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 18.104.22.168 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. 22.214.171.124 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.
Bridges - --- CbssA or B - Class A A or 70R 1 tmckrd Class A A or 7OR(wheoled 1- 1 Figure 19.4 :Impact Fraction Factor for RC Highway Bridges Class A loading is to be normally adopted on all roads on which permanent bridges or culverts are constructed. Class B loading is to be adopted for temporary structures, timber bridges, and for bridges in specified areas. 19.2.7 Arrangement of Live Load on a Bridge The loading should be so arranged as to produce maximum BM and SF in the component under consideration. In deciding the arrangement of vehicles on a bridge, the following guidelines should be followed : The vehicles are to be aligned so as to travel parallel to the length of the bridge. When these vehicles are on the bridge, no other live load need be considered as acting over the unoccupied area. Vehicles in adjacent lanes are to be assumed moving in a direction producing maximum stresses. For multi-lane bridges and culverts, single train of Class AA tracked or wheeled vehicles shall be considered for every two-lane width. SAQ 1 (a) List the IRC codes to be used while designing road bridges on a National Highway. (b) Describe the IRC standard loadings and indicates the conditions under which each should be used. (c) What is the significance of Impact Factor and how is it estimated? 19.3 COMPONENTS OF CULVERTS AND T-BEAM BRIDGES The components of a culvert with reinforced concrete deck slab are the following : Deck slab Wearing coat, kerbs, hand rails etc. Abutments and wing walls Foundations The super-structure of a T-beam bridge consists of the following components : Deck slab Wearing coat, kerbs, hand rails, footpaths, if provided