00694 Pouyan Zarnani - Timber Design Society

More documents

Recommendations

Info

LVL groups Rivet penetration Table 3: Strength and failure mode predictions using the proposed method compared to experimental results on LVL Proposed wood strength P w (kN) Rivet strength P r * (kN) t block (mm) Proposed/Observed Proposed wood strength P w (kN) Connection strength (prediction/test result) Mean ultimate load † Failure mode L p (mm) t block =t ef,e t block =t ef,y P u (kN) BRG1-L 28.5 314 "461" 27.1(t ef,e ) / 23 - 314/358 Brittle/Brittle BRG2-L 28.5 362 "461" 27.1(t ef,e ) / 27 - 362/370 Brittle/Brittle BRG3-L 28.5 380 "461" 27.1(t ef,e ) / 24 - 380/375 Brittle/Brittle BRG4-L 28.5 376 "461" 27.1(t ef,e ) / 21 - 376/391 Brittle/Brittle BRG5-L 28.5 381 "461" 27.1(t ef,e ) / 28 - 381/402 Brittle/Brittle BRG6-L 28.5 378 "461" 27.1(t ef,e ) / 26 - 378/410 Brittle/Brittle BRG7-L 28.5 391 "461" 27.1(t ef,e ) / 26 - 391/435 Brittle/Brittle BRG8-L 53.5 419 "692" 45.5(t ef,e ) / 48 - 419/463 Brittle/Brittle BRG9-L 53.5 392 "519" 45.5(t ef,e ) / 43 - 392/384 Brittle/Brittle BRG10-L 53.5 440 "519" 45.5(t ef,e ) / 42 - 440/419 Brittle/Brittle BRG11-L 53.5 432 "519" 45.5(t ef,e ) / 44 - 432/427 Brittle/Brittle BRG12-L 53.5 432 "519" 45.5(t ef,e ) / 41 - 432/398 Brittle/Brittle BRG13-L 53.5 436 "519" 45.5(t ef,e ) / 41 - 436/456 Brittle/Brittle BRG14-L 53.5 440 "519" 45.5(t ef,e ) / 46 - 440/468 Brittle/Brittle BRG15-L 53.5 427 "519" 45.5(t ef,e ) / 47 - 427/437 Brittle/Brittle BRG16-L 53.5 434 "519" 45.5(t ef,e ) / 42 - 434/445 Brittle/Brittle BRG17-L 53.5 441 "345" 40.1(t ef,y ) / 50 362 345/290 Ductile/Brittle BRG18-L 28.5 237 "259" 27.1(t ef,e ) / 24 - 237/247 Brittle/Brittle BRG19-L 53.5 334 "388" 45.5(t ef,e ) / 46 - 334/315 Brittle/Brittle MIG20-L 78.5 436 "417" 26.7(t ef,y ) / 29 233 233/245 Mixed IV/Mixed IV MIG21-L 78.5 338 "278" 26.7(t ef,y ) / 27 176 176/207 Mixed IV/Mixed IV MIG22-L 53.5 255 "259" 45.5(t ef,e ) / 35 - 255/214 Brittle/Mixed III m MIG23-L 28.5 178 "172" 24.2(t ef,y ) / 19 166 166/159 Mixed III m/Mixed III m DUG24-L 28.5 505 "345" 24.2(t ef,y ) / - 498 - /345 Ductile III m/Ductile III m DUG25-L 53.5 515 "388" 40.1(t ef,y ) / - 479 - /388 Ductile III m/Ductile III m DUG26-L 78.5 419 "185" 26.7(t ef,y ) / - 213 - /185 Ductile IV/Ductile IV * The rivet strength in BRG and MIG groups are based on the rivet capacity derived from DUG tests. † Coefficient of variation (COV%) for brittle/mixed failure modes 4-9 % and for ductile failure modes 2-4%. Table 4: Strength and failure mode predictions using the proposed method compared to experimental results on glulam Proposed t block Connection strength Proposed wood Rivet Rivet (mm) (prediction/test result) wood strength Glulam penetration strength strength P w Mean P groups w P r (kN) ultimate (kN) (kN) Proposed/Observed load † Failure mode L p t block =t ef,e P t (mm) block =t u ef,y (kN) BRG1-G 53.5 340 "531" 45.5(t ef,e ) / 46 - 340/335 Brittle/Brittle BRG2-G 53.5 328 "398" 45.5(t ef,e ) / 45 - 328/301 Brittle/Brittle BRG3-G 28.5 188 272 * 27.1(t ef,e ) / 25 - 188/224 Brittle/Brittle BRG4-G 53.5 226 "266" 45.5(t ef,e ) / 50 - 226/315 Brittle/Brittle MIG5-G 78.5 222 "217 * " 29.7(t ef,y ) / 28 127 127/160 Mixed IV/Mixed IV DUG6-G 53.5 397 "298" 40.6(t ef,y ) / - 379 - /298 Ductile III m /Ductile III m † Coefficient of variation (COV%) for brittle/mixed failure modes 11-17 % and for ductile failure modes 8%. * Values are based on the tests conducted by Buchanan and Lai [16] on Radiata Pine glulam with the same density. One can note that the predictions using the other models are mostly constant for the tests with approximate capacities of 350 kN to 450 kN (Fig. 12b). These are the tests series conducted to observe the effects of bottom, edge and end distances. For instance, as the bottom distance d z gets larger due to increase in timber thickness, the capacity of the connection gets higher as asserted by Stahl et al. [6] as well. The predictions based on the proposed analysis are shown in Table 5 for test groups BRG1-L to BRG3-L which are identical in every parameter except the bottom distance. Thicker specimen with larger d z induces more stiffness for the resisting bottom shear plane. Therefore, the stiffness ratio of the resisting bottom plane increases and for the other resisting planes reduces. Subsequently, a higher proportion of the applied load transfers to the
Predicted Average Strength [kN] Predicted Average Strength [kN] bottom resisting plane and the maximum stress lowers on the lateral shear and head tensile planes in comparison to the stresses for a thinner specimen. Regarding the fact that in these test series the earlier failure belongs to the head tensile plane which gets first overloaded, so the connection capacity gets higher. The same behavior happens in respect of increasing the edge distance d e . However, the predictions by the O86 code show an opposite behavior with a decreasing trend in thicker specimens. The reason is the fact that the Canadian code includes a volume effect on the shear strength which affects the connection capacity. The results from current tests and Stahl et al. [6] disprove the size effect based on Weibull weakest link theory of brittle failure adopted by Canadian and also U.S. codes. In fact, the size of the wood surrounding the main loaded block affects mostly the stiffness of the resisting shear planes rather than the shear strength of the wood. For instance, the strength of a connection with minimal edge and bottom distances can be simply predicted as the (a) (b) 550 450 350 250 600 500 400 300 200 New Analytical M. r² = 0.91 MAE=-2.6% STDEV=9.8% 150 Radiata Pine-LVL Radiata Pine-Glulam Narrow connection 50 50 150 250 350 450 550 Observed Average Test Strength [kN] 700 Unsafe Conservative 100 Stahl Method O86-09 Code 0 EC5 0 100 200 300 400 500 600 700 Observed Average Test Strength [kN] Figure 12: Comparison of analyses and the current test data in brittle/mixed failure modes; (a) New analysis, (b) Stahl’s method, O86 code and EC5 tensile capacity of the head plane since shear planes have no significant stiffness to carry any load. Also, the predictions for narrow connections are overestimated by EC5 and Stahl’s models; on the other hand, underestimated by O86 code (Fig. 12b). Moreover, for mixed failures, the predictions using Stahl’s method are non-conservative. These overestimated values are due to the fact that Stahl’s equation considers the full length of the rivet penetration as the effective wood thickness and to the fact that the possibility of mixed mode failure is not included. Thus, the necessity for predicting the connection strength under a mixed mode of failure is required. 5.2 DATA AVAILABLE IN THE LITERATURE A similar comparison was made using data available in the literature and current test data (Fig. 13). Five sets of data were considered from the literature: tests performed by Foschi and Longworth [2] on Douglas Fir-Larch glulam, Buchanan and Lai [16] on Radiata Pine glulam, Karacabeyli et al. [17] on Hem-Fir solid timber, Stahl et al. [6] on Southern Pine glulam, and Marjerrison [18] on Douglas Fir-Larch and Spruce Pine glulam. It should be mentioned that the practice codes report the connection strengths at the 5 th percentile level. Therefore, a conversion was made to compare mean ultimate test strengths with 5 th percentile values from the code estimated at 75% confidence level. Coefficients of variation (COV) of 20% and 15% were considered for brittle/mixed failures on glulam and LVL correspondingly. In addition, a load duration factor was applied to the wood strength values taken from the codes to consider the short term loading effect for experiments. Based on the codes, a load duration factor of 1.25 was used for O86 values and 1.10 for EC5. By comparing the prediction models (Fig. 13), it can be deduced that there is more conformity between the predictions using the proposed analysis and the available test data. The predictive new analysis results in a higher correlation coefficient of 0.87 and less MAE of +0.9%. The method proposed by Stahl and the O86 code predictions are better than the ones using the EC5 model. The values obtained using Stahl’s method are scattered (Fig. 13b) which leads to higher STDEV compared to the one using O86 (Fig. 13c). There is considerable over prediction for the strength of a connection with large end distance by Stahl’s method and EC5. This sample supports the fact that by adding to the end distance, the load carrying capacity of the connection doesn’t increase correspondingly to the additional shear resistance surface due to larger end distance. On the other hand, O86 code’s values are underestimated and too conservative. Table 5: The effect of increasing the bottom distance on the stiffness ratio of resisting planes and the wood capacity Specimens Bottom distance d z Stiffness ratios of resisting planes Maximum load causing failure on each plane (kN) Capacity prediction (min P wi *2) Test result (mm) R h R b R l P wh P wb P wl (kN) (kN) BRG1-L 17 (0.3X b ) 58% 30% 12% 157 281 298 314 358 BRG2-L 35 (0.6X b ) 51% 39% 10% 181 214 344 362 370 BRG3-L 62 (1.1X b ) 49% 42% 9% 190 205 355 380 375
Page 1 and 2: PREDICTVE ANALYTICAL MODEL FOR WOOD
Page 3 and 4: the lateral blocks and is Atl 2tef
Page 5 and 6: Grain 1200 mm Load F [kN]nt mm thic
Page 7: Test groups with tightly spaced riv

00694 Pouyan Zarnani - Timber Design Society

Create successful ePaper yourself

Delete template?

Save as template?