Keywords: Parabolic tied arch, structural concepts, induced flexion, alternatives, geometric line, pressure line, horizontal force, efficient behavior, tensor

An almost 100-years-old three-spanned truss bridge located on Carcarañá River, near Inriville, Córdoba, Argentina, collapsed while a heavy vehicle was passing through it. A new structure had to be built. Many alternatives were evaluated. The structural concepts evaluated during the study of a bowstring arch bridge alternative, which would replace the old structure, are detailed in this paper.

It was carried out a careful hydraulic study in order to find the waterway governing parameters. As a result the date and value of the maximum discharge, were found. Regarding scour, they were compared, the present time and the hundred-years-ago sections, to find that the riverbed had good resistance. Piers were 100-years-old masonry ones, and were constricting the waterway opening. So to the present alternative, the piers idea was abandoned. One span bridge would be less problematic.

Concerning foundations it has to be pointed out that the soil conditions in the site, made it not viable to receive the horizontal forces of an arch bridge. In fact, the soil is a hardpan, a soft rock derived from the carbonates from the water and the hard clay from the riverbed, which behaves very well with compression loads, but does not with shearing stresses. This led to a tied arch bridge.

Due to this situation, it was projected a tensor, in order to receive the horizontal force from the arch, and to serve as support to the transverse truss beams. There are two tensors, one for each arch. The tension in the tensor, which is the horizontal force recycled, results useful in the roadway, otherwise it would be conducted to the abutments.

The principal bridge dimensions were deduced from the governing factors, which were: traffic and hydraulic needs.

At this stage loads were estimated. These included live loads and an estimation of the dead weight of the structure. To begin with, it was taken the simplest arch configuration, the circular arch. Despite the inconvenience of this choice it was studied to have the very first approach, rather than to adopt it. Some remarkable conclusions came.

Fig. 1. Arch bridge configuration

With the arch shape and the adopted loads it was possible to get the flexor moment diagrams, and through them, the preliminary bridge dimensions. As the cross sections were dimensioned, and the most important unions sketched, it were obtained the first performance results. It was now time to visualize how the loads traveled through the structure to the soil. Some messages came from the results since it was possible to see where a stress concentration was occurring, where the loads preferred certain path to another, which was projected for them to go, overloading part of the structure, etc.

It was found that the stresses were unequally distributed among the chords. The upper chord carried almost all the loads and then they wholly passed to the ends. It means that the lower chord was not working at all but just in the ends close to the abutments. This would lead to over dimensioning. Some measures were proposed.

Another pressure line (i.e. the locus where the loads travel through from application point to foundations) was chosen, a parabolic one. It was pursued one that would correspond to most of the load states, which in this stage were only from the adopted loads. Several configurations were tried with the well-known funicular polygon in order to get the best possible load distribution between the chords.

The pressure line shape, influences the chords shape. The better the chords follow the line shape, the better the load distributes itself on them, without overloading ones in detriment of the others.

The 3D analysis software was reloaded with the new information. For this first attempt it was found that the deflection at the crown was inadmissible. The reasons for such a behavior were found; it is known that the loads travel through the pressure line, which ideally must coincide with the geometric line. In separating these lines, it is generated a moment given by the load value times the separation of the lines.

In other words, it is produced a flexion, but as the system is a truss, they became axial stresses. Theses stresses were the cause of the bars’ shortening, and consequently, the bridge deflection.  Some successful actions were taken. The results were very good. 
Some another aspects with reference to the pressure line are emphasized.

Conclusion