Block Shear Strength Research Articles

Bonding Performance of Preservative-Treated Cross-Laminated Timber (CLT) Posttreated with CU-Based Preservatives

Abstract To expand the use of cross-laminated timber (CLT) to exterior applications, there is a need to protect the panels from biodegrading agents such as fungi and termites. Pressure treatments are effective methods of increasing the durability of wood and wood-based products; however, studies on pressure-treated CLT are limited. In this study, preservative-treated CLT samples from prefabricated CLT panels were prepared and impregnated with Cu-based preservatives through a conventional vacuum-pressure process. The effects of panel layup (3-ply parallel, 3-ply perpendicular, and 5-ply parallel) and preservative treatment (untreated [control], copper azole-type C [CA-C], and micronized copper azole-type [MCA]) on the bonding performance were investigated. Panel layup and preservative treatment had a significant influence on the block shear strength and percentage of wood failure (WFP) of the treated panels. Overall, approximately 60 percent of the block shear specimens had a WFP of >75 percent. However, fewer than 10 percent of the delamination specimens met the ASTM D2559 (2018) limitation of 1 percent for softwood used in outdoor applications. ASTM D2559 counts shallow wood failure as delamination, which could have been a reason for the high delamination rate. The percentage of wood failure and the high rate of delamination could be due to the moisture-induced adhesion failure resulting from the pressure-treatment process. The preservative pressure-treatment of the CLT panels increased the moisture content (MC) from 12–15 percent to approximately 85 percent MC, and the severe swelling of the panels during treatment might have imposed a high stress on the bond line. However, no noticeable delamination of the panels was observed during the actual treating phase of the study. These results show the feasibility of treating prefabricated CLT panels with CA-C and MCA preservatives without compromising the bonding strength.

Effects of material property and thickness on block shear strength in lean duplex stainless steel welded connections

Recently, studies have been carried out to apply lean duplex stainless steels, with less than 1.5% nickel, which provides higher material strength and lower cost compared with austenitic stainless steel for building structures. This study compared with existing experimental and finite element analysis results on the structural behaviours of cold-formed lean duplex stainless steel welded connections with different tensile strength (tensile strength-to-yield stress ratio) and plate thickness despite identical steel grade. The main variables are plate thickness and weld lengths. The material tensile strength of a 3.0 mm thick plate was 22% lower than that of a 1.5 mm thick plate. All specimens showed block shear facture in the base metal. Block shear strengths predicted by current design codes and the proposed equations from previous studies were compared with test strengths. KDS 41 31 00-19/AISC2016 and EC3 design specifications underestimated block shear capacity by on average 24%, 43%, respectively for 1.5 mm and 3.0 mm thick specimens, and equations suggested in previous studies of stainless steel welded connections did not sufficiently account for the influence of stress triaxiality by material property and plate thickness difference on the block shear strength of lean duplex stainless steel specimens. Therefore, thickness-dependent modified strength equations were recommended based on experimental and numerical studies of lean duplex stainless steel welded connections and comparison of carbon steel and other stainless steel connections.

Block shear strength of high strength steel staggered bolted connections

AbstractThis paper presents an experimental and numerical study of block shear strength of staggered bolted connections made of high strength steel (HSS). The study aims to examine the effect of low ductility and low tensile‐to‐yield strength ratio of HSS material on the behaviour and block shear strength of staggered bolted connections. Two Q690 HSS staggered bolted connections were tested and failed in block shear failure, which was characterised by tension fracture of the staggered net tension plane and significant shear deformation along the shear plane. The test observations were further interpreted by finite element (FE) analysis. The test results and FE analysis indicate that the block shear failure mechanism of staggered bolted connections is the fracture of the staggered net tension plane combined with the yielding of the effective shear plane. Finally, the design methods in various design codes for evaluating the block shear strength of bolted connections were assessed and the results showed that the existing design methods provided inconsistent predictions of the block shear strength of HSS staggered bolted connections. More experimental and numerical works are currently in progress to further understand the block shear strength of HSS staggered bolted connections.

