Publication: Experimental and computational study on load carrying capacity of load bearing plain walls for buildings
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Date
2021-12-01
Authors
Radhi Al-Salami, Hussein Abdul Ameer
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Abstract
Reinforced concrete wall is a common structural element used as load resisting element in structures. Other than load resistance, they also serve as space enclosing wallstructures. This study focuses on concrete walls with less than 0.4% of compression reinforcement, called plain walls or ordinary concrete walls (BS 8110-1:1997), which are traditionally used as non-structural member in frame buildings. The design formulas
for axial capacity of plain walls in BS 8110-97 neglect the contribution from the small percentage compression reinforcement. Taking advantage of progress made in concrete technology, plain wall is believed to provide alternative load bearing structural system due to the ease of using B.R.C mesh as reinforcement and economy due to relatively lower quantity of steel needed for construction in comparison with conventional reinforced concrete walls. The later point is critical for regions or countries where steel for construction needs to be imported. This study was carried out with the aim of investigating the influence of the low ratio of reinforcement on the behavior of plain walls. The possibility of adopting structural system using plain walls for tall buildings was also explored. Experiments were carried out to examine the effect of the arrangement of reinforcement - one layer, two layers, two layers with short hooks and two layers with long hooks, on behavior of plain wall specimens. A total of 40 plain wall
specimens measuring 1600 mm (height) × 540 mm (width) × 106 mm (thickness) cast with 0.3% compression reinforcement were tested under axial load with three eccentricities of 3 mm, 15 mm and 30 mm (Group A – 20 specimens) and axial load with maximum eccentricity of half wall thickness 53 mm (Group B – 20 specimens). Concrete mix design with characteristic strength less than 20 MPa (actual strengths achieved were 16 MPa and 18 MPa) was used in the casting of the wall specimens. Deformation pattern, failure load and the associated mode, load-deflection behavior and
strains in concrete and steel were recorded. Comparison of failure load with prediction based on BS8110-1:1997 was also made. A modified formula for predicting the axial capacity of plain wall considering the contribution of reinforcement was proposed. Simulation of plain wall behavior under axial load was also carried out using ANSYS. The outcome from the simulation work was used to obtain information about possible capacity of actual scale plain wall in buildings. Analysis and design using ETABS on series of models of tall buildings with typical floor layout of an actual project were
carried out with the aim of determining the maximum number of storey possible with appropriately arranged plain walls as the load resistance system. Aspects of length of wall, shape of wall (L-shape vs straight shape) and symmetry of layout were variables considered in the models. Three different sets of concrete strength were considered: 25 MPa, 25+40 MPa(40 MPa to lower half and 25 MPa to upper half of building) and 40 MPa. Results of experiments showed significance difference in failure load relative to the predicted axial capacity using BS 8110-1:1997 in the range of 1.78 to 3.56 for Group A specimens depending on the eccentricity of load. Comparison for Group B specimens was not possible due to limitation of the formula in BS 8110-1:1997. Effect of the two layers of reinforcement in comparison with single layer on axial capacity was found to be more significant for Group B specimens which failed due to excessive bending in the top portion of the wall specimen. However, the effect was less significant for specimens in Group A. Provision of hooks did not show any significant contribution to increase the axial capacity for both Group A and B specimens. The proposed modified formula for the prediction of axial capacity of plain wall yielded better prediction when compared to the experimental results. Simulation results using ANSYS showed good agreement with the experimental results. Outcome of analysis and design using ETABS showed that the maximum number of storey possible for tall buildings with the use of plain walls as structural system is 15 storey with 25 MPa concrete. The maximum number of storey may be increased to 25 with concrete strength (25+40) MPa and 40 MPa if plain walls of
appropriate length and combination use with L-shape walls were considered.Symmetrical layout of plain walls shows the best performance with the possibility of 25storey (with 40 MPa concrete) with relatively lower wall Demand/Capacity (D/C) ratios for the walls. This study has shown that compression reinforcement in plain wall contributed to the axial capacity of the plain wall. The outcome of this study has provided important information about the potential of plain walls with reinforcement ratio less than 0.4% to be used as load bearing members. Further study using full scale plain walls should be carried out in order to take more advantage of the potential of plain walls.