While the slightly skewed alignment arises from the structure’s urban situation, the asymmetric division of the bridge into two unequal length segments of 70 m and 100 m is in response to the uneven disposition of the tracks below.
Both bridge segments are structurally similar, lenticular space trusses and a centrally positioned walkway. The segments have different cross sections because of their unequal spans, but care was taken to minimise the effect of this on the striking overall impression created by the bridge’s continuously curving contours.
Emblematic steel structure
A system of laterally inclined planar trusses surrounds the walkway of the Be’er Sheva bridge. The trusses of the two unequal-length main spans are not of constant height, varying from 0.6 m at the ends to 7.5 m and 11 m at mid-span respectively. Because of their characteristic appearance, these types of structures are called lenticular trusses. They are very efficient for simply supported girders of this size. Despite the great structurally effective depth at mid-span, where the maximum bending moment occurs, they nevertheless possess a visually appealing height-length ratio.
Structural system for static effects
The Be’er Sheva bridge is defined by three static systems, corresponding with the three main loading axes. In the vertical direction, the total load is composed of self-weight and live load in a system comprising two independent single-span space trusses resting on hinged bearings. In spite of the continuous truss cords over the middle pier, the spans themselves can be considered as independent because the small cross-section at the pier is flexible in terms of its bending resistance, while the great structural effective depth at mid-span provides much higher bending stiffness. Therefore the support moment and rotational deformation of the middle pier are reduced to a minimum and can be disregarded. The largest loads in the longitudinal direction are caused by thermal expansion of the bridge. Therefore a statically-determinate system was selected so that these deformations do not cause additional imposed stresses. Both spans are supported at the bridge ends on steel pinned bearings at the base of the end supports designed to allow longitudinal movement. The middle pier is built into its foundation to provide the necessary longitudinal stiffness. Transverse forces from the wind, seismic loads and horizontal live loads are carried by the whole bridge acting as a continuous girder on three separate hinged bearings.
Dynamic effects
Long-span steel bridges tend to have a low natural frequency and can easily resonate in response to the footfall frequency of the crossing pedestrians. During the design process, the dynamic load behaviour was extensively analysed in cooperation with Prof. Izhak Sheinman at Technion, the Israel Institute of Technology in Haifa. To reduce the cost of using vibration dampers, the bridge deck was additionally fixed to the supports of the northern escalator. A friction bearing was designed for this task. This bearing resists the relatively small dynamic forces but behaves as a sliding bearing to allow slow thermal expansion movements.
Production and construction
The varying geometry of the complex space truss, with different cross sections and distances between the main chords, the changing lengths and connection angles of the cross beams preclude any kind of standardised element. However, the early design of a 3D model allowed the form-finding process to be optimised based on the structural engineering calculations and later permitted paperless, smooth working with the steelwork contractor. The individual components were prefabricated in the factory and preassembled close to the erection site into five segments. After constructing the middle pier and the two end supports, the smaller 230 tonne then the larger 430 tonne segment of the bridge were lifted into position and welded together. This overnight operation interrupted rail traffic for less than 24 hours. Following the primary structural elements, the bridge finishings were added, elevators, escalators, walkway lighting, landscaping etc. The bridge was officially opened in January 2016.
