Response of Reinforced Concrete Beams with …...Seven reinforced concrete T-beams were tested with two design parameters, including number of web circular openings (four or six) and

Fourth Asia-Pacific Conference on FRP in Structures (APFIS 2013) 11-13 December 2013, Melbourne, Australia

© 2013 International Institute for FRP in Construction

Response of Reinforced Concrete Beams with Multiple Web Openings to Static Load

Nazar K. Oukaili1 and Abeer H. Shammari2

1Professor, Department of Civil Engineering, University of Baghdad, Baghdad, Iraq, E-mail: [email protected]

2Ph.D. Student, Department of Civil Engineering, University of Baghdad, Baghdad, Iraq, E-mail: [email protected]

ABSTRACT Web openings in beams occur quite often in practice to provide convenient passage of electrical and mechanical ducts. Shear failure may dominate in cases where web opening created or the flexural strength has been increased. In such cases, increasing the shear capacity can prevent catastrophic shear failure. In this paper, the investigation of the response of simply supported reinforced concrete T-beams with multiple web circular openings is presented. An experimental research program was conducted on the shear behavior of simply supported beams subjected to an incremental concentrated static load at mid span. Seven reinforced concrete T-beams were tested with two design parameters, including number of web circular openings (four or six) and the method which was used to strengthen the member at openings (using internal deformed steel bars as in the case of the openings are planned before casting the beams or using carbon fiber reinforced polymer (CFRP) fabric as in the case of the openings will be created in the existing beams). Test results indicated that the strengthening of beams at openings may compensate the decrease of the beam capacity due to the existence of the openings under static loading. The compensation of the beam capacity depends on the strengthening method which was adopted. The gain in shear capacity of the tested beams ranged between 27% and 92% which also represents about 86% to 114% of the reference solid beam capacity under static loading. A nonlinear finite element program, ANSYS 12.1 release, was used to validate the results of the tested beams. Comparisons between the finite element predictions and experimental results in terms of failure loads and load-deflection relationships are presented and showed good agreement. KEYWORDS Concrete beams, multiple openings, strengthening, finite elements method. INTRODUCTION In some cases, it is necessary to create multiple openings in the web of the beam to permit a systematic layout of utility duct and pipes which resulted in a significant reduction in the story height (instead of placing these ducts and pipes underneath the beam soffit and, for aesthetic reasons, covering them by a suspended ceiling, which in turn creating a dead space and increasing the story height) and major saving in material and construction cost especially in multistory buildings. Creating openings in the web of the beam reduces the cross sectional area and therefore decreases the shear capacity and increases the deformation ability. If the openings are planned before casting the beam, internal deformed steel bars can be fabricated around the openings while if the openings will be created in the existing beam, strengthening by external steel plates or fiber reinforced polymer (FRP) materials can be used to restore the original capacity. The combination of low weight and high strength to weight ratio of fiber reinforced polymer (FRP) material are advantageous for this type of material. Moreover, FRP materials having high resistance to corrosion, hence they are satisfactory in terms of durability requirement. The FRP materials are usually exist in the form of sheets, strips, rods, laminates or fabric. In this study, Carbon Fiber Reinforced Polymer (CFRP) fabric was used as a strengthening system.

According to Somes and Corley (1974), a circular opening may be considered as large when its diameter exceeds 0.25 times the depth of the web because introduction of such openings reduces the strength of the beam. Moreover,

Mansur et al (1991) carried out an experimental investigation on eight reinforced concrete continuous beams, each containing a large transverse opening. Mansur et al. study showed that an increase in the depth of opening from 140 mm to 220 mm led to a reduction in collapse load from 240 KN to 180 KN. Abdallaa et al. (2003) used fiber reinforced polymer (FRP) sheets to strengthen the opening region in an experimental program. Meanwhile, El Maaddawy and Sherif (2009) investigated the use of FRP sheets for shear strengthening of reinforced concrete deep beams with square openings. However, to the researcher’s knowledge, there are very limited researches on studying the behavior of reinforced concrete beams with multiple web openings and no researches on using CFRP fabric to strengthen beams with multiple web openings. In this study, the experimental results of seven reinforced concrete T-beams are presented. The experimental program addresses the behavior of reinforced concrete beams when four or six circular openings in shear region are created, the loss of structural capacity due to creating the multiple web openings and the efficiency of using internal deformed steel bars or external CFRP fabric around the openings to restore the original capacity. The results in terms of crack patterns, ultimate loads, failure mode, load versus deflection and the strain distribution in the vicinity of openings for concrete and CFRP are presented. For validation, a nonlinear finite element program, ANSYS, was used to simulate the tested beams. EXPERIMENTAL PROGRAM In the experimental program, a total of seven reinforced concrete T-beams were fabricated and tested as simply supported beams under one incremental concentrated load at mid span until failure. The design parameters are the number of web openings (four or six) and the method used to strengthen beams at openings (using internal deformed steel bars or CFRP fabric) to re-gain the loss of structural capacity due to creating web openings. TEST SPECIMENS Designation of test specimens and their details are listed in Table 1. One of these beams was a solid control beam without openings, two of them were with unstrengthened openings and the remaining four beams were with strengthened openings.

