A distinctive new bridge in Croatia is designed to stimulate development in a run-down area of the capital city. Dave Parker reports.
Zagreb has never been a river city. The town was built on the slopes of the Medvenice Mountain, well away from the floodplain of the unpredictable River Sava. Even in modern times, when Zagreb has expanded south of the Sava, river crossings have remained relatively few.
So construction of the Domovinski Most, or 'homeland bridge', to the south east is of major significance. It will provide a new access point into the city and, the authorities hope, stimulate industrial and residential development in the area.
The far-sighted decision to build the crossing arose from a more pressing need, as Croatian Institute for Bridge & Structural Engineering chief designer Rajka Veverka explains. 'A new wastewater treatment plant for Zagreb was planned for the far side of the river. Wastewater had to get there from the city centre.
Engineers decided they would rather have the pipes above the river, where they could get at them in case of problems, than buried underneath it.'
A bridge just to carry wastewater would have been something of an extravagance. So it was decided that ultimately the new crossing should be capable of carrying road, light rail and pedestrian traffic as well. Consideration was even given to building the bridge in phases, with the first phase being a simple box girder containing the wastewater pipes and carrying two lanes of road traffic on top.
'Wings supported on steel props would be added each side to carry vehicles, then water mains would be hung on the outside of the box, ' says Veverka.
'In the end it was decided to build the whole cross-section at once, for the sake of long term durability.'
An unusually wide 34m crosssection was the result. The 840m length was determined by a number of factors, not least the maximum gradient climbing capacity of tramcars. On the north bank the abutment had to be set far enough back to allow the construction of a new flood relief channel sometime in the future.
Verveka's original competitionwinning design had some very unusual features. But low piers on the approach spans and the excellent ground conditions on the floodplain meant that span by span insitu construction off falsework was the obvious construction method.
With spans of up to 72m and plans to work on a span on each bank simultaneously, a lot of falsework was needed. Falsework supplier SGB design engineer Evzen Haluzik says the priority was to reduce the number of purpose made components to the minimum.
'Basically we designed Cuplock towers using standard components and linked them together with conventional tube and couplers, ' he explains. 'This gave the contractor plenty of tolerance and buildability, and made it easy for them to move on to spans of different length.'
Myriad precast concrete pad foundations support the falsework. Phenolic-coated plywood makes up the actual shuttering.
The C45 concrete in each span will be placed in three 'layers':
first the bottom slab and its tapering wings, then the vertical internal walls and diaphragms, followed by the top deck itself.
Typical pours will be in excess of 1,000m 3, to be placed non-stop over 48 hours.
After months of snow, progress is picking up as the weather improves. The first pour is due and a new landmark for the Croatian capital will soon make its mark.