It is certified that the work contained in this thesis entitled "Tear behavior of columns in masonry filled RC frames under lateral loads" by Mr. At the same time, the realistic evaluation of shear failure of columns requires suitable materials and analytical models.
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Schematic representation of the analytical model and the plastic hinge. a) Comparison of experimental and analytical lateral load response; Rn = nominal shear strength of aggregate according to MSJC (2013) Rs = sliding shear strength of aggregate.
I NTRODUCTION
In the first phase, the effectiveness of current code recommendations in preventing shear failure in columns was evaluated (Table 1.1). From the experimental investigation, the shear failure of the piles due to the backfill effect has been evaluated and the current code recommendations have been evaluated.
R EVIEW OF L ITERATURE
Lateral Strength
It was observed that the strength of the infilled frames with AAC blocks was found to be approximately 1.2 to 1.5 times that of the corresponding bare frame samples. Brokken and Bertero (1981) and Zarnic and Tomazevic (1985) (as mentioned in CEB 1996) reported that the experimental results showed no significant influence of the infill reinforcement on the lateral strength of the infilled frames.
Lateral Stiffness
Similarly, Valiasis and Stylianidis (1989) reported that the presence of an axial compression load on the columns improved the lateral stiffness of the studied system. From previous literature, it was also observed that the strong and stiff infill improves the lateral load performance of the infilled frame system.
Energy Dissipation
Stylianidis (2012) reported that the use of strong mortar leads to a slight increase in the energy dissipation capacity of infilled frames. They further reported that the energy dissipation performance of the infilled frame was not significantly affected by window openings, while the energy dissipation performance of the infilled frames was found to decrease in the case of door openings.
Modes of Failures of Infilled Frames
The description of failure modes together with their analytical relationships with respect to fill strength are reviewed in this section. The flexural failure of the columns was developed by observing the plastic hinges at the ends of the columns [Fig.
Macromodels
The constitutive force-displacement relationship of the masonry panel has a significant dependence on the type of failure mechanisms of the panel. The authors reported that only the multi-strut model can represent the brittle failure mechanism occurring at the nodes due to the presence of the masonry infill.
Micromodelling
However, the main difficulty of the double model is the placement of non-parallel equivalent columns and the distribution of shear force and bending moment due to the application of the concentrated force near the resultant stresses. Furthermore, the analysis of large buildings requires high computational effort, which has been found unacceptable for practical engineering purposes.
Summary
Local adverse effects are related to the damage to the individual members due to the presence of fillings, which will ultimately lead to the overall failure of the structure. Recommendations from the various earthquake standards regarding both global and local adverse effects of backfilling are discussed in the following sections.
Global Adverse Effects of Infills
The detrimental effect is created due to the irregular distribution leading to catastrophic damage to the entire structure. The other alternative was to provide the symmetrical shear walls designed for 1.5 times the design story shear force in both directions of the building.
Local Adverse Effects of Infills
Second, the captive-column effect where infill is in contact over a fraction of the full height of the column. If the clear height of the opening is short, the calculated design shear force can be very large.
Summary
No detailed study on the evaluation of nonlinear stress-strain characteristics of fly ash masonry is available in the literature. Therefore, the following chapter provides details on the material characteristics of fly ash bricks under pressure, shear and tension.
M ATERIAL C HARACTERISATION OF
B RICK M ASONRY
Determination of WA and IRA of Fly Ash Brick Units
The water absorption capacity of fly ash bricks is highly variable and location dependent as observed in several previous studies where the WA was found to range from 12.5 to 37% (Malaviya et al.). However, fly ash bricks are a newly emerging technical material. and are very water absorbent; the proposed limits for brick may not apply to fly ash bricks.
Compressive Stress-Strain Characteristics of Fly ash Brick Units
3.3(b) shows the change in modulus of elasticity (Eb) of fly ash brick units with compressive strength. The tensile strength of fly ash bricks (fbt) was determined using the split tensile test according to ASTM C1006-07 (ASTM 2007).
Compressive Behavior of Fly Ash Brick Masonry Prisms
Failure mechanisms of fly ash masonry prisms: (a) vertical cleavage cracks (1:3 strong mortar); (b) vertical splitting and crushing of bricks (1:4 medium mortar); The modulus of elasticity for strong and weak fired clay brick prisms was found to be approx. 1.5 times (4200 MPa) and 1.6 times (2300 MPa) of fly ash brick prisms.
Analytical Estimation of Masonry Prism Strength
The low tensile strength of fly ash brick masonry may be due to the smooth surfaces of the fly ash brick units, which could have led to weak interfacial bonding. Khalaf (2005) also reported that the tensile strength of the calcium silicate bricks tested for different types of mortar was found to be smaller (0.1-0.14 MPa) than the clay bricks MPa) due to the smoothness of the surfaces of the calcium silicate bricks.
Initial Shear Strength of Fly Ash Brick Masonry
It was clearly observed that the initial shear strength was dependent on the quality of the mortar and was found to increase as the strength of the mortar increases. The strength of the fly ash brick masonry triplets was found to be lower compared to the burnt brick triplets tested by Singhal and Rai MPa in the previous study; COV of 0.23) and Alecci et al.
Shear Strength of Fly Ash Brick Masonry
The average compressive strength of Class-I and Class-III fired claystone units were found to be about 19.2 MPa [COV of 0.20] and 6.3 MPa [COV of 0.28]. It was observed that the average WA (18.3%) of fly ash brick units was higher than that of burnt clay bricks.
P RELIMINARY S TUDY ON L ATERAL L OAD
B EHAVIOR OF M ASONRY I NFILLED RC F RAMES
Ductile and Non-Ductile Bare Frame (Specimens 1 and 2)
The crack pattern observed in both ductile and non-ductile bare frames was similar, but the amount of cracks initiated in non-ductile frames was higher, with cracks developing at 0.46% drift (lateral load of 15 kN). Finally, it was determined that although significant shear cracking developed in non-ductile frame columns, both ductile and non-ductile bare frames failed due to the flexural mechanism in the columns.
Ductile and Non-Ductile Infilled Frames (Specimens 3 to 8)
Since the shear span (moment to shear ratio) for the braced columns is short, diagonal shear cracks (diagonal tensile cracks) developed/initiated along the free length of the columns at a smaller displacement level. As the slip level increased, widening of the diagonal shear cracks followed by slippage of the concrete between the widened cracks was observed.
Lateral Load Response of Frames with Improved Shear Capacity
In the first in-filled frame with enhanced shear capacity throughout the column length (Specimen 9), 8 mm diameter bars (fy = 460 MPa) with 3-leg stirrups spaced 90 mm apart throughout the column length were provided. Energy dissipated by frame with improved shear capacity along the length of the column and in critical areas was similar to the energy dissipated by the previously tested infilled frames [Table 4.3 and Fig.
A SSESSMENT OF S HEAR F AILURE IN C OLUMNS OF
M ASONRY I NFILLED RC F RAMES
- Calculation of Shear Capacity of Column Sections
- Evaluation of Masonry Infill Parameters
- Local Effects of Infill Based on Codal Approaches
- Modelling Parameters of RC Members
- Masonry Infill Modelling Parameters
- Analytical Results of Bare Frame and Infilled Frames
- Application of the Improved Strut Model
- Verification of Improved Analytical Model
The equations for evaluating the shear capacity of the column sections are presented in Chapter 2. In the current study, the shear demand on the column due to the effect of infill is taken as the capacity of the infill.
I MPROVEMENT IN S HEAR B EHAVIOR OF
I NFILLED F RAMES : D ESIGN E NHANCEMENTS
Case I: Enhancement of Transverse Reinforcement
To avoid failure of members due to shear compression, IS 456 (BIS 2000) limits the maximum shear that a section can withstand to τcmaxbd where τcmax. In order to increase the shear capacity of the column section to meet the shear demands due to the infill effect, ρsh was found to be much higher than the maximum reinforcement limit criteria.
Case II: Increasing Dimensions of Column Sections
Comparison of analytically obtained lateral load response and plastic hinge formation stages in the infilled frame with extended column test sections. From the parametric analysis using the improved analytical model, it was found that the frames with expanded column test sections were able to prevent shear failure due to the negative infill effect.
Description of the Test
The strengthening details of the RC frame with raised column section are shown in Figure. From the analytical investigation, it was observed that the lateral load capacity of the infilled frame with improved column section was higher than the previously tested infilled frames (preliminary study), but the capacity was smaller than the capacity (250 kN) of the servo-controlled hydraulic actuator used.
Strong Floor
Hysteretic Response
Evaluation of Influencing Parameters
The lateral load capacity of IF-EC was found to be approximately 1.2 times that of IF-FB2 (Table 6.4). The energy dissipated by IF-EC was found to be higher compared to other filled frameworks and was approximately 2.3 times that of IF-FB2.
Crack Pattern and Failure Mechanisms
At the same degree of reliance, the lateral load capacity of the filled frame was also achieved. While in the case of IF-EC, the initiation of shear cracks was delayed and also the performance of the infilled frame was achieved at a higher degree of drift.
Lateral Load-Strain Response
A detailed experimental evaluation of Methodology II was discussed in the following sections. There must be a compromise between the type of infill used to delay the shear failure of the columns without affecting the main functional requirements of the infill.
Description of the Test
In the case of IF-CC2, class III burnt clay bricks were used in the construction of infill wall. From the material characterization of class I and class III bricks, it was observed that class III bricks were low strength and soft (low elastic modulus) compared to class I fired clay bricks.
Hysteretic Response
Evaluation of Influencing Parameters
The secant stiffness of IF-CC1 was found to be higher at the initial drift levels compared to IF-CC2 and IF-FB2. The contribution of infill in the case of IF-CC2 in the post-peak was significantly higher compared to that of the IF-CC1.
Crack Pattern and Failure Mechanisms
The capacity of the IF-CC2 also reached a higher drift level together with the initiation of shear cracks near the column corners. In the case of IF-CC1, it was observed that flexural cracks were marked at several locations along the length of the column, which was not observed in the previously tested infilled frame specimens.
Lateral Load-Strain Response
From the lateral load–strain response of IF-CC1, it was confirmed that the performance of rebar started in the post-peak regime at 1.38% drift after the initiation of major damage events (flexural and shear cracks). Whereas, in the case of IF-CC2, it was observed that the yielding of rebar was initiated before the lateral load capacity of the infilled frame.
I MPROVEMENT IN S HEAR B EHAVIOR OF I NFILLED
F RAMES : D ECREASING F RAME -I NFILL I NTERACTION
- Linear Parametric Analysis
- Nonlinear Analysis using Improved Analytical Model
- Description of the Test
- Hysteretic Response
- Evaluation of Influencing Parameters
Six types of collector beam configurations were considered: (1) central horizontal collector beam; (2) two horizontal collector beams; (3) three horizontal collector beams; (4) central vertical collector bar; (5) central horizontal and vertical collector beams; and (6) two horizontal and one vertical collector beams. Hysteretic responses of the bare frame with collector beam (BF-CB) and filled frames with collector beams (IF-CB1, IF-CB2 and IF-CB3) are shown in Fig.
