Deflections from Strain Gauge Data

191 where a, b, c, d, e, and f are constants. This polynomial function is used to fit discrete curvature data points from each pair of strain gauges along the shaft depth, by using the least-squares method. The deflections (y) are found by double integration of the fitted curvature. Two boundary conditions are needed to obtain the two constants in the double integration. Yang (2005) and Dunnavant (1986) describe the following boundary conditions that can be applied
1. y
0
, y
tip
= 0 fora long drilled shaft (i.e., length to diameter ratio, L/D, ≥ 10)
2. y
0
, y
fixity
= 0 fora short shaft (i.e., L/D < 10)
Where:
y
0
= Measured deflection at the ground line.
y
tip
= Deflection at the drilled shaft tip.
y
fixity
= Deflection at a fixity point, defined where the deflection is approximately zero from inclinometer data. When the boundary condition y
fixity
= 0 is not available, then the condition θ
0
can be used instead fora short drilled shaft, where θ
0
is the measured shaft tilt at the ground line. This demonstrates why it is important to measure the deflection of the head of the pile or drilled shaft with dial gauges and/or LVDTs bearing against a reference beam. Using two gauge positions, one set vertically above the other at the head of the element provides a means to measure the tilt at the ground line. The importance of including an inclinometer or SAA in the test setup is also apparent from study of these boundary conditions, because it may enable the depth of a fixity point to be defined.

Download 6.03 Mb.

Share with your friends: