The diagonals should be at an angle between 50° and 60° to the horizontal. For the lateral bracing of the chords, where a "Saint Andrew's Cross" type system as shown in Figure 8a is adopted, the nodes of the lateral system will coincide with the nodes of the main trusses. The anchorages are also under tension, but since they are held firmly to the earth, the tension they experience is dissipated. The truss bridges consist of a grouping of triangles that are manufactured from straight and steel bars, according to the truss bridge designs. In a simple truss it is common for only some of the diagonals to be influenced by fatigue. The truss as a structural form dates back to Roman times. When a concrete slab is cast in place to support the highway or the railway, the horizontal forces caused by the shrinkage of the concrete should be taken into account in the design of the lower chord connection joints. river (or whatever the obstacle is) and the deck is suspended from these Design principles are presented, e.g. The serviceability check is not required for tension members or for some slender compression members. For trusses with spans greater than about 100m, the chords will usually be box shaped so allowing the ideal disposition of material to be made from both economic and maintenance viewpoints. Turley, S., Savarkar, S.G., Williams, J., Tweed, R.J.C., "Design, Fabrication and Erection of Ganga Bridge, Mokameh, India", Paper 6425, Proc. In the principal buckling mode of a "Diamond" lateral bracing system, one half of the diamonds have all their members in tension (see Figure 9). Avoid eccentricity of loading and connections to reduce secondary stresses. For long and medium spans, the lateral members are frequently made from two rolled channel sections connected by lacing to give an overall depth the same as the chords. At the connections of all tension members and elements, care has to be taken in the arrangement of bolt holes to ensure that the critical net section area of the section is not so small that fracture will govern. On short spans single laterals often suffice. A Beam bridge consists of a rigid horizontal structure (such as a beam) with two supports on the opposite ends. The Historic American Engineering Record illustrates 32 separate bridge truss geometries in its 1976 print shown in Figure 16.1 [1]. 3.3 Geometry. Even where continuous trusses show savings in the use of steel, they may not be economic. Economic design of the top compression chord leads to sub-division with a post. Layfield, P., Taylor, G., McIlroy, P., King, C., Casebourne, M., "Tyne and Wear Metro: Bridge N106 over the River Tyne". Eiffel built lattice trusses in France (Figure 2f). "Sealed" sections should be provided with a drainage hole at the lowest point. distribute its load and dissipate the compional and tensional forces. The web members are then all narrower than the chords and the chord splice is offset from the node. is a suspended roadway, the cables transfer the compression to the towers, which span ranges, span-to-depth ratios and arrangement of diagonals. The effects of the primary axial loads and the secondary moments are combined by the use of suitable interaction formulae. Crevices formed at gussets unless special precautions are taken. For shorter spans the choice is between the Warren and the Pratt configuration. They should be so disposed, with respect to the cross-section of the member, as to transfer the load in proportion to the respective parts of the section (Figure 12b). Generally trusses have stiff joints. For short and medium spans, it will generally be found economic to use parallel chords to keep fabrication and erection costs down. Modern suspension bridges have two tall towers which the cables are Undamaged parts of a truss bridge can easily be re-used after an accident or the effects of war. Birds will nest and roost in the most unlikely places! As with compression members, box sections would be preferable for ease of maintenance but open sections may well prove cheaper. Box sections make painting easier, but rolled hollow sections leave nasty crevices at gusset positions, unless the joints are welded. As a result of the interaction, the lateral system may carry as much as 6% of the total axial load in the chords. b. Eccentric loads not in the plane of the truss, such as loads from cross girders. A check should always be made on the economic depth for a given bridge. Through and semi-through truss bridges are used when the depth of deck construction is very limited, for instance, when a highway or a railway crosses a canal. The spans are usually between 60 and 120m which is the normal economic range. Roberts, G., Kerensky, O.A., "Auckland Harbour Bridge, New Zealand Design", Paper 6528, ICE Proc., Vol 18, April 1961, pp. It should be noted here that the design loading has a considerable effect on the truss configuration. Typically the top chords of the bridge will be in compression. Strung from. Where the spans of railway bridges are long the economic depth is usually great enough to allow bracing to be provided above the loading gauge level. However, these secondary effects are generally insignificant. Different sections for chords and diagonals are shown and discussed together with the connections between the members. In this unit, students will apply their knowledge of transversal/parallel relationships to the construction of a truss bridge. For long bridges continuous trusses may be the economic solution, but remember that least weight of steel does not necessarily mean least cost. However, beam brides are mainly effective for short amount of distances as they are weaker for longer spans of distances. The structure is composed of top and bottom chords triangulated with diagonals and/or verticals in the webs so that each member carries purely axial load. Flexural stresses due to any eccentricity at joints have to be taken into account by sharing the moments due to eccentricity between the members meeting at joints in proportion to their rotational stiffness. When the length of the gap to be crossed makes the use of a multiple span bridge unavoidable, it is cheaper and usually possible to raise the road line and build another type of bridge requiring a greater depth under the deck. These supports, called the abutments, carry the load and keep the ends of the bridge from spreading out. This fact can make welded members more economic, particularly on the longer trusses where the packing operation might add a significant amount to the erection cost.