. The Bell System technical journal . n modulus of abeam of uniform cross-section is constant and readily determinable. Thesection modulus, however, of a beam of nonuniform cross-section, such as acrossarm, varies because of the different cross-sectional shapes and dimen-sions involved. In this study the following five different shapes were recognized: (1) Roofed section between pinholes (2) Roofed pinhole section (3) Roofed brace bolt hole section (4) Rectangular pole bolt hole section (5) Rectangular section without bolt holes The dimensions of the sections investigated were as follows: Sect
. The Bell System technical journal . n modulus of abeam of uniform cross-section is constant and readily determinable. Thesection modulus, however, of a beam of nonuniform cross-section, such as acrossarm, varies because of the different cross-sectional shapes and dimen-sions involved. In this study the following five different shapes were recognized: (1) Roofed section between pinholes (2) Roofed pinhole section (3) Roofed brace bolt hole section (4) Rectangular pole bolt hole section (5) Rectangular section without bolt holes The dimensions of the sections investigated were as follows: Section of Arm Dimensions Minimum Nominal Roofed section, except at end of arm Roofed section at end of arm Unroofed sections (Inches) 3^ X 4^ 3^x4 3Ax4A (Inches) 3ix4^ 3ix4A3ix4i 23 24 BELL SYSTEM TECHNICAL JOURNAL Since there is little, if any, engineering interest in the strength of structuralmembers of maximum size, no investigations were made of sections ofmaximum dimensions. 50 U 20 z GRAPH 1 1 \ 1 i 1 1 -_————— __^ ^ . 1. 20 30 40 DISTANCE FROM CENTER OF ARM-INCHES 50 60 Tz: -^ 7^Z ^ ^ ^=71 ^r T ^ Fig. 1—Moment diagram for T}pe A southern pine and Douglas fir crossarms per Specification AT-7075:Graph 1—Resisting moments of arms of nominal dimensions, straight grained and free from knots. (Fiber stress 5000 psi)Graph 2—Resisting moments of arms of minimum dimensions, having maximum slantgrain (1 in 8), and containing knots of the maximum sizes permitted (viz.,sizes shown at bottom of arm sketch). (Fiber stress 3250 psi)Graph 3—Bending moments from a load of 50 pounds at each pin position. Section modulus calculations were made of each shape of minimum andnominal size, both with and without knots. Tests have shown that, be- RELATIVE BENDING STRENGTH OF CROSS A RMS 25 cause of the distortion of the grain that occurs around them, knots are fullyas injurious to the strength of structural timbers as knot holes.^ Therefore,in dealing with sections containing knots
Size: 2142px × 1167px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjecttechnology, bookyear1
