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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.

Prestress Kerns Defination

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.

Equation :

Kb = Zt/A

Kt = Zb/A

Where,

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 Cables.

The following calculations show how this is affected :-

Determine Prestress Kern Positions

 

Determine Section Properties 

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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng)
MEC Engineers, Civil & Structural Engineer
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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng) MEC Engineers, Civil & Structural Engineer HP: 016-8672189, Email: Widget: email cloaker