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
- RCC Structures I1 Cantilever portion Longitudinal girders Cross beams Standard details are used for kerbs and hand rails. Wearing coat can be of asphaltic concrete or cement concrete of 1 : 1.5 : 3 mix with an average thickness of 75 rnm. 19.4 ANALYSIS OF SLABS CARRYING WHEEL LOADS The live load on a bridge consists of wheel loads acting on the contact area of wheels of a standard IRC vehicle with the surface of the road. This type of loading is not easy to deal with while analysing bending moment and shear force in the deck slab because of its highly indeterminate nature. The Lse of elastic theory for varying .position of a wheel load acting on a slab results in equations whose solution is very time consuming and impracticable. The analysis is done in a semi-empirical manner by modifying the results of elastic analysis suitably. There are, in general, three approaches employed for this purpose, namely (a) effective width method, (b) use of Pigeaud's coefficients, and (c) Westergaard's method. The first two approaches, normally used in practice, have been described here. 19.4.1 Effective Width Method This method is applicable for the following two support conditions of a rectangular slab : Slab simply supported on two opposite edges Slab supported on all four edges and aspect ratio (B / L) very large When a point load acts on a slab, it deflects forming a saucer. Since the slab gets curvature in the plane of the span as well as at right angles to it, it is obvious that bending moments in the slab are created in the plane of span as well as normal to it. It is not only the strip of the slab immediately below the load that bears it but even strips on either side of the load take part in supporting the load. The bending moments are, therefore, much smaller than they would be if only the strip of slab below the load was acting singly. It is therefore assumed that the load is supported by a certain width of slab, known as the effective width and the load is assumed to be distributed over this entire width thus converting the indeterminate problem into a determinate one. If the effective width is known, then bedding moments along the span can be easily calculated statically. However, Bending Moment (BM) at right angles to the span are not given by this method but taken empirically as the sum of 20% of the BM due to dead load and 30% of the BM due to live load calculated along the span. The Indian Road Congress has recommended certain formulae to obtain effective width of slabs as given below. (a) Slabs Supported on Opposite Edges For a single load, the effective width 'b' is given by where, x = distance of the centroid of wheel load from any support, L = effective span = clear span for Simply Supported (SS) slabs, for continuous slabs, a = contact length of wheel with the surface of road parallel to supports after dispersion through wearing coat = (g + 2h), g = length of area of contact of the wheel with the road surface parallel to supports, h = thickness of wearing coat,