BEHAVIOR OF INFILL MASONRY WALLS STRENGTHENED WITH FRP MATERIALS

Transcription

1 BEHAVIOR OF INFILL MASONRY WALLS STRENGTHENED WITH FRP MATERIALS D.S. Lunn 1,2, V. Hariharan 1, G. Lucier 1, S.H. Rizkalla 1, and Z. Smith 3 1 North Carolina State University, Constructed Facilities Laboratory, 2414 Campus Shore Dr., Campus Box 7533 Raleigh, NC, 27695, USA 2 Corresponding Author: dslunn@ncsu.edu 3 Fyfe Co. LLC., 838 Miralani Drive, Suite A, San Diego, CA, 92126, USA ABSTRACT The objective of the research program presented in this paper is to evaluate the effectiveness of strengthening infill masonry walls with externally bonded glass fiber reinforced polymer (GFRP) material to increase their outof-plane resistance to lateral loads. The experimental program comprised fourteen full-scale specimens, including four un-strengthened (control) specimens and ten strengthened specimens. All specimens consisted of a reinforced concrete (RC) frame (which simulates the supporting RC elements of a building superstructure) that was in-filled with solid concrete brick masonry. The specimens were loaded by out-of-plane uniformly distributed pressure in cycles up to failure. Parameters investigated include the aspect ratio, FRP coverage ratio, number of masonry wythes, and type of FRP anchorage. The type of FRP anchorage was found to greatly influence the failure mode. Un-strengthened specimens failed in flexure. However, strengthened specimens without overlap of the FRP onto the RC frame failed due to sliding shear along the bed joints which allowed the walls to be pushed out from the RC frames in a rigid body fashion. In the case where GFRP sheets were overlapped onto the RC frames, the aforementioned sliding shear caused delamination of the GFRP sheets from the RC frames. Use of steel angles anchored along the perimeter of the walls as shear restraints allowed these walls to achieve three times the service load without any visible signs of distress. GFRP strengthening of infill masonry walls was found to be effective when proper anchorage of the FRP laminate is provided. KEYWORDS FRP, infill masonry walls, strengthening, flexural, debonding, sliding shear, arching action. INTRODUCTION Collapse of unreinforced masonry structures, including infill masonry walls, is a leading cause of property damage and loss of life during extreme loading events. Many existing structures are in need of retrofit to bring them in compliance with modern design code provisions. Conventional strengthening techniques are often timeconsuming, costly, and add significant weight to the structure. These limitations have driven the development of alternatives such as externally bonded fiber reinforced polymer (FRP) strengthening systems, which are not only lightweight, but can be rapidly applied and do not require prolonged evacuation of the structure. Several studies have explored the application of FRP strengthening systems to masonry walls subjected to out-of-plane loading, however these studies focused on simply supported masonry structures and did not directly address the boundary conditions of masonry infill walls, which typically consist of a mortar interface between the masonry infill and the supporting structural elements. The objective of this research program is to evaluate the effectiveness of strengthening infill masonry walls with externally bonded glass fiber reinforced polymer (GFRP) sheets to increase their out-of-plane resistance to loading by accurately simulating the boundary conditions of infill masonry walls. EXPERIMENTAL PROGRAM The experimental program comprised fourteen full-scale specimens (244 mm high and 244 mm to 39 mm wide), including four un-strengthened (control) specimens and ten strengthened specimens. All specimens consisted of a reinforced concrete (RC) frame (which simulates the supporting RC elements of a building superstructure) that was in-filled with solid concrete brick masonry having an average compressive strength of 13.2 MPa. The strengthened specimens were reinforced with externally bonded GFRP sheets applied to the 53

