OBSERVATION REGARDING THE BEHAVIOUR OF EXISTING STEEL TRUSS GIRDER BRIDGES WITH LARGE SPANS

Luiza Toduţi Ioan Badea Edward Petzek Radu Băncilă UDK: 624,3 DOI:10.14415/konferencijaGFS2017.002 Summary: At the end of the 19th and the beginning of the 20th century many of steel truss girder bridges – for large spans, were conceived as semi parabolic truss girders. Some of these structures are still in operation on the railway network, or isolated as highway bridges. The live loads increased in time; for the railway bridges in present the UIC 71 convoy and for highway bridges the EC 1 loads are required. The ratio between the dead load (which remained the same) and the live load (which increased), can lead to some interesting observations, regarding the stresses in some bridge elements. This situation can be observed in many existing bridges situated in South – Est of Europe.


PARABOLIC TRUSS GIRDER BRIDGES, WITNESSES OF THE PAST -INTRODUCTION
Parabolic or semi-parabolic (half-parabolic) truss girder bridges were used in the majority of cases at the end of the 19 th century and the beginning of the 20 th [1].Due to the classical advantages of these structures, they covered a large domain, from spans of 30-100 m.For larger spans, in these time cantilever truss girder were chosen.The general form of these structure is presented in Figure 1; the upper (lower) chord is curved, the diagonals are descending ones (tensioned).
Compared to a truss with parallel chords, there is an increase in the fabrication cost, but for medium and large spans the additional cost may be balanced by saving in material.
The aesthetic appereance of these structure is pleasant, some of the parabolic truss girder bridges, are monuments of the engineering art., was calculated and represented in Fig. 2. In this way the present situation of the bridge can be easy appreciated [2].

STATICALLY BEHAVIOUR OF PARABOLIC TRUSS GIRDER BRIDGES [3]
In Figure 3 a simple parabolic truss girder is presented.Usually M 4 > M 3 > 0; the sign of the effort in the diagonal bars depends on the variation of bending moment M and the form of the girder.Generally (1): Where M i and M i+1 are the bending moments of similar simple supported girder.
For a girder with parallel chords (Figure 4) the efforts in the diagonals are: sin ∝ With: gthe dead load and pthe live load (convoy), the efforts in the diagonals are: The aria of the influence line for D i : and for D i+1 : The result gives the unusual form for the main truss girder presented in Figure 5.For the aesthetic reasons in the central panels of the truss girder parallel chords are disposed.In the German literature this structure are called "Schwedler trusses".For large spans it is possible that the central descending diagonals are compressed; for this reason counter diagonals are recommended, disturbing the aspect of the structure (

CASE STUDIES
The first analyzed structure is the railway bridge in Mehadia (main line from Timisoara to Bucharest), erected in Figure 7.

Figure 7. The Mehadia bridge during a test realized in 1987
The structure is a half parabolic truss girder with a large span of L=101,76 m.The central diagonal of the bridge (Figure 8) under the self weight and the UIC 71 Convoy is compressed.With a very high slenderness it is obviously that the diagonal does not resist in compression.An analysis performed with rigid joints, leads to a similar result.If the structure resists, it is due to the redistribution of the efforts on the level of deck network girders.Generally, after 1919, these type of structures were executed with counter diagonals (Figure 9)

Figure 1 .
Figure 1.Different formsA large number of these structures, having an age of 100 or even more years, are still in use on the main railway lines and as isolated structures on the highway network.

Figure 2 .
Figure 2. Historical convoys action on railway bridges related to UIC71 load model.For a rapid evaluation of the carrying capacity of the structure the ratio

Figure 4 .
Figure 4. Truss girder with parallel chords.The load evaluation was performed for two spans: L = 50 m and L = 100 m.For the dead load g, the expression[1]:  = 26 + 1700, was used.The UIC71 convoy was considered for the live loads p, with a dynamic coefficient of 1,2.

Figure 9 .
Figure 9. Railway bridge with counted diagonalFor highway bridges the situation is more favorable.The possibility of compression efforts in the central diagonal can be avoided by providing a stronger diagonal.In Figure10the bridge in Bocsig is presented; the cross section of the central diagonal is made up by four angles.