601. General
The function of piers and abutments is to transmit the live load and the dead load of the superstructure to the foundations. The details of the loading to be considered in the design of substructure are contained in the IRS Bridge Substructure Code and Bridge Rules and include, inter alia, impact effect of the live load and the longitudinal forces exerted by it, transverse loading caused by wind action on the substructure and the live load and due to the pressure exerted by back fill etc.
602. Importance of aesthetics in Construction
1. Apart from functional requirements, aesthetics merits serious consideration as the piers and abutments are exposed to view. In combination with the substructure, the entire bridge should provide a pleasing view and harmony with the surroundings. Thus a design which is appropriate for viaduct may be out of place in a built up area. 2. Reinforced and prestressed concrete permit adoption of piers and abutments in various forms such as A,Y, etc.
3. Though the main dimensions and choice of spans are largely determined by functional and economic considerations, proper proportioning of the various elements of a bridge (i.e. height, width and length of piers, length of spans etc.) is also important. Variations within reasonable limits should be allowed for in design to enable proper proportioning of the bridge.
4. The final shape of a structure should also highlight the special qualities of the materials used for construction. For example, stone masonry generally goes well with an arch bridge, while the use of prestressed concrete girders with a flat decking and tall or thin piers. A bridge should not intrude into the environment and look heavy.
5. While constructing road over bridges or flyovers in heavily built up areas, the aim should be to avoid too many piers in the middle so that a road user can have a clear unobstructed view. In viaducts, it is advisable to have slender and tall piers.
603. Material of construction
1. For stone masonry, the proportion of cement mortar used should be minimum 1: 4.
2. When mass cement concrete is used the mix shall be minimum M.20 grade. It shall be preferably design mix, using 40 mm aggregate. 3. Reinforced cement concrete, used in the form of thin piers or as a framed structure, can be adopted for viaducts, fly overs and road over bridges. Cellular piers are suitable if the heights are considerable. For reinforced cement concrete structure, the mix concrete shall be minimum M-25 grade.
4. Prestressed cement concrete can be used for all piers of viaducts. The mix to be adopted should be according to the design requirements.
604. Piers, abutments, wing walls and approach slabs 1. Piers, abutments, and wing walls :
a) The size of piers and abutments depends on the construction materials used.
b) Masonry piers are provided with a batter varying from 1 in 24 to 1 in 12. Their width at the top is determined keeping the minimum space required for seating of the bearings of girders as also to provide sufficient distance on the outside of the bearings to resist diagonal shearing.
c) For masonry abutments, a front batter of 1 in 16 to 1 in 10 is used: a flatter slope or steppings are provided in the rear as per design requirements.
d) When piers are reinforced cement concrete, typical sections used are shown in Annexure 6/1.
2. Wing Walls
a) The abutments can either be of the conventional type with the front face exposed or of the buried type when waterway requirement is not the main consideration.
b) In the former case, wing walls are necessary to retain the slopes of the approach banks. Wing walls can be of the splayed, straight, square or ‘box’ type (Annexure 6/2). Butt joints should be provided between wing walls and abutment, wing walls and return walls and for the various tracks, when the bridge is for more than one track to cater for differential settlement in case of poor soils.
c) Wing and return walls also require provisions of weep holes as in abutments.
3. Approach Slabs :
In order to reduce impact effect and to obtain improved running, properly designed approach slabs may be provided on both the approaches of non-ballasted deck bridges having spans of 12.2 m or more. One end of the approach slab may be supported on the abutment and other end on the formation. Length of the approach slab shall be minimum 4 m.
605. Construction aspects - General
1. When the ground is dry, construction of piers and abutments will not require any special arrangement. For their construction in water either coffer dams or temporary sheet piles may be used to divert the water as indicated in Para 404.
2. Construction of tall reinforced concrete piers can be expeditiously done with the slip form construction.
3. In abutments, weep holes should be provided at vertical intervals of 1 m and horizontal intervals of 1 m in a staggered manner. Behind the abutment and wing walls boulder filling and back fill material should be provided for the full height. The boulder filling should not be less than 600 mm thickness. The back fill material should consist of granular material of GW, GP, SW groups as per IS 1498 and should be free of clay and cement (Annexure 6 / 3).
606. Important points in regard to construction of substructure and superstructure
1. As regards construction in brick and stone masonry, the relevant specification prescribed by the Chief Engineer shall be followed.
2. When concrete is used in construction, the important points to be observed are given in the subsequent paras.
3 i). Concreting shall conform to the requirements specified in IRS Concrete Bridge Code and IS.456.
ii) Proportioning of the concrete mix shall preferably be done by weigh batching.
However, for concrete upto M-20 grade, mixing on equivalent volume basis may be permitted with necessary correction for bulkage. For concrete richer than M-20 proportioning shall be done by weigh batching only.
