Chapter 18General Design Requirements

18.1 General

The general design requirements for design of prestressed concrete structures shall be as per 17 to 23 of Subsection 5A(C) of Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code except as modified and supplemented in 18.2 to 18.6.5.

18.2 Effects of Prestress

The effects of prestress shall also be taken into account in assessing loads and forces.

The deductions for prestressing tendons as in 18.3.1 shall be considered for the determination of area, centroid and moment of inertia of the cross-section.

18.3.1 In calculating area, centroid and moment of inertia of a cross-section, deduction (or addition in case of transformed area of tendons) for prestressing tendons shall be made as follows:

a) In the case of pre-tensioned members, where the prestressing tendons are single wires distributed on the cross-section or strands of wires of relatively small cross-sectional area, allowance for the prestressing tendons need not be made. Where allowance is made, it shall be on the basis of (m-1) times the area of the

prestressing tendons, $m$ being the modular ratio.

b) In the case of post-tensioned members, deductions shall invariably be made for prestressing tendons, cable ducts or sheaths and such other openings whether they are formed longitudinally or transversally. These deductions need not, however, be made for determining the effect of loads applied after the ducts, sheaths or openings have been grouted or filled with concrete. Where such deductions are not made, a transformed area equivalent to $(m-1)$ times the area of the prestressing tendon shall be taken in calculation, m being the modular ratio.

NOTE — m shall be calculated as $E_{s}/E_{c}$ ; for values of $E_{s}$ and $E_{c}$ , see 4.6.1.2.1 and 5.2.3.1, respectively. Wherever necessary, creep effects shall also be taken into consideration.

c) In case of multi-directional prestressing, loss of area due to presence of ungrouted ducts/sheaths placed at an angle to the prestressing tendons under consideration may be made at full area (100 percent) of such ducts. In case, such ducts placed at an angle have been grouted prior to stressing of tendons 50 percent of area of such ducts can be considered as effectively available.

In evaluating the slenderness effects during lifting of slender members following factors require consideration:

a) Member geometry,

b) Location of lifting points,

c) Method of lifting, and

d) Tolerances in construction.

All beams, which are lifted on vertical or inclined slings, shall be checked for lateral stability and lateral moment on account of tilting of beam due to inaccuracies in location of lifting points, and due to the lateral bow.

For calculating the factor of safety against lateral instability $(\gamma_{\mathrm{j}})$ references may be made to specialist literature; the factor shall not be less than two.

For determining the lateral moment due to tilting, realistic values which are not likely to be exceeded in practice shall be assumed for the eccentricity of lifting points and the lateral bow. The maximum tensile stress for $\gamma_{i}/(\gamma_{i}-1)$ times the lateral moment due to tilting shall not exceed $1.5\ N/mm^{2}$ .