' I UNIT 19 BRIDGES Structure 19.1 Introduction Objectives 19.2 IRC Loading II 19.2.1 19.2.2 19.2.3 19.2.4 19.2.5 19.2.6 19.2.7 Class AA Loading Class 70R Loading Class A Loading Class B Loading Impact Effect Selection of Loading for the Design of a Bridge Arrangement of Live Load on a Bridge 19.3 Components of Culverts and T-beam Bridges 19.4 Analysis of Slabs Canying Wheel Loads 19.4.1 Effective Width Method 19.4.2 Pigeaud's Coefficients 19.5 Culverts 19.6 T-beam Bridges 19.6.1 Proportioning 19.6.2 Analysis of Longitudinal Girder 19.7 Summary 19.8 Answers to SAQs I 19.1 INTRODUCTION A bridge is a structure providing passage over an obstacle without closing the way beneath. The required passage may be for a road, a railway, pedestrians, a canal or a pipe . line. The obstacle to be crossed may be a river, a road, a railway or a valley. According to the material of construction of the super-structure, bridges can be classified as timber, masonry, steel, reinforced concrete, prestressed concrete, composite, or aluminium bridges. Reinforced Concrete (RC) is now a universally used material for the construction of bridges because of its durability, economy, ease of construction, and its adaptability to create pleasing designs. Reinforced concrete is well suited for the construction of highway bridges in the small and medium span range. The usual types of reinforced concrete bridges are : Slab bridges (culverts), Girder and slab (T-beam) bridges, Hollow girder bridges, Balanced cantilever bridges, Rigid frame bridges, Arch bridges, and Bow-string girder bridges. The selection of type of bridge for a certain location depends mostly on cost considerations and also on natural conditions, e.g. nature of foundations, water-way to be provided and difficulties of construction. Experience is invaluable in selecting the proper type of bridge. Generally speaking, the cost of a bridge will be least when the length of spans is so chosen that the cost of main girders in one span is equal to the cost of pier and its foundation. First two types of RC bridges will be covered in this unit. The design will be confined only to the design of super-structure. The design criteria as laid down by the Indian Road
RCC Structures - 11 Congress (IRC) Code of Practice will be gener:.lly followed. The relevant extracts of the code are given in the text. Objectives After studying this unit you should be able to design * reinforced concrete deck slab of slab culverts for IRC loading, and the super-structure of reinforced concrete T-beam bridges for IRC loading. - 19.2 IRC LOADING Vehicles crossing a bridge are the live loads that are transient in nature. These loads can not be estimated precisely, they may also change in future, and the designer has very little control over them once the bridge is opened to traffic. However, hypothetical loadings that are reasonably realistic need to be evolved and specified to serve as design criteria. IRC, for these reasons, has standardized the live loads to be considered in the design of a highway bridge. The loads have been classified under four categories namely Class AA, Class 70R, Class A, and Class R loading. For simplifying the analysis, the contact area of wheels with the pavement has been assumed to be rectangular. The four categories of IRC live loads are described in the subsequent sub-sections. 19.2.1 Class AA Loading There are two sub categories in this loading namely tracked vehicle of 700 kN and wheeled vehicle of 400 kN with dimensions shown in Figure 19.1. The tracked vehicles represent battle tanks that move on chains. ( a) TRACKED VEHICLE PLAN - ( b ) WHEELED VEHICLE Mir~imumValue of C-(cm) e Lane Bridge -- 1 Clear Carriage W a y I P ' i @- '-7 3.8 moment and .above / L 30 / GO -- Multiple Lane Bridge b n 5 5 - m o m e n t C+ Minimum clear distance of wheel from inner edge of the kerb Figure 19.1 :IRC Class AA Loading 1I
Bridges 19.2.2 Class 70R Loading This loading consists of a tracked vehicle of 700 kN or a wheeled vehicle of total load of 1000 kN.The tracked vehicle as shown in Figure 19.2 is siinilar to that of Class AA. The wheeled vehicle as shown in Figure 19.2 is 15.22 m long and has seven axles with loads totaling 1000 kN. This loading was originally included in the Appendix to the bridge code for use for the rating of existing bridges. In recent years, there is an increasing tendency to specify this loading in place of Class AA loading. etwO3 1 :;; L.bb55 0v.r 5 5 I a l TRACKED VEHICLE eoolr LOADINS W H E E L SPACING I b l WHEELED VEHICLE , Figure 19.2 :IRC Class 70R Loading 126.96.36.199 Class A Loading Class A loading consists of a train of wheel loads carrying a driving vehicle and two trailers as shown in Figure 19.3. 188.8.131.52 Class B Loading This loading also comprises a driving unit and two trailers similar to that of Class A loading but with smaller axle loads as shown in Figure 19.3. 19.2.5 Impact Effect Moving vehicles produce higher stresses than those which would be caused if the vehicles are stationary. It is mainly because of the impact caused by vehicles during motion on an uneven surface of the road. In order to take into account the increase in stresses due to dynamic action and still proceed with the simpler statistical analysis, an impact allowance is made for impact. The impact allowance is expressed as a function of the percentage of the applied live load, and is computed as below : (a) For IRC Class A or B Loading I = 0.5 for L 13m I = 0.088 for 3mIL545m for L 2 45 m
- RCC Structures I1 where, I is the impact fraction factor, and L is the span in meters. (b) $or IRC Class AA or 70R Loading Tracked Vehicles : I = 0.25 for L 1 5m + 0.0375 (9 - L) I = 0.088 + (45 - L)/3000 for 5 m I L I 9 m for 9 m I L I 45 m I = 0.888 for L 1 45 m I = 0.1 Wheeled Vehicles : I = 0.25 I = 4.5 6 + L for L 1 12m for 12mILS45m I = 0.088 for L 2 45 m The impact fraction factor, I, can alternatively be read from Figure 19.4. 0 KK I zr bb . . =3" 3% 3 3 3 AXIAL LOAD (kt4 CLASS A z CLASS B 1 AXIAL LOAD IkN) CONTACT WIDTH B W Ira) lmn) 250 500 3w 300 100 175 w 114 6) 41 27 16 100 150 150 125 .DRIVING VEHICLES C LEARANCES Over 7.5 Figure 193 :IRC Class A and B Loadings 19.2.6 Selection of Loading for the Design of a Bridge The following points should be considered while deciding the loading to be considered in the design of a bridge. The Class AA or 70R loading is to be adopted for bridges located within certain specified municipal localities and along specified highways. Normally structures on National Highways and State Highways are provided for these loading. Structures designed for Class AA or 70R loading should also be checked for Class A loading, since under certain conditions, more severe s e s s e s may be obtained under Class A loading. Class 70R loading should oniv be considered when it is soecificallv snecified.