A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
by welding channels or angles for enclosure of stain gages.
Strain gages should always be installed in pairs located back to back on
the same piece of steel. For instance they may be placed back to back on
either side of web of H-piles. Only one gage mounted on the cross section
of H-pile is not too useful because it may be affected by an unknown
degree by bending moment. Hence, the results of axial load may appear to
be doubtful.
25. There is an old rule that “the area of a follower should be one-fifth
of precast concrete pile”. Why?
The rules of wave mechanics suggested that to avoid reflection of stress
wave caused by different impedance values, acoustic impedance should
be the same for the follower and precast concrete piles. As such, it
enhances smoothest driving and prevent follower from bouncing on the
head of piles which is undesirable as it may damage the piles and lowers
the efficiency of driving.
Acoustic impedance = Elastic Modulus (E) x Area of Pile / wave velocity
Wave velocity (c) = Elastic Modulus / Density of Pile
For normal steel, E=205GPa, c=5,100m/s
For normal concrete, E=30GPa, c=3,800m/s
For same acoustic impedance,
Area of concrete/Area of steel = (205/5,100)/(30/3,800) = 5
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