Published at : 25 Apr 2019
Volume : IJtech Vol 10, No 2 (2019)
DOI : https://doi.org/10.14716/ijtech.v10i2.2467
|Made Sukrawa||Faculty of Engineering, Universitas Udayana, Kampus Bukit Jimbaran - Bali 80361, Indonesia|
|Ida Ayu Made Budiwati||Faculty of Engineering, Universitas Udayana, Kampus Bukit Jimbaran - Bali 80361, Indonesia|
Openings in the walls of infilled frame structural systems are very common. Reinforcement around the wall openings confines and strengthens the wall, making an infilled frame with a confined opening (IFcO) a reliable structural system for seismically active regions. To encourage the application of IFcO, an analysis method is proposed by introducing a simple equivalent diagonal strut formula with reduced width due to the wall opening. Finite element models using shell elements were used as reference to develop strut width formula for IFcO with varying opening ratios (r) and diagonal angles (?). The formula was verified against previous test results and then applied for the design of 3 and 5-story buildings, consisting of IFcOs with r of 30, 60 and 80% to represent medium, large and very large openings. The seismic responses of the strut models were then compared to those of the shell and the bare frame models. The effect of the opening on internal forces, frame reinforcement, wall stresses and soft story mechanisms were also investigated.
Confined opening; Diagonal strut; Infilled frame; RC frame design; Seismic load
Infilled reinforced concrete frames (IFs) are one of the most common types of structure used in multi-story buildings, including in areas of high seismicity. The composite action between the brittle masonry wall and ductile RC frame produces a stiffer and stronger structure than a bare frame (BF) alone. Openings of various sizes and locations in the infill wall are very common in house windows and doors, reducing their stiffness. Surendran and Kaushik (2012) reviewed many research papers on infilled frames with opening (IFOs) and summarized the importance of considering the effect of openings in the wall in analysis and design. As most earthquake design codes do not address this issue in detail, they suggest the need for the development of a uniform method of analysis and design of IFO structures, which are constructed almost everywhere in the world. A similar study by Nicola et al. (2015) suggests the inclusion of infill walls in modelling RC frames because the seismic behavior of the structure could be affected positively or negatively by the infill distribution on the frame.
It is widely known that confinement in concrete compressive members will increase their strength and stiffness (Boonpichetvong et al., 2016). Similarly, an infill wall panel with confinement along its four edges performs better under compression. An experimental study by Sigmund & Penava (2014) on infilled frames with wall openings, with and without confinement, IFcO or IFO demonstrated that the confinement around the openings was capable of preserving the lateral strength, stiffness and ductility of the tested IFO, as in the case of a solid
IF. The confining element used in the experiment was in the form of a practical RC tie-column and beam with the same thickness as the wall, which would prevent early failure of the wall due to stress concentration around the opening. Provision of confinement around the wall opening is also recommended by the Euro Code (EC, 2009). The EERI (2011) also outlines the importance of confinement in the wall to improve the seismic resistance of a confined masonry structure. In addition, Sigmund and Penava (2013) propose a design method to analyze the European practice of IFO with infill employing a diagonal strut model by correcting the strut width for solid infill using complex correction factors dependent on the damage state and type of opening.
With regard to IFOs, Kakaletsis and Karayannis (2009) report IFO test results without confinement, in which the wall opening significantly reduces the lateral strength, stiffness and energy dissipation capacity of the IFO. Based on these results, Asteris et al. (2012) subsequently proposed a correction factor to reduce the strut width of solid infill using the complex strut width formula recommended by FEMA (1998). Further analytical methods have been proposed by researchers, including use of a multi-strut model with a shear spring (Crisafulli & Carr, 2007) and a micro model based on detailed modeling of the brick and mortar (Penava et al., 2014). All of these models are intended to mimic the behavior of IF up to the point of failure, instead of for use in IFO design. The lack of practical design guidance has discouraged structural engineers from considering infill walls in the design of RC frames, especially when the wall contains large openings. This lack of awareness of infill walls is one of the reasons for many soft story IF failures when subjected to strong earthquakes.
This paper proposes a simple elastic approach to the analysis and design of IF with confined openings (IFcO) based on the test results of Sigmund and Penava (2014). A new formula for diagonal strut width has been developed using finite element (FE) models for reference, with consideration of practical aspects of infill construction in Indonesia and other countries with warm climates, such as thin infill walls and low concrete strength for confinement. After validation of the formula, it was applied to the design of 3- and 5-story IFcO with varying wall opening ratios. The seismic response of the IFcOs was compared to that of the BF and FE models.
A simple diagonal strut model for infilled frame with confined openings (IFcO) has been proposed using a formula of reduced strut width, Wsco (Eqs. 1 and 2), driven using a shell element model for reference. The proposed model includes the effect of the angle of diagonal ?, the strength of the frame concrete f’c, and the opening ratio r.
The equation has been validated against previous test results and applied in design examples, leading to the following conclusions: (1) The type and location of openings in the wall of an infilled frame produced similar seismic responses and design results. For the purpose of designing IFcO, the proposed equation of diagonal strut width using a single correction factor (C) can be used, irrespective of the opening type or location. The confinement in the wall equalizes the response of the IFcO models; (2) The importance of including an infill wall in the analysis was justified, showing that the IFcO models were much stiffer and stronger than the bare frame (BF) models, even if the opening ratio in the wall was as high as 80%. If the opening in the wall is too large, however, the wall will act like a wing wall, which requires stronger material to prevent damage to it; (3) The maximum stress on the corners of the opening in the wall justifies the importance of a tie-column and beam around it to reinforce the opening, as well as to confine the wall; (4) Using the same wall strength and thickness along the height of the building resulted in increasing wall stresses from high to low floor levels. This may cause a soft story mechanism, initiated by failure of the infill wall on the lower floor; and (5) The soft story models, without infill walls on the ground floor, exhibited soft story mechanisms, even when the opening in the wall was very large.
The important contributions of infill walls with confined openings to the lateral stiffness and strength of frames suggests that walls with openings should be considered as part of the structural system to check for possible soft story mechanisms, as well as to obtain more accurate and efficient results. Future analytical research on IFcO should include taller structures using walls of varying thickness. Experimental research is necessary to study the actual behavior of IFcO with large confined openings in order to address the trends of large windows and doors in modern architecture.
The research was partly funded by Udayana University of Bali through a research grant of DIPA PNBP No. 2044.2/UN 14.2.5.V.1/LT/2017. Special thanks go to Widiana Surya, Agus Putra and Prastha Bhisama, structural engineers at Vilamas, who assisted the authors in the computer modeling.
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