Lateral resistance of anchor-bolt joints between timber sills and foundations II. Effective lateral resistance of multiple anchor-bolt joints

Monte Carlo simulations were conducted to estimate the effective lateral resistance of multiple anchor-bolt joints with ordinary specifications of Japanese post and beam constructions. Basic lateral load-slip curves of single anchor-bolt joints required in the simulations were determined from the test results of our earlier report. The effective lateral resistance of multiple anchor-bolt joints was estimated for some combinations of loading direction, length/diameter ratio of anchor bolts, lead-hole clearance, and number of anchor bolts. The principal results of the simulations are: (1) anchor-bolt joints loaded perpendicular to lateral forces are not recommended to be counted as supplementary resisting elements because their supplementary shares are far less than those expected from their allowable lateral resistance; (2) multiple anchor-bolt joints with small length/diameter ratios have comparatively lower effective resistance ratios than multiple anchor-bolt joints with large length/diameter ratios; (3) the effective resistance of multiple anchor-bolt joints is affected not only by lead-hole clearance or number of bolts but also variance of load-slip characteristics of single anchor-bolt joints. Part of this work was presented at WCTE2004, Lahti, June 2004     

Effective lateral resistance of combined timber joints with nails and bolts

An experimental study on combined steel-to-timber joints with nails and bolts is conducted in this study. Principal results are as follows: The initial stiffness and effective allowable resistance of combined joints depend obviously on clearances in predrilled bolt-holes. The combined joints with nails and bolts have high potential of energy capacity to resist strong earthquake forces. There are upper limits of clearances in predrilled bolt-holes that allow advantages of considering the synthetic resistance of combined joints in practical structural design. Combining the joint components with appropriate design will give higher performance against strong earthquakes increasing the safety margin and energy capacity until the failure. The combined joints should be designed under the restrictions of particular specifications in closed design systems because the advantages of combining the joint components are influenced obviously by various actual conditions, which are too difficult to consider in detail in open design systems.     

A comparison of the withdrawal resistance load of nails using experimental and interference approaches

Abstract

The objective of this study was to determine and predict the withdrawal resistance or pull-out load of common wire nails embedded in radial, tangential and cross-sectional grain orientation of Douglas fir (Presudotsuga menziesii) and sugar maple (Acer sacharum) samples. Four lead-hole diameters of 1.5, 2.0, 2.5 and 3.0 mm were used to create various interference fits. Nails with a diameter of 3.38 mm were driven into the samples to a depth of 10 mm for the experiments. The overall withdrawal resistance of Douglas fir samples was found to be lower than that of maple samples. Strength values of each sample increased with decreasing lead-hole diameters. No significant difference was found between withdrawal resistance values from radial and tangential sections in either species. However, significantly lower values were obtained for the cross-sections of the samples than for the two other sections. Ratios between predicted pull-out load values of the nails from both species based on the finite element numerical interference approach were very close to experimental measurements, with ratios ranging from 0.93 to 1.09. The results provide better understanding of the behaviour and performance of pull-out resistance for building systems.     

Nonlinear superposition model for the behavior of bolted joints

Summary A previously developed model to predict the load-slip relationship for mechanical joints using one bolt subjected to lateral loading (Part 1) was extended to incorporate the rotational resistance of joints containing two bolts. The rotation is about the long axis direction of the bolts, and considers the wood members oriented at arbitrary angles to the grain. The model utilizes nonlinear translational springs to represent the parallel and perpendicular to grain components of the reaction force present on each bolt resisting the applied moment. A series of experiments were conducted to determine the spring constants of bolted joints in axial loading and to verify the predictions of the mathematical model. Bolted joints subjected to a pure rotation were tested using combinations of steel plates and wood side members experiencing bolt reaction forces at various angles of load to grain. The results indicated an excellent agreement between theoretical predictions and experimentally obtained data.The authors wish to extend thanks to the Colorado State Agricultural Experiment Station for their financial support and to the Fulbright-Hayes Foundation for the educational scholarship and research funding provided to the project     

Fracture of multiply-bolted joints under lateral force perpendicular to wood grain

