Chapter 14 Roadway Bridge
contents • Roadway Bridge Floor • Side walks and Railings • Bridge Bracings • Design of lateral support at top chord of through pony bridge • Cross Sections for wind Bracing • End X-frame in deck bridges • Transmission of the braking forces the bearing
Truss Bridges A. Types of bridge trusses B- Determination of forces in truss members c. Proportioning of truss members D- Box section for bridge trusses Top chords Lacing bars, batten plates Bottom chords Diagonals Verticals Design of compression member Design of Tension Members
Design of Bolted Joint • Design of Battens and Diaphragms • Design of End Portals
Roadway Bridge Floor • The floor of a Roadway Bridges consists of: • 1. A wearing surface or Roadway Covering. • 2. Sub floor transmitting the loads to the stringers and X-girders. • The sub floor is similar to the solid floor of a ballasted Railway Bridges. It may be timber, steel floor or R.C. floor. Back
· Timber floor (Type 1) • For bridges, generally two layers of flanks are provided. For calculating these flanks we assume that the maximum wheel load is distributed over two flanks.
· Reinforced concrete floors (R.C. floor) It may be supported by the main girders only, the X.G. only or by stringers and X. girders. The span of the slab may be 2.5 to 3.5 m, and thickness of slab to be 20 cm nearly. The R.C. slab reinforcement, generally 12 bars are used at least per one meter.
· Wearing surface · The wearing surface for roadway covering consists of timber blocks, hard bricks, asphalt bricks, stone blocks, asphalt or concrete. The choice of material depends on the traffic, the span of bridge, the cost and climate
Side walks and Railings The side walks are placed either inside or outside of the main girder. If they are arranged outside, they must be supported on cantilever brackets situated in the plane of the X.G. so that the –ve bending moment of the bracket is transmitted to the X.G. The floor of these side walks should be a precast R.C. slab (6cm) thick resting on the side walk stringers. The wearing surface is a 2 cm layer of asphalt. In through bridges the curb should be at least 50 cm inside the main girder Back
Hand railings and brackets withstand the effect of a transverse horizontal force of 150 kg/ m’ in cases of Railway bridges, Roadway bridges, and foot bridges, supposed acting at top level of hand rail. This horizontal is transmitted from the hand rail to the main posts and from their connections to the cantilever brackets. Side walks parts 1. Slab Take strip 1 m and statical system as continuous beam supported on side walk stringer (one way slab), take t = 8 cm and get As. The applied loads are considered 500 kg/m2 or one concentrated load 5 t.
1. Side walk stringer Simple beam span distance between two brackets (take channel X. sec.) 2. Hand rail Simple beam span distances between two brackets (take angle or channel X. sec.).
4 Post Cantilever beam (take 2-angle or 2-channel X. sec.).
5 Connections Double shear bolts
6 Bracket Calculate M& N& Q at center of bracket. In case of beam loaded alone we must calculate Fl.t.b and the check that the actual stress Fc is less than the
allowable stress Fp.b. For ST 37 If 100 If 100 = l/ i , where I for compression flange only. · for bracket l/ b 2 l/ b · assume unequal angle 80120 · Bolts subjects to shear
Bolts subjects to tension 0.8 Ft
Bridge Bracings • The bridge is provided with horizontal and vertical bracings:- • 1. The stringers are connected together by stringer bracing given before. • 2. The chords of the main girders are jointed together by an upper and lower horizontal bracing called wind bracings.
These transmit to the bearings of the bridge; • a. The lateral forces due to wind. • b. The lateral shock 6t. • c. Centrifugal force. • 3 - Special horizontal bracings for the braking forces. • 4-Two vertical and transverse bracing called X-frames or portals (in case of through) transmitting reaction of the upper lateral bracing to the bearings of bridge. • 5- Some intermediate vertical transverse bracings called intermediate X-frames or intermediate portals for the rigidity of the structure. • It isn’t necessary to find all these bracings in every bridge, there existence depend upon the type of the bridge, the span and the floor.
I-The Deck Bridge The upper wind bracing transmits the wind pressure WT on the train & WF on the floor & ½ WG on the wind ward side of the main girder.
The lower wind bracing transmits the wind pressure ½ WG on the wind ward side of the main girder. W = 100 kg/ m2 in case of loaded bridge W = 200 kg/ m2 in case of unloaded bridge * The wind pressure WT on the train produces in addition to horizontal loading of the upper wind bracing. a vertical loading, to the main girder.
In case of a truss bridge, only the exposed area of the members is considered. This area is equivalent to 40 % of the hole area of the surface of the truss. In all bridges with an upper and a lower wind bracing, their shall be provided at each end a X-frame to transmit to the bearings,the horizontal reactions of the upper wind bracing. The horizontal reactions of the lower wind bracing are transmitted directly to the bearings
· The end X-frames in deck bridges shall be of rigid type. In all railways and in roadway deck bridges there shall be intermediate transverse bracing at least at every third panel point to increase the stiffness of the bridge. These intermediate X-frames will release the end X-frame from a part of the horizontal reaction of the upper wind bracing. Yet it is recommended not to consider that release unless the bridge as the space structure.
