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Tests of Reinforced Concrete Shear Walls with Confined Boundary Elements using Welded Wire Reinforcement

By M.  Saatcioglu  and M.  Navidpour Department of Civil Engineering  University of Ottawa, Ottawa, CANADA

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The primary objective of the research project is to assess performance of welded wire reinforcement (WWR) in concrete shear wall boundary elements as transverse reinforcement.  The objective also includes development of design information for use of WWR in earthquake resistant flexural walls.  The scope consists of tests of large-scale reinforced concrete shear walls under reversed cyclic loading, as well as material testing of WWR. The scope also includes FEM analyses of walls with different properties of WWR.

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Boundary elements of earthquake resistant reinforced concrete shear walls require closely spaced transverse reinforcement to prevent buckling of compression reinforcement and to confine concrete for improved ductility. Closely spaced transverse reinforcement, with the amount of steel required in building codes, results in congestion of steel cages especially in high-strength concrete boundary elements.

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Alternative to conventional ties is the use of WWR Grids with sufficient strength and deformability. While earlier research at the University of Ottawa indicated favorable performance of WWR for use in columns, its use in shear wall boundary elements is needed.

Use of WWR Grids  as transvers reinforcement

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Previous Research on  BauGrid  Welded Wire Reinforcement Grids

Mongi  M.  Grira , “Innovative Approaches to Column Confinement”, PhD Thesis (1998), University of Ottawa Saatcioglu  M.,  Grira  M., “Confinement of Reinforced Concrete Columns with Welded Reinforcement Grids”, ACI Structural Journal, Technical Paper 96-S4, V. 96, No. 1, January-February 1999, pp. 29-39.

Previous material tests conducted at the University of Ottawa exhibited satisfactory performance without premature failure of grid welds.

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Previous Research on  BauGrid  Welded Reinforcement Grids

Previous research on reinforced concrete columns tested under simulated seismic loading showed excellent column performance.

Drm
——sz4
-200
-120
Displacemmt,
P = 190/0 Po
p, = 1.26%
p, = 0.83mc1
s = 76mm
1m
(mm)

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Standard coupon tests were also conducted on wire samples that included welded joints within the coupons.

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ASTM A1064/A1064M – 12: “Standard Specification for Carbon-Steel Wire and Welded Wire Reinforcement, Plain and Deformed, for Concrete”  “… The minimum average shear value in pound-force shall not be less than 35,000 multiplied by the nominal area of the larger wire in square inches [in Newton shall not be less than 241 multiplied by the nominal area of the larger wire in square millimeters]…  “

Horizontal Wire
Vertical Wire
Section A-A

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Specimen ID Failure Load kN ) Yield Capacity ( kN ) Failure Load / Yield Capacity (%) J1 30.1 39.0 77.2% J2 31.7 39.0 81.2% J3 32.0 39.0 82.0%

Min. ASTM Requirement: 17  kN  (44% of Yield) (241 times the area of steel in Newtons)

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Specimen ID Load at Failure (kN) Average Load Per Bar ( kN ) Yield Strength (kN) Average Load/ Yield Strength (%) SH-2-1 24.6 12.3 39.0 31.5% SH-2-2 24.5 12.25 39.0 31.4% SH-8-1 33.6 11.2 39.0 28.7% SH-8-2 31.6 10.53 39.0 27.0%

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Specimen ID Load at Failure (kN) Angle  (Degrees) F1 (kN) F2 ( kN ) Fy (kN) F1/Fy (%) F2/Fy (%) B-2-1 14.9 68 13.82 5.58 39.0 35.4% 14.3% B-2-2 16.0 68 14.83 5.99 39.0 38.0% 15.4% B-3-1 10.2 73 9.75 2.98 39.0 25.0% 7.6% B-3-2 12.9 73 12.34 3.77 39.0 31.6% 9.7% B-8-1 9.6 68 8.90 3.60 39.0 22.8% 9.2% B-8-2 9.5 68 8.81 3.56 39.0 22.6% 9.1%

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Concrete Prisms with Cast-in WWR Grids Direct Shear Tests

