STRENGTH REQUIREMENTS FOR ROUND CONDUIT 



753 



l)edding eft'ect, the possible maximum bending moment at the bottom 

 of the tube was obtained by doubling the values of the side points. 

 Figs. 12 and 14 show clearly that the maximum bending moments occur 

 in wet claj^, and also that improvement is obtained by an increase in 

 the height of backfill or a decrease of the applied load. These figures 

 apply to 4-inch diameter tubes. In conversion of results obtained using 

 4.o-inch diameter tubes, it was assumed that the eciuivalent two-point 

 load is directh' proportional to the tube diameter. 



DYNAMIC LOAD 



A study was made to determine the effect of moving loads on the 

 maximum bending moment of the conduit. For this purpose trucks were 

 driven over the backfill at a speed of approximately 20 miles per hour, 

 the strains measured and then compared with those obtained by static 

 loads. The results show that for a clay backfill, the bending moments 

 due to dynamic loads were equal to or even smaller than those obtained 

 by static loads. For sand backfill, however, the dynamic loads caused 

 an increase of the maximum bending moments of approximatel}^ 10 per 

 cent. These results are in close agreement with dynamic load tests con- 



lOOOr 



900- 



600 



u. 

 \ 



CD 



- 700 



o 

 < 

 o 



z 

 o 



0. 



1 



o 



9 



z 



UJ 



> 



§ 200 



600 



500 



400 



300 



100 



CO 



o 



<t 

 o 



4 

 Z 



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X 



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a. 

 a. 



BACKFILL. ^' GRAVEL 

 WIDTH OF trench: 22 IN. 

 CONDUIT MEAN DIAMETER : 4IN. 



\ 

 \ 

 \ 



15000 LBS \ 



12000 LBS 



9000 LBS 



6000 LBS 



3000 LBS 

 BACKFILL 



10 



20 



30 



40 



50 



60 



HEIGHT OF COVER (IN) 



FiK. 14 — Equivalent two-point load versus height of cover for gravel backfill. 



