Joined: 01 Jul 2009
|Posted: Sun May 24, 2020 4:49 pm Post subject: Requirement of major changes for Prestress concrete codal provisions in upcoming IS 456
Many of us have attended the webinar on "proposed changes to IS 456 and IS 1343" organized by SEFI on 14th may 2020. We are expecting major changes in existing codal provisions and also inclusion of many new topics in upcoming version of IS 456 which also includes prestressed concrete now. However, I observed that majority of changes are applicable to reinforced concrete only and there are no significant revisions for prestressed concrete. I believe after this major changes and many new topics, upcoming 456 will compete with international standards, especially for reinforced concrete. There are certain complexities, which can be discussed and resolved without affecting purpose of the clause.
Prestressed concrete needs major revisions in below three broad areas:
(1) Components of prestressing force and its combination with other loads for SLS and ULS:
* In statically indeterminate structures like buildings, applied prestressing force/moment have two components namely balanced load/moments and secondary (hyperstatic) forces/moments.
For SLS, prestressing force shall be combined with dead and live loads for stress and deflection check. Seismic loads being factored loads, SLS checks (stress and vertical deflection) for combinations including seismic load is not appropriate. I do not even advocate for SLS checks (stress and vertical deflection) for combinations including design wind load, even with factor 0.8. However, lateral deflection and drift for wind will be necessary.
For ULS, sections are always cracked and load balancing does not exists, So, only secondary (hyperstatic) forces with load factor 1 shall be combined with dead, live, wind and seismic loads with code specified load factors. Secondary (hyperstatic) forces are generated due restraining effect of foundation and supporting vertical members (columns, walls etc) to free movement/shortening of prestressed member. Secondary forces can be extracted from global mode and combined with other loads to check safety of vertical supporting elements. Considerable redistribution can be observed for secondary forces during construction and during service life of building. So, combining secondary forces extracted from single level model (floor elements with column/wall modeled above and below the floor) with other loads may give reliable results while checking safety of vertical elements. Many times it is observed that secondary forces reduces effects due to other loads.
(2) In present IS 1343 many requirements related to durability, design, maximum and minimum prestressing reinforcement limits, spacing, permissible punching stresses, effective flange width etc. are not specified for prestressed members, which requires to follow IS 456 requirement of reinforced concrete members for prestrresed concrete members, as IS 1343 considers IS 456 as mother code. Separate requirements shall be specified for each clause for RC and PT members. Requirements for PT members can be further separated for bonded and unbonded system as mentioned below.
(3) Six limit states namely safety, serviceability, durability, robustness, redundancy and restorability are specified in upcoming IS 456. There are two systems available for post-tensioning in structures/buildings namely bonded and unbonded. Presently in IS 1343, both this systems are treated similarly for majority of requirements related to safety, serviceability, durability, robustness, redundancy and restorability. Behaviour and requirement of bonded and unbonded system for above limit states are different. Majority of present IS 1343 requirements are applicable to bonded system and no separate requirements are specified for unbonded system. In such cases, structures designed and constructed using unbonded system as per IS 1343 may have durability or safety issue in future. Many times unbonded structures are designed with ACI codes or by combining indian code with ACI code without necessary modifications. I believe, ACI code have different philosophy and approach and shall not be adopted fully or partly without necessary modification in India.
Some of the requirements which shall be separately specified for unbonded system are as below:
(A) For Durability:
(i) Encapsulated system (where the whole system is completely enclosed in a watertight covering from end to end, including prestressing steel, anchorages, sheathing, posttensioning coating, sleeves, and an encapsulation cap over the strand tail at each end) shall be mandatory for unbonded system irrespective of environmental exposure conditions for corrosion protection and durability. Separate code shall be available which specify requirement for encapsulated system including requirement for material and minimum thickness/sizes for various components (cables, PT coatings, PT sheathing, Anchorage coatings, connecting sleeves, end caps, coupler etc. ) in the system.
(ii) For bonded system, requirements for durability of grout and grouting procedure are specified. However, no procedure are specified to ensure watertight encapsulation of unbonded system. Such test procedure as available in other codes shall be available.
