Importance of Type of Reinforcement (TMT Vs TOR) in Earthquake Resistant Design...

Importance of Type of Reinforcement (TMT Vs TOR) in Earthquake Resistant Design of RC Structures

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types of reinforcements
Pankaj Agarwal
Professor, Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand (U.K.), India
VVS Surya Kumar Dadi
Assistant Professor, Department of Civil Engineering , Institute of Technology, Guru Ghasidas Vishwavidyalaya, Bilaspur (C.G.), India

Abstract: Performance evaluation of two types of reinforcements i.e. Thermo Mechanically Treated (TMT) reinforcement and conventionally used Twisted Ore Reinforcement (TOR) are carried out under monotonic and cyclic loadings. The stress history, strength decay, hysteresis behavior, strain energy and failure mode under uni-axial and cyclic loading with different strain rates are compared.  Comparative behavior of both types of reinforcement in RC components comprising of beam specimens and beam-column joint specimens are also evaluated under pushover and cyclic loading. The post yield behavior of these specimens have been compared after flexural tensile yielding of reinforcement.

Non-linear performance evaluation of a G+2 moment resisting reinforced concrete (RC) frame building with TMT and TOR type of reinforcement has been evaluated as per ASCE/SEI 41-06 in SAP 2000. The results signify the effects of types of reinforcement characteristics for seismic evaluation of strength and ductility parameters. However, the structural performance of the G+2 building is more governed by the seismic design philosophy/ mode of failure rather than type of reinforcement.

Introduction

Reinforced Cement Concrete (RCC) has been mostly adopted nowadays as construction material in India. Developed and developing sectors of Indian habitats along with infrastructural developments witness its pronounced use. It is also the common man’s construction material as it is the material composite of concrete and steel due to its cast in-situ characteristics. Flexibility in material transport with no geometric constraints in casting makes it more useful for all terrains. The advancement in technology provides an opportunity to understand more clearly, the behavior of materials which in turn necessitates to optimize the designs or to create new materials with the required behavior. The designs are to be modified with a futuristic view of the material scarcity that is going to occur due to the depletion of the material sources day-by-day. The solution for material scarcity can be imposed with higher ratio of strength/volume of the material. When the reduction of the member section is attempted by using high strength concrete, reinforcement congestion can be avoided by the use of high strength reinforcing bars leading to easier construction practice and quality control. Similarly use of higher strength reinforcing steel bars with strength greater than that available will also result in saving the quantity of steel required. Hence the need for new concrete construction practice using new materials of relatively higher strengths is felt necessary. Therefore, production of high strength reinforcing steel is the need of the hour to fulfill the growing needs of the coming up multistoried constructions.

In seismic prone areas, building structure is designed in such a way that adequate reserve strength should be available to prevent failure in the case of a major earthquake. This means that the structure should be able to undergo post-elastic deformations without losing a large percentage of its strength. The ability of the system to undergo plastic deformation (also called reserve strength)  is characterized by ductility which is a property of paramount importance for earthquake resistant structures, as it gives the designer the choice to design the structure for much lower forces than consideration of an elastic system would require. Thus, the basic provisions of all modern structural codes refer to providing sufficient strength, and a corresponding sufficient ductility. Therefore, for the ductile performance of a multistoried building, a flexure mode of failure is desirable in which the reinforcement may yield. Two types of reinforcement are in general used in the construction of RC structures; particularly in India i.e. (i) High Yield Strength Deformed (HYSD) reinforcement also called (TOR) reinforcement and (ii) Thermo Mechanically Treated (TMT) reinforcement. The basic difference is in their mechanical properties i.e. strength and ductility parameters.

Mechanical Properties of TMT Vs TOR Reinforcing Bars

Monotonic and cyclic behavior of the TMT and TOR type of reinforcement is studied in this section.  The aim of the study is to highlight the effect of reinforcement characteristics on the structural performance evaluation. Uni-axial tensile tests have been carried out on the 10mm reinforcing bar coupons of length 300mm with a gauge length around 200mm of different types of reinforcement in 100 kN capacity Universal Testing Machine (UTM) under 3 mm/min rate of loading to determine the monotonic stress-strain diagram as well as the mechanical properties of reinforcement. On the same type of specimen, cyclic tests have also been carried out under different amplitudes of sinusoidal loading (or strain amplitude) ranging from 0.5mm to 5mm. The comparative typical plot of TMT vs TOR reinforcing bar under uni-axial tensile test and cyclic test along-with fracture strain energies is shown in Figure 1 while Figure 2 shows comparative plot of strength  decay of 10 mm TMT vs TOR reinforcing bar specimens under cyclic test at different strain rates. The comparative strength decay and fracture strain energy plot of 10 mm reinforcing bar of TMT Vs. TOR are  presented in Figure 3.