The crack initiation and propagation of multiplybolted joints subjected to lateral forces perpendicular to the grain were analyzed. Two types of bolted joint were subjected to lateral loads perpendicular to the grain. One had joints of two bolts aligned with the wood grain (type H), and the other had joints of two or three bolts aligned perpendicular to the grain (type V). The crack initiation and propagation were analyzed by means of the average stress method (ASM) and linear elastic fracture mechanics (LEFM), respectively. The maximum loads calculated by LEFM agreed comparatively well with the experimental results, and it was proved that the LEFM was an appropriate tool to analyze the fracture of multiply-bolted joints subjected to a force perpendicular to the grain. It was also found that the multiply-bolted joints failed with the fracture of the wood before the joints yielded, and that it caused a considerable decrease of the maximum loads. The reduction of strength should be considered in the design of multiply-bolted joints subjected to lateral forces perpendicular to the grain.Part of this work was presented at the annual meeting of the Architectural Institute of Japan, Hikone, September 1996     

Bending moment resistance of dowel corner joints in case-type furniture under diagonal compression load

We investigated bending moment resistance under diagonal compression load of corner doweled joints with plywood members. Joint members were made of 11-ply hardwood plywood of 19 mm thickness. Dowels were fabricated of Beech and Hornbeam species. Their diameters(6, 8 and 10 mm) and depths of penetration(9, 13 and 17 mm) in joint members were chosen variables in our experiment. By increasing the connector's diameter from 6 to 8 mm, the bending moment resistance under diagonal compressive load was increased, while it decreased when the diameter was increased from 8 to 10 mm. The bending moment resistance under diagonal compressive load was increased by increasing the dowel's depth of penetration. Joints made with dowels of Beech had higher resistance than dowels of Hornbeam. Highest resisting moment(45.18 N·m) was recorded for joints assembled with 8 mm Beech dowels penetrating 17 mm into joint members Lowest resisting moment(13.35 N·m) was recorded for joints assembled with 6 mm Hornbeam dowels and penetrating 9 mm into joint members.     

Numerical analysis on tensile performance of bolted glulam joints with initial local cracks

Under varying climate conditions, cracks are commonly observed in bolted joints, owing to the shrinkage of wood and confinement from slotted-in steel plates and bolts. A three-dimensional finite element model was developed to investigate the mechanical behavior of bolted glulam joints with initial cracks. Wood foundation was prescribed in the model to simulate the local crushing behavior of wood surrounding the bolts. The behavior of wood in compression and the foundation were defined as transversely isotropic plastic in the software package ANSYS. Cohesive zone model was applied in the numerical analysis to consider the propagation of initial cracks and brittle failure of wood in the bolted joints under tension load. The numerical model was validated by the experiments conducted on full-scale specimens and it is indicated that the numerical model has good ability in predicting the failure modes and capacity of tension joints with local cracks. To further investigate the influence of crack number, length and locations, a parametric study was conducted with the verified model. Moreover, to study the effects of cracks on the behavior of bolted joints with different failure modes, another bolted joint including bolts with different strength grades and diameters was designed and analyzed in the parametric study, which was expected to have bolt yielding failure mode. It was found that the initial cracks can decrease the capacity and initial stiffness of tension joints by up to 16.5 and 34.8%, respectively.     

Effects of pretension in bolts on hysteretic responses of moment-carrying timber joints

The adoption of a concept similar to the prestressing technique used in laminated wood decks of bridge structures might increase the initial stiffness or ultimate resistance of dowel-type timber joints by applying pretension to their bolts. This study investigated the effect of pretension in bolts on hysteretic responses and ultimate properties of moment-carrying timber joints with steel side plates. A pretension of 20 kN that yielded a prestress level of 1600 kPa or about 90% of the allowable long-term end-bearing strength of spruce species was applied to the bolts of prestressed joints. The superiority of the prestressed joint over the non-pre-stressed joint was proved by very high hysteretic damping, equivalent viscous damping ratio, and cyclic stiffness. At any given rotation level, hysteretic damping reduction and moment resistance decrement due to continuously reversed loads were found to be small because bolt pretensioning minimized the pinching effect. This study showed that the hysteresis loop of the prestressed joint can be obtained by adding the frictional hysteresis loop due to pretension force into the hysteresis loop of the non-pre-stressed joint. Despite a great increase of initial stiffness, only slight increments in ductility coefficient and ultimate moment resistance were found in the prestressed joint.     

One-year stress relaxation of timber joints assembled with pretensioned bolts

In our previous study, great increases of hysteretic damping and initial slip resistance of timber joints were attained by applying axial pretension to the steel fasteners. To evaluate the effectiveness of this method, 1-year stress-relaxation measurement was carried out. Nine prestressed joints were prepared and three of them were restressed after 3 and then 6 months after the initial prestressing. All joints were exposed to indoor conditions, and relaxation of the pretension was regularly measured from time-dependent decreases of axial strain of the bolts. After measurement, the joints were subjected to cyclic and monotonic loading tests until failure. The average ratio of residual stress to the initial prestress after 1 year was about 0.23 and 0.66, respectively, for joints without restressing and those with restressing. A simulated stress-relaxation curve developed from the four-element relaxation model predicted 3% of the initial stress after 5 years. Without a regular restressing program, the initial prestressing effect therefore must be considered negligible. However, about 20% of the pre-stress level can be reasonably assumed if restressing is carried out annually. This small residual stress was found to introduce suffi cient frictional damping to signifi cantly increase the equivalent viscous damping ratio of the joints. Part of this study was presented at the 10th World Conference on Timber Engineering, Miyazaki, June 2008     

Effects of density profile of MDF on stiffness and strength of nailed joints

Nail-head pull-through, lateral nail resistance, and single shear nailed joint tests were conducted on medium density fiberboard (MDF) with different density profiles, and the relations between the results of these tests and the density profiles of MDF were investigated. The maximum load of nail-head pull-through and the maximum load of nailed joints were little affected by the density profile. However, the ultimate strength of lateral nail resistance, the stiffness, and the yield strength of nailed joints were affected by the density profile of MDF and showed high values when the surface layer of the MDF had high density. It is known that bending performance is also influenced by density profile. Therefore, the stiffness and the yield strength of nailed joints were compared with the bending performance of MDF. The stiffness of nailed joints was positively correlated with the modulus of elasticity (MOE); in the case of CN65 nails, the initial stiffness of joints changed little in response to changes in MOE. The yield strength of nailed joints had a high positive correlation with the modulus of rupture (MOR). The stiffness and the yield strength of nailed joints showed linear relationships with MOE and MOR, respectively.     

Technological heterogeneity of adhesive bonds in wood joints

 Gluing of wood is among the most effective methods for the permanent joining of furniture elements or building woodwork manufactured from wood. Technological errors occurring during the preparation process of the glue material may lead to variations in the strength of adhesive/wood joints. The purpose of the described research project was to investigate the effect of the heterogeneity of the glue bond on the distribution of tangential stresses in furniture joints, especially the effect of gas cavities, faulty glue bonds and glue outflows on the distribution of tangential stresses in adhesive bonded overlap, cross and angle wood joints. Using developed numerical models, it was shown that shear stresses in bonds of cross and angle joints reach their maximum values in corners of joints. The torsion center of cross joints is situated in the geometrical center of the bond, while in angle joints – it is found half-way through the length of one of the perpendicular edges of the joint. It was also proven that gas cavities present in the glue bond contribute to increased stresses in the neighbourhood of the source of heterogeneity. This phenomenon initiates a process of de-cohesion and, hence, reduces the overall strength of the joint. Faulty gluing, similar to gas cavities, constitutes a potential source of stress-breaking processes and reduces the strength of joints. On the other hand, glue outflows present in wood bonds increase their strength by expanding the initiation threshold of fractures even in situations where technological heterogeneity of the glue bond occurs. In furniture constructions as well as in large-size building woodwork constructions or, wherever grace and elegance of the finished product is of lesser importance, glue outflows can be treated as a positive and desirable phenomenon. Received 13 March 1999     

