400 70 300 200 50 100 0 -200 .. -300 0 or Undamaged 400 0 10 20 3040 50 60 70 0 Cycle Number 00 4.8 Fig.4.Sequential phased displacement loading protocol Fig.5.Nail-failure trend an driven nails displayed a fairly uniform array of failure moc h specimen is listed with nails overdriven32 mm.A de in of Table ctive initial stiffess was calculate be due simply to the difficulties associated with distinguishing (1) e as her to the As the overdr first cycle during pulling of the specimen. mpc cese(foofe(FS and ete.)is giver as the nai overdriven 4.8 mm behavior of a nail e and more elike that of a shor quakes in shear walls with flush-driven nails.an indication that the ity of the wall (the nails in the specimens with naved like short ca ting the possibility of the force ould be required to d ong ca ed is th nail withdra ne di wal is eliminatec was o and na ved for the specimens tested.Such ani the deeper overdriven ailand the nail-head holding a increased, the exr ected the p网 and even eliminated hecause the effective thickness of the nane depth in Th 0a0 red loadf regardless of loading direction (push or pull). for each serie Results of tests is the not required (S ral Engineers As em Cal 1:the overdriven nail depth(OD)i fornia 997).In column 6 of Table the ultimate load (P)fo JOURNAL OF STRUCTURAL ENGINEERING/JULY 2002/90 200Specimens assembled with nails overdriven to any depth dis￾played predominantly pull-through, tear-out, and pull-tear modes ~or a combination thereof!, while those assembled with flush￾driven nails displayed a fairly uniform array of failure modes. Occurrence of these modes increased from approximately 28% for the specimens with flush-driven nails to about 58% for the specimens with nails overdriven 3.2 mm. A decrease in occur￾rence of these modes was observed for specimens with nails over￾driven 4.8 mm. The decrease, however, is not significant and may be due simply to the difficulties associated with distinguishing between modes of failure as nails are highly overdriven. For the specimens with flush-driven nails, approximately 20% of the nails experienced fatigue. As the overdriven nail depth increased from flush to 4.8 mm, that percentage decreased almost linearly to 0%. Nail withdrawal was uncommon and was only seen in specimens with flush-driven nails. The percentage of slightly damaged nails increased from approximately 38% for those specimen with flush￾driven nails to about 46% for specimens assembled with nails overdriven 4.8 mm. The trends shown in Fig. 5 are helpful in understanding nail behavior. The first trend observed is that nails pull through, tear out, or pull tear as overdriven depth increases from flush to 4.8 mm. These modes of failure have been observed during earth￾quakes in shear walls with flush-driven nails, an indication that the sheating is the limiting factor on the capacity of the wall ~the sheathing is failing, not the nails!. This problem is accentuated as nails are overdriven. The panel thickness is essentially reduced by overdriving the nails, virtually eliminating the possibility of the panel developing the full strength of the nails. Another trend observed is that nail withdrawal is eliminated and nail fatigue decreases as overdriven nail depth increases. Nail withdrawal is eliminated because more energy is required to with￾draw the deeper overdriven nail, and the nail-head holding capac￾ity is decreased. Since the sheathing is tearing out prematurely, nail withdrawal is eliminated. Nail fatigue is significantly reduced and even eliminated, because the effective thickness of the panel with overdriven nails is too thin, and the nail tears out the sheath￾ing before it is worked to its fatigue strength. Results A summary of results is presented in Table 1. The specimen des￾ignation is listed in column 1; the overdriven nail depth ~OD! is given in column 2. The overdriven nail depth for the specimens with flush-driven nails, FLUSH-1 and FLUSH-2, is zero. The effective initial stiffness (Keff! of each specimen is listed in column 3 of Table 1. Effective initial stiffness was calculated using Eq. ~1! Keff5P1,pull2P1,push D1,pull2D1,push (1) where P1,pull5force corresonding to the displacement peak of the first cycle during pulling of the specimen, D1,pull , and P1,push 5force corresponding to the displacement peak of the first cycle during pushing of the specimen, D1,push . In column 4, the average effective initial stiffness (KE) for each series of tests ~FLUSH-1 and FLUSH-2, etc.! is given. A slight increase in initial stiffness is observed as the nail overdriven depth increases. Overdriven nails densify the sheath￾ing panel under the nail head, which contributes to an increased initial stiffness. In addition to the sheathing consolidation, the behavior of a nail becomes more and more like that of a short cantilever beam as the nail overdriven depth increases. Compara￾tively, the nails in the specimens with flush-driven nails behaved like long cantilever beams, while the nails in the specimens with nails overdriven 4.8 mm behaved like short cantilever beams. Initial stiffness should, therefore, increase as nail overdriven depth increases, because a larger force would be required to de￾form ‘‘short cantilevered nails’’ than to deform ‘‘long cantilevered nails’’ to the same displacement level. A slight increase in initial stiffness was observed for the specimens tested. Such an increase would be even more noticeable, except that the bearing strength of the sheathing panel became the controlling factor as the nail overdriven depth increased, offsetting the expected increase in stiffness. Due to overstress of the panels, nail tear-out or edge failure of the sheathing panel became dominant as nail overdriven depth increases. This trend is clearly shown in Fig. 5. Maximum load (Pmax) for each specimen is listed in column 5 of Table 1. Pmax is simply the maximum measured load for a test, regardless of loading direction ~push or pull!. Pmax for each series of tests is within a 10% range, indicating that the testing was accurate and highly predictable and that a third test per series was not required ~Structural Engineers Association of Southern Cali￾fornia 1997!. In column 6 of Table 1, the ultimate load (Pult) for Fig. 4. Sequential phased displacement loading protocol Fig. 5. Nail-failure trend JOURNAL OF STRUCTURAL ENGINEERING / JULY 2002 / 903 Downloaded 05 Jan 2009 to 222.66.175.206. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyright