Piers.pptx- empirical design of substructures for bridges involves utilizing established principles

The empirical design of substructures for bridges involves utilizing established principles and past experiences to determine the dimensions and materials for foundational elements such as abutments, piers, and foundations. This method leverages historical data and simplified assumptions to achieve safe and economical designs, especially when detailed geotechnical investigations or advanced computational methods are not feasible.

1. EMPIRICAL DESIGN OF SUBSTRUCTURES AND FOUNDATIONS. School of Civil Engineering SANJAY RAJ. A

2. SUBSTRUCTURES

3. BRIDGE ELEMENTS

4. substructures The substructure of a bridge comprises the piers and abutments which are located below the level of the bearings and rest above the foundations.

5. BED BLOCK A reinforced concrete bed block resting over the top of the piers and abutments is generally provided to evenly distribute the dead and live loads on to the pier and abutments. The bed block is generally cast with M-15 grade concrete and reinforced with steel bars of area equal to 0.3 per cent of the cross sectional dimensions and distributed as mesh reinforcement near the top and bottom surfaces of the bed

6. MATERIALS FOR PIERS AND ABUTMENTS Mass concrete of M-10 grade corresponding to mix proportions of 1: 3: 6 with 40 mm maximum size aggregates. Reinforced concrete of M-15 grade corresponding to mix proportions of 1: 2: 4. Coursed Rubble masonry in cement mortar of proportions 1: 4. Brick masonry in cement mortar of proportions 1: 6. Prestressed concrete for piers particularly in viaducts with tall piers. Concrete of M-30 to M-40 grade is the minimum requirement for prestressed concrete piers.

7. TYPES OF PIERS

8. Dead load of superstructure and pier. Live loads of vehicles moving on the bridge. Effect of eccentric live loads. Impact effect for different classes of loads. Effect of buoyancy on the submerged part of the pier. Effect of wind loads acting on the moving vehicles and the superstructure. Forces due to water current. Forces due to wave action. Longitudinal forces due to tractive effort of vehicles. Longitudinal forces due to braking of vehicles. Longitudinal forces due to resistance in bearings. Effect of earthquake forces. Forces due to collision for piers in navigable rivers. FORCES ACTING ON PIERS The various forces to be considered in the design of piers are as follows:

9. DESIGN OF PIER

10. SUB-SOIL INVESTIGATIONS : Scope : To determine the nature, extent and engineering properties of soil/rock strata and depth of ground water table for development of a reliable and satisfactory design of bridge foundation. Guidance of the following standards with latest edition may be taken : ( i ) IS:1892 “Code of Practice for Sub surface Investigation for Foundations” may be utilised for guidance regarding investigation and collection of data. (ii) IS:6935 “Method of Determination of Water Level in a Bore Hole.”

11. SUB-SOIL INVESTIGATIONS : (iii) IS:2720 – “Method of Test for Soils.” The tests on undisturbed samples shall be conducted as far as possible at simulated field conditions to get realistic values. (iv) IS:1498 “Classification & Identification of Soils for General Engineering Purposes.” (v) IRC:78 “Standard Specification and Code of Practice for Road Bridges, Section VII – Foundation and Sub-structure.”

12. Sub-surface investigations Sub-surface investigations to be carried during three stages viz. ( i ) Reconnaissance Survey; (ii) Preliminary Survey; and (iii) Final Location Survey.

13. SUB-SOIL INVESTIGATIONS : Reconnaissance Survey : At reconnaissance stage, obviously bad locations for bridge foundations, such as, unstable hill sides, talus formation (i.e. soil transported by gravitational forces consisting of rock fragments), swampy areas, peaty ground etc, are avoided. The reliable data from geological and topographical maps and other soils surveys done, in the past are scrutinised Preliminary Survey : The scope is restricted to determine depth, thickness, extent and composition of each soil stratum, location of rock and ground water and to obtain approximate information regarding strength and compressibility characteristics of various strata. The objective of the exploration is to obtain data to permit the selection of the type, location and principal dimensions of all major structures.

14. SUB-SOIL INVESTIGATIONS : Final Location Survey : During the final location stage, undisturbed samples are collected to conduct detailed tests, viz, shear tests, consolidation tests etc, to design safe and economical structure. The exploration shall cover the entire length of the bridge and also extend at either end for a distance of about twice the depth below bed of the end main foundations, to assess the effect of the approach embankment on the end foundations

15. SUB-SOIL INVESTIGATIONS : During sub-surface investigations, the following relevant information will be obtained : ( i ) Site Plan – showing the location of foundations and abutments, etc. (ii) Cross Sections along the proposed bridge, indicating rail level, top of superstructure, high flood level (HFL), low water level (LWL), founding levels etc. (iii) Load conditions shown on a schematic plan, indicating design combination of loads transmitted to the foundation; (iv) Environmental factors – Information relating to the geological history of the area, seismicity of the region, hydrological information, etc