Bending moment resistance of corner joints constructed with spline under diagonal tension and compression

We determined the effects of the penetration depth and spline material and composite material type as well as joining method on bending moment resistance under diagonal compression and tension in common wood panel structures. Composite materials were laminated medium density fiberboard (MDF) and particle board. Joining methods were buttand miter types. Spline materials were high density fiber board (HDF).The penetration depths of plywood, wood (Carpinus betolus) and splinewere 8, 11 and 14 mm. The results showed that in both diagonal compression and tension, MDF joints are stronger than particle board joints,and the bending moment resistance under compression is higher compared with that in tension. The highest bending moment resistance under tension was shown in MDF, butt joined using plywood spline with 8 mm penetration depth, whereas under compression bending moment resistance was seen in MDF, miter joined with the HDF spline of 14 mm penetration depth.     

Shear resistance and failure modes of nailed joints loaded perpendicular to the grain

Shear tests were conducted on nailed joints in wood that were loaded perpendicular to the grain; these joints had 21 specifications depending on different combinations of wood species, nail dimensions, number of nails, and edge distances of the main members, and their effects on the shear resistance of the nailed joints were also investigated. The nailed joints with CN75 nails had higher initial stiffness than the joints with CN50 nails, provided the initial stiffness of nailed joints connected with 3 or 5 nails was not always a simple product of the number of nails and the initial stiffness of nailed joints connected with a nail, and instead depended on the combination of wood species of the main member and nail dimensions. When the edge distance decreased, the maximum load and energy capacity decreased, thereby affecting the energy capacity. The maximum load of the nailed joints with CN75 nails may be smaller than those with CN50 nails depending on the combination of wood species and nail dimensions. When the edge distance of the nailed joints was less than 26 mm, the energy capacity of the nailed joints with CN75 nails was less than or similar to those with CN50 nails.     

Relationship between clamp force and pull-out strength in lag screw timber joints

This study empirically examines the relationship between clamp force and pull-out strength in lag screw joints of timber members, using data obtained in tightening tests and pull-out tests. Maximum clamp force per unit screw length as determined from the tightening tests was lower than the lower bound for the 95% tolerance range for pull-out strength per unit screw length as determined from the pull-out tests. Moreover, X-ray CT (computed tomography) observations of anchor members from both tests revealed that failure behavior clearly differed between the tightening test and the pull-out test: tightening caused damage to the wooden, female thread in addition to major splitting damage in the wood perpendicular to the grain near the tip of the lag screw.     

A new method for estimating stiffness and strength in bolted timber-to-timber joints and its verification by experiments (II): bolted cross-lapped beam to column joints

Bolted cross-lapped joints (BCLJs) are one of the basic jointing methods used in Japan and European countries. There are, however, some problems in the design of BCLJs. With increasing use of large-scale wooden frame structures in Japan, it is necessary to establish proper estimating methods for predicting actual characteristics. A new approach was developed, using Saint Venant torsion theory, to estimate the performance of bolted timber joints in a more practical manner than using computer simulations. The calculated values were compared with the experimental results, indicating that the rotational stiffness and yield moment of BCLJs would be precisely predicted using the proposed theory. It was also found that the rotational stiffness calculated using the design method rooted on Coulombs torsion theory is about two times higher than the experimental results in the case of a rectangular arrangement of bolts.     

Basis and monitoring of methoxyfenozide resistance in the South American tomato pinworm Tuta absoluta

