170 THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 
4. The horizontal forces, whether tensile or compressive, acting at a joint of a panel 
are directed partly along the longitudinal and partly along the wire (or wires) which meet 
at the joint, provided the wire is not slack. For each wire may be substituted an equivalent 
fictitious bar of the same material as the longitudinal and of an area depending on the mate- 
rial of the wire and on the angle which it forms with the longitudinal. If great accuracy is 
desired, a correction should be applied to this area due to the fact that the tension of the wire 
corresponds to a location of the fictitious bar midway between the longitudinals which it con- 
nects. This is explained in Section VII. The forces directed along the fictitious bar and the 
longitudinal at each joint are in the same proportion as their areas. 
5. The aggregate effect of a longitudinal and adjoining wires, cooperating as explained 
in (4), is the same as that of a girder of an area equal to the sum of that of the longitudinal 
and of the fictitious bars equivalent to the wires meeting at the joint. That area is the “‘effec- 
tive area” at the joint to be used in calculations of longitudinal strength. 
6. Initial tension in the wires prevents or greatly reduces slackness and enables the coun- 
terwires to act as effective strength members under many circumstances where they would 
otherwise be slack. It increases the rigidity of the ship as a whole, reducing in particular 
the extreme limberness under shearing which is characteristic of wired structures. The final 
stresses in the structure under shearing forces large enough to slacken the counterwires are 
the same as if there were no initial tension. Under dynamic actions initial tension lessens 
the violent strains which are apt to occur when the external forces are reversed and slack 
wires suddenly tauten up. 
7. The bending method can be safely applied with a fair degree of approximation to 
airships as now usually constructed, and is particularly valuable as a means of comparison 
between different ships. It should be applied to sections taken through the center of the 
joints of the frames rather than to sections between the frames. 
8. In applying the bending method to an actual airship, it is recommended to deal inde- 
pendently with each frame section as indicated above. Find first by the ordinary bending 
formula the stresses at the joints in the frame under consideration. Calculate the total forces 
at those joints due to bending. Find the forces called forth by shearing in the wires and 
longitudinals of the adjacent frame space which it is desired to consider and superpose on 
these the components of the forces due to bending. This will give the total forces acting at 
any given frame and on structural members adjacent to the frame. 
9. The method of shears is fundamentally in error when used as the sole means of de- 
termining the general stresses in the structure of an airship by a process of summation, be- 
cause it neglects the elastic strains in the girders. 
In conclusion I wish to express my indebtedness to the Bureau of Aeronautics and in 
particular to Commander J. C. Hunsaker for permission to make use of the various material 
and data which have been made available to me by that bureau and which have served as a 
basis for my investigations. 
My thanks are due also the aeronautical engineer af the Bureau of Aeronautics, Mr. 
C. P. Burgess, who introduced me to the subject and who has made several valu- 
able suggestions. 