(iii) Larger clear cover shall be specified for unbonded system for durability, necessary fire resistance and protection from accidental damage due to drilling etc. Bonded strands are sticked to remote face of duct at critical points, resulting in higher cover and thus higher durability and fire protection. Sheathing of unbonded cable have limited insulating values.
(B) For Safety:
(i) Presently IS 1343 requires to limit stress in tendon to effective stress, unless rigorous analysis is carries out to determine flexural resistance. However, ACI code is followed for determination of flexural capacity of unbonded system which allows stresses higher than effective stress for capacity determination. I believe now separate method will be available to determine moment capacity of unbonded system in upcoming IS 456.
(ii) Prestressing force lost due to restraing effect of stiff support (e.g. basement wall) is more for unbonded system and may result in failure if sufficient non-prestressed reinforcement is not available.
(C) For Serviceability:
(i) Minimum non-prestressed (bonded) reinforcement required for one- way slab, beam and two-way slab for unbonded system shall be more than that for bonded system to ensure flexural behaviour at ULS rather than tied arch behaviour and to control crack width and spacing due to overloading, temperature and shrinkage. For bonded system, bonded strands/cables are included along with non-prestressed reinforcement in calculation of minimum non-prestressed (bonded) reinforcement.
ACI 318 requires minimum 0.4% of non-prestressed reinforcement for one-way slab and beam calculated on area between flexural tension face and centroid of gross section for this purpose.
(ii) As unbonded tendons does not have bond with concrete, unbonded tendon shall be excluded from sectional properties (area, moment of inertia etc) used in calculation of stresses.
(iii) Bonded strands can be included to determine spacing of non-prestressed reinforcement, while calculating crack width.
(D) For Redundancy:
Capacity of unbonded system is due to integrity of anchorages only, while capacity of bonded system is due to both, bond with concrete and integrity of anchorages. Any damage to unbonded cable or anchorage of unbonded cable may result in complete loss of capacity of the cable. While in bonded system loss of capacity is limited to local zone in case of damage to cable or anchorage. So, redundancy of bonded system is more compared to unbonded system. Unbonded system requires higher non-prestressed reinforcement for redundancy.
UBC 97 requires minimum non-prestressed steel in unbonded members sufficient to resist unfactored sustained load equal to DL+0.25LL, to prevent progressive collapse in case of failure of unbonded prestressing anchorages at ULS.
Generally in our country, post construction modifications are not controlled and carried out without informing the authority. Any damage to unbonded prestressing cables due to such modification may result in loss of capacity in all panels containing these cables. larger non-prestressed reinforcement is necessary to prevent failure and to increase redundancy.
(E) For seismic resistance and ductility:
Though present discussion is limited to upcoming IS 456 only, requirement related to seismic resistance and ductility can't be considered separately, as IS 456 can be referred completely for OMRF in zone 2 only and it shall be followed along with IS 13920 in zone 3, 4 and 5.
Unbonded post-tensioned members do not inherently provide large capacity for energy dissipation under severe earthquake loadings because the member response is primarily elastic. So, due to lesser ductility, unbonded post-tensioned structural members should be assumed to resist only vertical loads and to act as horizontal diaphragms between energy dissipating elements under earthquake loadings.
American code (ACI 318) requires to limit the contribution of unbonded Prestressed reinforcement to 25% of the positive or negative flexural strength at the critical section in a plastic hinge region of beams of SMRF. This limit allows use of response reduction factor similar to that of RC SMRF.
Newzealand and australian code prefers bonded system in limited ductile and ductile frame and requires to limit contribution of unbonded Prestressed reinforcement to 20% of flexural strength at the critical section in a plastic hinge region of beams for unbonded system. It also requires to ensure that anchorage failure or cable de-tensioning is prevented under seismic loads for unbonded system.
Chinese code prefers bonded system and restricts use of unbonded system in high seismic zone and in Grade 1 frames (SMRF). It allows unbonded system in Grade 2,3 frames (IMRF,OMRF) in moderate seismic zone if contribution of unbonded reinforcement in flexural strength in plastic region is limited to 35% and if anchorages are protected from failure during seismic actions.