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beam, or chord, resist this. The vertical tie strut is usually a
steel rod which has a very high |
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tensile strength to weight ratio. The legs of the A-frame are solid
wooden beams which |
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have high compressive strength. Thus the Kingpost design makes very
efficient use of the |
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strengths of its structural members. |
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The strength of the triangular A-frame truss design depends on the
height, |
 |
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H,
of its apex in relation to the bridge's span, S. The larger
the H/S |
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ratio the greater the A-frame's inherent strength. In the
figure to the right |
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members that experience compressive stresses are colored red,
tension |
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blue. Fig 156 graphs the maximum load vs. H/S ratio determined by |
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destructive testing of four model A-frame structures made from
various |
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combinations of Polymorf small square (SS) and rectangular (REC)
panels. |
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. |
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 |
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. |
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Fig.
156 - Graph of load, L, vs. H/S ratio of A-frame models shown in Fig. 157
below |
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a) Large triangle |
b) Right triangle |
c) Isosceles triangle |
c) Small triangle |
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A-frame (LT) |
A-frame (RT) |
A-frame (IT) |
A-frame (ST) |
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(all REC) |
(2 SS, 1 REC) |
(2 REC, 1 SS) |
(all ST) |
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. |
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H/S ratio = .89 |
H/S ratio = .5 |
H/S ratio = 1.31 |
H/S ratio = .86 |
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Loadmax
= 6176 g |
Loadmax
= 3846 g |
Loadmax
= 9274 g |
Loadmax
= 5462 g |
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. |
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Fig. 157 - Maximum loads for A-frame trusses with different H/S
ratios (demonstration models) |
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Back to
Knowhere |
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Page 101
- Building stability - A-frame structure |
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