Glulam Structures Research Articles

Experimental study and theoretical analysis on mechanical properties of prestressed bolted glulam connections under cyclic loading

Due to the development of assembled buildings and public environmental awareness, glulam structures have been widely concerned nowadays which have the advantages in terms of energy efficiency and environmental protection, compared with concrete and steel. However, the beam-to-column connections connected by bolts with slotted-in plates in glulam frame structures generally have some disadvantages, such as the low load-bearing capacity, low stiffness, low ductility, and poor energy dissipation capacity. To improve the mechanical performances of conventional glulam beam-to-column connections against seismic effect, a prestressed bolted glulam connection was proposed in this paper. Twelve connection specimens from four groups were tested and numerically studied under cyclic loading. The results show that the application of prestressing force can improve the brittle failure of connections. Compared with the traditional glulam bolted connections, the application of prestressing force can improve the load-bearing capacity, energy dissipation capacity, ductility and initial stiffness of the connection, and slow down the stiffness and strength degradation. However, excessive prestressing may cause an early failure of the connection. The numerical analysis shows that the bolt diameter and the number of steel bars have significant positive influence on the load-bearing capacity of the connection. Even though increasing bolt diameter would be more efficient for the mechanical behavior of the connection, the transverse splitting failure of glulam would be more likely to occur. A calculation formula for the load-bearing capacity and a theoretical model for the moment-rotation relationship of the prestressed bolted glulam beam-column connections are proposed respectively for practical design. The rotation behavior evaluation of the prestressed bolted glulam beam-column connections indicates that the proposed connections are classified as semi-rigid connections, which can be used for ductility design.

Structural health monitoring of glulam structures: analysis of durability and damage mechanisms

In today’s environmental context, the use of glulam or Glued Laminated Timber (GLT) as an alternative to conventional building materials could reduce the carbon footprint of engineering structures. However, this material is sensitive to outdoor exposure with moisture content variations inducing internal stresses and cracks and high moisture content increasing the risks of decay. This study therefore focuses on the development of a protocol to evaluate the effect of climatic conditions on the mechanical performance of the material. For this purpose, GLT samples were equipped with embedded sensors. Moisture and deformation sensors can accurately track wet-dry (W/D) cycles and their effects on deformation at adhesive joints. Samples are stored outdoors and mechanical tests are carried out after 6 months of aging. The results show an average reduction in flexural strength of about 10% compared to unaged specimens. Shear tests on the adhesive joints show a decrease in strength of more than 20%. The study of the fracture mechanisms also indicates a link between the type of fracture and the aging conditions of the specimens. These tests also validated a monitoring protocol that will allow, in the long term, to evaluate the impact of these cycles on the mechanical performance of GLT.

Cyclic behaviour of exterior bamboo scrimber beam-to-column connections

With the advance in manufacturing technology, hot-pressed bamboo scrimber has been produced and widely utilised in construction practices in China. However, no design methods are available for bamboo scrimber members at present. Even though design equations for glulam structures are often used for bamboo scrimber members and connections, significant differences exist in the mechanical property of glulam and bamboo scrimber, which might lead to inaccurate estimations of the behaviour of bamboo scrimber structures. This paper describes the cyclic behaviour of exterior bamboo scrimber connections connecting I-beams and box columns. Four beam-to-column connections with welded steel angles and T-stubs were studied under reversed cyclic loading, and the influence of connection type and the number of bolts on the beam flange was considered. The load-displacement curve, failure mode, load capacity, ductility, stiffness degradation and viscous damping coefficient were obtained from the experimental results. Test results showed that connections with steel angles generally developed better ductility than those using T-stubs connecting the beam and column. When the number of bolts on the beam flange was increased, the energy dissipation of connections could be significantly increased. Besides, the shear resistance, distortion and strain of panel zones were studied, which could gain insight in the shear design of exterior connections. The contributions of the beam, column and panel zone to the total deformation of connections were analysed. Design recommendations were developed finally for application of bamboo scrimber beam-to-column connections in building structures. The experimental study provides valuable data for the design and practical application of this type of beam-column connection in building structures.

