Civil Engineering Reference
In-Depth Information
with the formwork during production. Finally, a third problem concerned the
occurrence of cracks in the base frame of the formwork which as a result
became less stiff and slightly deformed.
The 11 mm problem
The 11 mm problem gets its name from the fact that the combined length of
the B, C and K stones was short by 11 mm around the circumference: the B and
C stones were each 4 mm too short, whereas the keystone was 3 mm too
short. Since for each type of segment special moulds were used, it was clear
that the respective formwork did not have the correct dimensions.
How was it possible that by using a checking system, intended to record
deviations of tenths of millimetres, could pass over an error of 11 mm? The
answer is just as simple as it is humiliating: the system did not notice the
error because it was designed on the basis of the same (incorrect) informa-
tion that also lay at the foundation of the design of the formwork.The meas-
uring system in other words had exactly the same error as the formwork
with which the segments deviated from the design.
As soon as the formwork and the checking system were ready, it could not
be concluded otherwise than that the formwork was good and that the
measuring system also worked.
This meant that segment production could start in August 1998 with the form-
work and a measuring system both of which were designed on the basis
of the same incorrect interpretation of the drawings of the segments, … a
fundamental error: the measuring system and the formwork should have
been developed separately.
Fig. 9.14
Checking the
dimensions
Deformation of the longitudinal joints
A large number of segments appeared to be curved at the longitudinal
joints. During the construction of the rings, this could result in cracks form-
ing in the concrete because undesirable concentrations of forces occur
when the joints are not optimally flat. The curving was the result of tighten-
ing a bolt connection too much when closing the formwork. The formwork
Search WWH ::
Custom Search