December 2013
CONSTRUCTION WORLD
70
Professional Services
C
Okavango River Bridge, Namibia
T
he Okavango River bridge is a 150m long three-span incremen-
tally launched bridge constructed in the 1970s to carry one 50
tonne army tank at a time. With the improvement of the road
connecting Walvisbay Harbour to Lusaka and the Zambian copper
belt, traffic over the bridge has increased significantly, especially
heavy vehicles.
Abnormal vehicles could not use the route due to the capacity
limitation of the Okavango Bridge. The bridge has therefore been
widened and strengthened to carry abnormal vehicles and to safely
carry two lanes of traffic as well as pedestrians at the same time.
During the design of the widening and strengthening, various
options were investigated including carrying the pedestrians on
a separate structure, adding piers, extending the abutments and
various forms of widening.
In investigating the capacity of the exisiting deck, it was found
that the stress range in the concrete was within allowable limits if a
concrete overly was used to widen the deck to two lanes. However, it
was found that the shear and bearings were inadequate. Fortunately
the substructure could carry the increased loads.
Additional prestressingwas required to carry the heavier abnormal
loads. These were added to the structure as external prestressing,
all within the box of the box-girder deck. These were anchored via
concrete blocks inside the box.To transfer loads from the prestressing
to the concrete blocks and into the deck cross-section, shear action
and dowel action was used.
The normal component for shear was achieved by stressing the
blocks to the box-section using Dywidag bars. The shear capacity of
the webs was increased by installing vertical Dywidag bars inside the
box immediately next to the webs.
The design had to include staged construction as the bridge had
to widening while under full traffic. This was a particular challenge as
the bridge width before widening was 5,5 m, leaving some 2,5 m to
widen the structure and 3 m for traffic and pedestrians.
Project information
• Name of project: Okavango Bridge Project
• Company entering: SMEC SA
• Project start date: August 2009
• Project end date: May 2012
• Client: Namibian Roads agency
• Consulting engineer: SMEC SA
