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Chapter Eight 
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Chapter 8: CONCLUSIONS AND RECOMMENDATIONS FOR 
FURTHER WORKS 
8.1 CONCLUSIONS MADE FROM EMBEDMENT STRENGTH TEST 
SERIES 
EC 5 recommendations predict the embedment strength of local timber species 
reasonably well with a little discrepancy between the EC5 predictions and the 
experimental results. 
Difference between EC5 predictions and experimental results increase with 
increasing of bolt diameter for parallel to grain loading. 
Difference between EC5 predictions and experimental results decrease with 
increasing of bolt diameter for perpendicular to grain loading. 
Results of linear regression analysis also verify above-mentioned conclusions. 
Equation 6 .1 and Equation 6.2 are proposed to be used for the determination 
of embedment strength of local timber species. 
Proposed new model does not have a considerable effect on either joint 
strength or the failure pattern of joints tested in joint strength test series. 
8.2 CONCLUSIONS MADE FROM JOINT STRENGTH TEST SERIES 
European Yield Model provides more realistic design method than other 
empirical and analytical models, which are used to design, nailed and bolted 
timber joints, considering the effects of all geometric and material parameters. 
As a whole, EYM seems to be good in predicting the strength and the failure 
patters of joints tested. 
EYM predicted every type of failure patterns for the joints tested in this test 
programme. 
Every type of failure patterns were observed for the joints loaded in parallel to 
grain direction. 
Only failure patterns of mode lb , mode 2 and mode 3 were observed for the 
joints loaded in perpendicular to grain direction. 
It seems that the EYM predictions for failure mode are reasonably good 
because percentage of joints, which were predicted accurately by the EYM, is 
close to 50%. 
The best accuracy of prediction of failure pattern is with the bearing failure of 
central member (mode lb) . 
68 
Thus the EYM can be regarded as good for predicting the joints, which fails 
due to bearing failure than the bending failure. 
Experimental joint strengths were predicted well by E Y M when thickness 
ratio is close to unity. 
Factor of Safety increases in considerable level when increasing the main 
member thickness while keeping the side member constant in thickness. 
Some of these factors of safety are too high and resulting in more conservative 
design. 
Although the experimental strength of the joint increases in considerable level 
with increasing of the thickness ratio, EYM predictions are almost same for 
every thickness ratio when considering a particular bolt diameter and side 
member thickness. 
It seems that the effect of central member thickness on the strength of joint is 
not critically considered by the EYM theory. 
Although the joints which have larger central member thickness were 
predicted to fail in mode 2 and mode 3 failure types, in which central member 
thickness is less critical factor, they were failed in mode l b failure type, in 
which central member thickness is more critical factor. 
This behaviour was not observed by early researches. Most of early researches 
were conducted using unique thickness ratio (side to central) that is equal to 
2.0. 
A modification factor based on the joint geometry is proposed to reduce this 
discrepancy between prediction and experimental results and bring the average 
factor of safety approximate to 2.0, which brings the factor of safety of the 
joint to a reasonably conservative value. 
Equation 7.4 can be used to calculate the modification factor according to the 
ratio of central member thickness to bolt diameter (t2/d) and this modification 
factor independent from member density, member thickness and loading 
direction. 
After modifying the EYM predictions using the proposed modification factor, 
the average factor of safety is around 2.0 and thus the design has reasonably 
conservative factor of safety value. 
RECOMMENDATIONS FOR FURTHER WORKS 
Proposed model for embedment strength (Equation 6.1 and Equation 6.2) is 
based on the experimental results obtained from this test series. Therefore, it is 
necessary to conduct a verification test programme with a wide range of 
parameters that affect the embedment strength to check the reliability of this 
proposed model and c a n y out necessary modifications. 
69 
Specimens of embedment strength test were loaded in parallel and 
perpendicular to grain directions only. Applicability of Equation 3.25 can be 
further checked by conducting embedment strength tests varying the loading 
angle from 0° to 90°. 
Only side members of the joints, in which the joints tested in perpendicular to 
grain direction, were subjected to the loading in perpendicular to grain 
direction. Therefore, it is necessary to expand the test programme as central 
members too are subjected to perpendicular to grain loading. 
Because of the concept of applying an adjustment factor based on joint 
geometry has not been proposed by earlier researchers, it is strongly 
recommended to carry out further investigation with wider range of joint 
geometry. 
Further, this research programme used lower bolt diameters and member 
thickness although much larger size bolts and very thick members are often 
used in the construction industry. Internationally, bolts of the size of more than 
50mm are also used with suitable member sizes for heavy constructions. 
Therefore, it is required to check the applicability of EC5 recommendations to 
the joints made form local timber species with these larger bolts sizes too. 
Also, to develop the proposed adjustment factor, this test programme used 
only local timber species, which belongs to hardwood category. Therefore, it 
is necessary to carry out more researches with both softwood and hardwood 
categories to reach a reliable decision. 
This test programme was limited to the two local timber species. Because, 
number of species tested in this research programme is not enough to reach a 
reliable decision, conducting a further test programme with other timber 
species, in which density ranges from low to high, is required. 
It is recommended to conduct joint strength tests loading both side and central 
members in the directions other than parallel and perpendicular to grain. 
For future test programmes, it is necessary to select joints geometries such the 
way that there is a wide range of central member thickness for a particular bolt 
diameter and side member considered. This is required to carry out analysis 
plotting the normalized joint strength against the ti/d ratio. 
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V 
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