Development of lignin-reinforced polyurethane adhesive for glued laminated timber

Glued-laminated timber (Glulam), a structural engineered wood product, is mostly manufactured using structural adhesives including one-component polyurethane (PUR). Nevertheless, the concerns associated with PUR such as unsatisfactory gap-filling properties and lower resistance to delamination create opportunities for newer adhesives with enhanced bonding performance in outdoor environments. In this study, the use of fractionated lignin as a modifier in a polyurethane-based adhesive system was explored. Herein, the effect of lignin content (1, 2, and 3%) on the block shear strength (BSS), wood failure percentage (WFP), and delamination were evaluated. The results indicated that all the lignin-reinforced PUR specimens showed better adhesion performance compared to the controls (without lignin). Markedly, the lignin-PUR adhesive formulation containing 1% lignin addition exhibited superior adhesion properties than those with 2% and 3% lignin content. The statistical analyses also revealed that the lignin content influenced the BSS and WFP of the glulam specimens. Moreover, the lignin-modified PUR specimens showed increased delamination resistance and met the requirements for delamination stipulated in the ASTM D2559 standard, regardless of the lignin content. The lowest percent delamination (0.07%) was obtained from specimens bonded with 1% lignin. The presented data suggest that specimens bonded with 1% of lignin provided better bonding strength compared to other lignin-filled specimens. Thus, this study demonstrated the technical feasibility of fabricating glulam with enhanced adhesion performance using lignin-modified PUR adhesive.

Block shear and tear-out in bolted connections as limited by curling

In the literature, the block shear and tear-out phenomena and strengths of cold-formed steel bolted connections have been well understood and thoroughly investigated. However, the strength of these failure modes in the bolted connections between thin and thick plates may be governed by the effect of out of plane curling of the thin plate, and this phenomenon has not been studied in detail yet. It is apparent that the curling not only affects the shear strength but also changes the behaviour of the bolted connections. In particular, the design shear strength of the bolted connections remains unchanged with high ductility even though the end distance between the bolt holes and the free edge is becoming larger. An experimental program on the block shear and tear-out behaviour of bolted connections was performed at the University of Sydney in order to provide a better understanding on the effect of the curling. A total of 48 tests including thicknesses of thin plates (1.0 mm, 1.2 mm and 1.9 mm) and 10 mm thick plates was conducted at the University of Sydney with three configurations of bolted connections in conjunction with two different hole diameters (9 mm and 14 mm) and various end distances as the main variables. Three different failure modes being block shear, tear-out and curling were observed in the tests. In addition, different material properties of the cold-formed sheet steels (i.e., G300, G450, G500 and G550) were also considered in this program to create a large database for the investigation. Consequently, a design equation for block shear and tear-out strength accounting for the curling effect is proposed in this paper. It provides a better correlation with the experimental results than those predicted in the current Australian and New Zealand Standard (AS/NZS 4600) and the North American Specification for cold-formed steel structures (AISI S100). In addition, a calibrated Finite Element Model (FEM) was subsequently developed using the commercial finite element program ABAQUS to verify against the test results. The reliable data set of the FEM models is further utilised to extend the data range in a parametric study and is finally used for the verification procedure of the proposed formula over a wider range of data.

Experimental and numerical investigations of block shear failure in gusset plates welded to double angle members

Double angle braces are used extensively in structures to resist lateral loads such as wind, earthquake, and blast loads. A possible failure mode in the gusset plate when all-around weld is used for the angles is the block shear failure. To date, only limited test data on welded gusset plates failing in this mode have been reported in the literature, and all of them only considered concentrically loaded gusset plates. This paper investigates the block shear failure in welded gusset plates experimentally and numerically when connected to double angle members with possible in-plane load eccentricity effects. The test results, including failure loads, fracture sequences, and load-displacement responses are reported in detail. Results showed that tensile fracture was initiated adjacent to the angle heel weld and then propagated along the tension plane. Due to existing in-plane load eccentricity, shear planes did not rupture simultaneously and the shear plane adjacent to the heel weld failed earlier. Following the testing, a numerical investigation using nonlinear finite element analyses with ductile damage capability was performed on the tested specimens. Subsequently, a parametric study was carried out to generate further numerical data over a wide range of gusset and angle dimensions, welding configurations, and connection lengths. Accordingly, the existing design equations for block shear strength as given by AISC specification were evaluated. It was shown that AISC provides excessively conservative and scattered failure loads because of ignoring stress triaxiality and using shear yield instead of shear rupture in its strength equations. In addition, equations specified for welded gusset plates by previous studies were considered and shown to be unsafe for general use. To address these shortcomings, an improved block shear strength equation is proposed in this paper and shown to predict the gusset strength in block shear with improved accuracy.

Bonding performance of cross-laminated timber-bamboo composites

Bamboo is one of the most important renewable resources in China, leading to the development of bamboo products as well. Alternative applications of bamboo need to be developed in the field of construction, especially in cross-laminated timber (CLT) construction, due to better mechanical performance of bamboo and the increasing popularity of CLT construction in recent years. The bonding performance of cross-laminated timber-bamboo (CLTB) composites fabricated with Spruce-pine-fir (SPF) dimension lumber and bamboo scrimber boards was investigated here. L9 (34) orthogonal experiments with four factors (adhesive type, adhesive spreading rate, clamping pressure, and clamping time) and three levels were applied to evaluate their impacts on the bonding indicators of CLTB through block shear and delamination tests. Results showed that adhesive type had the most critical influence on the bonding performance. The optimal manufacturing parameters were adhesive of phenol resorcinol formaldehyde (PRF), adhesive spreading rate of 250 g/m2, clamping pressure of 1.0 MPa, and clamping time of 420 min, respectively. PRF adhesive yielded higher block shear strength, wood failure percentage, and lower rate of delamination than one-component polyurethane (PUR) and emulsion polymer isocyanate (EPI), especially under wet or boiling condition.

Effect of pressing parameters on dimensional stability and bonding performance of thermally compressed coconut wood

Previous research on thermally compressed (TC) coconut wood has shown potential for structural applications. However, its dimensional stability needed to be improved. This study aimed to improve the dimensional stability of TC coconut wood for further use in laminated products by using relatively high pressing temperatures to make it more hydrophobic. TC specimens with a target compression ratio of 55 % were manufactured from low-density (LD, 332 ± 25 kg/m3) wood, and TC specimens with a target compression ratio of 25 % were manufactured from medium-density (MD, 514 ± 28 kg/m3) wood. Two pressing temperatures (170 °C and 200 °C) and three pressing times (1 h, 2 h and 3 h) were applied. Chemical changes, dimensional stability, physical and mechanical properties, and bonding performance of the specimens were examined. Thermal degradation of the LD specimens was higher than that of the MD specimens. Differences in chemistry between the surface and the interior could explain bonding behavior. Pressing temperature had a greater impact on wood degradation than the pressing time. Dimensional stability could be achieved at 200 °C pressing temperature. Most physical and mechanical properties of the TC specimens had decreased due to thermal degradation. However, compressive strength parallel to the grain and modulus of elasticity were not affected at the chosen pressing parameters. Bonding performance determined by block shear tests was dependent on the original material, panel assembly, and the sanding process. Longitudinally laminated panels made from MD specimens with sanded surfaces provided the highest block shear strength.

The Dimensional Stability and Bonding Performance of Hybrid CLT Fabricated with Lumber and COSB

The differences of physical and mechanical properties of different laminations, such as softwood, hardwood or other structural composite lumber, in hybrid cross-laminated timber (HCLT), lead to their dimensional stability and bonding performance more complex than generic cross-laminated timber (CLT). In this paper, the spruce-pine-fir (SPF) dimension lumber and construction oriented strand board (COSB) were employed to fabricate HCLT. The effects of four configurations and three adhesives on the dimensional stability and bonding performance of CLT and HCLT were evaluated in term of the water absorption (WA), thickness swelling (TS), block shear strength (BSS), wood failure percentage (WFP) and rate of delamination (RD). The results showed that with the increase of the COSB laminations, the WA of HCLT specimens decreased, and the values of TS, BBS and WFP increased. The configuration had a significant influence on the dimensional stability, BBS and WFP of the specimen. The adhesive had a significant influence on the dimensional stability and some bonding performances of the specimen. The phenol resorcinol formaldehyde (PRF) specimens had the lowest average RD value compared with the one-component polyurethane (PUR) and emulsion polymer isocyanate (EPI) specimens. Failures were prone to occur in the middle of the thickness of COSB lamination during block shear and delamination tests. The outcome of this paper could help the engineering application of HLCT.

Post-fire block shear strength of thin-walled carbon steel bolted connections

Steel structures that survive a fire can be restored afterwards, but it is necessary to ensure that they have sufficient strength and stability to serve for the rest of the projected life. This paper is concerned with the post-fire behaviour of thin-walled carbon steel bolted connections, which has received limited attention to date from the research community. The experimental programme of this study includes testing thirty-four specimens of bolted connections and thirty-four coupons. The specimens and coupons were exposed to heating of 400 °C or 700 °C for a specified time (30 min, 60 min, 90 min, and 120 min), and cooled either by air or water. The results presented that some physical changes occur after the steel was exposed to high temperatures and it was mainly a reduction in the thickness. The yield stress and the modulus of elasticity of the heated coupons were affected mainly by the heating temperature and time, and an insignificant effect was recorded on the method of cooling (the maximum difference was 4%). The strength of the post-heated bolted connections was improved significantly in air-cooled specimens and reduced clearly in water-cooled specimens. The opposite behaviour was noticed for the connection ductility. The failure modes of the coupons and the bolted connections were not influenced by the heating temperature and cooling method. The coupons failure was a typical tensile fracture and the bolted connections failure was block shear with curling. The post-fire mechanical properties were used successfully to predict the post-fire strength of the connection.

Studies on Strength of Hollow concrete Block Masonry Externally Bonded with Textile Reinforced Mortar Composites

The aim of present study is to determine the mechanical properties of hollow concrete block masonry externally bonded with coir and glass textile reinforced mortar composites. The properties of textile composites were determined by testing TRM Coupons. A total number of 39 prisms were cast. The block masonry prisms were externally bonded with coir and glass textile fibre mesh with mortar. The elastic properties of block masonry, shear and flexural strength were determined for both unreinforced and strengthened specimens. The results showed that increasing the number of layers of textile fibers, enhanced the flexural capacity of TRM coupons. The provision of connectors also enhanced flexural strength. The stiffness of unreinforced masonry prisms was found to increase by strengthening with textile reinforcements even though the increase in compressive strength is marginal. There is a significant enhancement in flexural strength and shear bond strength when compared to unreinforced masonry.

Investigation of block shear strength of high strength steel bolted connections

This study presents an experimental and numerical investigation of block shear strength of high strength steel (HSS) bolted connections under double shear. A total of 16 HSS (grades Q690 and Q960) and eight mild steel (MS) (grade Q345) bolted connection specimens with various geometric configurations were tested and failed in block shear failure mode. The test and finite element (FE) analysis results confirmed that the block shear failure is a combination of fracture of the net tension plane and yielding of the effective shear plane, which is located between the net and gross shear planes. The effects of material properties (such as ductility and tensile-to-yield strengths ratio (fu/fy)) of steel materials on the block shear strength were comprehensively investigated based on the validated FE models. It was found that the tensile stress across the net tension plane could be fully developed irrespective of the steel grade due to effective stress redistribution along the net tension plane and the beneficial effect of the biaxial tensile stress state. However, the shear stress magnitude along the shear plane is influenced by the ductility and fu/fy ratio of steel materials and the length of the shear plane. The low ductility and fu/fy ratio of HSS materials did not allow for effective stress redistribution along the long shear plane and the shear yielding could only partially develop along the shear plane at the ultimate state. For the HSS specimens with short shear planes, the shear stress could develop up to the shear yield strength in general. Finally, the accuracy of the existing design equations in various design codes (i.e., prEN 1993–1–8: 2021, ANSI/AISC 360–16, and CSA S16–19) and relevant literature for predicting the block shear strength of bolted connections was evaluated and design recommendations were provided.

Block Shear Strength of Coped Beam Connections with Double Bolt Lines

AbstractCoped beam connections are commonly used in the construction of steel structures. Existing block shear provisions for these connections, with multiple bolt lines, account for the presence o...

Block shear performance of double-line bolted S690 steel angles under uniaxial tension

We investigated the block shear performance of 16 steel angle specimens connected by double-line bolts. Among these specimens, 10 were made of S690 high-strength steel and 6 were made of normal-strength steel. The angle specimens were fabricated using two hot-rolled steel plates through groove welding. Two angle sections, i.e., 125 × 65 × 6 and 125 × 85 × 6 mm (long leg length × short leg length × thickness), were considered in the test. All angles were connected to the long leg. Apart from steel grade, the test parameters included bolt rows, parallel pitch, transverse pitch, edge distance, and unconnected leg length. Typical block shear of specimens were observed, and different fracture patterns were characterised. The test results confirmed that the block shear strength of the tension angles could be improved by increasing the tension plane area with the increase of the transverse pitch and edge distance and increasing the shear plane area with the increase of the bolt row number and parallel pitch. However, the test results showed that the block shear strength of the angles was not affected by the length of the unconnected leg. Subsequently, numerical models were built to further investigate the block shear behaviour of the double-line bolted angles, and the analysis parameters were the end distance, unconnected leg length, and connected leg length. According to the experimental and numerical results, the accuracy and adequacy of design specifications in the United States, Europe, Canada, and Japan and design equations documented in the literature for evaluating the block shear performance of double-line bolted steel angles were evaluated.

Block shear strength in thin-walled lean duplex stainless steel fillet welded connection with base metal fracture

This paper investigates the accuracy of different design equations given in design specifications and literatures in determining the block shear strength of the lean duplex stainless steel fillet welded connections with base metal fracture. It points out the problem inherent in the block shear provision specified in the North American cold-formed design specification, which does not consider shear strain hardening. The previous test results have found that full shear strain hardening can be assumed for the carbon steel and ferritic stainless steel welded connections, but not applicable to the thin-walled lean duplex stainless steel welded connections with a higher ratio of the tensile strength to yield stress. Based on the results of finite element analysis including ductile damage simulation, this study reveals that the block shear strength of the lean duplex stainless steel specimens can be determined accurately using the mean between the shear yield stress and the shear ultimate stress. This paper also investigates the effect of stress triaxiality incorporated in the block shear equations found in the literature through comparative analysis with the bolted connection. The effect is shown to be negligible for the thin-walled lean duplex stainless steel welded connection, although higher stress triaxiality is developed in welded connection than in bolted connection. This paper recommends a block shear equation assuming the normal tensile stress in the net area in conjunction with partial shear strain hardening. The recommended equation was found to be more accurate than other equations for determining the block shear strength of the thin-walled lean duplex stainless steel welded connections.

Block Shear Strength of Double-Lap Welded Connections in Mild Carbon Steel Plate

Block Shear Strength of Double-Lap Welded Connections in Mild Carbon Steel Plate

Recommendation on block shear strength equation of double shear four-bolted connection in cold-formed mild carbon steel

In this study, experimental research was performed to investigate the structural behaviors such as the ultimate strength and fracture mode of double shear four-bolted connection fabricated with cold-formed mild carbon steel. It shows that bolted connections failed by either block shear fracture or net-section fracture at test end. Finite element (FE) analysis model was developed to replicate experimental response. After successful verification, it was employed for further analysis such as stress and strain distributions that could not be seen through the physical testing. Three paths on the plate model of bolted connection were established to investigate the critical shear plane and the shear stress distribution on the critical shear plane at ultimate state. A modified block shear fracture strength equation of the bolted connection was proposed considering the shear stress factor (average of ultimate shear stress and yield shear stress) at the critical shear failure plane (midway of gross shear section and net shear section; effective shear plane), and block shear fracture strengths by test and analysis results were compared with those by the suggested equation. It is concluded that the proposed equation is able to provide more accurate prediction in determining block shear strength of double shear four-bolted connection with mild carbon steel.

