Page 1 of 3 V1.0 3 01/2008 Reduction in Shear Capacity Due to Overdriven Fasteners Diaphragms and shear walls, constructed with wood structural panels such as oriented strand board (OSB) and plywood, provide the primary lateral load resisting system in many types of construction. The ability of these systems to resist and transfer shear load is highly dependent on the strength and behavior of the sheathing fastener connection. The allowable shear loads provided in the building codes for wood-framed diaphragms and shear walls are based on the assumption that the fasteners used to attach the sheathing to the framing members are driven so that their heads or crowns are flush with the surface of the sheathing.
During construction of these assemblies, however, it is common to have at least a portion of the fasteners with their heads overdriven below the surface of the panels. This can be especially true when power-driving equipment is used to install the fasteners. Improper installation of the fasteners may potentially reduce the shear capacity of the shear walls and diaphragms.
Recent studies indicate that overdriven nails reduce shear wall strength from 5% to 22% depending on the depth of the overdriven nail-head. The bar chart on the ... more. less.
following page provides an estimate of the percent reduction in shear capacity for various percentages of fasteners overdriven to three different depths. This chart is based on an analytical model developed from research by Judd and Fonseca (1, 2), and Jones and Fonseca (3) involving in part the pseudo-dynamic testing of the 8 by 8 9 shear walls utilizing 7/16 d thick OSB and 8d cooler nails.<br><br> Review of the chart indicates that shear capacities are reduced when fasteners are overdriven. The reduction in shear capacity depends on the magnitude of the overdriven depth of the fasteners and the percentage of total fasteners overdriven. For example, if 30% of the fasteners in a diaphragm or shear wall are overdriven by up to 1/16 d, the estimated reduction in the shear capacity is only approximately 3%.<br><br> If 100% of the fasteners are overdriven up to 1/16 d, the estimated reduction in shear capacity is approximately 9%. Similarly, if 20% of the fasteners are overdriven by up to 1/8 d, the estimated reduction in the shear capacity is also approximately only 3%. Depending on the design requirements of the diaphragm or shear wall, reductions in shear capacity of up to 5% might be considered negligible and therefore possibly ignored.<br><br> If the full design shear capacity is required, additional fasteners must be added to take the place of the overdriven fasteners. Care must be taken to ensure that the additional fasteners, together with the original fasteners, do not violate minimum spacing requirements and therefore induce splitting of the framing members. This chart (see Chart 1) can be used to estimate the reduction in shear capacity if fasteners are overdriven by varying percentages and depths.<br><br> Page 2 of 3 V1.0 3 01/2008 Example Assume that 20% of the nails in a shear wall are driven flush with the sheathing and the remaining nails are overdriven as follows: " 30% between flush up to 1/16 d, " 40% between 1/16 dup to 1/8 d, and " 10% between 1/8 d up to 3/16 d. Using the chart, an estimate of the reduction in shear capacity based on the number and magnitude of overdriven nails would be: " 20% flush = 0% reduction " 30% up to 1/16 d H 2.6% reduction " 40% between 1/16 d up to 1/8 d H 6.7% reduction " 10% between 1/8 d up to 3/16 d H 2.5% reduction Total Reduction H 11.8% In order to restore the shear capacity to within approximately 3% of the design capacity, additional fasteners are required to take the place of all the fasters that have been driven by more than 1/16 d. Care should be taken to ensure that the additional fasters are driven flush and not overdriven.<br><br> Additional Considerations The allowable shear capacity of diaphragms and shear walls depends on several other factors including fastener type, size, spacing, and amount of penetration into the framing members. In addition, wood species, width of framing members, panel grade, thickness, layout, and presence or absence of panel edge framing support influence the allowable shear capacity. Each of these factors must be considered when evaluating the effects of overdriven fasteners.<br><br> For instance, if the required shear capacity is based on a fastener spacing of 6 d at the supported edges of the panels but the actual nail spacing is 4 d, the effective shear capacity may still be adequate even with a portion of the fasteners overdriven. Similarly, if the required shear capacity is based on a sheathing grade panel, but a Structural I sheathing grade panel has been installed instead, the effective shear capacity may still be adequate even with a portion of the fasteners overdriven. Another factor to consider is whether or not the panels have been wetted, thus causing the fastener heads to have become embedded in the panel due to thickness swell as opposed to being overdriven.<br><br> If the nail embedment is due to panel swelling and not overdriving, no shear strength reduction of the panels needs to be considered. Page 3 of 3 V1.0 3 01/2008 Chart 1. References  Fernando S.<br><br> Fonseca, Johnn P. Judd. (2004).<br><br> cEffect of Overdriven-Nail-Depth Combinations on Wood Shear Walls d. Proceedings of the 8 th World Conference on Timber Engineering, June 14-17, Lahti, Finland  Johnn Judd, Fernando S. Fonseca.<br><br> (2004). cShear Wall Strength Reduction from Overdriven Nails. d Proceedings of the IX ENCONTRO BRASILEIRO EM MADEIRAS E EM STRUCTURAS DE MADEIRA, CUIABA- JULHO De 2004  Scott N. Jones, Fernando S.<br><br> Fonseca. (2002). cCapacity of Oriented Strand Board Shear Walls with Overdriven Sheathing Nails d.<br><br> Journal of Structural Engineering, v. 128, n. 7, p-898-907 Additional Information  CUREE- Caltech Wood-Frame Project.<br><br> (2002). Richmond, CA  Karacabeyli, E., Ceccotti, A. (1998).<br><br> cNailed wood-framed shear walls for seismic load: Test results and design considerations d. Proceedings of the Structural Engineers World Congress. San Francisco, paper No.<br><br> T207-6  Monique C. Hite and Harry W. Shenton III.<br><br> (2002). cModeling the Non-Linear Behavior of Wood Frame Shear Walls. d Proceedings of the 15 th ASCE Engineering Mechanics Conference, Columbia University, June 2-5, New York, NY  Zacher, E. G., Gray, R.G.<br><br> (1989). cLessons learned from dynamic tests of shear panels. d Proceedings of the Sessions Related to Design, Analysis and Testing at Structural Congress, A. H-S Ang, ed., ASCE, New York, 134- 142<br><br>