Important Questions and Answers - 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 = A_pb A_ps (d-d_n)

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