16. SUB-SOIL INVESTIGATIONS : (v) Geotechnical Information – Giving subsurface profile with stratification details, engineering properties of the founding strata, e.g. index properties, effective shear parameters, determined under appropriate drainage conditions, compressibility characteristics, swelling properties, results of field tests, like static and dynamic penetration tests; (vi) Modulus of Elasticity and Modulus of sub grade reaction;

17. SUB-SOIL INVESTIGATIONS : (vii) A review of the performance of a similar structure, if any, in the locality; and (viii) Information necessary to assess the possible effects of the new structure on the existing structures in the neighbourhood

18. FOUNDATIONS: FOUNDATIONS: As far as possible, foundations should be located on a firm ground having stable strata. This would not always be possible and, therefore, the foundations must be designed adequately against any expected failures. Following basic requirements should be fulfilled. ( i ) Safety against strength failure: Foundation should be safe against catastrophic failures caused by foundation pressures exceeding the “Bearing Capacity” of foundation soil. It is basically the strength failure of the supporting soil mass.

19. FOUNDATIONS: (ii) Safety against deformations and differential settlements: The foundation should deform within acceptable limits of total and differential settlements. These acceptable limits depend on the type of structure and sub-strata involved and should be decided judiciously. The settlement shall not normally exceed 25 mm after the end of the construction period for bridges with simply supported spans. Larger settlement may be allowed if adjustment of the level of girders is possible so as to eliminate infringements to track tolerances. In case of structures sensitive to differential settlement, the tolerable settlement limit has to be fixed based on conditions in each case.

20. FOUNDATIONS: (iii) Allowable Bearing Pressure : The allowable bearing pressure for foundation supported by rock or soil mass, based on the above two criteria, shall be taken as lesser of the following : Net ultimate bearing capacity divided by factor of safety of 2.5, or (b) The allowable pressure (maximum) to which the foundation of the structure may be subjected without producing excessive settlement (i.e. more than 25mm) or excessive differential settlement of the structure. (iv) In case of open foundation, the resultant of all forces on the base of foundation (for rectangular foundation) shall fall within the middle third if the structure is founded on soil. Depth of foundations in soil strata shall not be less than 1.75 m below the anticipated scour level. Foundation shall not normally rest on compressible soils.

21. Choice of Superstructure

22. Choice of Superstructure

23. Choice of Superstructure

24. Choice of Superstructure

25. Choice of Superstructure

26. Longitudinal Analysis of Girders The distribution of live loads among the longitudinal girders can be estimate by any of the following rational methods.  Courbon method  Guyon Massonet method  Hendry Jaegar method

27. CourboN’s method: Courbons method: Among these methods, courbon method is the simplest and is applicable when the following conditions are satisfied: a) The ratio of span to width of deck is greater than 2 but less than 4 b) the longitudinal girders are interconnected by at least five symmetrically spaced cross girders. c) The cross girder extends to a depth of at least 0.75 times the depth of the longitudinal girders. Courbon method is popular due to the simplicity of computations as detailed below: The center of gravity of live load acts eccentrically with the center of gravity of the girder system. Due to this eccentricity, the loads shared by each girder is increased or decreased depending upon the position of the girders.

28. Courbon‟s method: This is calculated by courbon theory by a reaction factor given by, Ri =[ P x Ii / ∑Ii] x [1+(∑Ii / ∑Ii di 2 ) x e x di ] P= total live load ( kN ) Ii=moment of inertial of longitudinal girder ( i ) e=eccentricity of the live load (m) di = distance of girder ( i ) from the axis of the bridge.

29. Guyon-Massonet : Guyon-Massonet : This method has the advantage of using a single set of distribution co-efficient for the two extreme cases of no torsion grillage and a full torsion slab thus enabling the determination of the load distribution behavior of any type of bridge. Mmean = (M/n) Design bending Moment=(1.10 x K x Mmean x I.F.) K=distribution co-efficient (which is depends on flexural parameter and torsional parameters) they are: θ =b/2a [ i /j]0.25 α = G( io+jo )/(2E√ij) 2a= span of the bridge 2b=effective width of the bridge i =second moment of area per unit transverse width j=second moment of area per unit longitudinal width

30. Hendry and Jaegar : Hendry and Jaegar : Assume that the cross beams can be replaced in the analysis by a uniform continuous transverse medium of equivalent stiffness. According to this method, the distribution of loading in an interconnected bridge deck system depends on the following three dimensionless parameters. A= (12/∏4 ) x (L/h)3 x ( nEIT /EI) F=(∏2 /2n) x (h/L) x (CJ/EIT) C=2E (1+ μ) =0.4E----------------- (where μ=0.15) Where L= the span of the bridge h=spacing of longitudinal girders n=number of cross beams EI, CJ=flexural and torsional rigidities, respectively, of one longitudinal girder EIT=flexural rigidity of one cross beam

31. Guyon-Massonet The parameter A is the most important of the above three parameters. It is a function of the ratio of the span to the spacing of longitudinal and the ratio of transverse to longitudinal flexural rigidity.

32. Discussion

33. Thank you