The development of welded reinforcement grids as confinement reinforcement for concrete structures

Reinforced concrete structures are rising; including California’s tallest rein-forced concrete structure—a 58-story building—which is under construction currently in San Francisco. Although the upsurge in tall building construction along the West Coast reflects the global trend, California, in particular, faces the challenge of being a highly seismic region.

While the design approach of many structures adhere strictly to the seismic design and detailing provisions of building codes, some of the more complex structures are breaking ground on the strength of performance-based design. In his text book, Seismic Design of Reinforced and Precast Concrete Buildings, Robert E. Englekirk, Ph.D.,S.E., said, "Ductility is an essential attribute of a structure that must respond to strong ground motions." And the requirements for transverse reinforcement of concrete frames and shear walls play an especially vital role in the ductile performance of the structure.

Transverse reinforcement in reinforced concrete members serves three main functions: stability of longitudinal reinforcement, enhancement of shear (diagonal tension) and torsional capacities, and confinement of core concrete.

When properly designed and detailed, transverse reinforcement prevents shear failures, premature crushing of unconfined concrete, reinforcement bond failure, and longitudinal bar buckling. Traditionally, this has been achieved by using closely spaced hoops, crossties, and overlapping hoops as transverse reinforcement, in addition to applying stringent code requirements for seismic bends, hooks, and hook extensions. Unfortunately, quite often conventional ties designed according to these provisions result in labor-intensive construction, reinforcement congestion, and concrete placement problems.

An alternative to using conventional transverse reinforcement are welded reinforcement grids (WRG); see Figure 1 and "The reinvention of transverse reinforcing" sidebar below. The benefits of WRG in construction include the following:

* a reduction in confinement steel weight;
* labor and time savings in cage assembly and installation;
* reduced reinforcing congestion;
* more efficient concrete placement and consolidation;
* increased speed of construction and improved constructability; and
* improved seismic performance through superior ductility.

WRG research and development

Experimental research and actual field reports conducted on reinforced concrete columns and shear walls indicate that WRG confinement systems offer superior performance and easy cage assembly when used as transverse reinforcement. The grid pattern improves concrete confinement and results in enhanced deformability in the inelastic range. This feature makes WRG especially suitable for the construction of seismic-resistant structures. As part of the industry’s ongoing research, a full-scale, seven-story concrete structure was constructed using WRG as confinement reinforcement in the boundary elements of the shear walls and tested at University of California, San Diego (UCSD) Englekirk Structural Engineering Center’s out-door shake table in 2005; see Figure 3. This simulation used an acceleration that was 2.7 times greater than the Northridge Earthquake—which caused extensive damage in 1994.

The test was proposed by Dr. Robert E. Englekirk, Ph.D., S.E., and successfully proved the theory that ductility is the key to a building’s superior seismic performance. The results indicate that with the ensured higher ductility, as supplied in this study by the use of WRG, it is safe to construct mid-rise concrete structures with less reinforcement steel than present building codes require.

Welded reinforcement grids have been used in construction as well as ongoing research for more than 15 years.The earliest work,Cyclic Behavior of Shear Wall Boundary Elements Incorporating Pre-fabricated Welded Wire Hoops by Bertero, Thompson & Miranda of the Earthquake Engineering Research Center, University of California, Berkeley, in January 1990 showed improvement in seismic performance and significant reduction in the labor requirements for cage fabrication. These conclusions were validated in the field and documented in later work, Concrete Columns Confined with Welded Reinforcement Grids by Saatcioglu & Grira of Ottawa Carleton Earthquake Engineering Research Center of the Department of Civil Engineering at the University of Ottawa in September 1996, which stated that "construction advantages, combined with superior performance observed in column tests make welded reinforcement grids a viable alternative to conventional ties."

More recently, WRG tests with high-strength concrete (HSC) up to 12 kips per square inch (ksi) (80 MPa) have also been conducted for columns and core wall boundary elements. The challenge to overcome with higher-strength concrete is that the material is quite brittle if not properly reinforced; however, achieving the required reinforcement with conventional ties is often prohibitive from either a constructability or a cost perspective. Recent tests conducted at the University of Ottawa by Professor Murat Saatcioglu and at Berkeley by Professor Jack Moheile on HSC specimens reinforced with WRG have shown excellent ductile performance. The results of these tests allowed the use of WRG in what will be the tallest reinforced concrete structure in California when it is completed in 2008.