2 exterior (tension) face of the outer wythe of the masonry infill. The GFRP laminate has a design ultimate tensile strength, in the primary direction of the fibers, of 46 MPa, a tensile modulus of.9 GPa, a rupture strain of.176 mm/mm and a thickness of 1.3 mm, as provided by the manufacturer. The parameters investigated include the aspect ratio, FRP coverage ratio, number of masonry wythes, and the three types of FRP anchorage shown in Figure 1. The aspect ratio, width to height, was varied from 1. to 1.6 to explore the varying degrees of two-way action. The FRP coverage ratio is the percentage of the surface area of the exterior face of the outer wythe that was covered with unidirectional GFRP in the vertical (V) and horizontal (H) directions respectively. This percent coverage varied from 5% in both directions to % in both directions. Both single and double wythe specimens were tested. For double wythe specimens, the collar joint between the two wythes was either intentionally filled with mortar or left empty for comparison purposes. Three different anchorage systems were used. The first provided anchorage by overlapping the GFRP sheets onto the RC frame. For comparison and in consideration of the fact that not all existing masonry infills are flush with the supporting RC boundary elements, the second anchorage system terminated the GFRP sheets at the outer edge of the masonry with no overlap onto the RC frame. After it was determined that these specimens failed prematurely in a shear sliding mode, the final five specimens were tested with the third anchorage system in which mechanical anchorage was provided by a steel shear restraint anchorage system applied to three sides. L L GFRP GFRP GFRP Steel Angles Tube Steel Assembly FRP Overlap onto onto Shear Restraint Anchorage System Figure 1: FRP Anchorage Systems A profile view and a photograph of the test setup are shown in Figure 2. The test specimens were loaded in one direction out-of-plane with a uniformly distributed pressure to simulate the differential pressure (suction) induced by a tornado. An airbag was used to apply static pressure in increasing cycles up to failure. The airbag was placed between the brick walls and the laboratory reaction wall. The laboratory reaction wall is a strong wall fixed to the laboratory strong floor, both of which are extremely rigid compared to the test specimens. The concrete frames were secured to the reaction wall using high strength steel bars spaced 915 mm on center. This system was used to simulate the rigidity of existing RC structural members. Test specimens were supported by a 46 mm deep steel wide flange beam to achieve alignment with the holes in the laboratory reaction wall. Reaction Wall Strengthened Figure 2: Test Setup 54

3 TEST RESULTS For each specimen, the elastic pressure limit and the ultimate applied pressure were determined. In this paper, the elastic limit corresponds to the pressure which induced a major loss in stiffness. The magnitude of the elastic limit was determined graphically based on the change in stiffness of the brick wall, as defined by the slopes of the pressure-deflection curve. The ultimate applied pressure was determined experimentally as the maximum pressure sustained before total collapse of the brick walls. The elastic limit and ultimate pressure are shown schematically in Figure 3 for a typical pressure-deflection relationship. Elastic Limit Applied Pressure Ultimate Pressure Deflection Figure 3: Elastic Limit and Ultimate Applied Pressure There were three failure modes observed in testing: Flexural, shear sliding, and GFRP debonding as shown in Figure 4 and Figure 5. All observed failures were the result of one or more of these modes. The type of FRP anchorage was found to greatly influence the failure mode. Unstrengthened (control) specimens failed in the flexural mode characterized by the formation of a main horizontal or vertical crack (or both). The out-of-plane displacement profile suggests that the double wythe control specimens developed arching action spanning the horizontal direction. Strengthened specimens in which the FRP reinforcement was terminated at the outer edge of the masonry with no overlap onto the RC frame failed in the shear sliding mode characterized by a large relative slip between the masonry and the RC frame. The FRP stiffened the wall panel and held it together as a single unit that was then able to slide out of the frame in a rigid body fashion. Strengthened specimens with the GFRP sheets overlapped onto the RC frame failed in a debonding mode characterized by the delamination of the GFRP sheets beginning at the interface between the masonry and the RC frame. Debonding observed in this experimental program was always the result of the relative slip between the masonry infill and the RC frame due to shear sliding. Specimens that were mechanically anchored using the steel shear restraint anchorage system withstood over three times the design pressure without any visible signs of distress. The influences of the various test parameters are shown in Figure 6. Overlapping the GFRP onto the RC frame more than doubled the lateral load carrying capacity of the double wythe control specimen, however, the specimen without overlap provided no significant increase in strength. Both double wythe specimens had vastly greater strength than the single wythe specimen, however, there was little difference between the double wythe specimens with and without fill in the collar joint. Specimens with an aspect ratio of 1.2 had a greater stiffness and a higher load carrying capacity than the corresponding 1.6 aspect ratio specimens. (a) (b) Main Horizontal Crack RC Frame Main Vertical Crack Δ Figure 4: Failure Modes I: (a) Flexural Failure, (b) Shear Sliding Failure 55