The South American tomato pinworm, Tuta absoluta (Meyrick, 1917) (Lepidoptera: Gelechiidae), is heavily targeted by insecticide applications. Methoxyfenozide is highly effective against T. absoluta with reduced side effects to natural enemies. This work aimed to (1) genetically and biochemically characterize resistance of the T. absoluta GBN population to methoxyfenozide, (2) establish cross resistance profiles with other insecticide groups and (3) monitor resistance in populations with the goal of improving T. absoluta insecticide resistance management (IRM). Methoxyfenozide resistance was completely recessive, polyfactorial and autosomal. Effective dominance revealed that 10 mg methoxyfenozide/L would be enough to eliminate susceptible homozygotes and heterozygotes, thus used to diagnose resistance in field populations. The synergism of methoxyfenozide toxicity in the resistant population for PBO (SR?=?95×), DEF (SR?=?51×) and DEM (SR?=?45×), suggested monooxygenases, esterases and glutathione S-transferases as resistance mechanisms. However, only monooxygenase activity appeared to be involved in methoxyfenozide resistance. Resistance ratio for methoxyfenozide (2352-fold) after selection and cross-resistance ratios of a lab-selected GBN strain (“GBN-Sel”) were significant relative to a susceptible strain “JDR1-Sus” for tebufenozide (656-fold), cartap hydrochloride (10.68-fold), deltamethrin (4.70-fold), abamectin (2.65-fold), lufenuron (2.22-fold) and indoxacarb (1.92-fold), with negative cross-resistance to spinetoram (0.32-fold). Evidence of control failures was observed in 10 field populations of T. absoluta (mortalities between 13 and 76%), and all populations showed frequencies of resistant phenotypes (percentage survivorship ranging between 4 and 96%). A rational basis for managing resistance to bisacylhydrazines is discussed, along with details of recommended T. absoluta resistance management tactics.

     

Shear strength of beam splice joints with glued-in rods

Splitting failure in beam splice joints with glued-in rods parallel to grain in endwood subjected to pure shear is considered. A simple theoretical expression based on beam-on-elastic-foundation theory and quasi-non-linear fracture mechanics is presented for calculation of the joint strength. Tests were conducted on jointed beams in a four-point bending test setup in which the joints were located at the point of pure shear force. Hardwood dowels with a diameter of 12mm and a glued-in length of 120mm were used as rods, and various beam cross sections and dowel configurations were tested. The theory presented is found to agree well with test results in all cases in which the edge distance of the glued-in rods is relatively small. Some test results indicate that the theory may be conservative in case of large edge distances.     

Strength performance of mortise and loose-tenon furniture joints under uniaxial bending moment

We determined the effects of adhesive type and loose tenon dimensions （length and thickness） on bending strength of T-shaped mor- tise and loose-tenon joints. Polyvinyl acetate （PVAc） and two-component polyurethane （PU） adhesives were used to construct joint specimens. The bending moment capacity of joints increased significantly with increased length and thickness of the loose tenon. Bending moment capacity of joints constructed with PU adhesive was approximately 13% higher than for joints constructed with PVAc adhesive. We developed a predictive equation as a function of adhesive type and loose tenon dimensions to estimate the strength of the joints constructed of oriental beech （Fagus orientalis L.） under uniaxial bending load.     

Estimation of stiffness and strength in timber knee joints with adhesive and verification by experiment

The use of adhesive joints is gradually increasing, especially those with glued-in steel rods (GIRs). There are, however, some problems with the design methods when used for moment-transmitting applications. In this article, design methods for GIRs and cross-lapped glued joints (CLJs) are proposed. For CLJs, we made a hypothesis that both rotational deformation of CLJ and stress of the glue line occurred with bending and shearing deformation of the timber. Using this hypothesis and Kelvin’s theorem, a mechanical model of CLJ is proposed. For GIRs, the axial stress component of the rod and the lateral stress component of the rod were taken into account using the theory of a beam on an elastic foundation. From the comparisons between calculations and experimental results, it was recognized that the stiffness and strength of CLJs and GIRs could be predicted precisely using our proposed models.     

不同播种密度对楸树砧木梓树幼苗的生长影响研究

对梓树3个种源当年生幼苗进行砧木培育,通过不同密度的播种和移栽,选出在天水地区以种源A最为适宜培育,播种移栽密度以19株/m2为宜,当年生苗木平均地径可达8.85 mm;苗木地径与平均侧根长、地上部鲜质量、地下部鲜质量、地上部干质量、地下部干质量呈正相关性,与苗高、叶数、根数、主根长无显著相关性;各种源苗木生物量与平均侧根长呈正相关,与主根长无显著相关性。