Joint Reinforcement for Shear Strength Research Impacts Code

Ralph O Johnson

Horizontal wire reinforcement research determined efficient for masonry shear walls

Advances made possible as a result of masonry joint reinforcement research and testing can be translated into cost savings for owners, labor savings for the contractor, predictable and confident strength number for the engineer and a change to the building code.

Concrete and steel have been working together in walls for quite some time. The concrete masonry unit (CMU) – or block – wall is no different. In concrete block walls, where and how should steel placement be in order to maximize its shear strength advantage?

Iowa State University Research Recently completed testing on concrete masonry panel walls at Iowa State University deter mined the effectiveness of masonry joint reinforcement (MJR) as shear reinforcement for CMU walls in high seismic areas. Results of these tests have provided important design information. In the past, MJR was not allowed to be considered for use as shear reinforcement for designers working with the Strength Design method for masonry. These tests showed that, strategically placed, MJR can be as strong as or stronger than bond beams. In fact, MJR performed better than bond beams, due to distribution of steel throughout the wall, as opposed to the large concentration of steel typically associated with reinforced bond beams. MJR puts a substantial amount of steel through out the wall, giving it more elasticity and ultimate strength when compared with widely-spaced, reinforced bond beams.

Ten different full-scale walls were built and tested. Eight were built using partial grouting and two were constructed using full grouting. Walls with only bond beams were constructed to provide base comparison values. Other walls were constructed using MJR where the amount of shear reinforcement provided by the MJR was varied by changing the number of side rods provided at consistent vertical spacing. In each test set, block walls were constructed with #4 bars in bond beams at 48″ oc vertically, 3/16″ ladder (heavy duty) MJR at 8″ oc vertically or they were constructed with double 3/16″ (4 wires) at 8″ oc vertically to compare results. The area of vertical wall reinforcement was held generally consistent but minor changes were made to the distribution.

Research objectives included the primary goal of determining the appropriateness of using joint reinforcement as the primary horizontal shear reinforcement in CMU walls along with parameter comparisons that could lead to property and formulation criteria for use in masonry structural codes. Testing included recorded loads, wall displacements, reinforcement strains and observations of behavior including digital video and photography. Subsequent analysis included comparisons of wall strengths at peak load and at three levels of drift. Reinforcement strains were recorded and compared to reinforcement material tests. Reinforcement was inspected after wall tests for deformation and fracture. Cracking was observed and used to help evaluate conditions within regions of the walls. Load/displacement plots were used to define displacement ductility. Sliding at base of the wall was monitored and measured in bed joints above base bond beams and beneath top bond beam.

Exceeding Expectations Results from these tests greatly exceeded expectations. Tests showed that MJR should be a major consideration as primary horizontal shear reinforcement in block walls.

These 10 panels were constructed over a period of three years. Each panel was built under controlled conditions, cured and loaded to failure through cyclically increasing loads that represented loads that could be applied to shear walls during seismic events. After a panel was built and tested, another panel was constructed and the process continued. Data generated will prove invaluable to designers and engineers in the future.

Results show that MJR is as effective as bond beams in control ling shear stress. By using MJR, the mason can continue working instead of stopping to construct bond beams. This results in labor savings and savings in material costs. Such heavy MJR was used, in some cases, that they exceeded normal specifications. When some tests were completed, the wire products were all that was holding the wall together and, in no case, did any wire product fail. Also noted in these tests were superior properties of crack control. MJR walls limited the size of cracks as compared to larger cracks noted in the bond beam walls. Once a large crack develop ed in the bond beam wall, test results were skewed because of the weakened wall.

Several previous tests on shear strength in block walls had resulted in tension failure in the MJR. This resulted in an assumption by members of the code council that MJR was either not strong enough or too brittle. A softer wire had been suggested that would elongate with the load while giving the tension needed during loading without failure. As a result, MJR was not considered on the strength side of the code, only considered as crack control. A goal of this research was to prove that MJR is a very important component in the CMU wall and can be as effective as bond beams in resisting shear stress.

