ANALYSIS OF INFLUENCING FACTORS ON COMPOSITE ACTION FOR REINFORCED GROUTED CONCRETE BLOCK WALL-BEAM

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1 ANALYSIS OF INFLUENCING FACTORS ON COMPOSITE ACTION FOR REINFORCED GROUTED CONCRETE BLOCK WALL-BEAM Zhai, Ximei 1 ; Guo, Yanfeng 2 ; Gao, Song 3 1 PhD, Professor, Harbin Institute of Technology, China, School of Civil Engineering, xmzhai@hit.edu.cn 2 Postgraduate, Harbin Institute of Technology, China, School of Civil Engineering, @qq.com 3 Postgraduate, Harbin Institute of Technology, China, School of Civil Engineering, gaosong66@gmail.com The nonlinear Finite Element Analysis (FEA) program ANSYS is adopted to research the mechanical behaviors of wall- made of reinforced grouted concrete block masonry wall and reinforced concrete (RC) supporting. Based on the verified simulating means by comparison between test and simulation results of six reinforced grouted concrete block walls, an extensive parametric study is conducted to analyze the effect of principal factors on bearing capacity for the reinforced concrete block wall-, including the material strength of grouted block masonry wall, the size and position of the wall opening, the height, the span and the reinforcement ratio of longitudinal steel for the RC supporting, the rebar ratio of the grouted wall. The results show that block masonry wall and cast-in-place RC supporting work together by core columns with vertical reinforcement, and the reinforced block wall- acts like a RC deep. The compressive strength of grouted block masonry wall, span and the reinforcement ratio of longitudinal steel of the RC supporting are main influence factors of bearing capacity for wall-. The opening can weaken the arch action and change the arch transfer path which leads to the reduction of its capacity. For the simply-supported supporting, the depth-to-span ratio can be reduced to at least 1/14. Key words: concrete block, wall-, reinforced masonry, bearing capacity, finite element analysis 1. INTRODUCTION Nowadays, large open space is demanded at the lower floors of high-rise reinforced grouted concrete masonry shear wall structure for commercial use. RC frame or RC frame-shear wall structure can better meet this requirement for the lower floors. Hence, reinforced grouted concrete block wall carrying load on the top and resting on a cast-in-place RC supporting spanning a opening space serves not only as a load transferring media but also acts a composite part of the supporting. Built up with grouted block, reinforcement in the vertical cores as well as horizontal grooves, the reinforced grouted concrete block wall and RC supporting may be considered as a composite system to work together and named as reinforced grouted concrete block wall-, which will be further studied in this paper. Despite the fact that reinforced grouted concrete masonry shear wall structure has been widely used in the practical projects, the research on working mechanism, mechanical behaviours for the reinforced masonry wall- at home and abroad are still at the initial stage, especially for wall- with the opening. For that reason, the engineering designers usually regard the

2 supporting as RC frame-supported, ignoring the cooperative working performance between the reinforced grouted masonry wall and the RC supporting. Obviously, it is conservative and unreasonable. Not only is the depth of the RC supporting unnecessarily increased, but the available architectural space is decreased as well. Namely, it is a waste of material and effective space. Based on the above background, the objective of this paper is to gain better understanding of composite performance between reinforced grouted concrete masonry wall and RC supporting under vertical uniform load, define the principal factors influencing the ultimate bearing capacity for such wall-, and propose an appropriate depth to span ratio for a supporting by FE analysis 2. SUMMARY ABOUT WALL-BEAM TEST Six simply-supported wall- with various depths of the RC supporting, opening dimensions and positions under uniform distribution load were tested, including five walls with opening and one wall- without opening. The bearing capacity, the loaddeformation relationship, force-transferring path, and failure are obtained. The test results show that the reinforced grouted block masonry wall has a significant structural effect rather than being a dead load over its supporting reinforced concrete s, there is a structural interaction between the wall and the supporting, and the structural performance of reinforced grouted concrete block wall- is a typical tie-arch and it is similar to a RC deep. Design details and test data of these specimens are shown in Table 1 and Figure 1, see the reference from Zhai and Totoev (2009) for more details. Table 1: Specimen Details Ultimate Ultimate RC Supporting Wall of opening steel from test simulation steel in Beam Dimension Vertical load load from Horizontal No. in Wall /kn /kn Wall depth Longitudinal Stirrup (mm) WB A12@ A12@ B20 A8@100 WB A12@ A12@ B20 A8@100 WB A12@ A12@ B20 A8@100 WB A12@ A12@ B20 A8@100 WB A12@ A12@ B20 A8@100 WB6 Solid A12@ A12@ B20 A8@100 Note: A-low strength plain steel bar; B-medium strength deformed steel bar. 3. FE MODEL Zhai and Totoev (2009) researched the deformation behavior, fracture process, failure pattern and other characteristics of wall-s through testing. As there is a limitation of experimental conditions and the of specimen numbers, the nonlinear FEA program ANSYS with the benefit of parameters analysis is adopted in this paper, to make up for the deficiency of test data, expand the analysis parameter scope and define the principal factors influencing the ultimate bearing capacity for such wall-. Solid65 unit is used to simulate the grouted concrete block masonry and concrete supporting, Link8 unit to simulate steel bar in this paper. The constitutive stress-strain curve of the grouted concrete block masonry from the results of Zhang (2002) is accepted, as following equations:

3 (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 1: Test Specimens: a) WB1; b) WB2; c) WB3; d) WB4; e) WB5; f) WB6; g) Sectional Steel for Supporting Beam; h) Sectional Steel for Top-Beam; i) Bolting Steel Plate Upward section Downward section σ ε ε 2 = 2( ) ( ) σ max ε0 ε0 (1) σ ε = 0.16( ) σ ε (2) max 0 where ε0 =0.002 is for the compression strain corresponding to the maximum stress. For the ultimate compression strain εcu, the advice of εcu =0.003 from Chinese Code for design of masonry structure GB (2001) is adopted. Also the value of for the compression strain is the indication to stop FE simulation calculation, presenting the ultimate bearing capacity for wall-. The constitutive relation of steel bar refers to the MELAS l from ANSYS, which is a multiple fold-line elastic one to describe the stress-strain curve. Two straight lines are adopted in this paper, using a horizontal line as the strength of steel bar after yielding to ignore the hardening stage. Fig. 2 shows the FEA l. Based on the above-mentioned material parameters and failure criteria, the adopted l will be appropriate if the FEA results, including bearing capacity, steel bar strain, fracture distribution, stress distribution and etc. present similar to the experimental results.

4 =1 (AVG) =0 =4.662 = =3.686 MX 1 (AVG) Y Z X Y Z X MN Y Z X Figure 2: The FEA l Figure 3: The distribution of principle stress of W Figure 4: The distribution of principle stress of W6 Fig. 2 and Fig. 3 show the distribution of principle stress of reinforced masonry wall-, from which it can be seen that the uniform load on the top of the wall- is mainly transferred through the arch path. The ends of the supports for the RC supporting are in compression while all the other parts are in eccentric tension. This arch transfer path with tie by the RC supporting is similar to the test results from references (Hu and Tan 2007; Prodromos 2003; Tan and Tong 2003; Tang and Tan 2004), which provided some experiment researches on RC deep s. Fig. 5 shows the crack distribution of wall- WB4 with opening by finite element simulation, which is approach to the experimental result of WB4, see Fig. 6. Figure 5: Crack distribution of WB4 from EFA Figure 6: Crack distribution of WB4 from test results The test results of the bearing capacity of wall-s are similar to what FEA has presented, as Table1. The above-mentioned comparison between simulation and test results verified the rationality and applicability of FEA. 4 ANALYSIS OF INFLUENCING FACTORS ON THE BEARING CAPACITY FOR THE WALL-BEAM Analysis parameters are as follows in this paper: (i) the material strength of grouted concrete block masonry wall; (ii) the size and position of the opening; (iii) the depth and span of the RC supporting ; (iv) the ratio of longitudinal reinforcement in the RC supporting ; (v) the ratio of distribution reinforcement in grouted masonry wall. 4.1 The material strength of grouted block masonry wall

