-9
-
Note that the total strain, at any point in the cross section, is the algebraic sum of the
bending strains and the axial strain. It will be seen that the strains in the outer corners of the
flange can be a lot higher than the strains measured on the web and that failure of the
section can be initiated at these points, hence the importance of analyzing the bending
moments.
The above consideration would also seem to lead to the conclusion that, from the point of
view of obtaining the best measure of the maximum strains, the best location for the strain
gages would be on the outer corners of the flanges as shown in Figure 6B. The
disadvantage of having the gages located here lies in the difficulty of protecting the gages
and cables from accidental damage. But a much more
serious problem
can arise from the
fact that
each of the 4 gages can be subjected to localized bending forces which affect
only one gage, but not the others.
It is not uncommon for welding to take place at points
close to a strain gage and often this produces large strain changes in any strain gage close
by. Also it is not uncommon for local blocking (e.g. tunnel arch supports) and the addition of
struts to cause strain changes on only a single gage close by.
It is always best to locate gages in pairs, one on either side of the neutral axis of the
part of the I beam to which the gage is attached
. This is why the configuration of 6A is
preferable. There is the added advantage that gages located on the web, as shown in
Figure 6A, are much easier to protect.
Figure 6B - Strain Gages Mounted on Flanges
(Not Recommended on Tunnel Arches)
If, for reasons of economy, it is decided that only two strain gages per cross-section are to
be used, then the configuration of figure 6C will give the axial strains and the bending
moment around the minor YY axis only.
Figure 6C - Axial Strain Measurement and Bending Moment about YY axis only.
A XIS YY
AX IS X X
2 a
2 b
1
2
c
d
3
4
A XIS YY
AX IS X X
2 a
2 b
1
2
d