The stress-strain diagram of TMT reinforcing bars clearly and differentially manifests the yield point, yield plateau, strain hardening region as well as strain softening regions which are essential for a ductile type of reinforcing steel. The strain energies of all the TMT 10mm specimens are in the range between 94 and 119 MJ/m3 with an average of 104.30 MJ/m3. The number of cycles required to fail the TOR specimens as well as corresponding strain (or fracture) energy required to fail the specimens are also much lower at the same amplitude of loading as compared to TMT.

Performance Evaluation of TMT Vs TOR Type of Reinforcement in RC Components

Nonlinear performance of a multistory moment resisting frame (MRF) RC building under severe earthquake mainly depends upon the response of its structural and non-structural components. The advisable mode of failure of any component in seismic design is always ductile. To obtain the ductile response of building components, it is essential that the component fails in flexure rather than any other brittle mode of failure. This is only possible, when the yielding occurs in reinforcement. Therefore, the non-linear behavior of components actually will be based on the characteristics of the reinforcement used in the components. The aim of this section is to present the performance evaluation test results on beam components and exterior beam-column joints components with the TMT/TOR types of reinforcement under different proportion.

Pushover and Cyclic Performance Evaluation of Beam Specimens

The performance of beam specimens with TMT and TOR type of reinforcement in varying % have been carried out under pushover and cyclic loading. The actual size of beam specimens is 275mm x 275mm x 1.78m with span to depth ratio (l/d) 6.5. The details of the reinforcement in the beam specimens are shown in Figure 4 while the complete test setup for pushover loading and cyclic loading are shown in Figure 5. The salient features of test set-up consist of  (i) Strong Floor to fix the test setup firmly with the help of high tension bolts, (ii) Reaction Wall to apply the pushover and cyclic loads on the beam specimen with the help of, (iii) Two Servo-Controlled Hydraulic Actuators in synchronized mode. The one end of the actuator is connected to the middle of beam specimen and the other end is connected to reaction wall and (iv) Mechanical Jacks to restrain the beam specimens at both the ends so that no translation movement is possible; only rotation can occur as in case of simple supported conditions. The loading history under pushover testing consists in the form of ramp loading of gradually increasing amplitude. In case of cyclic testing, loading is applied in the form of displacement control sine sweep wave at a very low frequency (f= 0.0083 Hz), as also shown in Figure 6.  The amplitude of loading increases gradually in two phases i.e. phase 1 consists of 5mm to 50mm with an interval of 5mm and phase II consists of 50mm to 150mm with an interval of 10mm.  These specimens have been tested up to failure and its complete non-linear behavior in the form of load deformation diagram under both types of testing have been recorded with the help of load cell and LVDT mounting on the actuator itself. The resultant load is the sum of the individual load of both the actuators while the resultant displacement is the average of individual displacement. The pushover and hysteresis plot of tested beam-specimens with different types of reinforcement under pushover and cyclic loading are shown in Figure 7 and 8 for the different % of reinforcement. It shows that the beam specimens with TMT reinforcement have an over strength ratio of 1.12 with an ultimate ductility of about 8.0 irrespective to % of reinforcement. The specimens with TOR reinforcement show a brittle failure with a comparatively low ductility as well as energy dissipation. The load-deformation curves with varying % of reinforcement clearly depict the respective performance of reinforcement in their pushover curve. The cyclic performances of beam specimens with TMT reinforcement have a ductile pattern and more energy dissipation as compared to specimens of TOR reinforcement. The ductility obtained from the cyclic testing from its envelope curve is much less corresponding to pushover testing. A comparison of pushover curve and envelope of load deformation curve under cyclic testing indicate that beam specimens have achieved approximately the same strength in respective reinforcement and % of reinforcement however ductility obtained from the pushover curve is much higher than the ductility obtained from the envelope of load deformation curve.

