Anchor Bolts in Masonry
Are you specifying or using the best bolts and correct placement procedures?
Not all anchor bolts are created equal - not even close. Surprisingly enough, common practice within the masonry industry has been to use weaker anchor bolts, when stronger anchors would cost about the same and could give much better performance.
Are you using the strongest anchor bolts? Are you installing your anchor bolts properly? Read on to see how you may be able to achieve better performance with no increase in material or construction costs.
The companion specification to the MSJC Code (the Specification for Masonry Structures - ACI 530.1/ASCE 6/TMS 602), requires that plate and bent-bar anchors comply with ASTM A 36/A 36M, and that other anchor bolts comply with ASTM A 307, Grade A. Although the steels prescribed by these specifications can be quite similar chemically, their mechanical characteristics can differ. In particular, the bent region of bent-bar anchors can be brittle. This means that under load, the leg of a bent-bar anchor can suddenly fail, causing the anchor to pull out of the masonry. This does not happen with headed anchors.
Research studies show that headed anchors in masonry, loaded in tension, fail either by yield and fracture of the anchor shank, or more typically by breaking out a piece of masonry.
Differences in performance between headed and bent-bar anchors are recognized in other design provisions. For example, the draft MSJC strength design provisions include different formulas for calculating tensile anchor capacity as governed by yield and fracture of the anchor shank, by tensile breakout, and by pullout. The pullout failure mode applies to bent-bar anchors only. Pullout capacity is calculated assuming that no special measures are taken to remove oil from the anchor shank. The capacity of bent-bar anchors is always controlled by pullout, and is quite low. In contrast, the capacity of headed anchors is controlled by breakout, is significantly higher, and increases with embedment depth. As embedment depth increases further, a headed anchor will eventually fail by yield and fracture of the anchor steel. Headed anchors can have more than 4 times the capacity of otherwise identical bent-bar anchors.
Accordingly, the MSJC is considering separating the allowable-capacity equations for headed and bent-bar anchors to reflect the higher capacity of headed anchors. When this is done, a given number of bent-bar anchors could in many applications be replaced by a fewer number of headed anchors.
Even before the allowable-stress provisions of the MSJC Code are updated in this way, however, designers and contractors are encouraged to use headed anchors instead of bent-bar anchors. Calls to various distributors show that the two types of anchors cost nearly the same, and that their installation expenses should also be quite similar.
When asked why bent-bar anchors have been so popular, Jim Painter (Vice-President, Painter Masonry Inc., Gainesville, Florida) and Larry Breeding (Senior Laboratory Technician, National Concrete Masonry Association, Herndon, Virginia), noted that they are traditional in masonry construction, and that contractors have not had a good reason to switch to other types of anchors. Both respondents noted that to the casual user, bent-bar anchors have probably looked stronger because of the large leg embedded in the grout. Both respondents, however, could also see how these anchors would be weakened by the bending process, and enthusiastically supported recommending the use of headed anchors if they were stronger. They saw no strong reason why masons would not embrace headed anchors, since stronger anchors could mean fewer bolts in the wall in many applications. Painter's only concern was how to hold headed anchors in the correct location and at the correct embedment depth in the tops of masonry walls; he felt fairly sure that a clip could easily be made that would do this, and he expected that new products would quickly be created to help masons place anchors quickly and easily.
Section 126.96.36.199 of the MSJC Code provides minimum requirements for the embedment, edge distance and spacing of anchor bolts. Anchor bolts should be placed as specified, since the capacity of an anchor bolt will be reduced if its embedment depth is too small, or if it is placed too close to a free edge or to another anchor bolt.
If bent-bar anchors must be used, contractors should remember that their capacity in tension depends on their pullout resistance. While coating bent-bar anchors with oil or other materials to prevent corrosion is admirable, it nevertheless reduces bond and the resulting pullout strength of the anchor. It is therefore critically important to clean dirt and oil from bent-bar anchors before they are placed in the masonry.
Brown, R. H. and Whitlock, A. R. (1983), "Strength of Anchor Bolts in Grouted Masonry," ASCE Journal of Structural Engineering, vol. 109, no. 6, June 1983.
Hatzinikolas, M., Longworth, J. and Warwaruk, J. (1979), "Strength and Behavior of Anchor Bolts Embedded in Concrete Masonry," prepared for the Alberta Masonry Institute, 1979.
Shaikh, A. F. (1996), "Design of Hooked Anchor Bolts in Concrete and Masonry: Proposed Code Provisions and Commentary," prepared for National Codes and Standards Council, July 1996.
Tubbs, J. B., Pollock, D. G., Jr., McLean, D. I. and Young, T. C. (1999), "Performance of Anchor Bolts in Concrete Block Masonry", Proceedings, 8th North American Masonry Conference, Austin, Texas, June 6-9, 1999.
About the Authors
Phillip Samblanet is a professional engineer, and the Executive Director of The Masonry Society. He served as the secretary for the MSJC Inspection Task Group.
Richard E. Klingner, Ph.D. is the L. P. Gilvin professor of civil engineering at the University of Texas at Austin, where he specializes in the behavior and design of masonry, particularly for earthquake loads. He is also the author of Masonry Structural Design and the former chair of the Masonry Standards Joint Committee (MSJC).