The
graph shows there is a direct, linear relationship between the maximum
load applied |
to the
apex of the A-frame models and their H/S ratios. This is due to the
fact that the |
greater
the H/S ratio the more parallel the legs of the A-frame are to the
downward vertical |
force of
the load and the upward force of the reactions at the base. And the
more the |
legs are
aligned with these forces the less compressive stresses are induced in the
legs |
and the
less tensile stresses are induced in the base of the A-frame. Thus
the tall, narrow |
IT
A-frame model carried a heavier maximum load at the point of failure
than the shorter, |
wider RT
model. The weakest links in the structure were the joints joining
the legs to the |
base which failed
due to excess tensile stresses. The following figure shows the
estimated |
compressive
(red) and tensile (blue) stresses induced in the RT and IT models by the
actual |
test loads. Notice that although the IT A-frame withstood a
maximum load that was nearly |
 |
 |
|
|
Figure 158 |
Estimated compressive |
and tensile stresses in |
A-frame models with |
different H/S ratios |
|
|
|
two and one-half times the load of the RT structure, the estimated
tensile stresses |
experienced by their base members were about the same. This
verifies the destructive |
testing results of the models. |
|
Note: Gaining a deeper understanding of the forgoing empirical
observations involves |
|
doing a
static analysis of the A-frame structure. This has to do with how forces
and reactions |
are
quantified and added together as vectors. You are encouraged to
click here to see how |
|
. |
Exercise: 1) Construct four A-frame models like those shown in
Fig. 157. Examine their |
|
load bearing characteristics when a) the base tie member is a rubber
band, |
|
and b) when the base is a panel. Push down on the apex to test their
strength. |
|
2) Do destructive testing of the models built in 1b).
Graph and compare your |
|
|
3) Advanced - Test the holding strength of the pinge joint
used in the A-frame |
|
models. Use this value to predict the maximum load the models
should be |
|
able to bear based on a static analysis. Compare your
results to the test data. |
|
4) Advanced - Using the load test results from 2)
compute the maximum tensile |
|
stress experienced by the base member at the point of failure using
static |
|
analysis. Compare this value with the actual test data of 3). |
|
. |
Since
longer spans require the A-frame truss of the Kingpost bridge to be
taller, there is a |
limit to
the distance it can span before the legs of the truss get too tall, heavy,
and costly. |
. |
Back to
Knowhere |
 |
Page 102
- Building stability - A-frame structure |
 |
|