is given by, |
Section crack in flexure [ IS 1343-2012, Clause no. 23.4.2, Pp-33] The ultimate shear resistance of a section cracked in flexure, is given by Where, =effective prestress after all losses have occurred, which shall not be taken as greater than 0.6 = characteristic strength of prestressing Steel =Ultimate shear stress capacity of concrete obtained from IS 1342-2012,table 8 PP-33 b=breadth of the member, which, for flanged sections, shall be taken as the breadth of the web d=distance from the extreme compression fibre to the centroid of the tendons at the section considered =moment necessary to produce zero stress in the concrete at the depth =is the stress due to prestress only at a depth of d and distance Y from the centroid of the concrete section which has second moment of area I V and M= shear force and bending moment respectively, at the section due to ultimate loads. should be taken as not less than The value of calculated at a particular section may be assumed constant for a distance equal to d/2, measured in the direction of increasing moment, from that particular section. For a section cracked in flexure and with inclined tendons, the component of prestressing force normal to the longitudinal axis of the member should be ignored.
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When V, the shear force due to the ultimate loads, is less than , the shear force which can be carried by the concrete, minimum shear A/F should be provided in the form of stirrups such that Where, = total cross sectional area of stirrups let's effective in share b= breadth of the member which for T, I and L beam should be taken at the breadth of the rib = stirrup spacing long the length of the member, and = Characteristic strength of the stirrup reinforcement which shall not be taken greater than 415 However, share reinforcement need not be provided in the following cases a) Where V is less than 0.5 b) In members of minor importance When V exceeds , share reinforcement shall be provided such that, In rectangular beams, at both corners in the tensile zone, a stirrup should pass around a longitudinal bar, a tendon or a group of tendons having a diameter not less than the diameter of the stirrup the depth is then taken as the depth from the extreme compression fibre either to the longitudinal bars or to the centroid of the tendons whichever is greater. The spacing of stirrups along member should not exceed nor 4 times the web thickness for flanged members. When V exceeds the maximum spacing should be reduced to the lateral spacing of the individual legs of the stirrups provided at a cross section should not exceed |
Concrete grade | M-30 | M-35 | M-40 | M-45 | M-50 | M-55 and over |
Maximum shear stress N/mm² | 3.5 | 3.7 | 4.0 | 4.3 | 4.6 | 4.8 |
IS :1343-2012 Cl No. 19.6.21-a recommends that the permissible stress on concrete, after accounting for all losses due to relaxation of steel, elastic shortening, creep of concrete, sleep in anchorages shall be Whichever is smaller Where, cube strength at transfer =bearing area =punching area - Permissible stress may be increased by 25% at the time of transfer provided the value does not exceed - The effective punching area shall only be the contact area of the anchorage devices which is circular in shape commercial be replaced by a square of equivalent area the bearing area shall be the maximum area of that portion of the member which is geometric li similar and concentric to the effective punching area. - In case of embedded anchorages coma bearing stress shall be investigated after accounting for the surface friction between the anchorage and the concrete. - Where a number of encourages are used, the bearing area shall not overlap, where there is already a compressive stress prevailing over the bearing area comedy total stress shall not exceed the limiting values specified above. - For stage stressing of the cables, the adjacent unstressed anchorages shall be neglected while determining the bearing area. |
Bursting tensile force[IS 1343-2012, Clause no. 19.6.2.2, pp-26] a) The bursting tensile forces in the end blocks, or reasons of bonded post tensioned members should be assessed on the basis of the tendon jacking load. For unbonded members, the bursting tensile forces should be assessed on the basis of the tendon jacking load or the load in the tendon at the limit state of collapse, whichever is greater. The bursting tensile force, existing in an individual square and block loaded by a symmetrically placed square anchorage or bearing plate, may be derived from the equation below: Where, bursting tensile force load in the tendon assessed as above side of loaded area side of end block b)The force will be distributed in a region extending from from the loaded face of the end block. Reinforcement provided to sustain the bursting tensile force may be assumed to be acting at its design strength (0.87 times characteristic strength of R/F) except that the stress should be limited to a value corresponding to strain of 0.01 when the concrete cover to the R/F is less than 50 mm. c)when circular anchorage or bearing plates are used, the sides of the equivalent square area should be used. Where groups of anchorages or bearing plates occur, the and blocks should be divided into a series of symmetrically loaded prisms and each prism treated in the above manner. For designing and blocks having a c/s of the beam, reference should be made to specialist literature |