Precast prestressed concrete structure and Prestressed Articulation Joint System are introduced.
About Precast prestressed
Prestressed concrete is one of structural concrete introduced compressive stress by tendons. The tendons are composed of high strength prestressing strands or bars. Prestressed concrete has performances that controlling cracks and high durability. And compressive stress makes the member restorable.
Precast prestressed concrete structure
-KANSETU KOHO-
Prestressed concrete is one of structural concrete introduced compressive stress by tendons. Prestressed concrete has performances that control cracks and flexibility. Precast concrete is prefabricated at a plant. Frame systems of precast prestressed concrete refer to assemblies of beams and columns. Prestressing tendons connect precast beams and columns.
Prestressed Articulation Joint System
Structural members are precast prestressed components. Secondary tendons connect the components. Corbels at columns transmit shear forces from beams to columns. Beams ends and corbels are carefully confined to prevent damages during the rotational movement. Tendons are stretched up to 50% of their nominal yield point stress. Corbels help to avoid vertical slip and improve the join rotational behavior. Gaps may appear during earthquakes but they will close after the seismic activity finishes.br
This method has the advantages as follows:
High grade of durability is assured
Energy absorbing capacity is improved.
Residual horizontal drift is negligible
Evaluation of losses caused by earthquakes is easy
About regular PCaPC method, Tendons are tensioned to 80-90% of their nominal yield point stress. Consequently, the tendons have not spare capacity for non-deterministic forces such as seismic or accidental impact loads.
When gaps are generated, the contact area is reduced and consequently the beams may slip down and produce damages in the members and failure of tendons as well. This system has no corbels and it has the chance to occur a gap at joint during an earthquake. Loss of friction causes great damages on the ability to absorb earthquake energy.
PAJS includes primary and secondary prestressing tendons and beams-supporting corbels. This system allows rotations at joints, protects the floors against falling, and avoids permanent gaps between beam-column connections. Beam-column connections behave as a rigid joint under gravity loads and earthquake Level 5 (moderate earthquake, 50-year return period) and the interfaces remain in compressive state, i.e. no tensions are present in the components. Beam-column connections rotate elastically under gravity loads and earthquakes Level 6 (heavy earthquake, 500-years return period), and Level 7 (extra heavy earthquake, 1000-years return period).
Primary tendons are anchored at the member ends and tensioned to 80% of the nominal yield point stress. Secondary tendons are tensioned to 50% of the nominal yield stress to keep an elastic behavior under the seismic loads Level 6 and 7. Corbels transmit shear forces to columns and facilitate the end rotation of the beams.
Seismic Structural System with PAJS
Two laboratory tests were performed at Tokyo Institute of Technology on November 2002 and November 2003. Research results from laboratory tests were obtained as follows.
PAJS showed a great resistance for a level 7 earthquake. Damages were reduced by the elastic joint deformations. No structural damages were observed. The precast concrete member resisted to the level 7 earthquake and posed only minor damages. The special characteristic of the precast concrete member is the performance to restore the deformations after the force was applied.
A reinforced concrete test member was tested together with a precast concrete member. The test result showed that the reinforced concrete member yielded severe cracks for a level 6 earthquake.
The test results have been presented at the meeting of Architectural Institute of Japan and at the fib (International Federation for Structural Concrete) symposium.
A list of committee members is presented as follows: