INFLUENCING PARAMETERS ON THE EXPERIMENTAL DETERMINATION OF THE WITHDRAWAL CAPACITY OF SELF-TAPPING SCREWS

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1 INFLUENCING PARAMETERS ON THE EXPERIMENTAL DETERMINATION OF THE WITHDRAWAL CAPACITY OF SELF-TAPPING SCREWS Andreas Ringhoer 1, Gerhard Schickhoer 1 ABSTRACT: Depending on physical and geometrical boundary conditions, withdrawal and/or steel tensile ailure govern the structural design o axially loaded sel-tapping screws. Thereore needed parameters such as the withdrawal strength ax or the steel tensile capacity tens are commonly determined by tests and given in technical approvals and standards. Consequently, it is o major importance to apply a test coniguration and procedure guaranteeing a general validity or the broad bandwidth o screw applications. Due to the act that the mechanical steel behaviour o modern timber screws (i) is comparable with those o similar steel products and (ii) has a typically low dispersion, we concentrate in this paper on eects inluencing the withdrawal strength representing the composite behaviour timber-screw. Thus, we show and discuss results o an experimental campaign, aiming to investigate the inluence o dierent load path and supporting conditions as well as o varying loadings rates and sample preparations on the withdrawal perormance o sel-tapping screws. KEYWORDS: Axially loaded sel-tapping screws; test conigurations; load path variation; rate o loading, pre-drilling 1 INTRODUCTION 1 Sel-tapping screws are nowadays commonly used to connect or reinorce linear or laminar structural components out o wood based products such as solid timber (ST), glued-laminated timber (GLT) or crosslaminated timber (CLT). Thereby, inclined or longitudinal positioning relatively to the load direction lead to an optimised way o application reaching the maximum load bearing capacity; see Figure 1. Reason thereore is the resulting primary axial load condition o the screw mainly governed by two ailure mechanisms: (a) steel ailure in tension and (b) withdrawal ailure. For case (b) the characteristic withdrawal capacity (in direction o the screw axis) o the single astener reachable is given as ollows: regression unction, e. g. design equation (8.39) according to EN [1], or (2) by using constant values (mainly in dependence o d), reerred to a characteristic density ρ k o 350 kg/m³ and recommended by screw manuacturers in their product related European Technical Assessments (ETAs, ormerly known as European Technical Approvals). α α Fax,k kax ax,k d l, (1) e where k ax considers the eect o insertion angles α o screw axis to grain direction, deviating rom 90, ax,k the characteristic withdrawal parameter at, d the diameter and l e the inserted threaded part o the screw. In addition, there are two main possibilities determining the withdrawal parameter ax,k : (1) by using an empirical 1 Institute o Timber Engineering and Wood Technology, Graz University o Technology, Ineldgasse 24, 8010 Graz, Austria. andreas.ringhoer@tugraz.at Figure 1: Common screw applications; let: as connection; right: as reinorcement; above: inclined positioned; below: longitudinally positioned Both alternatives principally base on experimental investigations subjected to certain boundary conditions. Especially in case (2), determination o ax,k values ollows an internationally regulated procedure with the aim to avoid irreproducible deviations caused by dierent test and assessment standards. Thereby, technical guidelines such

2 as European Assessment Documents (EADs; or screws Common Understanding o Assessment Procedure, CUAP ormerly had to be applied) rule the way (number o tests, which standards have to be considered, etc.), mechanical parameters o e. g. asteners or use in timber engineering have to be determined. In case o ax (o sel-tapping screws), this document reers to the standard EN [2] or determination o the characteristic 5 % percentile and especially to EN 1382 [3] or test procedure. Fixed boundary conditions in the latter mentioned document are i. the position o the astener relative to the annual ring orientation at radial as well as tangential arrangement has to be considered; ii. iii. the angle ε o the screw axis to the sample s surace it is ixed to 90 (not to be conounded with α); the distance o the screw axis to the supporting minimum three times the diameter; iv. the rate o loading (RoL) withdrawal ailure should appear in 90 ± 30 s with a constant loading velocity v; and v. the type o loading monotonic and deviating rom EN [4] without an initial hysteresis. Subsequently, we may separate those ive points in dependence o their impact on the withdrawal capacity as well as their treatment in previous studies. Firstly, it is well known that the type o loading (monotonic or cyclic), doesn t inluence the maximum test loads (F max acc. to EN 1382 [3], used to determine ax ) in a major way, see e. g. [5,6]. Secondly, conclusions made in [7,8] indicate that the position o the screw axis to the annual ring orientation at (radial or tangential) has no signiicant inluence on ax. Thus, the necessity o testing both directions has to be discussed. Thirdly, and with regard to a time to ailure area o interest, lasting rom 0 s (roughly) to 300 s (as target value or shear strength acc. to EN 408 [9]), but excluding impact impulse loads caused by pendulum impact hammers or drop weight impact testing machines, there are two deviating opinions concerning an RoL related inluence existing. On the one hand, Rosowsky and Reinhold (tested small screws and nails both subjected to axial and lateral loads) [10] as well as Aoki and Tsuchimoto (tested bolted timber joints) [11] conclude no (or in special cases just little) impact o RoL on the bearing capacity. On the other hand, Jansson documents in [12] (clear wood bending tests, class 1 material ) an increase o ailure stress by decreasing time to ailure o about 13 % between 0.2 s and 60 s. Furthermore, Girhammar and Andersson [13] observe a substantial increase in the dynamic strength (v test = 75 mm/min - 75,000 mm/min) i compared to a reerence static strength (v test = 2.0 mm/min) o laterally loaded nailed timber-to-timber connections. Fourthly, investigations, where the angle o the screw axis to the sample s surace or its distance to the supporting have been varied systematically, are hardly to be ound. In addition to these boundary conditions discussed, there are urther parameters not ruled in detail in EN 1382 [3] but also probably inluencing the bearing resistance o axially loaded sel-tapping screws. They are (i) the load and support conditions (push or pull), (ii) the shape and direction o the load path in the sample (push/compression, pull/tension or pile/shear), (iii) the test preparation (with or without pre-drilling) and (iv) the climate conditions (moisture content and temperature) o the sample at time o testing. Concerning point (i) and (ii), previous studies ocusing on sel-tapping screws are rare again. Bejtka states in [14] that both loading alternatives push and pull lead to the same withdrawal capacities. A very important act considered in all content-related investigations ollowing on. In contrast to that, Gehri and Haas theoretically assume in [15] signiicant impacts on ax caused by compression and shear (push and pile) load path situations in the timber specimen, see Figure 2. This statement is one o the main motivations or carrying out the experimental campaign described in section 2. Figure 2: Dierent loading and load path situations; let: pull; right: push; above: compression; below: shear; acc. to [15] With regard to test preparation (iii), neither in current standards and assessments/approvals (c. EN , section 10 [1], e. g. ETA-11/0190 [16]) nor in previous studies also ocusing on this topic (ST: [17]; GLT and CLT: [18]) signiicantly dierent withdrawal capacities o pre-drilled and not pre-drilled screws have been observed. In addition, in [17] the ST and GLT specimen s temperature (iv) has been varied between -20 C and 50 C with no essential inluence on ax, too. In contrast to that the behaviour in dependence o the specimen s moisture content u: Hereby, results o withdrawal tests carried out with dierent types o timber products indicate a

3 remarkable inluence o u on ax, independently rom the product and the way, specimen have been conditioned (reaching target moisture content beore or ater inserting the screw). Dierent correction unctions or 0 % u 20 % are provided in [19] and thus not discussed in detail in the rame o this paper. In brie: Based on the status quo concerning inluencing parameters on the experimental determination o the withdrawal capacity o sel-tapping screws, their dependency on varying load path situations, the distance to the supporting, the rate o loading as well as on the angle o the screw axis to the sample s surace is either unknown or dierent opinions exist regarding these topics. Consequently, in our paper we exactly ocus thereon studied in the rame o three experimental campaigns, which are described in section 2 and section 3 more in detail. Finally, in section 4 we summarise our indings related, especially with regard to regulations currently given in EN 1382 [3]. 