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Simple Curve Layout - 14070_246
VERTICAL CURVES - 14070_248

Engineering Aid 1 - Advanced Structural engineering guide book
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the remainder of the stations in the same way as you would if the transit was set over the PC. If the setup in the curve has been made but the next stake cannot be set because of obstructions, the curve can be backed in. To back in a curve, occupy the  PT. Sight on the  PI and set one half of the  I angle  of  the  plates.  The  transit is now oriented so that, if the  PC is  observed,  the  plates will  read  zero,  which  is  the  deflection  angle  shown  in the notes for that station. The curve stakes can then be set in the same order shown in the notes or in the reverse  order.  Remember  to  use  the  deflection  angles and chords from the top of the column or from the bottom of the column. Although the back-in method has been set up as a way to avoid obstructions, it is also  very  widely  used  as  a  method  for  laying  out curves. The method is to proceed to the approximate midpoint  of  the  curve  by  laying  out  the  deflection angles and chords from the PC and then laying out the remainder of the curve from the  PT. If this method is used, any error in the curve is in the center where it is less  noticeable. So far in our discussions, we have begun staking out curves by setting up the transit at the PI. But what do you do if the PI is inaccessible? This condition is illustrated in figure 11-11. In this situation, you locate the  curve  elements  using  the  following  steps: 1. As shown in figure 11-11, mark two intervisible points A and B on the tangents so that line AB clears the obstacle. 2. Measure angles  a and b by setting up at both A and B. 3. Measure the distance  AB. 4. Compute inaccessible distance  AV and BV using the formulas given in figure 11-11. 5.  Determine  the  tangent  distance  from  the  PI to the PC on the basis of the degree of curve or other given limiting  factor. 6. Locate the PC at a distance  T minus AV from  the point A and the PT at a distance T minus BV from  point B. Field Notes Figure 11-12 shows field notes for the curve we solved and staked out above. By now you should be Figure  11-12.—Field  notes  for  laying  out  a  simple  curve. 11-11







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