Nov 30, 2019

International Codes for PT Beams


ALL INTERNATIONAL CODES REQUIRES TO SATISFY ALL DUCTILITY REQUIREMENTS OF RC BEAMS FOR PT BEAMS ALSO. THERE ARE CERTAIN ADDITIONAL REQUIREMENTS IN SOME CASES.

Seismic design of post-tension beams as per various codes:

SECONDARY ACTIONS DUE TO PT SHALL BE CONSIDERED IN ALL LOAD COMBINATIONS.





1 Newzealand code :

*PT system shall be preferably bonded. If the unbonded system selected contribution of cables in moment strength in the plastic region shall be limited to 20%.

Comment: the capacity of the unbonded system is mainly due to the integrity of anchorage unlike in the bonded system where capacity is due to bond with concrete and integrity of anchorage. Changes of loss of prestress due to damage to anchorage during a seismic event are more in the unbonded system so the contribution of the unbonded cable shall be reduced by lowering high point and by reducing the number of cables. This may result in beam size similar to that of RC.

* Duct for the bonded system shall be corrugated to ensure bond with concrete.

* Minimum compression reinforcement shall be 15% of total compressive force on the section.
Comment: it helps in stress reversal and increases ductility.

* Depth of neutral axis shall be limited to 0.25D to ensure ductile tension controlled failure.

*in high seismic zones duct of the bonded system can be specifically arranged in a layer in plastic hinge region and in joint to ensure proper confinement of beam-column joints.

Comment: As there is no bond with concrete, a recent version of ACI requires to exclude (deduct) area of unbonded cables from the total sectional area in the calculation of stresses. This reduces effective area and moment of inertia and increases stresses. Also, due to the presence of unbonded cables in the beam-column joint effective joint area will be less and joint may be weaker.

* spacing of stirrups shall be limited to 6d in the confined zone.



2 American code ACI 318:

* Contribution of unbonded cables in moment strength of section in plastic hinge region shall be limited to 25% in beams of SMRF.

* There are also some other requirements related to maximum precompression and testing of anchorages.



3 Chinese code:

The seismic design of the post-tension structure is extensively coveted in Chinese codes of seismic load, seismic design of concrete and prestressed structures.

* Unbonded system is not allowed in GRADE 1 (our SMRF) frames. The unbonded system is allowed in GRADE 2 & 3 (IMRF) frames if

-its contribution in moment strength is less than 35%

-in cantilever beam total moment strength is achieved through reinforcement.

-seismic overturning moment of the unbonded frame in shear wall building is less than 35%.

* Depth of neutral axis shall be limited to 0.25D in grade 1 frame and 0.35D in grade 2 & 3 frames.

*Prestress intensity ratio shall be less than 0.6 for grade 1 frame and 0.75 for grade 2 & 3 frames.

* Bottom reinforcement in the plastic hinge region shall be at least 0.2%.

* stirrups spacing in the confined zone shall not be more than 100 mm. The minimum dia of stirrups shall be 10 mm.





(Source- Social media)

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