2 MATERIALS AND METHODS 2.1 GENERAL OVERVIEW As already mentioned in section 1, the test programme being described is separated into three individual series and was carried out at Lignum Test Centre (LTC) at Graz University o Technology. While the irst and main series primary ocuses on dierent load path situations (Ia), supporting possibilities (also Ia) and their distance to the screw axis (Ib and Ic), additional series II and III concentrate on dierent loading rates and surace angles. Furthermore, series I has been carried out in chronological order: Ater inishing (and based on) data assessment o Ia, series Ib and Ic () were planned and tested. Thus, only two o our conigurations were investigated regarding supporting distances. Table 1 and Figure 3 to Figure 5 give an overview regarding all conigurations tested (note: series II coniguration is standard push-pull and thereore not shown graphically). For reasons o accuracy, series I screws had been pre-drilled. Although, studies mentioned in section 1 indicate no signiicant impact, one control-group as part o subseries Ia was conducted without pre-drilling; a comparison is given in section 3. Table 1: Overview o test series I-III Series Subseries α [ ] ε [ ] d d PD n * I Ia Ib Ic II III * outliers included i ii iii iv Figure 3: Test coniguration o series Ia: (i) push-pull; (ii) push-pile; (iii) pull-pull; (iv) pull-pile (schematic) i d h 2d 3d 4d 5d 6d 9d Figure 4: Test coniguration o series Ib (let) and Ic (right) i 90 iv ii 90 Figure 5: Test coniguration o series III (schematic) iv a s 2d 3d 4d 5d MATERIALS The timber specimen used or the experiments were made out o solid timber (I and II) and GLT (III) beams o Norway spruce (Picea abies) with a total length o about 4 m. To achieve homogenous material conditions (location parameters as well as distribution o density ρ, assumed to be normal distributed) or each series, which avoids any density related correction o test result aterwards, the principle o matched samples has been applied in the rame o specimen production. The sel-tapping screws used or the experiments were partially threaded with d = 6 (III) and 8 mm (I and II). Geometrical details are given in Table 2.

4 Table 2: Geometrical details o the screws used or the tests Diameter d Total length Thread length Eective length l e Technical Approval ETA- 12/0373 [20] ETA- 11/0190 [16] 2.3 METHODS The tests determining the withdrawal capacities o series I- III were perormed on Allround-Line testing machine by manuacturer Zwick GmbH & Co. KG. The time to ailure o all experiments (deormation controlled), except series II, resulted in the range o 90 ± 30 s acc. to EN 1382 [3]. Series II loading protocols are given in Table 3. When tests were inished, 4d x 4d x l e clear wood samples were cut out around the screw hole and went to urther treatment determining the density by measuring the physical dimensions as well as the moisture content by perorming oven dry method. Furthermore, all samples were cut in the middle and observed regarding knots inluencing the axial load bearing behaviour and - i they had - thus marked as outliers. Data assessment was carried out by spreadsheets and the sotware package R [21]. Table 3: Loading conditions or test series II Subseries Targeted time to ailure [s] Rate o Loading [mm/min] n * * outliers included 3 TEST RESULTS AND DISCUSSION 3.1 GENERAL COMMENTS AND FINDINGS For a better overview test results and their discussion are thematically divided up into the ollowing subsections. Deviating rom EN 1382 [3], withdrawal strength ax o all results shown in section 3 has been determined by F ax ax d l, (2) e where F ax is the maximum orce measured or each test, in (N). Finally, it is worth mentioning, that moisture content o all series varied in a range o max ± 2 % per series. 