Table 1. Details of specimens used in experimental program

Beam number Beam symbol Number of openings Type of strengthening at opening regions

1 BSW Without openings --- 2 BSO4 4 No strengthening 3 BSO6 6 No strengthening 4 BSO4S 4 Internal steel bars 5 BSO6S 6 Internal steel bars 6 BSO4F 4 CFRP fabric 7 BSO6F 6 CFRP fabric

All openings were of circular shape with diameter equal to 110mm. The ratio of the opening diameter to the web depth was 0.48 which was enough to cause a sizeable reduction in shear capacity and to evaluate the efficiency of different strengthening techniques (web containing a circular opening of three-eighths the web depth dose not reduce the strength of the specimen, ACI-ASCE committee 426). The distance between the support and the beginning of the first opening was 130 mm which equals about half the effective depth. Specimens with multiple web openings fail in the post (the element between two openings when these openings are placed closed to each other) when the width of the post is smaller than 3/8 of the web depth (ACI-ASCE committee 426). In this study, the width of the post was about 5/8 of the web depth. In order to investigate the shear behavior of the test specimens, the solid control beam was designed to fail in shear (the flexural capacity was designed to exceed the shear capacity of the control beam) according to ACI 318-M building code requirements (2011). A schematic diagram of the test specimens showing the dimensions and reinforcement details is depicted in Figure 1a. The length of the test specimen was 2000mm with an effective span

of 1800mm; the effective depth (d) was 256mm. Identical T-shape cross-section was used for all specimens. The tension steel reinforcement consists of two 20mm, the compression reinforcement consists of four 6mm. Stirrups consist of 4mm at 130mm center to center. This reinforcement was used for the control beam, beams with unstrengthened openings and beams with CFRP fabric strengthened openings as shown in Figures 1b-1d. The rest beams were strengthened with pre-fabricated internal steel bars consisting of three 8mm as diagonal bars around the openings, two 8mm horizontal bars below and above the openings and concentrated stirrups of 4mm at (130/3) mm center to center; above, below and beside the openings, see Figures 1e-1f.

Sec. A-A

(b) BSW

(e) BSO4S

(c) BSO4, BSO4F

(d) BSO6, BSO6F

Figure 1. Details of beam specimens (unit: mm). (f) BSO6S

CFRP STRENGTHENING CONFIGRATION Beams BSO4F and BSO6F were strengthened with CFRP fabric having uni-directional fibers (SikaWrap-230C type) which cut into segments and placed around the openings in two layers, the first layer was of horizontal and vertical segments of CFRP fabric placed around openings while the second layer was of diagonal segments of CFRP fabric placed beside openings with an angle of inclination of 45°, see Figure 2.

CFRP Second layer

CFRP first layer

Figure 2. Alignment of CFRP fabric.

In general, the concrete surface beneath the CFRP fabric was roughed and then cleaned prior to the adhesive application using epoxy resin (Sikadur-330 type) with average thickness of 1 mm.

MATERIAL PROPERTIES Properties of concrete, steel reinforcement and fiber reinforced polymer used in this study are given in Table 2.

Table 2. Material properties Material Diameter, (mm)

Thickness, (mm) Yield strength,

(MPa) Compressive

strength, (MPa) Tensile strength,

(MPa) Modulus of

elasticity, (GPa) Concrete --- 27 3.2 23

Steel

20 600 --- 747

200 6 420 --- 570 4 400 --- 553 8 450 --- 602

CFRP fabric 0.131 --- --- 4300 238 Epoxy resin --- --- --- 30

7 days at +23°c 4.5

7 days at +23°c

TEST SETUP AND INSTRUMENTATIONS All beam specimens were tested as simply supported beams under a monotonic concentrated load at mid span up to ultimate load using a 3000 kN capacity hydraulic universal testing machine (MFL SYSTEM). The following instrumentations were used in this study: 1- Electrical strain gages were used to record strains of concrete and CFRP fabric surface; locations of strain gages with their coordinates are shown in Figure 3.

