Withdrawal strength of self-tapping screws in tropical hardwood
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Abstract
Self-tapping screws are popular tools used in timber engineering. The axial withdrawal strength of self-tapping screws could be used in timber structures as an active or passive application. An example of an active application is the use as a fastener in wall to wall connections. An example of a passive application is the use as reinforcement perpendicular to the grain, to prevent cracking along the grain. Existing methods to calculate the withdrawal strength of self-tapping screws are mainly based on tests done on softwood. Some screw manufacturers offer calculation methods for the use of their screws in hardwood from temperate regions in so called European Technical Assessments (ETAs). But the Eurocode and the ETAs offer no calculation methods for high density tropical hardwood species. Tropical hardwood species are generally used in The Netherlands for hydraulic and outside structures such as lock-doors, bridges, weirs and jetties. Tropical hardwood species are used because of their strength and durability in outside conditions. There are no well-established methods to calculate the withdrawal strength of self-tapping screws in tropical hardwood perpendicular to the grain and the purpose of this thesis is to gain more knowledge on this topic. The central question of this thesis is: What is the withdrawal strength of self-tapping screws in tropical hardwood species perpendicular to the grain and is it possible to develop a verification model for engineering purposes?
To find answers to the main question first a literature review has been conducted. Literature on timber anatomy, self-tapping screw properties and available calculation methods have been reviewed. This is followed by a literature review on possibilities to drive self-tapping screws into high density woods. This is problematic because screws could fail already during insertion or cause splitting of the timber. Screw modifications such as cutter tips could be used to reduce the insertion moment, but also pre-drilling of the timber could be done to avoid torque failure of screws or the splitting of timber. The literature study is finalized with an investigation to the influence of material properties on the withdrawal strength. Tropical hardwood species are often used in outside or wet structures so literature on the influence of moisture content has been investigated. In research done by Ringhofer et al. [22] it was found that if the moisture content increases above 12 % the withdrawal capacity significantly decreases. Pre-drilling of the timber helps to insert self-tapping screws more easily into timber, but this also removes material and could therefore reduce the withdrawal strength. Research done by Brandner, Ringhofer, and Reichinger [5] found that if the pre-drilling diameter is smaller or equal to 80 % of the nominal screw diameter pre-drilling had no influence on the withdrawal strength. The mentioned research however has been done on temperate hardwood and the question is whether this is the case for tropical hardwood as well. According to EN 1995-1-1:2011 pre-drilling is necessary if the characteristic density of the timber is larger than 500 kg/m3. With increasing timber density the withdrawal strength also increases (Hubner, Rasser, and Schickhofer [13]). Regarding the screw diameter, with increasing diameter the withdrawal strength (fax, MPa) decreases as was found by for example Hubner, Rasser, and Schickhofer [13]. With increasing diameter the withdrawal capacity (Fax, kN) also increases. With increasing effective length of self-tapping screws inserted in wood the withdrawal strength (fax, MPa) does not increase or decrease (Xu et al. [30]). The withdrawal capacity (Fax, kN) however increases, a linear relationship between the effective length withdrawal capacity can be observed in research done by Xu et al. [30].
Findings in the literature study were the basis of how the test series were designed. EN1382:2016 describes how to test and determine the withdrawal strength of screws in timber. This standard also gives rules on the dimensions of the test piece. The width of the available tropical hardwood pieces was to small according to EN1382:2016 which states that a minimal edge distance of 5d has to be used. Methods from the EN 1995-1-1:2011 and ETA-11/0030:2020 from Rotho Blaas have been used to make a selection of screws that will be used in testing. The goal is to have withdrawal as main failure mode. The following screw diameters have been selected: 6, 8 and 11 mm. The diameter 11 mm screw has been tested in a pre-testing series with pre-drilling diameters: 0,7d and 0,8d; with insertion depth 90 mm; Also three tropical hardwood species with the highest density were selected to test the limits of the materials. It was found that the smaller edge distances did not cause any problems. Further because of pre-drilling no damages occurred on the timber or screw, and there were no problems during screw insertion. No significant difference in withdrawal capacity was found between pre-drilling 0,7d and 0,8d so this was changed to pre-drilling 0,8d and 0,9d in the main test series. Also screw failure was observed for insertion depth 90 mm, this was changed to a maximum of 70 mm in the main test series to make sure screw withdrawal is the main failure mode.
For the main test series 6 tropical hardwood species were used (Kanda, Lati, Longhi, Tali, Limbali and Mukulungu), and also the softwood Spruce as a reference material. The diameter 6 mm, 8 mm and 11 mm screws were used with pre-drilling diameter 0,8d and 0,9d. Conditioning of the wet test pieces has been done by putting the timber blocks underwater. On every test piece or timber block four tests were done: two withdrawal strength tests (according to EN 1382:2016) and two stiffness tests (according to EN 26891:1991). Also the insertion moment was measured. After testing the density of every test piece was determined together with the moiture content.
The main test series provided a broad range of data. The following was measured: the density, the moisture content, the withdrawal capacity, the insertion moment and slip modulus. All results can be found in annex A. The test piece geometry was not according to EN 1382:2016, in general larger edge distances were required. Nevertheless did this not lead to any problems, no cracks or screw failure occurred during the insertion of the self-tapping screws nor was it any problem to insert the screws at all. This had all to do with the pre-drilling diameters of 0,8d and 0,9d what was used. Because of this very low insertion moments were measured. Also during and after withdrawal tests no significant damage was observed. Even in extreme situations that were tested during pre-testing there was no evidence that a larger edge distance was needed. In the analysis it was found that with increasing density the withdrawal strength also increases, this is in line with literature. It was also found that the species does influence the withdrawal strength, as was found by calculating the withdrawal strength to density ratio per species. With increasing screw diameter the withdrawal strength decreases as was found in this analysis, and also in line with literature. Only in the case of pre-drilling 0,9d for the diameter 8 mm screw this was not the case when comparing it to the diameter 6 mm screw, this probably was a result of pre-drilling accuracy. Regarding the influence of pre-drilling it was found that on average the withdrawal strength is reduced by a factor of 0,68 when the pre-drilled hole is 0,9d instead of 0,8d. In literature it was found that the effective screw length does not influence the withdrawal strength. No clear influence of effective length has been found, except for screw diameter 6 mm and 8 mm when pre-drilled 0,9d. The diameter 6 mm and 8 mm screws are HSBH screws with a larger inner diameter and it might be that the withdrawal strength is not optimal for these types of screw when inserted in 0,9d pre-drilled holes. During testing wet and dry pieces were tested. It was found that the moisture uptake rate is very different for the various tropical hardwood species that were tested. It was also found that as expected the withdrawal strength is negatively influenced by an increased moisture content. First of all it has to be mentioned that the scatter in data is larger when compared to the test data from withdrawal tests, this has to be taken into account. In the analysis it was found that with increasing density the slip modulus also increases. Also the timber species influences the slip modulus just like in the case of the withdrawal strength. No clear influence of screw diameter was found on the withdrawal strength. Just like for the withdrawal strength is the slip modulus influenced negatively when pre-drilling 0,9d has been used instead of 0,8d. This negative influence increases with increasing diameter. With increasing effective screw length also the slip modulus increases, this would be expected and was also found in the test results. Also a higher moisture content has a negative influence on the slip modulus, this was also as expected. A larger influence was observed in the case of timber species with the highest densities measured (Tali and Mukulungu), while for these species the lowest moisture content was measured in wet conditions compared to other species.
Finally two models have been proposed to better predict the withdrawal strength of self-tapping screws perpendicular to the grain in tropical hardwood. For the first model withdrawal strength $f_{ax}$ parameters are determined per species, per timber condition and per pre-drilling diameter. The second model is one formula based on the density, screw diameter and effective screw length for all tested tropical hardwood species. Both proposed models show a good fit to the test data.