3.2 STATISTICAL EVALUATION To ind out i a test series speciication essentially inluences the withdrawal capacity, we generally tested median values ( ax,med ) and mean values ( ax,mean ) or signiicance deviation using Wilcoxon-Mann-Whitney test and Student s t-test, which postulates normal distribution (ND) o our results. Test results are shown and compared graphically in orm o notched boxplot diagrams (95 % - conidence interval (CI) determined by Wilcoxon-Test) and error bars (95 % - CIs as t-test results). We assume our data to be lognormal distributed (2pLND), whereore we tested ln( ax,i ) by applying Shapiro-Wilk-Test (SW) beore. In principle, there are two reasons or this assumption: (i) 2pLND constraints only positive data values as they are common or physical and mechanical properties such as strength and stiness (c.. [22]), while ND doesn t eature such restrictions; (ii) standard EN [2] also assumes 2pLND or data assessment. For reasons o general validity (except series III, where test numbers are too small): SW test results indicate that the assumption o 2pLND withdrawal strength is valid or all test results determined (see Table 4 to Table 8). Furthermore, we evaluated the possibility o signiicantly dierent deviations by testing the resulting coeicients o variation (CV[ln( ax,i )]) with the modiied McKay approach [23]. 3.3 IA - LOAD PATH SITUATIONS Main results In Figure 6 and Figure 7 as well as in Table 4 location parameters o withdrawal strength are given in dependence o the prevalent load path situation o the test. Additionally, Figure 8 and Figure 9 compare resulting CV[ln( ax )] o all subgroups. As shown in the corresponding igure, results o test series iii at are plotted with dashed lines, CI error bars are missing here. Reason thereore is the act that a signiicant number o screw tests ailed by tension perp. to grain instead o withdrawal (c.. Figure 3). Consequently, this data set has to be seen as right censored. Location parameters θ = (μ y, σ y ) were thus determined using maximumlikelihood estimation (rcmle) ln L xi max ln L x i, with n d L xi X xi i 1 F i X x i i i 1 1 di, where d i diers between 1 and 0, which includes whether withdrawal ailure has been reached or not. The resulting point estimations or ln( ax ) and CV[ln( ax )] are drawn as red points without CIs. With regard to all corresponding igures and Table 4, two points are worth to be mentioned: Firstly, the expectations o test series densities as well as their dispersions (CV[ρ]) only vary in the comparatively small scheduled bandwidth, avoiding any correction o (3)

5 withdrawal strength. Secondly and as given by completely overlapping notches and error bars, neither the logarithmic withdrawal strength nor its dispersion are signiicantly inluenced by dierent test conigurations and/or load path situations α = 0 CV[ln( ax )] 0.20 ln( ax ) i ii iii iv Figure 9: CIs o CV[ln( ax )]; test series Ia, 1.00 i ii iii iv Figure 6: CIs o logarithmic withdrawal strength; test series Ia, α = 0 ln( ax ) i ii iii iv Figure 7: CIs o logarithmic withdrawal strength; test series Ia, CV[ln( ax )] 0.20 α = 0 Table 4: Density and withdrawal strength o series Ia, E[ρ] [kg/m³] CV[ρ] E[ ax ] [N/mm²] p * CV[ ax ] n α = 0 i ii iii iv i ii iii iv * the p-values are the Shaprio-Wilk-Test results or ln( ax ) Pre-drilling As already mentioned in section 2.1, one control group per α was carried out without pre-drilling. Thus, in Table 5 the location parameters o density and withdrawal strength as well as pairwise statistical test results are given. Again, the expectation as well as the dispersion o density are similar or all series. In case o α = 0, Wilcoxon-Test as well as student s t-test p-values indicate a signiicantly higher withdrawal strength as consequence o pre-drilling while in case o dierences are negligible. Compared to previous studies (predominately ocusing on ; only in [17] one ratio or α = 0 is given), the result or α = 0 was not expected. When considering the delicate situation or this orm o application (long-term tests indicate a very poor DoL behaviour o end-grain screwed connections, see [24]) this positive eect o pre-drilling should be ocused in uture investigations. i ii iii iv Figure 8: CIs o CV[ln( ax)]; test series Ia, α = 0

6 Table 5: Comparison regarding pre-drilling or series Ia α [ ] 0 90 pre-drilling N Y N Y E[ρ] [kg/m³] CV[ρ] E[ ax ] [N/mm²] Shapiro-Wilk- Test p-value CV[ ax ] n Wilcoxon-Test p-value F-Test p-value T-Test p-value IB and IC - SUPPORTING DISTANCES IB - Varying supporting plates In this subsection, Figure 10 and Figure 11 as well as Table 6 illustrate the results o withdrawal strength in dependence o a varying hole diameter d h (o the supporting plate used or common push-pull tests). Again, nearly equal mean densities and CVs enable direct comparability o ax. Congruent to the behaviour observed or dierent load path situations in section 3.3, results indicate no signiicant deviations or dierent d h, while at α = 0 a