STRENGTH REQUIREMENTS FOR ROUND CONDUIT 



739 



Fig. 2 — Test set. 



External loads acting upon the conduit produce circumferential bend- 

 nig moments in the conduit wall. The magnitude and distribution of 

 these bending moments have been determined in tests conducted recently 

 at the Outside Plant Development Laboratory, Chester, New Jersey, 

 and at Atlanta, Georgia. These tests were made with gravel, sand, and 

 clay as backfill, in trenches of ^'arious width and depth and under con- 

 ditions simulating, as nearly as possible, those encountered in the field. 



TEST APPARATUS AND PROCEDURE 



A test method was developed which permitted the determination of 

 the circumferential bending moments in thin-walled conduits under 

 field conditions. 



The test device consisted of thin-walled aluminum or steel tubes of 

 one foot length. The steel tubes used had an outside diameter of 4 inches 

 and a wall thickness of 0.062 inches; the aluminum tubes had an out- 

 side diameter of 4.5 inches and a wall thickness of 0.065 inches. SR-4 

 strain gages were attached to the inside surface of the tube. Each tube 

 was efjuipped with four equispaced SR-4 strain gages (type A-o), (Fig. 

 1). One aluminum tube contained, in addition, sixteen SR-4 strain 

 gages (type A-8), which were ec^ually distributed around the internal 

 circumference of the tube. By means of an SR-4 strain indicator and an 

 Edin brush recorder it is possible to measure the strains caused by static, 

 as well as by dynamic loads. The strains could })e measured with an 

 accuracy of drlO X 10~^ inch per inch. 