5 Five material strengths of grouted concrete block masonry wall are selected and the EF results of the bearing capacity are presented in Table2. Table 2 shows that the bearing capacity of the wall- increases linearly with increasing compression strength of the grouted concrete block masonry. Sample No Table 2: The FE results for varying material strength Compressive Depth of Dimension strength of Bearing supporting of opening grouted block capacity masonry wall /kn N 2 JD Shear JD Shear JD Shear JD Shear JD Shear J None Shear J None Shear J None Shear Note: longitudinal reinforcements in the supporting are 6B20, all the openings are centrally located. 4.2 The opening size In order to obtain the influence degree of the opening size on the bearing capacity, simulation results of six FE samples with varying opening size are gained based on the sample JD-11.57, as shown in Table 3. It notes that when the opening ratio of wall- decreases from 23.8% to 14.3%, the bearing capacity increases 19%. But when the opening size continues to decrease, the bearing capacity remains fixed. It is results in that the masonry wall near supports above the RC supporting suffers the local compression failure, making the bearing capacity unchanged with the decrease of the opening ratio at the late loading stage. It can be noted from FE analysis that centrally located opening weakens the arch action, however, the dominating arch transfer path for load does not change. Hence, when the opening size decreases to some extent, the influence on the mechanical behavior and the bearing capacity for the wall- could be neglected. Table 3: The FE results for varying opening size Sample No Dimension of opening Opening ratio Compressive strength of grouted block masonry wall N 2 Bearing capacity /kn JD % Shear JD % Shear JD % Shear+Local JD % Shear+Local JD % Shear+Local JD None Shear+Local Note: longitudinal reinforcements in supporting are 6B20, all the openings are located centrally, all the depths of the RC supporting are 200mm. 4.3 The position of the opening Table 4 shows the FE result of five samples with the same opening size but varying opening position. It notes that as the opening position gradually moves to the wall edge, the bearing capacity decrease significantly. The opening position of sample JD-650 deviates 400mm from the sample JD-1050, with the bearing capacity decreases of 43%. The non-centrally located

6 opening changes the loading transfer path of wall-, making short wall leg get into high compression stress and destroy ahead of long wall leg, which causes the decrease of the bearing capacity of the wall-. Sample No Table 4: The FE results for varying opening position Dimension of opening Distance between opening center and support Compressive strength of grouted masonry wall N 2 Bearing capacity /kn JD Shear JD Shear JD Shear JD Shear JD Shear Notes: the longitudinal reinforcement of supporting in simulated samples is 6B The span and the ratio of longitudinal reinforcement of RC supporting The span of RC supporting and the ratio of longitudinal reinforcement are main influence factors when the wall- comes to the bending failure. Table 5 shows the FE results of the bearing capacity for ten samples without opening and for six samples with the opening. Table 5: The FE results for varying span and longitudinal reinforcement ratio of the supporting Longitudinal Compressive strength Dimension Bearing reinforcement of grouted block Sample No Span of opening capacity of supporting masonry wall N 2 /kn W B None 2649 Bend W B None 2399 Bend W B None 1952 Bend W B None 1765 Bend W B None 1505 Bend W B None 2754 Bend W B None 2500 Bend W B None 2383 Bend W B None 2168 Bend W B None 1786 Bend WD B Bend WD B Bend WD B Bend WD B Bend WD B Bend WD B Bend Note: the openings are all in the central position. The reinforcement ratio in masonry wall is the same for all simulated samples.

7 From Table 5 it is found that the bearing capacity of the wall- significantly decreases from 35% to 43%, as the span of the wall- without the opening increases from 2.1 to 3.7. The smaller the ratio of the longitudinal reinforcement is, the faster the reducing extent of bearing capacity. When the longitudinal reinforcement ratio of the supporting increases from 1.24% or (6B12) to 2.43% (6B14), the bearing capacity for the wall- samples with the opening increases 36%~53%, whereas the bearing capacity for the wall- samples without the opening increases 4%~17%. The effect of the longitudinal reinforcement ratio for the bearing capacity is sensitive for the wall- with the opening, and the longer the span is, the apparent the influence of longitudinal reinforcement ratio is. Therefore, the longitudinal reinforcement ratio and the span of wall- are the main influence factors on the bearing capacity of reinforced block wall- when it occurs to bending failure. 4.5 The supporting depth To find how the RC supporting depth influences the bending and shear capacity of the reinforced masonry wall-, ten samples with varying RC supporting depth are simulated. From Table 6, it is found that the bearing capacity merely increases 8.8% for shear failure and 15.8% for bending failure, while the depth-span ratio enhances from 1/14 to 1/6. RC supporting, as the tie rod of the tie-arch l, bears the eccentric tension mainly by the longitudinal reinforcement in it. Thus, the longitudinal reinforcement ratio of RC supporting is more crucial compared with the depth of it. In conclusion, the RC supporting depth can be reduced to 1/14 of its span, which can avoid the failure of the wall- caused by the supporting when there is enough longitudinal reinforcement in the supporting Sample No Table 6: The FE results for varying supporting depth Supporting depth depth-span ratio of supporting Longitudinal reinforcement of supporting Compressive strength of grouted block masonry wall N 2 Dimension of opening Bearing capacity /kn J-t /14 6B Shear J-t /10.5 6B Shear J-t /8.4 6B Shear J-t /7 6B Shear J-t /6 6B Shear W-t /14 6B Bend W-t /10.5 6B Bend W-t /8.4 6B Bend W-t /7 6B Bend W-t /6 6B Bend Note: the openings are all in the central position. 4.6 The ratio of the distributing bar in the grouted masonry wall Test results from Zhai and Totoev (2009) show that the horizontal distributing reinforcements in the reinforced grouted concrete block masonry wall provided effect by appearing yield stress when the shear failure of wall occurred. In addition, the longitudinal