non-signiicant but decreasing trend o withdrawal strengths with decreasing d h has to be observed. Since this behaviour can t be explained by mechanical eects (in act, we exactly assumed the opposite), we additionally evaluate these test results with a prediction model or ax,pred given in [17] especially or screw axis parallel to grain direction: ax,pred exp d (4) ln( ax ) α = 0 prediction experiment d h [d] Figure 10: CIs o logarithmic withdrawal strength and mean model results; test series Ib, α = 0 ln( ax ) d h [d] Figure 11: CIs o logarithmic withdrawal strength; test series Ib, When comparing mean values o ln( ax,pred ) (red symbols) with our experimental expectations (blue symbols) in Figure 10, the model values exactly result among the experiments showing that neither bigger subseries results (d h = 5 and 6 d), nor lower ones (d h = 2 4 d) signiicantly deviate rom the predictions. To conclude: Also or α = 0 an inluence o d h on ax can be excluded with high reliability. Moreover and independently rom α, a signiicant inluence o d h on the dispersion o ax cannot be observed, see Figure 12 and Figure 13. CV[ln( ax )] 0.20 α = d h [d] Figure 12: CIs o CV[ln( ax )]; test series Ib, α = 0

7 0.20 a s CV[ln( ax )] σ t,90 reinorcement d h [d] (e. g. suspending loads) Figure 13: CIs o CV[ln( ax )]; test series Ib, Table 6: Density and withdrawal strength o series Ib, d h [d] E[ρ] [kg/m³] CV[ρ] E[ ax ] [N/mm²] p * CV[ ax ] n α = * the p-values are the Shaprio-Wilk-Test results or ln( ax ) IC - Varying supporting screws As demonstrated in Figure 4, not only hole diameters o supporting plates (compression load paths, c.. Figure 2) but also the distances o the supporting to the test screws (shear load paths) have been varied. In contrast to the campaign shown in section 3.4.1, a airly smaller program in orm o our dierent distances a s = 2 5 d (only in grain direction and or ) was executed. Test results shown in Figure 15 and Table 7 indicate constant withdrawal capacities with increasing a s (which means increasing dimension o the shear ield), while those o subseries a s = 2 d, closest to the test screw, are remarkably smaller than the rest. This eect is easily explained by an already harmed timber-screw composite area. Based on this inding, it is worth mentioning that minimum distances (e. g. acc. to EN [1]) should be also considered or asteners with dierent load-bearing tasks, see Figure 14. Finally and equal to the previous comparisons, a signiicant inluence o d h on the dispersion o ax cannot be observed, see Figure 16. Figure 14: Example o dierent load-bearing tasks in one joint detail ln( ax ) a s [d] Figure 15: CIs o logarithmic withdrawal strength; test series Ic CV[ln( ax )] a s [d] Figure 16: CIs o CV[ln( ax )]; test series Ic

8 Table 7: Density and withdrawal strength o series Ic a s [d] E[ρ] [kg/m³] CV[ρ] E[ ax ] [N/mm²] CV[ ax ] n * the p-values are the Shaprio-Wilk-Test results or ln( ax ) 3.5 II - RATE OF LOADING In addition to dierent conditions or loading and supporting, this section presents the withdrawal strength results in dependence o a varying loading rate. Similar to section 3.4.2, the experimental programme had been reduced to, investigations with screws inserted and loaded parallel to grain are thus seen as a task or the uture. Nevertheless, Figure 17 and Table 8 give an overview o the subseries withdrawal capacities and densities, which again underline the un-necessity o any correction. Back the ax results, we can notice a remarkable (but non-signiicant) increase at target time to ailure (tt) o ~ 0 s, while the rest indicates no inluence o tt on withdrawal capacity or our ield o interest. Once again, Figure 18 demonstrates that CV[ln( ax )] is not aected by a varying loading rate. Following previous studies concerning RoL, Figure 19 compares ax and tt in a semi logarithmic scale, demonstrating that (i) changes o tt with a actor smaller or equal to 10 1 s don t aect the withdrawal capacity in a major way and (ii) an accurate model describing the behaviour o ax in dependence o RoL needs at least one more sampling point at e. g. tt = 5 s or 10,000 s. Anyway, Equation (5) and Equation (6) propose a irst linear approach in dependence o tt (also drawn in Figure 19) ax,i ax,re log ( tt ) (5) and in dependence o v (mm/min) ax,i ax,re log ( v), (both R² = ). (6) Table 8: Density and withdrawal strength o series II 10 E[ρ] [kg/m³] CV[ρ] E[ ax ] [N/mm²] CV[ ax ] n * the p-values are the Shaprio-Wilk-Test results or ln( ax ) p * p * ln( ax ) E[tt] target tt [s] Figure 17: CIs o logarithmic withdrawal strength; test series II CV[ln( ax )] target tt [s] Figure 18: CIs o CV[ln( ax)]; test series II ax [N/mm²] log10(tt) Figure 19: Test series II: Withdrawal strength vs. log10(tt)