Conclusion

The technology of WRG creates opportunity to reduce the size of components without compromising strength and performance. Additionally, the technology permits improvements upon the blast and seismic performance of larger and more complex structures while allowing for more efficient concrete placement and construction. By seeking products and methodologies to improve design and construction, engineers are creating the opportunity for emerging technology that is essential to the concrete industry. They are providing their clients with safer and better-performing buildings, resulting in significant time and cost savings.

Hanns U. Baumann, S.E., has been practicing structural engineering in Southern California since 1961. He has been personally involved in the development of more than 30 new construction products, related mainly to reinforced concrete construction. In 1992, he won the Construction Innovation Forum’s Nova Award for his invention of the BauGrid WRG system. He founded BauTech, Inc., which develops and markets his inventions. He can be reached at Hanns@BauTech.com. Al Anvari, P.E., is the president of Delta Building Systems, which he founded in 2004 to manufacture and distribute advanced building systems in the Asian and North American markets. He has more than 25 years of experience in building design, construction system development, marketing, manufacturing, and distribution in North America and Asia. He can be reached at Al@BauGridUSA.com.

 The reinvention of transverse reinforcing
An introduction to welded reinforcement grids

The patented and proprietary system, BauGrid welded reinforcement grids (WRG), can replace many pieces of cut and bent steel reinforcement bars with a single piece. Made with high-strength steel, the WRG enhances the performance of the structure through improved structural integrity and improved ductility.

Since the system is created as single pieces, the WRG provides a method for more accurate and much faster placement of confinement reinforcement. The WRG comprises two layers that are usually welded perpendicular to each other, but other configurations, angles, and bent conditions can also be accommodated. WRGs are custom manufactured to the engineer’s design requirements to within an 1/8-inch (3mm) tolerance, with sizes up to 42 inches (1067mm) in width and up to 96 inches (2440mm) in length; see Figure 2.

WRGs are manufactured using high-strength, cold-drawn wire with 3/8-inch (9.5mm), 1/2-inch (12.7mm), 5/8-inch (15.88mm), and 3/4-inch (19mm) diameters. The yield strength of the wire achieves a minimum of 75 kip per square inch (ksi) (520MPa) or greater, up to 85 ksi (585 MPa). The cold-drawn wire is placed in two layers in specially designed jigs and fuse-welded at every intersection using resistance welding techniques. The manufacturing procedure and quality assurance program ensure that the WRG exhibits weld strengths and elongation characteristics that assure the development of reliable plastic hinges. This is based on research undertaken by Saatcioglu & Grira of Ottawa Carleton Earthquake Engineering Research Center of the Department of Civil Engineering at the University of Ottawa, who addressed specifically the ductility and elongation characteristics of both the component steel and the resulting WRG in their report, "Material tests for welded reinforcement grids," published in October 1997.

In addition to these ductility requirements specified by the manufacturer, the resulting WRGs also meet the requirements of ASTM A185 for Tensile, Shear and Reduction of Area. The results of these tests accompany the product documentation for each shipment of BauGrid WRG.

WRG in construction

The benefits of WRG have been proven through the use of BauGrid WRG for more than 15 years. Because many in the industry still design with conventional ties, it is common to convert designs to WRG after initial design phases are complete. The process is simple; start with direct substitution as shown in Figure 1. In this example, direct replacement of the conventional ties with the WRG will reduce the weight of steel by 25 percent.

Typically, the approach is to follow the substitution with an assessment of the spacing requirements given the higher yield strength of the WRG. In the example shown in Figure 1, the conventional #5 ties are specified at 5 inches on-center, and the recommended substitution is to replace the ties with BauGrid WRG of 5/8-inch diameter at 6 inches on-center. The basis for the assessment lies in the transverse reinforcement requirements for boundary elements of shear walls as detailed in the American Concrete Institute’s (ACI-318) Equation 21-4: A_sh = 0.09sb_c(f ’_c/f_yt), where f ’_c = 8 ksi, and f_yt = 75 ksi. If the area of #5 bar, A_sh is 0.31 square inches, solve for spacing, s, for the  increased transverse reinforcing spacing for the boundary elements in Figure 1; see Table 1 for calculations.

By increasing the spacing of the confinement steel from 5 inches on-center to 6 inches on-center, the weight of steel for confinement can be further reduced by as much as 40 percent.

Figures and Tables

Figure 1: Conventional hoops and crossties within the boundary element of traditional, transverse reinforcing (left) versus the 8-cell welded reinforcement grid replacement (right)

Figure 2: Plan and section of a custom-manufactured WRG for the boundary element of a core wall

Figure 3: Conceptual image of the shake table test facility at UCSD

Table 1: Because of the higher yield strength, WRG can result in larger spacing of the transverse reinforcing for the boundary element shown in Figure 1.