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HIGH STRENGTH REINFORCEMENT FOR GRAVITY AND SEISMIC APPLICATION

 
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hemalmistry
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Joined: 01 Jul 2009
Posts: 68

PostPosted: Sat Nov 21, 2020 4:40 pm    Post subject: HIGH STRENGTH REINFORCEMENT FOR GRAVITY AND SEISMIC APPLICATION Reply with quote

Dear SEFI Members,

IS:13920-2016 allows use of use of Fe 415 or lower grade reinforcing bars confirming to IS 1786 and satisfying requirements of clause 5.3.2 and 5.3.3 in Special moment resisting frames (SMRF) and in ductile walls.

Clause 5.3.2 requires ratio of actual yield strength to characteristic yield strength to be less than 1.2. Also, combining requirement of clause 5.3.3 of IS:13920 and clause 5.7.2.2 of IS:16700-2017, ratio of actual ultimate tensile stress (UTS) to actual yield stress (YS) shall be between 1.15 and 1.25.  
     
IS:13920-2016 also allows use of Fe 500 and Fe 550 reinforcing bars confirming to IS 1786 and satisfying requirements of clause 5.3.2 and 5.3.3 in Special moment resisting frames (SMRF) and in ductile walls if they are produced by thermo-mechanical treatment process and having elongation more than 14.5%. No other requirements are specified for use of these high strength reinforcing bars for use in SMRF and in ductile walls.

ACI 318-14 allows only Grade 60 (Fe 420) reinforcing bars confirming to ASTM A615 and ASTM A706 in SMRF and in special/ductile walls for flexural, axial, shear and torsion application.

In ACI 318-14 unique strain limits are specified to define transition and tension controlled section, as only Grade 60 (Fe 420) reinforcing bars are allowed. However, in ACI 318-19 definition of type of section (i.e. compression, transition or tension controlled) is based on strain limits which are reinforcement grade dependent like in Indian code.

ACI 318-14 requires to limit fy to 420 Mpa (even if reinforcing bar with fy > 420 Mpa are used) for shear and torsion application to control the width of inclined/diagonal cracks under service-level gravity loads for all structural members.

ACI 318-19 also allows use of Grade 80 (Fe 550) reinforcing bars confirming to ASTM A706 for SMRF and use of Grade 80 (Fe 550) and Grade 100 (Fe 690) reinforcing bars confirming to ASTM A706 for special/ductile walls for flexural, axial and shear application.

ACI 318-19 requires to limit fy to 420 Mpa (even if reinforcing bar with fy > 420 Mpa are used) for  torsion application and for shear application in members other than SMRF and special/ductile walls. For shear application in SMRF, fy is limited to 550 Mpa and for shear application in special/ductile wall, fy is limited to 690 Mpa with reduced stirrups/hoops spacing in plastic region. Service-level cracking is not a concern in members of the seismic-force-resisting system subjected to design-level earthquake forces.

ACI 318-19 (and also ACI 318-14) allows use of Grade 80 (Fe 550) and Grade 100 (Fe 690) reinforcing bars confirming to ASTM A615, ASTM A706 and ASTM A1035 for lateral support to longitudinal reinforcement and for concrete confinement.

Requirements similar to that of IS:13920-2016 like maximum ratio of actual to characteristic yield stress, minimum ratio of actual UTS to actual YS and minimum elongation are also specified for these high strength reinforcing bars in ACI 318-19. However, some additional requirements (rather penalties) are also specified for use of these high strength reinforcing bars for gravity and seismic applications. These additional requirements like reduced spacing of stirrups/hoops in plastic region of beams, columns of SMRF and boundary elements of special/ductile walls, increased minimum column dimension, requirement of high strength concrete, increased development length, reduced modulus of rupture for deflection calculation and restriction on location of mechanical splice (coupler) are summarized in attached document.  
     
Closer spacing of stirrups/hoops is required in plastic region of beams and columns of SMRF and in plastic region of boundary elements of special/ductile walls to prevent or delay buckling of high strength (Fe 550) longitudinal bars and for concrete confinement.

Larger column dimension/joint depth (26*Db-max) is required when beam with high strength (Fe 550) reinforcing bars frames into column of SMRF. Larger column dimension is required for reasonable control on amount of slip of beam bar in beam-column joint during major earthquake. Specified increased column dimension provides performance similar to column with Fe 420 beam bars.

Minimum concrete grade required for SMRF and special/ductile wall with high strength reinforcement is more to enhance bar anchorage and to reduce neutral axis depth for improved performance.

Research has shown that required development length increases disproportionately with increases in yield strength. So, larger development length is required for all structural elements (SMRF, IMRF, OMRF, Ductile and non-ductile wall, gravity elements etc) with high strength reinforcement.

For non-prestressed (RC) flat slab/plate, use of longitudinal reinforcement with fy > 550 MPa may result in larger long-term deflections than in the case of fy <550> 550 Mpa) is used for RC flat slab/plate.  

Mechanical splices on Grade 80 (Fe 550) and Grade 100 (Fe 690) reinforcement may not be capable of resisting the stress levels expected in yielding/plastic regions. The locations of these mechanical splices are restricted because tensile stresses in reinforcement in yielding regions can exceed the specified tensile strength of coupler.

Regards,
Hemal Mistry



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