148 THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 
The wire will elongate an amount Axc cos@ and hence: 
Fra AeGSOSIO TE 
Zsec ¢ 
This produces a horizontal pull at C,: 
3 
Tacos axcoe ? a IDS, 
assisting the c-bar in counteracting P. If we now put: 
Axc cos’ @ reE ahee iE 5 TB, 
l aa l 
it is seen that, as far as horizontal pull is concerned, we might replace the wire by a hori- 
zontal bar of duralumin of a sectional area a such that 
g= a Qo cos? ra) (1) 
where the ratio Ey for duralumin and wire steel has a value of about 2.87. 
D 
Substituting this fictitious bar for the wire at C,, we have: 
T cos p = S26 a Ep 
F= Ae 1B 
l 
a AN (a +a) Ep 
Hence = aa P sec o (2) 
py= ep 
Aes (3) 
These are important equations. They show how the pull at the joint is distributed be- 
tween the wire and the girder, and, evidently, if several wires are attached to the same joint, 
they can each of them be represented by a fictitious bar parallel with the girder. 
The rule expressed by (2) and (3) is applicable also in ordinary girder construction, 
where the diagonals may he flat bands of steel plates, angle bars or other profiles, the only 
condition being that the vertical frames or posts must be rigid and capable of préserving the 
distance between the girders. It applies in compression as well as in tension, but in case of 
wires only up to the point where the wires become slack. 
