Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 353

Minimizing errors in the measurement systems should be taken in the first
place whenever and wherever a structural behavior is measured. Reliable
field measurement methods or technologies are critical in control analyses.
In cases where a strain gage is used to measure stresses, nonglobal stress-
related strain such as temperature changes and creep and shrinkage strains
should be carefully investigated. Multiple strain gages usually are needed
to measure stress at one point, and its configuration should be studied
based on location. In the control of Yamen Bridge (a PC box girder cable-
stayed bridge with a main span of 338 m [1109′] and a single stay plane
in Guangdong, People's Republic of China), the girder stress at the neutral
axis can be simply derived from cable forces to calibrate the stress mea-
surements and to identify the strains due to creep and shrinkage.

11.3.2 Measurement of cable forces
Among the structural responses that determine the state after erection,
cable forces are the most important measurement, and obtaining them
is a relatively simple and reliable process. The fundamental frequency
method, which is fast and accurate, is widely used to measure the tension
force of cables. By collecting random vibration signals of cables under ambi-
ent excitation, the fundamental frequency f can be obtained by time- and
frequency-domain analyzers. The string vibration equation 11.13 can be
used to calculate the cable force:

T mLf

2

= 4 (11.13)

where:
f is the fundamental frequency of cable, in Hz
L is the length of cable, in m
m is the mass of cable, in kg
T is the tension force, in N

11.3.3 Construction errors
Construction errors, which may cause incorrect assumptions in structural
analyses, are due to the quality control of construction and may include the
following features:

1. Material properties such as errors in Young's modulus, temperature
expansion factor, and material densities

2. Sectional properties such as errors in girder dimensions due to instal-
lation or formwork deformations

3. Temporary construction loads

4. Creep and shrinkage properties for a concrete cable-stayed bridge,
and so on