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5.
GAUGE LOCATION
5.1 END EFFECTS
To avoid end effects, strain gauges should be placed away from the ends of
struts where they may be influenced by localized clamping or bolting distortions.
For most structural members a distance of five feet is sufficient. Alternatively,
end effects may be of some interest because they add to the load-induced
effects, and may be large enough to initiate failure at the ends of the structural
member, rather than in the middle.
5.2 WELDING EFFECTS
Arc welding close to the gauges can cause very large localized strains in the
steel member. Welding studs onto soldier piles to support lagging, shotcrete
reinforcing mesh, etc., can cause big strain changes. This is also true of welding
cover plates, protective channels, etc., over the gauges and cables. Always take
gauge readings before and after any arc welding on the steel structure so that
corrections can be applied to any apparent strain shifts.
5.3 BENDING MOMENTS
In the case of a steel structure, a strain gauge measures the strain at one point
on the surface, and this would be sufficient if it could be guaranteed that no
bending was occurring in the member. In practice, this will only occur near the
center of long thin members subjected to tensile loads. Elsewhere, bending
moments are the rule rather than the exception, and there will be a neutral
axis around which bending takes place.
Since bending effects must be taken into account, more than one strain gauge is
required at each cross section of the structural member. For a complete analysis
at least three gauges are required, and very often more than that are needed. On
a circular pipe strut, three gauges spaced 120 degrees apart around the
periphery of the strut would suffice (see Appendix F). On an H pile or I-beam, at
least four strain gauges would be required. On sheet piling, two gauges back to
back on either side of the pile would be sufficient. Where a member is subjected
to bending and only the front surface is accessible, e.g., a steel tunnel lining
or the outside of sheet pilings, the bending moments can be measured by
installing two vibrating wire gauges at different distances from the neutral axis
(see Appendix H).
Consider the example of an I-beam, as shown in the figure below.
20:
FIGURE 20: Strain Gauges Mounted on Central Web
16 | GAUGE LOCATION | GEOKON
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