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A23.4-09
©
Canadian Standards Association
November 2009
28.2.5 Stressing
28.2.5.1 General
28.2.5.1.1
An initial load shall be applied to the individual tendons to straighten them, eliminate slack, and provide a
starting or reference point for measuring elongation. The final load, for which elongation of tendons is
computed and measured, shall then be applied.
28.2.5.1.2
The force in the tendons shall be measured by elongation and by pressure gauge.
Note:
Stressing may be done to a calculated elongation using the force reading as a check against the calculated force or
vice versa.
28.2.5.1.3
Discrepancies between the elongation measurement and force measurement shall not exceed 5%, except
for jacking forces above 70% of the specified tensile strength, where the discrepancies shall not exceed
5 to 8%, varying proportionately between 70 and 80% of specified tensile strength. The elongation
measurement and the force measurement shall not vary by more than 5 to 8% (as applicable) from the
required design value.
Notes:
(1)
The force measurement should indicate that the proper force has been applied. A check of the elongation should
indicate that the correct size of tendon has been used and that operational losses are within tolerance limits. It also
provides a check on the force-measurement system.
(2)
Inaccurate and variable stressing can result in differential camber, lateral bowing of members, and reduction of the
cracking load.
(3)
The rotation of a tendon during stressing should be limited to one turn per
30 m
.
(4)
See ASTM A416/A416M.
28.2.5.2 Initial tensioning
Initial load shall be measured within a tolerance of ±200 N or 2%, whichever is larger.
Notes:
(1)
The same jack used for final stressing of single tendons may be used for initial tensioning if it has a proper system for
measuring the initial force (see
Clause 28.2.3.2
).
(2)
In single-tendon tensioning, the initial and final loads may be applied in immediate succession. When this is done, the
initial load is applied and held momentarily while reference marks are made for measuring elongation and slippage;
the load is then increased to its full value and the tendon is seated.
28.2.5.3 Elongation corrections
28.2.5.3.1
Elongation calculations shall be based on the actual modulus of elasticity of the tendon determined from
the stress-strain curve furnished by the tendon manufacturer.
28.2.5.3.2
Elongation calculations shall take into account the slippage of the tendon in the anchors and the seating of
the chucks for both the dead and live ends. Slippage and seating adjustments shall be based on average
values from tests. Testing shall be performed to determine average adjustment values.
28.2.5.3.3
Elongation calculations shall take into account the movement of anchorages and shortening of
stress-resisting forms. Compensation for movements shall be based on tests that simulate the appropriate
force and eccentricity.
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