead Loads
Following unit weights shall be assumed in the design as per IRC Codes.
Prestressed Concrete - 2.5 t/cu.m
Reinforced Concrete - 2.4 t/cu.m
Plain Cement Concrete - 2.2 t/cu.m
Structural steel - 7.85 t/cu.m
Dry Density of Backfill Soil - 2.07 t/cu.m Saturated Density of Backfill Soil - 2.2 t/cu.m
b) Superimposed Dead Loads
Wearing Coat: 65mm thick asphaltic concrete with total 0.2 t/sq.m (2.2 t/m for 11.0m wide c/way including allowance for an overlay).
Crash barriers: From design (i.e. 1.0 t/m per side) c) Live Loads
Carriageway Live Loads : The following load combinations will be considered in the analysis and whichever produces the worst effect will be considered.
- One/Two/Three lanes of IRC Class A.
- One lane of IRC Class 70R (wheeled/ tracked)
- One lane of IRC Class 70R (wheeled) with one lane of IRC Class A
- Minimum clear distance between 70R vehicle and Class A vehicle, when placed side by side in combination, shall be 1.2m for design.
- Resultant live load stresses shall be reduced by 10% in case all the three lanes are loaded i.e. in case of three lanes of IRC Class ‘A’ or one lane of IRC Class 70R with one lane of IRC Class A.
- Impact factor shall be as per Cl. 211 of IRC:6 for the relevant load combinations. For simplicity in design, Impact factor for continuous structures shall be calculated for the smallest span of each module and used for all the spans of that module.
d) Horizontal Forces
i) Longitudinal Forces due to live load Following effects shall be considered in the design.
- Braking forces as per the provision of Cl. 214 of IRC: 6.
- Distribution of longitudinal forces due to horizontal deformation of bearings/frictional resistance offered to the movement of free bearings as per Cl. 214.5 of IRC: 6.
ii) Horizontal Forces due to Water Currents
The portion of bridge, which may be submerged in running water, shall be designed to sustain safely the horizontal pressure due to force of water current as per the stipulations of Cl. 213 of IRC:6.
iii) Earth Load
1. Earth forces shall be calculated as per the provisions of Cl. 217 of IRC:6 assuming the following soil properties :
Type of soil assumed for backfilling : As per Appendix 6 of IRC: 78 with dry density of 2.07 t/cu.m and submerged density of 1.2 t/cu.m.
Angle of Internal Friction : = 30 Angle of Wall Friction : = 20
Coefficient of Friction ‘’ at base : tan (), while is the angle of internal friction of substrata immediately under the foundations.
2. Live load surcharge shall be considered as per the provisions of Cl. 714.4 & Cl. 715.1.5 of IRC:78 i.e. equivalent to 1.2m height of fill in case of abutments and return/wing walls and o.6m height when there is no live load on the span.
iv) Centrifugal Forces
For the road bridges situated on curve centrifugal forces shall have to be calculated as per the provisions of Cl.215 of IRC: 6 for a design speed applicable at horizontal curves.
v) Wind Effect
Structures shall be designed for wind effects as stipulated in Cl. 212 of IRC:6. The wind forces shall be considered in the following two ways and design shall be governed by the one producing the worst effect.
a. Full wind forces at right angles to the superstructure
b. 65% of wind force as calculated in (i) above acting perpendicular to the superstructure and 35% acting in traffic direction.
vi) Seismic Effect
The road stretch is located in in Seismic Zone-III as per the revised seismap of India(IS:1893- 2002). The seismic forces will be calculated as suggested by the modified clause for the interim measures for seismic provisions (Cl.222 of IRC:6-2000) published in Indian Highways, dated 28th May, 2009.
e) Other Forces/Effects i) Temperature Effects
a. The bridge structure/components i.e. bearings and expansion joints, shall be designed for a temperature variation of + 17 degree C considering moderate climate.
b. The superstructures shall also be designed for effects of distribution of temperature across the deck depth as given in Fig. 10 of IRC6-2000, suitably modified for the surfacing thickness.
Temperature effects shall be considered as follows :
a. Effects of non-linear profile of temperature shall be combined with 50% live load and full value of ‘E’ shall be considered.
b. Effects of global rise and fall of temperature shall be combined with 100% live load and full value of ‘E’ shall be considered.
ii) Differential Shrinkage Effects
A minimum reinforcement of 0.2% of cross sectional area in the longitudinal direction of the cast-in-situ slab shall be provided to cater for differential shrinkage stresses in superstructures with cast-in-situ slab over precast girders as per Cl 605.2 of IRC:22-1986.
However, effects due to differential shrinkage and/or differential creep shall be duly accounted for in the design. Additional reinforcements in the concrete deck shall have to be provided wherever found necessary.
iii) Construction Stage Loadings/Effects
A uniformly distributed load of 3.6 KN/m2 of the form area shall be considered to account for construction stage loadings in the design of superstructure elements, wherever applicable, as per Cl. 4.2.2.2.2 of IRC:87-1984.
iv) Buoyancy
100% buoyancy shall be considered while checking stability of foundations irrespective of their resting on soil/weathered rock/or hard rock. However, the maximum base pressures shall also be checked under an additional condition with 50% buoyancy in cases where foundations are embedded into hard rock. Pore pressure uplift limited to 15% shall be considered while checking stresses of the substructure elements.
f) Load Combinations to be considered in Design
All members shall be designed to sustain safely the most critical combination of various loads and forces that can coexist. Various load combinations as relevant with increase in permissible stresses considered in the design shall be as per Cl. 202 of IRC:6 and Cl. 706 of IRC:78.