9 3.6 III - SURFACE ANGLE The investigation presented in this section was originally carried out as single screw reerence test group in the rame o a research project dealing with the axial load bearing behaviour o screwed connections, see [25,26]. As shown in Figure 5, the angle ε o the test specimen s surace to the screw axis has been varied between 90 (acc. to EN 1382 [3]) and 45, which is a requently orm o application in practise. As consequence o relatively small numbers o tests (see Table 9), an investigation regarding the type o statistical distribution was not perormed. Thus, only notched boxplots shown in Figure 20 serve as indicators whether dierent values o ε inluence ax in major way. Based on the results given, we have to negate this question. In addition, non-signiicant deviations between load path situations i and iv at α = 45 are well in-line with those discussed in section ax [N/mm²] α = 45 ε = i iv iv Figure 20: CIs or withdrawal strength o test series III Table 9: Density and withdrawal strength o series III ε [ ] E[ρ] [kg/m³] CV[ρ] E[ ax ] [N/mm²] CV[ ax ] n i iv iv SUMMARY, CONCLUSION AND OUTLOOK In the rame o this paper we showed and discussed the results o a test campaign carried out to evaluate possible eects signiicantly inluencing the experimental determination o the withdrawal parameter o axially loaded sel-tapping screws. Based on the indings o corresponding previous studies, we assumed that especially dierent load path situations in the timber specimen lead to dierent withdrawal capacities. Moreover, we executed additional series aiming to investigate urther possible eects, such as pre-drilling, the distance o the supporting to the astener s axis, its angle to the specimen s surace as well as the time to ailure (or rate o loading). Our main interest was to observe the behaviour o expectation and dispersion (in orm o the coeicient o variation) o the parameter withdrawal strength in dependence o varying parameters mentioned beore. Consequently, we compared test series results by applying statistical tools or these location parameters such as Wilcoxon-Mann-Whitney-, Student s t-test and modiied McKay approach. The boundary condition o both latter mentioned test procedures is a ND data set, which is convenient or the natural logarithms o ax when the parameter itsel is assumed to ollow a 2pLND. With regard to this evaluation we conclude: I conditions, where too small distances o supporting screws already harm the timber-screw composite area (push-pile), or the screwed connection tested doesn t ail by withdrawal (e. g. pull-pull at ), are avoided, dierent loading and supporting conditions as well as the distance o the screw axis to the supporting won t inluence the withdrawal behaviour in a major way. Even though the number o corresponding tests is relatively low, the same indings can be stated or dierent times to ailure and surace angles, postulating that the ormer mentioned parameter varies in between a practical range (45 s tt 450 s). Finally and deviating rom the expectation at begin o testing, signiicant increase o ax as consequence o pre-drilling was ound or α = 0. Beside the latter mentioned observation, which should be ocused more in detail in uture investigations especially combined with duration o load (DoL) perormance, urther topics o interest are: The broadening o RoL tests series described in section 3.5 (one or two additional sampling points and the whole test campaign or α = 0 ) to develop an accurate RoL dependent model o ax as well as a comparison o test results or dierent load paths with a mechanical model (e. g. Theory o Volkersen [27]). ACKNOWLEDGEMENT The research project was conducted in the rame o the bridge project SCREWS at the Institute o Timber Engineering and Wood Technology at Graz University o Technology. All oundering and support by the involved partners and also the Lignum Test Centre has to be thankully acknowledged. REFERENCES [1] EN :2009, Design o timber structures - part 1-1: general - common rules and rules or buildings. [2] EN 14358:2007, Timber structures - Calculation o characteristic 5-percentile values and acceptance criteria or a sample. [3] EN 1382:1999, Timber structures - Test methods - Withdrawal capacity o timber asteners.

