BEAMS AND STRUCTURES 



83 



Allowable Stresses for Chord Sections 



For chord sections consisting of corrugated sheets, determine allowable stresses 

 as for columns as explained on pages 72-75. The column length of the corrugations 

 is taken as the distance between the transverse frames of the semimonocoque con- 

 struction. The coefficient of end restraint C is taken equal to one in the usual con- 

 struction. If the corrugations are covered with thin sheet, a value of C = 1.5 is used. 



Smooth Skin with Reinforcing Stringers 



The foregoing equations cannot be used for calculating a semimonocoque struc- 

 ture with fore-and-aft stringers. Application of the equation f^ = My/h would 

 imply that the sheet and stringers were stressed the same. This is true only to the 

 point of loading where the sheet begins to buckle. Beyond that load, the sheet con- 



b=8J5 



stringer area 

 =0./85sq.in. 

 k p=0.56 



Fig. 12. — Typical section with smooth skin covering reinforced on inside with continuous fore and aft stringers. 



tributes no further resisting, holding only its buckling load, and only the stringers 

 resist the further added load. 



The accompanying Fig. 12 is a typical semimonocoque construction, a smooth 

 skin covering reinforced on the inside with continuous fore-and-aft stringers of hat 

 section. The allowable P/A for the stringers must first be calculated. For the sec- 

 tion shown, the allowable P/A of the stiffener or stringer is calculated by the method 

 explained under the heading Calculation of Compression Members, page 72. As an 

 example, for the construction shown, assume 



Distance between transverse frames = 20 in. 



Coefficient of end restraint = 1.5 

 Radius of gyration of hat section, p = 0.56 



Calculations (see page 72) are as follows: 



L 



p 



B 



20 



= 35.7 



0.56 



45,000 X 35.7^ 

 1.57r^ X 10,500,000 