In addition, the stability of bridge supporting two superstructures (with an expansion joint) shall be checked under one span dislodged condition also.
g) Exposure Condition
Moderate exposure conditions shall be considered while designing various components of the bridge.
h) Design Codes
The main design criteria shall be to evolve design of a safe structure having good durability conforming to the various technical specifications and sound engineering practices.
Various Codes of Practices referred shall be as under : i) IRC:5-1998
ii) IRC:6-2000 alongwith the latest amendments i.e. upto 28th May, 2009 iii) IRC:18-2000
iv) IRC:21-2000 v) IRC:22-2000
vi) IRC:45-1972(reprint 1996) vii) IRC:78-2000
viii) IRC:83-1982 (Part I) ix) IRC:83-2000 (Part II)
x) BS 5400 – Part IX (For design of POT/POT-PTEE Bearings) xi) IS 1893-2002 – (Part-I)
i) Load combinations
The various load combinations to be considered will be as per the provision of IRC:6-2000. 7.16.7 Design Methodology
Superstructure
General
The superstructure is designed for various combination of Class A load and 70R load, severest of these load combination are chosen for design. The method of analysis and design of
superstructure depends on type of superstructure. Grillage analysis or any other suitable analysis is adopted for T Girder, I Girder, solid slabs, voided slabs, live load analysis for box girder a single line beam is idealised for longitudinal live load analysis. The superstructure is analyzed in the longitudinal direction for bending moment and shear, corresponding reinforcement or prestressing is provided for it. In the transverse direction deck slab is analyzed as continuous over girders and effect of differential bending of girders is also considered for deck slab design. The superstructure is also designed for temperature stresses, resulting from maximum and minimum temperature variations. The superstructure shall be RCC solid slab for spans upto 10.0 m. For spans ranging from 10.0 m to 25 m RCC T-girder and slab shall be provided. For spans from 20.0 m to 30.0 m prestressed concrete I-girders or prestressed concrete voided slabs shall be provided. For spans over 30.0 m PSC single cell or multi cell box girder shall be provided. RC Slab/RCC T- Beam & Slab Type Superstructure.
Based on the loads mentioned earlier, the bending moments and shear forces are worked out at the selected sections. Distributions of live load on longitudinal beams are worked out (in case of T-beam and slab type of superstructure). The sections are then designed as reinforced concrete sections subjected to the applied moments and shear forces. The design moments, shear forces and joint displacements can be worked out using Grillage method of analysis in STAAD-Pro, Rel. -2003 program, based on which structural design of various elements and checking of adequacy of different section can be done.
The RC Solid slab superstructures shall be analyzed using Grillage analogy method to obtain internal moments and forces based on which structural design shall be carried out.
Modelling & analysis of Superstructure
Modelling is substituting the actual structure to an equivalent mathematical structure, which is amenable to computer analysis. In modelling, the properties of the prototype are required to be correctly assessed and assigned to corresponding components of the model. Similarly support conditions are based on deformations permitted at the supports. Grillage modelling offers a good choice for a large variety of super structure forms.
The analysis is accurate only if the prototype is modelled accurately. We will pay special attention to the modelling / idealization aspect and if necessary will revise our model for greater accuracy.
We have suitable software for the analysis of bridges of all types for various IRC live loading, permanent dead loading and construction stage loading. These will be used in the analysis.
Design of Elements above Deck Level
The miscellaneous elements such as kerbs and parapets/railing are designed as reinforced concrete section for the loads and forces as per Cl. 209 of IRC: 6. - 2000.
Design of Bearing
The loads transferred from the superstructure to the bearings shall be taken from the earlier analysis of superstructure. Short and long term deformations shall be computed for the temperature, shrinkage and creep of concrete.
Elastomeric bearings shall be designed as per IRC: 83 (Part II) for these effects as reinforced multi-layer neoprene bearings. However, design loads and movements are to be supplied to the manufacturer to enable him to manufacture these bearings. The manufacturer’s details & design have to be got checked to ensure compliance with the design requirements.
Substructure and Foundation Piers
Pier will be wall/circular type with cantilever fixed at base, which is taken as top of foundation. The sections at various levels will be checked as sections subjected to axial thrust and bi-axial bending. In addition to dead load and live loads from superstructure, the pier substructure and its foundation will be designed for the loads due to seismic/wind and water current forces as appropriate.
Abutment
Abutments will be of non-spill through type. These shall be designed resting on open foundations, pile foundations or well foundations as per requirement and may have cantilever returns at top. In case the cantilever returns become too long independent RCC retaining walls shall be provided. For height of abutments greater than 8.0m counter forts shall be provided. Open foundation for piers and abutments shall be designed in reinforced concrete. The stability checks shall be carried out as per relevant IRC Codes.
Foundation
Foundation of bridge / ROB is to be conceptualized after evaluation of subsoil data such as type of soil and its safe bearing capacity at foundation level for abutment/pier/return-wall and footings. Thereafter suitable type of foundations is to be provided with respect to soil and type of superstructure. Adequacy of the size and depth of foundation will be ensured for the satisfactory performance of the structure. The structural design of the foundation is to be designed as per the latest computerized modeling. Particular attention is paid to stability checks and corresponding safety factors.
Open Foundation.
Design of isolated open foundation shall be based on complete sub soil investigations. The allowable bearing pressure shall satisfy the provisions contained in the clause 708 and the minimum foundation depth shall not be less than that specified in Clause 705 of IRC: 78-2000. The selection of the appropriate type of open foundation (counterfort type or cantilever type) depends on the magnitude and disposition of structural loads, allowable bearing capacity etc. However, if rock strata are encountered at shallow depth, it will be preferable to adopt open