10 [4] EN 26891:1991, Timber structures - Joints made with mechanical asteners - General principles or the determination o strength and deormation characteristics. [5] Flatscher G., Schickhoer G.: Monotone and cyclic tests on CLT single joints and walls. Submitted to Structures and Buildings, [6] Gavric I.: Seismic Behaviour o Cross-Laminated Timber Buildings. Dissertation. University o Trieste, [7] Plieschounig S.: Ausziehverhalten axial beanspruchter Schraubengruppen. Master Thesis. Graz University o Technology, 2010 (German). [8] Hübner U.: Mechanische Kenngrößen von Buchen-, Eschen- und Robinienholz ür lastabtragende Bauteile. Dissertation. Graz University o Technology, 2013 (German). [9] EN 408:2012, Timber structures - Structural timber and glued laminated timber - Determination o some physical and mechanical properties. [10] Rosowsky D. V., Reinhold T. A.: Rate-o-Load and Duration-o-Load Eects or Wood Fasteners. J. Struct. Eng., 125: , [11] Aoki K., Tsuchimoto T.: Priority o Factors to Decide the Splitting Strength o Bolted Timber Joints. In: 8th World Conerence on Timber Engineering WCTE 2004, , [12] Jansson B.: Impact Loading o Timber Beams. Master Thesis. University o British Columbia, [13] Girhammar U. A., Andersson H.: Eect o Loading Rate on Nailed Timber Joint Capacity. J. Struct. Eng., 114: , [14] Bejtka I.: Verstärkung von Bauteilen aus Holz mit Vollgewindeschrauben. Dissertation. Karlsruhe Institute o Technology, 2005 (German). [15] Gehri E., Haas P.: Schrauben au Ausziehen senkrecht zur Faser - Einluss der Prükoniguration. Test Report. Rüschlikorn, 2008 (German). [16] ETA-11/0190, Würth sel-tapping screws. European Technical Approval. Deutsches Institut ür Bautechnik, [17] Pirnbacher G., Brandner R., Schickhoer G.: Base parameters o sel-tapping screws. Paper CIB- W18 meeting, Dübendor, [18] Ringhoer A., Brandner R., Schickhoer G.: Withdrawal resistance o sel-tapping screws in unidirectional and orthogonal layered timber products. Mater Struct, DOI /s [19] Ringhoer A., Grabner M., Silva C. V., Branco J., Schickhoer G.: The inluence o moisture content variation on the withdrawal capacity o sel-tapping screws. Holztechnologie, 55, 2014 (in press). [20] ETA-12/0373, Sel-tapping screws or use in timber constructions. European Technical Approval. Österreichisches Institut ür Bautechnik, [21] R Core Team: R: a language and environment or statistical computing. R Foundation or Statistical Computing, Vienna, Austria, ISBN [22] Brandner R.: Stochastic system actions and eects in engineered timber products and structures. In: Schickhoer G., Brandner R. (eds.), Timber engineering and technology, vol 2. Monographic series TU Graz. Graz University o Technology, ISBN [23] Vangel M. G.: Distribution o the coeicient o variation and the extended t distribution. Am Stat, 50:21-26, [24] Pirnbacher G., Schickhoer G.: Zeitabhängige Entwicklung der Traglast und des Kriechverhaltens von axial beanspruchten, selbstbohrenden Holzschrauben. Research Report. holz.bau orschungs gmbh, 2012 (German). [25] Ringhoer A., Mahlknecht U., Schickhoer G.: SCREWS - Optimierung der Schraube hinsichtlich des Kratlusses in Verbindungen im Ingenieurholzbau. Research Report. Graz University o Technology, 2013 (German). [26] Mahlknecht U., Brandner R., Ringhoer A., Schickhoer G.: Resistance and Failure Modes o Axially Loaded Groups o Screws. In: Aicher S., Reinhardt H.-W., Garrecht H. (eds.): Materials and Joints in Timber Structures - Recent Developments o Technology, pages , [27] Volkersen O.: Die Schubkratverteilung in Leim-, Niet- und Bolzenverbindungen. Energie und Technik. Part 1-3, 1953 (German).