2- Demec point with an extensometer were used to record a flexural tensile strain at concrete surface at mid span, the location of this point is shown in Figure 3.

Strain gage

X- coordinate

Y- coordinate

SG1 480 80 SG2 740 80 SG3 1260 80 SG4 1520 80 SG5 1000 260 DP6 1000 20

Figure 3. Location of strain gages and demec point.

3- Two dial gages of 0.01 mm accuracy were used to measure vertical deflections at mid and quarter span. Figure 4 shows the test setup and instrumentation locations.

Figure 4. Test setup and instrumentation locations

Testing machine

Test beam

Strain gages

Dial gages

EXPERIMENTAL RESULTS AND DISCUSSIONS

Cracking Behavior, Failure Loads, and Mode of Failure A summary of first crack loads, ultimate loads and mode of failure is presented in Table 3. The loads recorded did not include the self-weight of the specimens. During each test, cracks were marked on one face of the tested specimen at discrete load increments; Figure 5 shows crack patterns for all specimens. The solid control beam, BSW, failed in shear mode. This specimen shows several flexural tension cracks at early stages of loading (at 40 kN) then (at about 40% of the ultimate load), number of inclined cracks initiated from these flexural cracks to form flexural-shear cracks. At the same load level, diagonal tension cracks appeared at nearly mid height of the beam, between the existing inclined cracks and the supports, and propagate towards the point of loading and the supports. Splitting cracks initiated from the diagonal tension crack (failure crack) oriented along the longitudinal reinforcement at an applied load of about 75% of the ultimate load. As the applied load was increased, these cracks widened and continued in propagation towards the point load and the supports till brittle shear failure took place at load of 160 kN. Unlike the solid beam, beams BSO4 and BSO6 showed fewer numbers of flexural cracks at load 40 kN, these cracks remained inactive. Several diagonal cracks initiated from the openings, passing through them and extended towards the point of the applied load and the supports; bringing about an abrupt shear failure at 112 kN and 95 kN for BSO4 and BSO6 specimens respectively. Beams BSO4S and BSO6S failed in shear mode which approximately similar to that of the solid beam at loads of 142.5 kN and 137.5 kN respectively. Beam BSO4F shows several flexural cracks at load 20 kN. Several inclined cracks initiated from flexural cracks to form flexural-shear cracks at load 40 kN and continued in growth in size and propagation towards the point load and supports until failure at load of 147.5 kN. Appearance of these cracks limited on the unstrengthened regions. Beam BSO6F did not show any crack since CFRP fabric covered almost whole surface area of the beam web. At failure, buckling of CFRP fibers was noticed where the concrete deteriorated above the path of the diagonal tension crack which appeared at advanced stage of loading. Ultimate load was 182.5 kN.

Table 3. Summary of the test results Specimen First crack

load, (kN) Ultimate load

Pu, (kN) Pu/ Pu of the

solid beam, (%) Gain in shear capacity, (%)

Crack and failure mode

BSW 18 160 100 --- Diagonal tension crack (shear failure)

BSO4 15 112 70 --- Shear cracks at opening regions (shear failure)

BSO6 12.5 95 59 --- Shear cracks at opening regions (shear failure)

BSO4S 20 142.5 89 27 Diagonal tension crack (shear failure)

BSO6S 15 137.5 86 45 Diagonal tension crack (shear failure)

BSO4F 20 147.5 92 32 Flexural-shear cracks (shear failure)

BSO6F 27.5 182.5 114 92 Diagonal tension cracks and buckling of CFRP fabric (shear

failure)

Figure 5. Crack patterns

Figure 5. Continue, crack patterns

Load-Strain Relationships Location at which strains were measured are shown in Figure 3.For beam BSW, the maximum measured compressive strain at concrete surface was 600 με ,this value is well below the crushing strain of concrete. Strains at the other four points were measured and plotted in Figure 6. The dotted lines indicated that strains were obtained by using demec points and extensometer due to losing of electrical strain gage readings which resulting from passing of cracks through these strain gages. For the same reason, the flexural tensile strain at concrete surface was obtained using demec points. Lorenzis and Nani (2001) pointed out that the recorded strain is not necessarily a maximum value as it is strictly related to the location of the gage with respect to the shear cracks. By the same procedure, strains were obtained for other beams and plotted in Figures 6. Concrete compressive strains remained below the crushing strain of concrete. For beams BSO4F and BSO6F, the maximum strain measured on CFRP fabric surface was 4000 με which represents about 26% the maximum tensile strain of this material.

