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Sunday, 7 October 2018

PRESTRESSING : Calculation of Elongation & Modified Elongation


The Stresses in a Pre-stressing strand normally vary along its length and decrease with time. The Principal factors affecting the distribution of stress along a strand are :
  • Friction looses during stressing
  • Shrinkage of concrete
  • Creep of concrete
  • Due to seating loss
  • Elastic shortening of concrete

Other factors such as changes in temperature in flexing of the structure under loading also affect the stresses in a strand, but these do not necessarily result in a permanent lowering of stress level and are not typically considered as stress loses.

a) Elongation Calculation :

1. Design load in N --- P
2. Length of Tendon in mm --- L
3. Area of Strand in mm2 --- A
4. Young’s Modulus in N/ mm2 --- E

Elongation (e) = P x L / A x E in “mm”

b) Modified Elongation Calculation :

1. Area of Strand in mm2 --- A
2. Young’s Modulus in N/mm2 --- E
3. Elongation in mm --- e
4. Modified Area of Strand (as per TC) in mm2 --- MA
5. Modified Young’s Modulus in (as per TC) N/ mm2 --- ME

Modified Elongation (Me) = e x A x E / MA x ME in “mm”
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