Research Trends in Hybrid Cross-Laminated Timber (CLT) to Enhance the Rolling Shear Strength of CLT

In this study, hybrid CLT research and development trends were analyzed to improve the low rolling shear strength of CLT, a large wooden panel used in high-rise wooden buildings. Through this, basic data that can be used in research and development directions for localization of CLT were prepared. As a way to improve the low rolling shear strength, the use of hardwood lamina, the change of the lamina arrangement angle, and the use of structural composite materials are mainly used. Rolling shear strength and shear modulus of hardwood lamina are more than twice as high as softwood lamina. It confirmed that hardwoods can be used and unused species can be used. Rolling shear strength 1.5 times, shear modulus 8.3 times, bending stiffness 4.1 times improved according to the change of the layer arrangement angle, and the CLT strength was confirmed by reducing the layer arrangement angle. Structural wood-based materials have been improved by up to 1.35 times MOR, 1.5 times MOE, and 1.59 times rolling shear strength when used as laminas. Block shear strength between the layer materials was also secured by 7.0 N/mm2,which is the standard for block shear strength. Through the results of previous studies, it was confirmed that the strength performance was improved when a structural wood based materials having a flexural performance of MOE 7.0 GPa and MOR 40.0 MPa or more was used. This was determined based on the strength of layered materials in structural wood-based materials. The optimal method for improving rolling shear strength is judged to be the most advantageous application of structural wood based materials with strength values according to existing specifications. However, additional research is needed on the orientation of CLT lamina arrangement according to the fiber arrangement of structural wood-based materials, and the block shear strength between lamina materials.

A Method of Measuring Wood Failure Percentage of Wood Specimens Bonded with Melamine-Urea-Formaldehyde Resins Using Image Analysis

Transparent and colorless melamine-urea-formaldehyde (MUF) resins make it difficult to identify the area of wood failure percentage (WFP) in the fracture surface of bonded wood specimens. Therefore, in this study, we develop a method of measuring WFP after the adhesion strength measurement of MUF resins under shear stress. The fractured wood surface of block shear strength (BSS) specimens bonded with cold-setting MUF resins at three melamine contents (20%, 30%, and 40%) was marked black, and then, WFP was accurately measured via image analysis. WFP values measured using this method consistently increased with BSS as the melamine content increased, showing the reliability of this new method. The results suggested that this new method is useful and reliable for measuring the WFP of the fracture surface of wood specimens bonded with colorless adhesives such as urea-formaldehyde, MUF, and melamine-formaldehyde resins.

Block shear behaviour and design of cold-formed staggered hole bolted connections

Block shear strength of cold-formed steel bolted connections has been thoroughly investigated in the literature, although the behaviour of block shear with staggered holes has not been studied in detail. It is more complex and apparent that a combination of tension and inclined shear behaviour due to the presence of the staggered holes affects the shear strength and behaviour of the bolted connections. This paper presents an experimental program on the block shear behaviour of staggered hole bolted connections to provide a better understanding on the failure modes. A total of 24 tests using G450 sheets was conducted at the University of Sydney with 1.9 and 2.4 mm thicknesses. Digital Image Correlation (DIC) was employed to capture both the full field of the displacement and the block shear failure mode of the staggered hole bolted connections during testing. A comparison of the experimental strength with existing design equations for block shear with staggered holes in AISI S100:2016 and AS/NZS 4600:2018 is included in the paper. A calibrated Finite Element Model is then developed by the commercial program ABAQUS to validate against the test results and to further provide detailed insights which could not be seen in actual tests (i.e, stress and strain distributions). In the FE simulation, to study the whole range of deformation of the staggered hole bolted connections (i.e., elastic, plastic and fracture stages), a plasticity model and a ductile damage model are applied to the cold-formed steel sheets to represent various stress states in the steel during failure.