 

 

Cyclic Performance Evaluation of Exterior Beam-Column Joint Specimens

A study has also been carried out on the cyclic behavior of external beam-column joints with TMT and TOR types of reinforcement in quasi-static testing facility. These joints are cast in the same % of reinforcement in beams as in previous section with a constant % of reinforcement in column. The details of reinforcement in beam and column of the joints specimens are shown in Figure 9. The schematic test set-up for the testing of beam-column joint specimens is shown in Figure 10. The loading history under cyclic testing is the same as applied in case of beam specimens.  In this testing program, the loading is applied at the free top end of the beam since the specimens have been prepared in inverted T-forms in which column is in horizontal position and beam in vertical position. The acquired load-deformation data in the form of hysteretic diagram for each specimen of beam-column joint specimens in different type and % of reinforcement are shown in Figures 11 and 12 along with the pattern of failure at maximum ductility. The cyclic testing of beam-column joints with two different types of reinforcement clearly manifest that the characteristics of reinforcement significantly influence the post yield performance of the joint particularly in case of “strong column – weak beam” mechanism at the joints.  However, the yield and ultimate capacity parameters are slightly higher in joint specimens with TOR reinforcement.  The energy dissipation in beam-column joint specimens also decreases as the type of reinforcement changes from ductile to brittle.

Performance Evaluation of TMT/TOR Types Reinforcement in RC Frame Buildings

The performance evaluation of a G+2 frame RC building designed as per IS 1893 (Part 1): 2016 and detailed as IS 13920: 2016 has been carried out in SAP 2000 with the two different types of reinforcement namely TMT and TOR. The plan and elevation of the building frame is shown in Figure 13. The non-linear performance of the G+2 building frame has been evaluated under different controlling actions/ mode of failures of beam and column as mentioned in ASCE 41-06. The matrix for different controlling actions/ mode of failures of RC components and the performance evaluation parameters of building frame is given in Table 1 and the performance curves are given in Figure 14. The effects of type of reinforcement on the performance evaluation of frame building under flexure mode have also been studied. The combination of types of reinforcement used in beams and columns of the frame building and results are listed in Table 2 and its non-linear pushover curve is shown in Figure 15. The performance of non-specific type of reinforcement as mentioned in ASCE 41-06 has also been plotted in the same Figures for comparison purpose. The results clearly manifest that the nonlinear behavior of structure is significantly influenced by the controlling actions /mode of failures of its components. The strength and ductility of the same building is drastically changed under different modes of failure of its components.

Conclusions

  • The advancement of metallurgical process of TMT reinforcement has proved to be successful in developing and producing high strength and high ductility reinforcing bars. In comparison to TOR type reinforcement as used in the earlier construction, TMT type of reinforcement show increased strength, ductility, energy dissipation under both the monotonic as well as cyclic type of loading up to fracture.
  • Beam specimens with TMT reinforcements under pushover loading reflect a ductile performance as compared to TOR reinforcement. The envelopes of load deformation diagram (hysteretic behavior) of beam specimens have also shown that the TMT reinforcements possess considerably higher strength and ductility as compared to TOR type of reinforcement. It is also conferred that pushover curve represents the approximate actual strength of the specimens but with higher ductility and also low energy dissipation as compared to cyclic testing. However, the specimens, in general, have shown large yield strength but low ultimate strength under cyclic testing. Similarly, the ductility under pushover testing is much higher as compared to cyclic ductility.
  • The effect of types of reinforcement in the beam-column joint specimens of the same % of reinforcement has been manifested in their load-deformation envelop diagrams. The energy dissipation in beam-column joint specimens increases with % of reinforcement but at the same time it decreases as the type of reinforcement changes from ductile to brittle. It is inferred that the failure of specimens i.e. plastic hinge formed in the beam of the joint reflects the performance of type of reinforcement.
  • The nonlinear characteristics of reinforcement have marginally influenced the global failure of building frame even in the flexure mode of failure which is responsible for the ductile response of the structure. The seismic design philosophy plays a significant and crucial role for the ductile response of the structure that is required for the earthquake resistant design of structure.

References

  1. IS 13920: 2016, “Indian Standard Ductile Detailing of Reinforced Concrete Structures subjected to Seismic Forces – Code of Practice”, Bureau of Indian Standards, New Delhi.
  2. IS 1893 (Part 1): 2016, “Indian Standard Criteria for Earthquake Resistant Design of Structures”, Fifth Revision, Bureau of Indian Standards, New Delhi.
  3. ASCE (2007), “ASCE/SEI 41-06: Seismic Rehabilitation of Existing Buildings”, ASCE Standard, American Society of Civil Engineers, Reston, USA.
  4. CSI (2006), “SAP2000: Integrated software for structural analysis & design, Version 10.0.5-Analysis Reference Manual”, Computers and Structures, Inc., Berkely, U. S. A.

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