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Figure 6. Continue, load-strain relationships

Load-Deflection Relationship The load versus mid span and quarter span deflection curves for all beams are presented in Figures 7-8. From these figures it can be noticed that introducing openings in the beams causing a reduction in the stiffness of them and this behavior depends on the number of the introduced openings. Strengthening regions around openings can compensate the reduction in the beam stiffness.

Figure 7. Load-mid span deflection relationships Figure 8. Load-quarter span deflection relationships

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FINITE ELEMENT MODELING

Element Types A solid element, SOLID65, is used to model the concrete in ANSYS. The solid65 element has eight nodes with three degrees of freedom at each node, translations in the nodal x, y, and z directions. A LINK8 element is used to model the steel reinforcement; two nodes are required for this element. At each node, degrees of freedom are identical to those for the SOLID65 element. An eight-node solid element, Solid45, is used to model the steel plates at the supports and loading area in the beam models. This element is defined with eight nodes having three degrees of freedom at each node-translation in the nodal x, y, and z directions. A layered structural solid element, SOLID185, is used to model the CFRP. This element has orthotropic material properties in each layer. The element has three degrees of freedom at each node, translations in the nodal x, y, and z directions.

Material Properties

Concrete: SOLID65 element is capable of predicting the nonlinear behavior of concrete materials using a smeared crack approach, William and Wranke (1975). Concrete is a quasi-brittle material and has very different behaviors in compression and tension. Compressive and tensile strength and modulus of elasticity of concrete which are needed as input data in ANSYS are given in Table 2. Poisson’s ratio for concrete was assumed to be 0.2, Bangash (1989).Also shear transfer coefficients are needed in ANSYS, these values range from 0.0 to 1.0, where 0.0 representing a smooth crack (complete loss of shear transfer) and 1.0 representing a rough crack (no loss of shear transfer). The shear transfer coefficients for open and closed cracks were selected equal to 0.3 and 0.5 respectively. Steel: elastic modulus and yield stress for the steel (reinforcement and plates) used in ANSYS follow the design material properties used for the experimental investigation. The steel for the finite element models is assumed to be an elastic-perfectly plastic material and identical in tension and compression. A Poisson’s ratio of 0.3 is used for the steel reinforcement. CFRP composites:input data needed for the FRP in the finite element model are given in Table4

Table 4.Smmary of material properties for CFRP composites, Products Catalog (2010). FRP

description Elastic

modulus, (GPa) Major

Poisson’s ratio Tensile

strength, (MPa) Shear modulus,

(GPa) Thickness of fabric, (mm)

Elongation at break, (%)

Woven carbon fiber

fabric

Ex = 238 xy = 0.22 4300

Gxy = 12.5 0.131

1.8 Ey = 19 xz = 0.22 Gxz = 12.5

Ez = 19 yz = 0.3 Gyz = 7.5

Modeling Methology

By taking advantages of the symmetry of beams, a quarter of the full beam was used for modeling with proper boundary conditions. This approach reduces computational time and computer hard space requirement significantly. Ideally, the bond strength between the concrete and steel reinforcement should be considered. However, in this study, perfect bond between concrete and steel was assumed. The CFRP was simplified in the model by ignoring the inclined segments of CFRP fabric present in the test beams. In ANSYS, the horizontal and vertical segments of CFRP fabric were modeled as volumes with a consistent overall thickness to avoid discontinuity which may develop high stress concentration at local area on the model. Figure 9 shows volumes created in ANSYS. Because only a quarter of the entire beam was modeled, planes of symmetry were required at the internal faces; rollers were used to show the symmetry condition. In support region, vertical movement was restricted while rotation was permitted to avoid excessive cracking of concrete at this region. Figure 10 shows FE mesh, loading and boundary conditions.