4 (a) FRP Debonding RC Frame (b) Tube Steel Assemblies Steel Angles Figure 5: Failure Modes II: (a) FRP Debonding Failure, (b) No Failure in Shear Restrained Specimens Overlapped (S1-1.2-O) (S4-1.2-NO)* Control (C2-1.2) (a) Influence of FRP Anchorage System 1.6 Aspect Ratio (S1-1.6-SR*) *Note: Shear Restraints Removed 1.2 Aspect Ratio (S5-1.2-SR*) Double Wythe - Solid Filled (C2-1.2) Single Wythe (C1-1.2) Double Wythe - No Fill (C3-1.2) (b) Influence of Number of Wythes *Note: Shear Restraints Removed 5% Coverage (S1-1.6-SR*) % Coverage (S2-1.6-SR*) (c) Influence of Aspect Ratio Figure 6: Influence of Parameters (d) Influence of FRP Coverage Ratio Summary of the experimental results is given in Table 1. All shear restrained specimens reached an applied pressure of 27 kpa with no visible signs of damage. These specimens are given the failure code NF for No Failure, because the test was terminated after they successfully resisted an applied pressure of 27 kpa. Four of these specimens (S5-1.2-SR, S6-1.2-SR,S1-1.6-SR, S2-1.6-SR) were re-tested without the shear restraints. In this second phase of testing, the specimens had the same type of FRP anchorage as those strengthened without overlap onto the reinforced concrete frame. This second phase of testing is denoted by an * following the specimen ID. 56

5 Table 1: Summary of Results Anchorage System Specimen ID Aspect Ratio Number of Collar Joint GFRP % Coverage Elastic Limit Ultimate Applied Failure Mode (w/h) Wythes (V/H) (kpa) Pressure (kpa) C Double Solid None FH-FV N/A C Single N/A None 6 6 FH (Control) C Double Solid None 21 3 FH-FV C Double No Fill None FH-FV Overlapped S1-1.2-O 1.2 Double Solid 5/ SS-D Onto RC S2-1.2-O 1.2 Single N/A 5/5 12 SS-D Frame S1-1.4-O 1.4 Double Solid 5/ SS-D S3-1.2-NO 1.2 Single N/A 5/5 SS S4-1.2-NO 1.2 Double Solid 5/ SS S5-1.2-SR* 1.2 Double No Fill 5/ SS Onto RC S6-1.2-SR* 1.2 Double No Fill 75/ SS Frame S1-1.6-SR* 1.6 Double No Fill 5/ SS S2-1.6-SR* 1.6 Double No Fill / SS S5-1.2-SR 1.2 Double No Fill 5/5 > NF S6-1.2-SR 1.2 Double No Fill 75/5 > NF Shear S7-1.2-SR 1.2 Double No Fill / SS Restrained S1-1.6-SR 1.6 Double No Fill 5/5 > NF S2-1.6-SR 1.6 Double No Fill / > NF Failure modes: FH Flexural with main horizontal crack; FV Flexural with main vertical crack; SS Shear Sliding; -D FRP Debonding; NF No Failure. WORKING STRESS ANALYSIS The working stress analytical approach considered the flexural behavior of the infill wall with respect to the reinforced concrete frame. The method specifies an allowable stress for both the masonry and the FRP material. For the masonry, the allowable stress is based on a 2/3 reduction of the measured compressive strength in accordance with the Standards Joint Committee (MSJC) Code (ACI 53-5 / ASCE 5-5 / TMS 42-5). The effective allowable stress for the FRP sheets is based on a reduction of the rupture stress based on a bond-dependent coefficient as described in detail in Lunn (9). The cross-section is analyzed in a state of pure bending for two failure cases: The first assumes that the masonry reaches its allowable stress while the strain in the FRP is below the allowable limit and the second assumes that the FRP material reaches its allowable stress first. The minimum of the moment resistances from the two cases is selected as the maximum allowable moment. Using this moment resistance, the uniformly distributed pressure, q e, to cause the maximum allowable moment is determined based on the geometry and boundary conditions of the wall. This pressure is calculated for two cases: the first assuming the worst case scenario in which the lateral support from the vertical edges is negligible and the wall is simply supported one-way in the vertical direction and the second assuming the wall behaves as a rectangular plate element simply supported by the four sides. Timoshenko & Woinowsky- Krieger (1959) provided the coefficient, β, governing the maximum internal bending moment for vertical bending of rectangular plates as a function of the aspect ratio width/height (w/h). The allowable applied uniformly distributed pressure, q e, for a given moment resistance, M r, and height, h, is given by Eq. 1. The results of both cases were then compared to the measured elastic limit to determine the extent to which the working stress approach could be used to predict the elastic limit for the various types of FRP anchorage systems. The allowable applied pressure based on the working stress analysis for both cases was compared to the 57

