Strength Requirements for Round Conduit 



By G. F. WEISSMANN 

 (Manuscript received October 16, 1956) 



Underground conduits arc subjected to external loads caused by the 

 weight of the backfill material and by loads applied at the surface of the 

 fill. These external loads will produce circumferential bending moments in 

 the conduit wall. The magnitude and distribution of the bending moments 

 have been determined by measurements of the circumferential fiber strains 

 in thin-walled metal tubes subjected to the external loads. The effects of 

 different backfill materials, different trench width, and trench depth have 

 been investigated. Bending moments caused by static and dynamic loa^s 

 have been compared. The bending moments are finally expressed in terms 

 of the required crushing strength. 



INTRODUCTION 



For many years, vitrified clay has been the principal material for 

 underground conduit used as cable duct by the Bell System. Vitrified 

 clay conduit has, in general, given excellent service. It has more than 

 adeciuate strength and durability for the wide variety of conditions 

 under which it must be used and for the long service life expected of it. 

 For this reason, relatively little attention has been given to the formula- 

 tion of special strength reciuirements for this type of conduit during 

 the period in which it has been standard for Bell System use. 



For some time other types of conduit, mainly in the form of single 

 duct, h&ye appeared on the market. Many of their properties make 

 them attractive enough to be considered for Bell System use. However, 

 to prevent possible failure or excessive deformation of the conduit under 

 field conditions each type of conduit should meet minimum strength 

 requirements, in order to provide the same reliable service that clay 

 conduit has given. The main purpose of this investigation was to deter- 

 mine the minimum strength reriuirement for round conduit under vari- 

 ous field conditions. 



An extensive investigation of the effects of external loads on closed 

 conduits has been conducted at the Iowa Engineering Experiment Sta- 



737 



