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Engineering Aid 3 - Beginning Structural engineering guide book
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of sight and a required point above or below it. This point may be a permanent elevation (bench mark), or it may be some natural or constructed surface. There are several types of leveling rods. The most  popular  of  all  is  the  Philadelphia  rod,  as shown in figure 11-48. it is a graduated wooden rod  made  of  two  sections  and  can  be  extended from 7 to 13 ft. In view A, each foot is subdivided into   hundredths   of   a   foot.   Instead   of   each hundredth being marked with a line or tick, the distance between alternate ones is painted black on a white background. Thus, the value for each hundredth is the distance between the colors; the TOP  of  the  black,  EVEN  values,  the  BOTTOM of  the  black,  ODD  values.  The  tenths  are numbered in black, the feet in red. This rod may be used with the level, transit, theodolite, and with the   hand   level   on   occasion   to   measure   the difference  in  elevation. Figure 11-48.-Philadelphia rod. The leveling rod may be read directly by the instrumentman  sighting  through  the  telescope,  or it  may  be  target-read.  Conditions  that  hinder direct  reading,  such  as  poor  visibility,  long  sights, and partially obstructed sights, as through brush or  leaves,  sometimes  make  it  necessary  to  use targets.  The  target  is  also  used  to  mark  a  rod reading when numerous points are set to the same elevation  from  one  instrument  setup. Targets for the Philadelphia rod are usually oval, with the long axis at right angles to the rod, and the quadrants of the target painted alternately red and white. The target is held in place on the rod by a C-clamp and a thumbscrew. A lever on the face of the target is used for fine adjustment of the target to the line of sight of the level. The targets have rectangular openings approximately the  width  of  the  rod  and  0.15  ft  high  through which the face of the rod may be seen. A linear vernier  scale  is  mounted  on  the  edge  of  the opening  with  the  zero  on  the  horizontal  line  of the  target  for  reading  to  thousandths  of  a  foot. When the target is used, the rodman takes the rod reading. The  other  types  of  leveling  rods  differ  from the  Philadelphia  rod  only  in  details.  The  Frisco rod, for direct reading only, is available with two or  three  sliding  sections.  The  Chicago  rod  is available with three or four sections that, instead of sliding, are joined at the end to each other like a  fishing  rod.  The  architect’s  or  builder’s  rod  is a two-section rod similar to the Philadelphia but is  graduated  in  feet  and  inches  to  the  nearest one-eighth in. rather than decimally. The upper section of the Lenker self-computing rod has the graduations on a continuous metal belt that can be rotated to set any desired graduation at the level of the height of the instrument (HI). To use the rod, you set the rod on the bench mark and bring the graduation that indicates the elevation of the bench mark level with the HI. As long as the level remains at that same setup, wherever you set the rod on a point, you read the elevation of the point directly. In short, the Lenker rod does away with the  necessity  for  computing  the  elevations. View B (fig. 11-48) shows the rod marked with metric measurements; the graduations of the rod are in meters, decimeters, and centimeters. The targets that are furnished with the metric rod have a  vernier  that  permits  reading  the  scale  to  the nearest millimeter. The metric rod can be extended from  2.0  to  3.7  meters. For high-precision leveling, there are precise leveling rods as well as precise engineer’s levels. A Lovar rod is usually T-shaped in cross section 11-43







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