6 measured elastic limit of the applied pressure from the experimental testing as shown in Figure 7. From the figure it can be seen that the elastic analysis for case II, in which the walls are treated as rectangular plate elements that are simply supported on the four sides, leads to a better prediction of the elastic limit of the applied pressure compared to the previous assumption of vertical bending only. The results of this analysis capture the differences resulting from the different aspect ratios much better than those of the previous assumption. This analysis under predicted the elastic limit of strengthened specimens with overlap of the FRP reinforcement onto the reinforced concrete frame and the specimen tested to failure with mechanical anchorage provided by the shear restraint system. However, the analysis over predicted the elastic limit of some of the strengthened specimens without overlap of the FRP reinforcement on to the reinforced concrete frame. This is due to the fact that these specimens failed prematurely due to shear sliding of the masonry out of the RC frame. Eq. 1 Allowable Exp = Theo Overlapped Shear Restrained Allowable Exp = Theo Overlapped Shear Restrained Measured Elastic Limit (kpa) (a) Case I: Vertical Bending Only Measured Elastic Limit (kpa) (b) Case II: Two-Way Plate Bending Figure 7: Comparison between the Measured Elastic Limit and Predicted Applied Pressure for the two cases CONCLUSIONS Simulating the actual boundary conditions of masonry infill walls was shown to be of great importance in understanding the actual strength gains that are achievable with FRP strengthening. GFRP strengthening of infill masonry walls was found to be effective, provided that proper anchorage of the FRP laminate was assured. Overlapping the FRP reinforcement onto the RC frame was very effective for double wythe specimens, but less so for single wythe specimens. Overlapping the reinforcement provides a level of lateral restraint of the mortar interface between the masonry infill and the RC frame. In double wythe specimens this appeared to delay the shear sliding mode of failure long enough to develop arching action, which further increased the resistance to shear sliding. In single wythe specimens however, due to the greater height-to-thickness ratio, arching action was not developed and thus the presence of the overlap did not increase the lateral load carrying capacity much beyond the shear sliding capacity. Mechanically anchoring the FRP using steel shear restraints was found to be very effective. All specimens strengthened in this way achieved over three times the service load without any visible signs of distress. It is not advisable to strengthen infill masonry walls with FRP sheets that terminate at the mortar interface between the masonry infill and the RC frame (i.e. without overlap), unless some additional anchorage or shear restraint is provided. The analytical study revealed that reasonably accurate prediction of the elastic limit of the applied pressure was achievable using a working stress analysis approach that analyses the infill wall as a plate element that is simply supported on all four sides. REFERENCES Lunn, D.S. (9) Behavior of Walls Strengthened with FRP Materials, MS thesis, North Carolina State University, Raleigh, NC. Standards Joint Committee (MSJC) (5). Building Code Requirements for Structures. ACI 53-5/ASCE 5-5/TMS 42-5, American Concrete Institute, American Society of Civil Engineers, and The Society, Farmington Hills, Reston, and Boulder, 5. Timoshenko, S., & Woinowsky-Krieger S. (1959). Theory of Plates and Shells. McGraw-Hill, New York, USA. 58