4. All works in PSC shall be done with weigh batching only. 5. Design of concrete mix shall be in accordance with any of the methods given in the recommended guidelines for concrete mix design, published by the Indian Standards Institution.
6. Ordinary Portland cement conforming to IS 269 shall be used for plain, reinforced and prestressed concrete work. Portland blast furnace cement conforming to IS 455 may also be used for plain and R.C.C. work but not for PSC work. Portland pozzolana cement (IS 1489) shall not be used for PSC and RCC works. It can be used only for foundation concrete and concrete works in Bridge substructure where reinforcement is not provided for structural strength. When Portland pozzolana cement is used it is to be ensured that proper damp curing of concrete is done at least for 14 days and supporting form work is not removed till concrete has attained at least 75% of design strength. High strength ordinary Portland cement conforming to IS 8112 (Grade 43) and IS 12269 (Grade 53) may also be used where required from consideration of mix design.
7. Reinforcement steel shall conform to one of the following specifications :
i. Grade I mild steel & medium tensile steel bars conforming to IS:432 (part-I).
ii. Cold twisted bars conforming to IS : 1786.
iii. Rolled steel made from structural steel conforming to IS: 2062 Grade A and Grade B.
8. The prestressing steel shall be any one of the following: i) Plain hard drawn steel wire conforming to IS : 1785 (Part I) ii) High tensile steel bar conforming to IS : 2090 and
iii) Uncoated stress relieved strand conforming to IS : 6006. 9. i) Reinforcement steel shall be free of loose mill scales, loose
rust and coats of oil, mud or other material ,while being used. ii) Cover and spacing of steel shall be uniform and as specified in
the drawings.
iii) All ends of binding wires shall be carefully turned inside so that they do not project out of concrete to induce rusting . iv) Reinforcement steel shall be adequately secured so that it
maintains its position during casting and vibration of concrete. 10. Aggregates : Aggregates conforming to IS : 383 shall only be used. They shall be clean. Marine aggregates shall not be used in concrete unless they are thoroughly washed in potable water and sulphur and chlorine content are low. The tests on aggregates shall be done in accordance with IS : 2386 (Part-I) to IS : 2386 (Part VIII)
11. Water used for mixing and curing concrete shall be clean and free from injurious amounts of oil, acids, alkalis, salts, sugar, organic materials or other substances which may be deleterious to concrete or steel. Potable water is generally considered fit for use in concrete. Further details can be seen in IRS Concrete Bridge Code.
12. Form work : Form work requirement shall be as per IRS Concrete Bridge Code including stripping time. In the case of PSC works, support shall not be removed till sufficient prestress has been imparted to the member.
13. Special attention shall be given to curing of concrete in order to ensure maximum durability and minimise cracking. The method of curing shall be as per IRS Concrete Bridge Code.
14. The appropriate value of minimum cement for different exposure conditions and maximum cement content for RCC and PSC works as well as the water cement ratio shall be as per the provisions given in the IRS Concrete Bridge Code. The equipment, material and the proportions of the mix to be used shall be submitted to and approved by the engineer before the work is started.
15. i) While transporting concrete from the mixer to the form work, no segregation shall occur nor should there be any loss of ingredients. Necessary precaution may be taken to ensure this.
ii) The concrete shall be deposited as nearly as practicable in its final position without rehandling. It shall be compacted before setting commences . It shall not be subsequently disturbed. The method shall be such as to avoid segregation. There shall be no displacement of steel or form work while placing concrete.
16. Compaction of concrete: All concrete shall be compacted by vibration. Generally internal vibration shall be used on all sections that are sufficiently large to admit them. The use of mechanical vibrators complying with IS:2505, IS:2506, IS:2514 and IS:4656 for compacting concrete is recommended.
The following techniques shall be followed for vibration:
i) Vibrators shall be distributed so that the concrete becomes uniformly dense and plastic mass.
ii) Vibrators shall be used for compaction only and not for moving concrete horizontally along the form.
iii) For horizontal and vertical operations of vibrators, the spacing of points of vibration shall be such that the zones of influence overlap.
iv) For concrete deposited in layers, the vibrators shall be inserted vertically and allowed to sink due to its own weight to the bottom of the layer and be slowly withdrawn. For succeeding layer, the vibrator shall penetrate the surface of the previous layer. For further details, IRS Concrete Bridge Code may be referred to.
17. Bearing areas for members shall be finished to true plane so as to give uniform bearing on the entire area. Bearing plane shall be horizontal even for the bridges on grades.
18. In major works, a field laboratory should be set up at the work site which should be equipped with necessary equipments to carry out the various tests on coarse and fine aggregates, cement, water and concrete.