In through bridge two horizontal wind bracings should be arranged if possible. In the plate girder through bridges we can’t arrange an upper wind bracing in the bony truss Roadway Bridge we have only a lower wind bracing which transmits all the wind loads to bearings. The force WF on the floor will be considered to act on a solid surface as the plate girder. The through Railway Bridge shown above is provided with two horizontal wind bracings.
The upper wind bracing transmits the wind pressure ½ WG on the wind ward side of the main girder. The lower wind bracing transmits the wind pressure ½ (WG), WT on the train & WF on the floor & on the wind ward side of the main girder. At the connection of each X-G to the main girder, stiffness bracket shall be arranged. Back
Design of lateral support at top chord of through pony bridge • C = force in flange = AfFt • The U-frame formed by the two vertical stiffeners and the horizontal stiff X-girder is acted upon by a horizontal transverse force = C/100 at the centroid of compression flange as well as the wind pressure between two consequence X-girders.The maximum stressed section is mn. The compression stress at point n ≯ Fltb. If the stress isn’t safe, we either increase the thickness of the bracket plate or add a stiffening angle.
The connection between the X-girder and bracket is designed on the shearing force A that between X-girder and the bracket and horizontal gusset of wind bracing on force B. if the X-G is built up section the bracket connection is designed as a web splice.
Cross Sections for wind Bracing • The diagonal of wind bracing The diagonal of wind bracing shall have stiff section to prevent vibration and to help in reducing the deflection of main girder due to eccentric loading (space frame treatment). The section should have a depth not less than L/40. The recommended sections are given in Fig.(5.).
The choice of the section depends more or less upon the span of the diagonal, the two channel section is convenient for too long spans executed in Banha and Samanood bridges. The two channels are connected together by latticing or batten plates. ≯ in compression & ≯ in tension
In case of one diagonal member only Case of the warren system which designed on a force S;
If the bracing is made up of crossed diagonal and struts, the calculation is made under the assumption that the tension diagonal are only acting. The struts here receive compression force. If the multiple systems of wind bracing a further reduction of 20 % in the allowable stresses given before, shall be made to account for approximation of solution that both systems (tension and compression member) equally share the lateral loads. in case of one angle in compression the allowable compression stress shall be reduced by 40% of Fc.
End X-frame in deck bridges The compression diagonal is assumed in acting and we design the tension diagonal, also we assume that the X-frame is resting at a movable support at one end and the hinge support at other end. Back
Transmission of the braking forces the bearing In Railway bridges especial bracing should be arranged to transmit the longitudinal forces from the stringers to the panel points of the main girder, hence; they are transmitted through the main girder to the hinged bearings. Some times a bracing is arranged at every panel point. But generally two bracings at the quarter points of bridge are sufficient. The braking bracing system shown in sketch is statically indeterminate but the loads are symmetric about perpendicular axes to X-X. Therefore the diagonal B’n & c’n are zeros since they correspond to themselves. Also, the loads are antisymmetric about axes Y-Y and thus members mb’ & mc’ & nb & nc are zeros, If special bracing of the longitudinal forces is omitted, these forces are transmitted
Truss Bridges b L/ 20, b h/ 3 Where, b = bridge width = distance between center lines of two main girders L = span of bridge The depth of trusses shall be chosen in such away that the elastic deflection due to L.L (without dynamic effect) shouldn’t exceed L/800 for Railway bridges and L/600 for Roadway bridges. Back
h≮ (simple ) & (continue) Road. h≮ (simple ) & (continue) Rail.
A. Types of bridge trusses Either both chords are straight and parallel or only one of them. In a through bridge the upper chord may polygonal, in a deck bridge the lower chord may be polygonal. Curved chord should not be used in bridge trusses on a account of the additional bending stresses. The loads are transmitted to the panel point of the truss by a system of stringers and cross girder. No load except the own weight of the truss members should act between the panel point.
1.Trusses with horizontal chords They suitable for span up to 60 m. the joints are simpler than in trusses with polygonal chords. The depth is h L/ 8 for Railway Bridge, or h L/ 10 for Roadway Bridge. For continuous and cantilever trusses the depth may be taken h L/ 10 for Railway bridges, h L/ 12 for Roadway bridges. Some times a greater depth is used to allow an upper wind bracing. The arrangement of web members may be N-system or warren system. The warren system trusses require generally less material than the N-shaped trusses, since the vertical members have smaller forces, the number of joints and changes of cross section in warren system are also less. Shop work for warren trusses will be cheaper.