The accommodate all six degrees of freedom, three
Posted On May 25, 2019
The right hand seaward bearing was removed by cutting out a section of
the supporting platform. The shore bearings were removed by cutting out a
section of the structure. Structural and metallurgical investigations were
carried out at the health and safety laboratories in Sheffield.
The dominant cause of the collapse quickly emerged from the design
calculations supplied to HSE by FKAB. It was thought that the resultant vertical
and horizontal forces applied to the bearing were at the centre of the bearing
pads. In reality, the reactions were at the outer edges of the pads, and the
bending moment on the axle at its connection to the disc would have been
calculated accordingly. This would have caused the axel connection to be hugely
overstressed. With regards to the welds the designer failed to carry out any calculations
and or fatigue assessments. Figure 3 shows where the
moment is calculated.
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It was thought that the total vertical load of the
structure would have been shared equally by all four feet. as the walkway was
supported on a moving pontoon at one end it was necessary for the design to accommodate
all six degrees of freedom, three translations and three rotations. However, the
body roll of the pontoon was overlooked causing the design to only actually address
five degrees of freedom. Instead the designer assumed without any apparent
checking that the torsional stiffness of the walkway was low enough that it
would be able to flex to accommodate the movement in the pontoon whilst
remaining equally loaded on all four of the feet. contrary to their assumptions
the framework of the walkway way actually so stiff that the amount of movement
was limited. This caused one or move of the four feet to lose contact with the
structure when the pontoon would roll longitudinally. Instead of the designers
assuming that all four of the feet would constantly be in contact with the
structure their assumptions should have included that two of the feet would
lose contact with the supports meaning only two feet would then have been
supported as the pontoon moved.
At one end the walkway was supported on a pontoon witch it shared with
the vehicle bridge. When vehicles passed over the bridge the pontoon visibly
moved as well as moving because of the tide and waves in the harbour. Therefor
it was assessed by HSE that fatigue loading was inevitable. Further checks
carried out by HSE found that the lifetime of the stub axel design present was
more likely to be months rather than years.
The findings of the health and safety laboratories found that the
calculations made by FKAB were very basic and with regard to the bending
stresses of the structure some parameters were fundamentally wrong. The states
that the use of realistic parameters in the calculations would have resulted in
values of stress in excess of normal design criteria. As a result of these
findings and other HSE investigations four companies were brought to trial by
the UK high court.