132 Vincent Street
PO Box 6345, Auckland 1141, New Zealand
T: +64 9 300 9000 // F: +64 9 300 9300
E: [email address] // www.beca.com
NZ Transport Agency
8 September 2009
PO Box 1459
Shortland Street
Auckland 1140
Attention: Tommy Parker
Dear Tommy
AHB Cycleway - Holmes Submission
As requested we have briefly reviewed the above submission, in conjunction with Hyder (UK.) and
have the fol owing comments.
The proposal is based on the concept of transferring peak live loads in the main navigation span
from the “overutilised” extension bridges to the “underutilised” truss bridge. To facilitate this
redistribution of loading it is proposed that two large diaphragm beams be constructed between the
east and west extension bridges, and passing beneath the truss bridge. The loads would be
transferred to the truss bridge through a “dynamic smart system”. This is understood to be an
active control system which transfers peak loads away from the extension bridge to the truss bridge.
The feasibility of this proposal depends on a number of significant issues which are discussed
below.
Firstly there needs to be sufficient reserve load capacity available in the truss bridge to cater for the
additional loading proposed to be transferred from the extension bridges. A recent study1 indicated
that the truss bridge has sufficient reserve load capacity to cater for future traffic load growth for the
next 10 to 20 years, and that further but limited strengthening of the bridge is possible. Given the
timeframe noted above, this additional strengthening of the truss bridge would likely be required
before the next harbour crossing is operational. It could be required earlier if legislation leading to
higher vehicle mass and load intensity comes into force.
The truss bridge and box girder bridges will require significant local strengthening to al ow the
transfer of load from the box girders to the truss bridge. The two diaphragm beams are large
structural members in their own right. The combined weight of the diaphragm beams, local
strengthening of three bridges and the load transfer system itself will place significant additional
load demand on the truss bridge, thus reducing its available capacity for live load.
Further strengthening is possible as noted above, but because this is limited then inevitably the
future capacity available for long term traffic growth wil be reduced. If as a result the truss bridge
capacity is insufficient to sustain the maximum expected traffic load growth until the next harbour
crossing is opened then this would be unacceptable. In principle a scheme which reduces the
available future level of service of the strategic truss bridge asset is very undesirable.
Our assessment is that there is a significant risk the proposal wil reduce the future load capacity of
the truss bridge to a level which is strategically unacceptable to NZTA.
1 “Assessment of Effect of Future Traffic Load Growth on AHB”, Beca, August 2009.
Our Ref: 3910504
NZ1-2112057-9 0.9
Page 2
8 September 2009
Secondly, the feasibility of the active control system needs to be established. Such a system will
include computer controlled active systems including a variety of mechanical and electrical
components. For such a system there are issues of reliability and a full failure mode and effect
analysis would need to be undertaken to determine the attendant risks, warning systems and
backup needed to ensure the bridge safety is never compromised. It can be expected that
extensive monitoring and safety systems will be needed and in the event the failure mode analysis
does not show a satisfactory solution is available, the system would not be feasible.
We are not aware of any similar systems on operational bridges and have asked Hyder (UK) to
comment based on their experience. They are not aware of any directly equivalent system but
noted two examples of simpler control systems. The most relevant example was a temporary
propping system devised by Hyder for the Avonmouth Bridge to support the approach spans of the
steel box girder bridge during the bridge strengthening. These could not be rigid as they would
have attracted too much load, and hence each was supported on a hydraulic ram connected to a
hydraulic accumulator, thus allowing the bridge to deflect under live load whilst providing a constant
propping load. As the loads were transferred by props directly to the ground and were not varied it
is very much simpler than the AHB scenarios proposed.
Finally, due to their required size the diaphragm beams will reduce the navigation clearance under
the main navigation span and will clash with the utilities carried by the bridge. They will also have
an effect on bridge aesthetics. These issues are however less significant than those noted above.
In summary, the proposed concept would reduce the future load capacity available from the truss
bridge and would introduce a complex actively controlled load transfer system into an already
complex structure. The long term impacts of the loss in truss bridge capacity, and the risks
introduced by the active load control system, would both need to be evaluated to determine the
feasibility of the proposal. The system would have a demanding maintenance and monitoring
regime and any evaluation would need to consider both the short term and long term costs which
are likely to be quite high.
Note that the additional costs of the proposed load transfer system would be additional to the costs
already identified for the cycleway/walkway.
Should you require any further clarification, please contact the writer.
Yours sincerely
s 9(2)(a)
on behalf of
Beca Infrastructure Ltd
s 9(2)(a)
Our Ref: 3910504
NZ1-2112057-9 0.9
