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Chapter: Civil - Prestressed Concrete Structures - Design Concepts

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Important Questions and Answers - Civil - Prestressed Concrete Structures - Design Concepts

Civil - Prestressed Concrete Structures - Design Concepts


PRESTRESSED CONCRETE STRUCTURES

 

DESIGN CONCEPTS

 

 

 

1.  What is meant by end block in a post tensioned member?

 

The zone between the end of the beam and the section where only longitudinal stress exists is generally referred to as the anchorage zone or end block.

 

2.     List any two applications of partial prestressing.

 

        Used in large diameter concrete pipes

 

        Used in railway sleepers

        Water tanks

 

        Precast concrete piles to counter tensile stress during transport and erection. used in bridges construction

 

3.     What is meant by partial prestressing?

 

The degree of prestress applied to concrete in which tensile stresses to a limited degree are permitted in concrete under working load. In this case, in addition to tensioned steel, a considerable proportion of untensioned reinforcement is generally used to limit the width of cracks developed under service load.

 

4.  Define degree of prestressing

 

A measure of the magnitude of the prestressing force related to the resultant stress occurring in the structural member at working load.

 

5.  Define Bursting tension.

 

The effect of transverse tensile stress is to develop a zone of bursting tension in a direction perpendicular to the anchorage force resulting in horizontal cracking.

 

6.  Define Proof stress

 

The tensile stress in steel which produces a residual strain of 0.2 percent of the original gauge length on unloading.

 

7.  Define cracking load.

 

The load on the structural element corresponding to the first visible crack. 8. Define Debonding.

 

Prevention of bond between the steel wire and the surrounding concrete. 9. Write formula for Moment of resistance in BIS code.

Mu = Apb Aps (d-dn)

10.                    What are the types of flexural failure?

        Fracture of steel in tension

 

        Failure of under-reinforced section

 

        Failure of over-reinforced section

        Other modes of failure

 

 




The end block of a post-tensioned PSC beam, 300 x 300 mm is subjected to a concentric anchorage force of 832.8 kN by a Freyssinet anchorage of area 11720 mm2. Design and detail the anchorage reinforcement for the end block.

 

Prestressing Concrete,P= 832800 N

 

 

Average compressive stress, fc= (832800/300x300) =9.3 N/mm2

 

2ypo = (?/4xd2) (1/2) = ?(11720x4/?mm   =   123

 

2yo = 300/2 = 150mm

 

Ypo/yo = (123/300)=0.41 Fc = P/A = 832.8/(300x300) = 9.25N/mm2

Tensile stress Fv(max) =fc(0.98 -0.825 ypo/yo) = 9.3(0.98 -0.825x0.41) = 6 N/mm2

 

Bursting tension Fbst = p(0.48 -0.4 ypo/yo)

 

=  832800(0.48-0.4x0.41)

=  264000N

 

Using 10mm diameter mild steel links with yield stress of 260 N/mm2

 

Ast = Fbst/0.87fy = (260x103)/(0.87x260) = 706.85mm2

 

Number of reqd =(264000/(0.87x260x79))= 15

 

The reinforcement is to be arranged in the zone 0.2 yo=(0.2x150) =30mm

 

 

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