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4. GAGE LOCATION
4.1. End Effects
If end effects are to be avoided then strain gages 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 5 feet is sufficient. On the other hand, 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 rather than in the middle of the structural member.
4.2. Welding Effects
Arc welding close to the gages can cause very large localized strains in the steel member.
Thus, welding studs onto soldier piles to support lagging, or shotcrete reinforcing mesh, etc.,
can cause big strain changes as can welding cover plates or protective channels, etc., over
the gages and cables. Always take gage readings before and after any arc welding on the
steel structure so that corrections can be applied to any apparent strain shifts
.
4.3 Bending Moments
In the case of a steel structure, a strain gage 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 then more than one strain gage is
required at each cross section of the structural member, and for a complete analysis at least
three gages are required and very often more. On a
circular pipe strut
three gages
spaced 120o apart around the periphery of the strut would suffice.(
See Appendix G
). On an
H pile or I beam
at least four strain gages would be called for, and on sheet piling two
gages back to back on either side of the pile would be required. (Where a member is
subjected to bending and
only the front surface is accessible,
for instance, a steel tunnel
lining or the outside of sheet pilings, the bending moments can be measured by installing
two vibrating wire gages at different distances from the neutral axis). (
See Appendix H
).
