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 Aframe 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 Aframe 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 Aframe'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 Aframe 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 Aframe models shown in Fig. 157
below 




a) Large triangle 
b) Right triangle 
c) Isosceles triangle 
c) Small triangle 
Aframe (LT) 
Aframe (RT) 
Aframe (IT) 
Aframe (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 Aframe trusses with different H/S
ratios (demonstration models) 

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 Building stability  Aframe structure 