8 reinforcement in the vertical core columns, at the same time, play a positive role in the arch transfer process as well as the composite action between masonry wall and the RC supporting. Therefore, in order to find the influence of distributing bars in the masonry wall, five steel diameters are selected for the FEA in this paper, and Table 7 shows the FE results. From Table 7 it can be found that the horizontal and longitudinal distributing reinforcement, with unobvious improvement on shear capacity, nearly do not provide influence on the bearing capacity. For Example, the distributing reinforcement ratio of JQ-16 is enhanced three times compared with JQ-8, the bearing capacity merely increases 10% for shear failure and 8.9% for bending failure. Therefore, to improve the bearing capacity of the wall-, what we should consider most is the material strength, span and the reinforcement ratio in RC supporting instead of reinforcement ratio in wall. Table 7: The FE results for varying distributing bar Sample No. Reinforcement of supporting Dimension of opening Longitudinal and horizontal distributing bar ratio of block masonry wall Bearing capacity /kn JQ B B8@200 (0.13%) B8@400 (0.07%) JQ B B10@200 (0.21%) B10@400 (0.11%) JQ B B12@200 (0.30%) B12@400 (0.15%) JQ B B14@200 (0.40%) B14@400 (0.20%) JQ B B16@200 (0.53%) B16@400 (0.27%) WQ B12 None B8@200 (0.13%) B8@400 (0.07%) WQ B12 None B10@200 (0.21%) B10@400 (0.11%) WQ B12 None B12@200 (0.30%) B12@400 (0.15%) WQ B12 None B14@200 (0.40%) B14@400 (0.20%) WQ B12 None B16@200 (0.53%) B16@400 (0.27%) Note: the openings are all on the central position for the samples with the opening Shear 2371 Shear 2426 Shear 2508 Shear 2563 Shear 2597 Bend 2617 Bend 2649 Bend 2690 Bend 2827 Bend 5 CONCLUSIONS 1) There is composition behaviour and structural interaction between the reinforced grouted concrete block masonry wall and the RC supporting by concrete core columns with vertical reinforcement, and the horizontal reinforcement in the block grooves can provide resistance to shear failure. The structural behaviour of reinforced grouted concrete block masonry wall- is a typical tie-arch. 2) For the shear failure of wall-, the span of the reinforced masonry wall- and the strength of the grouted masonry wall become the main factors for the bearing capacity. For bending failure, however, the longitudinal reinforcement ratio and the span of wall-

9 take the dominating position. 3) The opening on reinforced grouted block wall- can reduce the bearing capacity by weakening the arch action. The non-centrally located opening will change the shape of the arch and the force transferring path, which reduce its bearing capacity. 4) The horizontal distributing reinforcement can restrain the development of the inclined cracks, but has little influence on improving the bearing capacity. The stress of longitudinal reinforcement is relative small, but it can connect the supporting with the upper wall to a whole integrity, making them work together and avoid cracks between the supporting and the upper wall. 5) The depth-span ratio of simply-supported reinforced block wall- with the opening can be decreased to 1/14 under the condition of adequate reinforcement in the supporting. REFERENCE GB Code for Design of Masonry Structure. Standards China, Beijing, 2001, 151pp. (in Chinese) Hu, O.E. and Tan, K.H. Large Reinforced-Concrete Deep Beams with Web Opening: Test and Strut-and-Tie Results, Magazine of Concrete Research, 2007, 59(6): pp Prodromos, Z.D. Shear Compression in Reinforced Concrete Deep Beams, Journal of Structural Engineering, ASCE, 2003, 129 (4): pp Tan, K.H., Tong, K. and Tang, C.Y. Consistent Strut-and-Tie Modelling of Deep Beams with Web Openings, Magazine of Concrete Research, 2003, 55(1): pp Tang, C.Y. and Tan, K.H. Interactive Mechanical Model for Shear Strength of Deep Beams, Journal of Structural Engineering, 2004, 130(10): pp Zhang Y.J. The Research of Stress-strain Axial Compression Experiment of Concrete Block, Dissertation for the Master Degree, Harbin Institute of Technology, China, (in Chinese) Zhai, X.M., Totoev, Y.Z. and Stewart, M.G. Experimental Study on Structural Performance of Grouted RC Block Wall/Beam Composition, 11 th Canadian Masonry Symposium Proceeding, Toronto, 2009, pp