Results show sufficient areas of joint reinforcement can provide strength necessary to resist lateral loads and bridge cracks that form in shear walls confirming options designers can use for providing the amount of horizontal shear reinforcement in shear walls:

• Bond beams

• Joint reinforcement in each bed joint

Extrapolation of the results can also lead to consideration of additional means to achieve sufficient shear reinforcement:

• Joint reinforcement in every other bed joint or in an alternative bed joint spacing

• Combination of bond beams and joint reinforcement

The test report includes behavioral results including: in-plane shear force, location and area of reinforcement, displacement, drift, cracking, load/displacement plots and envelopes, ductility, strain energy, sliding, failure modes and other significant results important to shear wall design.

Placing joint reinforcing in bed joints of every course (or every other course) in lieu of bond beams helps control cracking and provide prescriptive requirement for horizontal reinforcement, allowing masonry walls to be constructed without slowing construction to place and grout bond beams. If sufficient area of join reinforcement is provided, reinforcement can also provide the tension capacity to span across cracks in shear walls and to act as primary shear reinforcement to carry in-plane shear forces. Presumption that the higher strength of the colddrawn wire would result in significantly reduced cross-sectional areas of steel led to the finding that the use of small to modest areas of well-distributed reinforcement or simple prescriptive reinforcement can also provide the required horizontal shear reinforcement.

These series of tests were funded by major MJR manufacturers, Wire-Bond and Hohmann & Barnard. Greg Baenziger, PE, graduate research assistant, and Max Porter, PhD, PE, Dist M ASCE, FSEI, FTMS, FACI, FACFE, DASFE, CFC, professor emeritus of Civil Engineering, both in the Department of Civil, Construction and Environmental Engineering at Iowa State University, supervised during the construction and testing of the wall panels. Special consultant was Mario Catani, former president of Dur-O-Wal, joint reinforcing manufacturer. An advisory committee from the Masonry Standards Joint Commit – tee board was also formed and consulted. All are concerned with building the best, strongest and most economical masonry walls that are possible with today’s techno – logy. These results will mean strong, aesthetically-pleasing and long-lasting masonry walls into the future.

Code change Recommendations following this research suggested a code change to allow joint reinforcing as primary shear reinforcement in Strength Design, similar to what was previously allowed for in Allowable Stress Design. With this inclusion, there should be appropriate limits placed upon material proper – ties to ensure the necessary strain capacity, compatibility and strength. The MSJC 2011 masonry code effectively restricts use of MJR as primary shear reinforcement in Chapter 3 by limiting actual reinforcement yield strength to 1.3 times a yield strength of 60 ksi (413.7 MPa0 or 78 ksi (537.8 MPa). Most MJR has a yield strength that exceeds this value, as does the MJR used in the test walls. Changes were considered during the deliberations of the 2013 MSJC Committee. Recommended changes were incorporated through several changes including:

• Acceptability of the use of joint reinforcement as primary horizontal joint reinforcement when the joint reinforcement uses wires of 3/16″ diameter with yield strengths up to 85 ksi;

• In SDCs A and B, MJR with single side wires of 3/16″ placed at 16″ oc may be used in partially grouted walls;

• In SDCs C through F, MJR with single side wires of 3/16″ placed at 8″ oc may be used in partially grouted walls;

• In SDCs C through F, MJR with double side wires of 3/16″ placed at 8″ oc may be used in fully grouted walls.

• Side wires of joint reinforcement used for primary horizontal shear rein forcement must be anchored around the vertical edge bar by either a cross- wire or 90º bends in the side wires.

To obtain the complete white paper entitled Joint Reinforcement as Primary Shear Reinforcement for Masonry Shear Walls by Greg Baenziger and Max Porter delivered at the 11th North American Masonry Conference, please contact The Masonry Society.

Ralph O Johnson, III, is President and Owner of Wire-Bond, one of the largest manufacturers of masonry joint reinforcement in the nation and family-owned for almost 40 years. He has nearly four decades of experience in the industry, in addition to holding 11 US patents. Currently, Wire-Bond operates two manufacturing plants based in Charlotte NC and Memphis TN. Johnson graduated from NC State University with a degree in Civil Engineering. | 704.525.5554

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