Splice Length Research Underway

These research programs are intended to provide additional information on factors which control reinforcing bar splice behavior including the length of the splice, the strength of the masonry, the strength of the reinforcing steel, and the structural cover depth. Historically, allowable stress design criteria have based the required splice length for reinforcing bars in masonry on the bar diameter and the stress in the reinforcing steel only. Neither the strength of the masonry nor the structural cover depth are currently included in the allowable stress design equations for the required splice length.

Research conducted by Schuller, Hammons and Atkinson (ref. 1) and Blake, Marsh and McLean (ref. 2) have confirmed that the strength of the masonry significantly affects the performance of lap splice lengths. Based on these and other research projects, splice criteria have been proposed for strength design and limit states design standards in several forums. These proposals would significantly increase the required lap splice lengths for typical reinforced masonry construction, particularly where the strength of the masonry is low and where the structural cover depth is small.

These longer lap lengths would increase the cost and difficulty of reinforced masonry construction since:

• the quantity of reinforcing steel would increase
• longer lap splices would make the use of closedend units more difficult, making open-end units necessary in many instances

/Public/News/911996230.gif" width="309" height="400" border="1" align="right" hspace="10" vspace="1"> Because of these concerns, the National Concrete Masonry Association began researching the effect of structural cover depth on the performance of spliced reinforcing bars as part of the NCMA Structural Cover Research Project. Additionally, the Northwest Concrete Masonry Association sponsored complimentary research at Washington State University.

Both of these research projects are using a similar test setup based on recommendations from a research team of Professor Richard E. Klingner, University of Texas at Austin; Professor David I. McLean, Washington State University; and Michael P. Schuller, Atkinson-Noland & Associates, Inc. (see Figure 1). This test apparatus loads parallel pairs of spliced reinforcing bars simultaneously in direct tension so that no eccentricity is applied to the spliced bars.

Two types of failure mechanisms could be expected from this research. The first, and most preferable, would be yielding and eventual rupturing of the reinforcing steel. Such a failure would indicate that the supplied lap splice length was adequate for the provided strength of masonry and structural cover depth. The second mode of failure would be a sudden brittle failure of the masonry. While this mode of failure is not as desirable as rupturing of the reinforcing steel, the research team recommended that acceptable splice performance be based on achieving a strength of 1.25 times the specified yield strength of the reinforcing steel. This criteria is based on code requirements for welded and mechanical splices.

The first phase of NCMA's research investigated the performance of spliced #4 through #9 bars at a structural cover depth of 2 in. The #4 reinforcing bars were spliced 18 in. (36 bar diameters). All the other bars were spliced for a length equal to 48 times the bar diameter. Preliminary results from the NCMA research program, for an average masonry strength of approximately 4000 psi, indicate:

• #4 reinforcing bars spliced 18 in. (36 bar diameters) failed by rupturing the reinforcing steel

• Masonry panels constructed with #5 through #7 bars failed by catastrophic cracking and spalling of the masonry at loads higher than 1.25 times the specified yield of the reinforcing bars

• Panels containing #8 reinforcing bars failed after yielding of the reinforcing steel but before achieving a load in excess of 1.25 times the specified yield of the reinforcing bars

• Panels with #9 bars failed prior to reaching the yield strength of the reinforcing bars

Results from the Washington State University research project are not yet available. However, based on the preliminary results of the NCMA research, NCMA's Structural Cover Task Group and the Engineered Concrete Masonry Design Committee have recommended that additional research be conducted which would include:

• lower strengths of masonry materials
• longer splice lengths for the larger bar sizes at a structural cover depth of 2 in.
• 48 bar diameter lap splices for larger bars at a structural cover depth of 3 in.

Additional research is being considered by the Council for Masonry Research which would include similar tests on clay masonry wall panels.

References

1. Atkinson, Richard H., Michael I. Hammons, Paul J., and Michael P. Schuller, "Masonry Research for Limit-State Design," U. S. Army Corps of Engineers, October 1994
2. Blake, John D., M. Lee Marsh, and David I. McLean, "Lap Splices in Flexurally Loaded Masonry Walls," The Masonry Society Journal, Vol. 13, No. 2, February 1995, pp. 22-36