Concrete volumes

CFRP volumes Steel support

volumes

Support condition

Equivalent applied load

Planes of symmetry

Figure 9. Volumes created in ANSYS for BSO4F model Figure 10. FE mesh, loading and boundary conditions

RESULTS COMPARISION

ilure Loads Fa Table 5 shows a comparison between the ultimate loads of the experimental beams and the final loads from the finite

Table 5.Comparision between experimental ultimate loads and finite element final loads Speci nce, (%)

element models. It is seen that the ANSYS models overestimated the strengths of the beams since finite element method deals with the modeled member as a rigid body which shows stiffer and stronger behavior than the real one. Micro-cracks produced by drying shrinkage are present in the concrete to some degree; these would reduce thestiffness of the actual beams, while the finite element models do not include micro-cracks.

men Experimental ultimate loads, (kN) Final loads from ANSYS, (kN) DiffereBSW 160 188.2 17 BSO4 112 129.5 15 BSO6 95 113.5 19

BSO4S 1 42.5 162.7 14 BSO6S 137.5 163.6 18 BSO4F 147.5 165.7 12 BSO6F 182.5 186.0 2

Load-De elationships

omparisons between load-mid span deflection relationships for the experimental and finite element analysis results

Figu ships

flection R Care presented in Figure 11. From these figures it can be seen that the load-deflection relationships from ANSYS agree well with the experimental relationships. ANSYS predicts the beam to be stiffer than the experimental one for all specimens; this behavior is also due to the assumed perfect bond between concrete and steel reinforcement.

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CONCLUSIONS

presented herein, the following conclusions have been drawn: - Introducing four or six un-strengthened circular openings with diameter of 0.48 the web depth in the shear zone

by 30% and 41% respectively,

of the solid beams.

by the load-deflection plots at mid span showed a good

9%. These differences are acceptable and therefore the finite element models can be reused for further

rkeya, A.M., Haggagb, H.A. and Abu-Amira, A.F. (2003). ”Design against cracking at openings in inforced concrete beams strengthened with composite sheets”, Composite Structures, 60, 197-204.

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ar Strength of Reinforced Concrete Members (ACI 426R-74) (Reapproved 980)”, Proceeding, ASCE, V.99, No.ST6, 1148-1157.

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erical modeling and applications”, Elsevier Science ublishers Ltd., London, England.

09).” FRP composites for shear strengthening of reinforced concrete deep beams ith openings”, Composite Structures, 89(1), 60-69.

ning of reinforced concrete beams with near-surface mounted ber reinforced polymer rods”, ACI Structural Journals, 98(1), 60–8.

continuous beams with openings1991”, ournal of Structural Engineering, 117(6), 1593-1605.

08.06.

bs of continuous beams”, American Concrete stitute, Detroit, MI, 359-398.

“Constitutive Model for the Triaxial Behavior of Concrete”, Proceedings, ternational Association for Bridge and Structural Engineering, Vol. 19, ISMES, Bergamo, Italy, pp. 174.

Based on the study 1of reinforced concrete T-beams reduces the strength capacity of these beamsincreases beam deflection and introduces intensive shear cracks around openings. 2- The application of strengthening technique introduced in this study greatly decreases beam deflection, control cracks around openings and increase the ultimate capacity of the beams. 3- The gain in shear capacity of T-beams with four and six strengthened circular openings by internal steel bars was 27% and 45%. Load carrying capacity of these beam were 89% and 86% 4- The gain in shear capacity of T-beams with four and six strengthened circular openings by CFRP fabric was 32% and 92%. Load carrying capacity of these beam were 92% and 114% of the solid beams. 5- Shear failure at opening regions of CFRP strengthened openings occurs due to a combination of shear cracking of concrete and buckling of FRP fabric glued to the concrete. 6- Finite element method using ANSYS program was used to simulate T-beams with multiple web openings. The general behavior of the finite element models represented agreement with the test data from the beam tests. However, the finite element models showed a stiffer behavior than the test data. The effects of bond slip (between the concrete and steel reinforcing) and micro cracks occurring in the actual beams were excluded in the finite element models, contributing to the higher stiffness of the finite element models. 7- The final loads from the finite element analyses were higher than the ultimate loads from the experimental results by 2% - 1studies by varying various parameters. REFERENCES Abdalla, H.A., Tore ACI 318M-11 Building code requirements for structural concrete and commentary (2011), Farmingto M ACI-ASCE Committee 426 (1973). ” She1 ANSYS, ANSYS User’s Manual Release 12.1, ANSYS, Bangash, M.Y. (1989). “Concrete and concrete structures: NumP El-Maaddawy, T. and Sherif, S. (20w Lorenzis, L.D. and Nanni, A. (2001). “Shear strengthefi Mansur, M.A., Lee, Y.F., Tan, K.H. and Lee, S.L. (1991). “Test on RCJ Sika Egypt –Products Catalog (2010) Edition 6, Version Somes, N.F. and Corley, W.G. (1974). “Circular openings in weIn William, K. J. and Warnke, E. P. (1975).In