beam, or chord, resist this.  The vertical tie strut is usually a steel rod which has a very high
tensile strength to weight ratio.  The legs of the A-frame are solid wooden beams which
have high compressive strength.  Thus the Kingpost design makes very efficient use of the
strengths of its structural members.


The strength of the triangular A-frame truss design depends on the height,

H, of its apex in relation to the bridge's span, S.  The larger the H/S

ratio the greater the A-frame's inherent strength.  In the figure to the right

members that experience compressive stresses are colored red, tension

blue.  Fig 156 graphs the maximum load vs. H/S ratio determined by

destructive testing of four model A-frame structures made from various

combinations of Polymorf small square (SS) and rectangular (REC) panels.



Fig. 156 - Graph of load, L, vs. H/S ratio of A-frame models shown in Fig. 157 below

a) Large triangle

b)  Right triangle

c) Isosceles triangle

c) Small triangle

A-frame (LT)

A-frame (RT)

A-frame (IT)

A-frame (ST)

(all REC)

(2 SS, 1 REC)

(2 REC, 1 SS)

(all ST)


H/S ratio = .89

H/S ratio = .5

H/S ratio = 1.31

H/S ratio = .86

Loadmax = 6176 g

Loadmax = 3846 g

Loadmax = 9274 g

Loadmax = 5462 g


Fig. 157 - Maximum loads for A-frame trusses with different H/S ratios (demonstration models)

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Page 101 - Building stability - A-frame structure

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