Specimen ID Load at Failure (kN) Average Load Per Bar ( kN ) Yield Strength (kN) Average Load/ Yield Strength (%) SH-2-1 45.8 22.9 39.0 58.7% SH-2-2 40.0 20.0 39.0 51.3% SH-3-1 42.5 21.3 39.0 54.6% SH-3-2 44.8 22.4 39.0 57.4% SH-3-3 41.4 20.7 39.0 53.1% SH-8-1 74.8 24.9 39.0 63.8% SH-8-2 69.9 23.3 39.0 59.7%

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Concrete Prisms with Cast-in WWR Grids Burst Tests

Specimen ID Load at Failure (kN) Angle  (Degrees) F1 (kN) F2 (kN) Fy (kN) F1/Fy (%) F2/Fy (%) BC-2-1 33.9 68 31.43 12.70 39.0 80.6% 32.6% BC-2-2 33.3 68 30.88 12.47 39.0 79.2% 32.0% BC-3-1 26.1 73 24.96 7.63 39.0 64.0% 19.6% BC-3-2 26.7 73 25.53 7.81 39.0 65.5% 20.0% BC-8-1 38.0 68 35.23 14.24 39.0 90.3% 36.5%

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Aspect ratio: 3:1  (4.35m H: 1.45m L) Aspect ratio of 2.0 is considered high-rise shear wall by ACI 318 1600  kN  axial load 82  MPa  concrete

4500 mm (15-0")
iddddddddddddddddi
Horizontal web Reinforcement: 10M @ 250 (10") oic TYP.
WWR Ties@75 mm TYP.

90
90
90
90
Vert.
Hor.
WWR Grids
@ 75mm (3") o/c
1450 mm (41-10 )
250
300 TYP.
250
Web Reinf. 10M @ 300 mm (12") o/c TYP.
Web Reinf. 10M @ 250 mm (10") o/c TYP.
8-15M Long. Bars
(Boundary Element)

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First grid weld failure at 3.5 % wall drift ratio.

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Lateral Drift (0/0)
-1.5 -1 -0.5 0 0.5 1
-5.5
5
-5
-4.5
-4
-125
-3.5
-3
-2.5
-2
1.5
5
2
2.5
3
100
3.5
4
125
4.5
150
5
-150
-10
7
5.5
15
Lateral Deformation at Top of the Wall (mm)

Grid wire strains reaching beginning of curved yield region of steel @ 2.3% wall drift ratio.

First grid weld failure  @ 3.5 % wall drift ratio prior to complete yielding of the grid wires. Subsequent weld failures beyond 3.5% drift cycles.

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VecTor2 Finite Element Analysis Software was used. As-built material properties were employed, matching laboratory data. Longitudinal bars were modelled individually. Horizontal reinforcement and transverse boundary element reinforcement were defined as smeared steel. Subjected to gravity load, simulated vertical load and reversed cyclic horizontal load.

Formworks -
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*1410

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Analysis based on  F y  =550 MPa, F u  = 640 MPa (no weld failure assumed)

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Analysis based on  WWR grid yield strength of F y  =241 MPa, F u  = 242 MPa ( ASTM 1064 limit)

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Analysis based on  WWR grid yield strength of F y  =400 MPa, F u  = 401 MPa  (Regular strength steel; weld failure at yield )

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Analysis based on  WWR grid yield strength of  F y  =500 MPa, F u  = 501 MPa  (Recoreded grid yield strength; weld failure at yield )

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Reinforced concrete shear walls with WWR grids used as boundary element transverse reinforcement can develop sufficient strength and ductility when designed by current North American Codes and Standards provided that the grids have sufficient strength and deformability. The WWR grids used in the current investigation have strengths close to the yield strength of wire used in manufacturing the grids. However, their joint weld capacity was not sufficiently high to allow the wires to yield. Their use in earthquake resistant construction requires careful assessment of grid material performance.

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The yield capacity of wire reinforcement can be used in design as representative of WWR grid capacity provided that the grids are subjected to burst tests and consistently demonstrate wire rupture in tension (as oppose to weld (joint) failure.  If the grid weld (joint) failure is observed during burst tests, the ASTM-1064 weld shear test can be used to estimate the grid capacity for use in design. Cast-in concrete grid burst tests, demonstrated in the paper, may also be used as an alternate test  procedure. A safety factor of  f w  = 0.85 is recommended for the grid design capacity. The additional capacity observed due to the redundancy of multiple-cell grids may be considered for additional safety.