Prestressed Kerns Positions
In design of Prestressed Concrete, locating the correct Kern Positions, Top and Bottom of a Section,
are very important calculations because it will determine the best tendon cable profile positions.
Kern Positions are defined as those positions Above & Below the Centroid of the Section where
Prestressed Tendons (or Force) can be located such that there will be NO TENSION in any part of the Section
under Prestressed Force ONLY.
Thus the Lower Limit of Prestressed Eccentricity is the Lower Kern denoted as "Kb" (or Kbot), while
the Upper Limit of Prestressed Eccentricity is the Upper Kern denoted as "Kt" (or Ktop). IF the prestress
is located within the Upper and Lower limits of these Kern Lines/positions then there will be no tension in the
section under any prestress force applied.
Thus, locating the Kerns NO TENSION ZONES is critical so that the tension tendons or cables can be located
within these Zones.
Kb = Zt/A
Kt = Zb/A
Zt = Section Modulus TOP
Zb = Section Modulus BOT
A = Cross-Section Area
Using Precast or Composite Properties ?
IF a precast section is used for prestress, and for service later, then the precast properties should be used
throughout. But if a Precast Section is used with a Composite Section, then the Composite Properties should be
used, without affecting excessively the precast properties. This means two cases precast and composite should be
considered together when determining the locations for Cable Profiles, particularly for Parabolic Post-Tensioned
The following calculations show how this is affected :-
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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE,
MEC Engineers, Civil & Structural Engineer