Deviations between planned and actual position of wood screws – consequences for minimum spacing

Based on insertion tests with 8mm self-tapping andself-drilling wood screws in spruce glulam theseconclusions are drawn:- Deviations between the actual and plannedscrew axis depend on the insertion geometry. Inunfavourable cases the magnitude amounts to 12%related to the insertion length. This maximum iscovered by tests without pre-drilling and with amaximum insertion length of 340mm.- If the intended angle between screw axis and graindirection is considerably less than 90°, the actualscrew axis tends to follow the grain direction.- The mainly positive Δ1 deviations in placement Iand particularly in placement II and III are likelyalso caused by insertion angles systematicallyless than the intended 45°. The reason for thissystematic inaccuracy is supposed to lie in theinsertion procedure using the wedge-shaped screwguides (cf. Trautz et al. 2007, p. 67).- If the entry point of the screw tip coincides withthe glue line, it seems to be unlikely that the axisof screws remains in the glue line. This may beexplained by the tendency of a screw to proceed inthe lamination with lower density.- Considering placement imperfections the necessaryspace for a single screw is cone-shaped. Acone model is therefore proposed in order toNEW ZEALAND TIMBER DESIGN » JOURNAL VOL 28 · ISSUE 2 determine minimum screw spacing. Due to limitedexperimental data, the formulation of the model isin a tentative stage.So far, consequences for screw spacing requirementsare:- Using the proposed cone model minimum screwspacing can be estimated such that harmful contactbetween adjacent screws is avoided. The modelapplies to four common insertion geometries.So far, its application is restricted to 8mm woodscrews, to spruce glulam and to an insertion lengthof approximately 400mm.---Screw spacing requirements should be discussedagainst the background of unintended deviationsand harmful contact.Requirements in Eurocode 5 and technicalassessments should at least be checked, amendedwhere necessary or a general note should be madein provisions that deviations have to be taken intoaccount.The results of the study are being used for aresearch proposal covering open questions. Severalissues to be examined concern the use of screwguides and their influence on the actual screw axis,wood species, connection principles, insertionlengths exceeding 340mm, the influence of thescrew diameter and other than the examinedinsertion geometries.