Experimental and numerical study on the mechanical behavior of curved glulam structures

Experimental and numerical study on the mechanical behavior of curved glulam structures

Assessment of the structural integrity of glulam using modal analysis and finite element updating

The increasing adoption of glued laminated timber (Glulam) as an environmentally conscious material in construction has been driven by its excellent structural properties and lower carbon footprint compared to other conventional materials. However, its organic nature underscores the need to ensure the long-term integrity of these glulam structures. This paper proposes a novel approach to non-destructive testing (NDT) through the combined application of modal analysis and updated finite element modelling. These advanced techniques allow a more accurate and detailed assessment of the structural condition of glulam. Modal analysis identifies changes in natural frequencies and vibration modes caused by potential material degradation, providing valuable structural health information without compromising the integrity of the material. To achieve this objective, the paper proposes to compare the real values measured in the modal analysis with those obtained from the numerical model by formulating an objective function that measures the error between the two. The differences between the two models are reduced using techniques based on Particle Swarm Optimization (PSO). The work presents a specific formulation aimed at achieving greater efficiency in the search for defects in this material. The results of the proposed method are verified by laboratory tests. For this purpose, glulam samples with different defects were tested and their identification was verified by updating the finite element models, demonstrating the ability and accuracy of the method to identify areas where the structural stiffness has decreased due to deterioration.

Investigation on the mechanical behavior of glulam beam string

The glulam beam string, which is a new type of self-balancing system in the field of large-span glulam structures, was investigated systematically in this study.A series of five-point bending tests were conducted while accounting for variations in string diameter. The failure mode of the specimen varied from lower steel cable breakage to upper glulam beam failure with the diameter increasing. Parametric analysis was performed utilizing numerical models, which accounted for the difference in string diameter, pre-camber, and number of strut members, and a series of design proposals were provided considering both the bearing capacity and economic effect. Furthermore, theoretical formulas based on the Rayleigh-Ritz method were found to be effective in predicting the failure mode and calculating the bearing capacity with accuracy.

Global buckling behaviour of bamboo scrimber box columns under axial compression: Experimental tests and numerical modelling

With the advancement of manufacturing technologies, bamboo scrimber members made of hot-pressed plates have been increasingly used in building structures in the form of thin-walled members. However, limited investigations have been conducted on global buckling behaviour of bamboo scrimber members. This paper presents experimental tests and numerical simulations of the global buckling behaviour of bamboo scrimber columns. Six groups of box columns with different slendernesses were tested under axial compression. The buckling resistance and failure mode of columns were obtained. Test results showed that due to the inelastic stress-strain relationship of bamboo scrimber, most of the box columns exhibited inelastic global buckling, with significant lateral deflections at failure. Comparisons were made between experimental results and design equations in existing codes for glulam structures to evaluate the applicability of the design equations to bamboo scrimber columns. Besides, numerical simulations were also performed for bamboo scrimber columns under axial compression, in which the nonlinear stress-strain relationship and geometric imperfections were incorporated. Numerical results showed that the model could predict the global buckling resistance of columns with reasonably good accuracy. Based on the numerical model, parametric studies were conducted on the effect of column slenderness on the buckling resistance. Finally, design recommendations were proposed for bamboo scrimber columns against global buckling.

Experimental Study of the Mechanical Behavior of Local Wood Terminalia Superba (Fraké) by Glued Wood Assembly According to the Beveled Configuration

The design of structural elements, the design of connections and supports, and damage have a significant influence on the technical characteristics of wood construction projects. An innovative experimental study was carried on locally obtained glulam beams. This study aimed to evaluate the load-bearing capacity of a finger-jointed Terminalia Superba (fraké) lamellae during routine use of an adhesive in the local area and to improve the connections in glulam structures. The achievement of this objective will allow to determine the influence of the technical characteristics of finger-jointing on the mechanical resistance, to maximize the mechanical resistance of the reconstituted beams, and eventually to minimize the losses due to sawing. All this will have a considerable impact on the technical and economic aspects of a wood construction project. The physical aspect of the species was studied and properties found. The influence of variation in density, the bonding surface, and joint efficiency on the bending strength (MOR) of Terminalia Superba (fraké) was studied. Mechanical properties were found and related to the optimum joint angle α, and the breaking point reads 0.20 mm. For the angular range of [0°–30°], adhesive failures are witnessed, and beyond this range, the failures are mixed. The 45° finger-jointing angle appeared to be better in the axial traction mode of rupture.

Structural Health Monitoring of glulam infrastructures with non-destructive embedded sensors

As an answer to actual environmental challenges, wood and specifically wood composites such as glued laminated timber (glulam) became a good alternative to “traditional” building materials such as concrete or steel. However, durability issues related to its environment, especially moisture, could be a imitating factor to the development of these structures. To allow glulam to be more widely used in infrastructures such as bridges we propose an embedded structural health monitoring system, measuring durability parameters in real time. It will allow a live assessment of the condition of the structure. Glulam moisture sensitivity is characterized by biological degradations at high humidity rates or cracks, delamination and internal stresses caused by humidity variations leading to a loss of mechanical properties. An integrated system, developed during two theses, allows us to record humidity rate with patch type sensors using resistive measurement method and stresses in glue joints using strain gauges, both embedded in the thickness of the glue. We are currently conducting long term experiments in real climatic conditions with loaded and unloaded beams exposed in an outdoor application to correlate the effects of real-life wetting/drying cycles combined with duration of load effects on the mechanical properties of our test samples. The running experiment started in august 2020 with 30x30x480mm model glulam beams, we are testing the bending strength of our samples to compare our results with tests conducted in climatic chamber which showed encouraging to predict the actual condition of glulam. We plan to use our data to create a predictive model to be able to determine structural performance of glulams based on live moisture measurements. This system will allow a live mapping of humidity in glulam structures, it could help glulam structures managers in preventing moisture induced pathologies and evaluating the remaining lifespan of the glulams.

Climate impact on reinforcements in wooden members – effective hygro-expansion coefficients

ABSTRACT Wood is a hygroscopic material. The moisture content changes due to ambient climate variations e.g. according to erection and service time. The moisture content adapts first on the surface, and moisture gradient develop over the cross section and results in moisture induced stresses (MIS). MIS occur in addition to normal stresses for the load-bearing resistance and can affect reinforcements. To achieve a reliable evaluation of the safety level of reinforced timber structures, the present paper shows results from experimental test series on the load, moisture, and deformation investigation of structural glulam members reinforced with internal mechanical fasteners. Stepwise climate scenarios were applied and considered climates experienced by the structural timber member before or after reinforcement. For the estimation of the swelling and shrinkage behaviour, effective hygro-expansion coefficients have been determined. The coefficients are significantly lower than previously assumed in the standards. This enables engineers to estimate the impact on reinforcements which is not neglectable and reduces damage in a wide range of applications. The research results promote the use of wood by ensuring quality, performance, and aesthetics in timber constructions.

Preload measurement of steel-to-timber bolted joint using piezoceramic-based electromechanical impedance method

Steel-to-timber bolted connections play an essential role in enabling glulam column and beam to work together. However, these connections are prone to bolt looseness due to shrinkage and creep effects of timber material during a long service life, which may cause consequent connection failures and structural safety hazards. Recently, detection and evaluation of bolt looseness have gained considerable attention, whereas the existing vision-based method is challenging to inspect early bolt looseness, and the percussion-enabled approach is susceptible to being affected by harsh environmental noises. Thus, in this study, the authors proposed a pre-tightening torque measurement method for steel-to-timber bolted connection based on variations of electromechanical impedance (EMI) of designed Lead Zirconate Titanate (PZT) sensors. A theoretical formula is derived first based on the piezoelectric equation and boundary conditions to illustrate the feasibility of using EMI variations to reflect looseness levels. Moreover, a sensor that can be easily integrated into the steel-to-timber connection and work as a washer is designed and fabricated. Consequently, the environmental performance against temperature and humidity of designed sensors is investigated by preliminary tests. Based on preliminary tests results, a frequency range that showed lower environmental sensitivity is selected as the excitation signal in the following bolt looseness measuring experimental studies. Experimental results well validate the effectiveness of EMI method in the monitoring of pre-tightening torque of steel-to-timber bolted connections, showing a great potential to inspect preload looseness of glulam structures in field applications.

Determination of charring thickness of wood by residual strength analysis

This study was conducted to propose an accurate measurement method of charred thickness in structural glued laminated timber (glulam) exposed to standardized fire temperature conditions for evaluation of the fire resistance of Japanese larch (Larix kaempferi) structural glulam. The compressive strength of the test piece collected from the structural glulam exposed to the standardized fire temperature for two hours was analyzed. The results showed that there was residual strength of 14% in the boundary area between sound and charred part. Therefore, the thickness of charred part should be calculated as approximately 4.5 mm less than what is measured visually. The cell wall thickness between sound-, boundary-, and charred-part exposed to high temperature was observed under a microscope. Higher temperature resulted in a thinner tracheid cell wall, and the radial tissue was broken down. The transformation of the boundary part of these microstructures progressed gradually until full carbonization of wood occurred. Based on compressive strength tests and microscopic observation in the boundary part of specimens exposed to high temperature, the current visual assessment of charred thickness measurement is safe in terms of structural aspect but not efficient in terms of wood utilization point of view due to exception of load carrying area of the member.

Seismic performance evaluation of glulam structures with either braced frames or knee-braced frames as the main lateral load resisting system

The glulam knee-braced frames and braced frames are both conventional structural systems that are generally adopted in engineering practice. However, it remains a question what are the advantages and challenges of the knee-braced frames compared with the braced frames and how to choose the appropriate lateral-resisting structures for the projects under a given design requirements. This paper provides numerical models of 3- story, 5- story, 6- story and 9- story glulam braced frames and glulam knee-braced frames with the same architectural plan view. Increment dynamic analysis (IDA) is performed to determine the limit state of MaxISDR (maximum inter-story drift ratio) for the glulam braced frames and the glulam knee-braced frames under different seismic hazard levels, i.e., IO (Immediate Occupancy), LS (Life Safety), CP (Collapse Prevention) and CD (Complete Destroy). Fifty earthquake records are adopted for the subsequent probabilistic seismic demand analysis based on the MaxISDR and ResISDR (residual inter-story drift ratio) damage indices. The fragility curves of these two types of structures are highlighted. The results show that, the limit state of MaxISDR for the glulam braced frames under the IO, LS, CP and CD hazard level are recommended to be 0.6%, 1.2%, 2.0% and 3.5% respectively. The limit state of MaxISDR for the glulam knee-braced frames under the IO, LS, CP and CD hazard levels are recommended to be 1.0%, 2.0%, 3.2% and 5.5% respectively. In addition, for the structural height that below 6 stories, the knee-braced frames have lower probability of MaxISDR exceedance than the braced frames under the earthquake with high PGA and lower probability of ResISDR exceedance than the braced frames. However, for the structural height of 9-stories, the recoverability of the knee-braced frames decreases rapidly. Adding some braces to the knee-braced frames can be a good solution to increase the lateral stiffness of the structures. The comparison results may provide technical reference for the structural design of glulam structures.

Experimental investigations into the mechanical performance of glulam dowel-type connections with either bolts or screws as fasteners

Dowel-type connections are the most common connections in glulam structures. Bolts are often used as fasteners for dowel-type connections. However, the clearance between the bolts and the pre-drilled bolt holes leads to low rotational stiffness and insufficient moment-resisting capacity. To achieve better mechanical performance, screws can be used as alternative fasteners for dowel-type connections. In this paper, monotonic and cyclic loading tests were conducted on glulam dowel-type connections with either bolts or screws as fasteners. The failure modes, moment-resisting capacity, ductility ratio, stiffness degradation, and equivalent viscous damping ratio of the specimens were analyzed and reported. Results showed that compared with traditional bolted connections, the screwed connections had larger moment-resisting capacity and better ductility. The hysteretic loops of the screwed connections were plumper, and the pinching effect was gentler compared to those of traditional bolted connections.

Strength Properties of Structural Glulam Manufactured from Pine (Pinus sylvestris L.) Side Boards.

The aim of this study was to assess the static bending strength of pine glulam manufactured when obtaining the main yield, i.e., structural timber or timber to be used in the production of structural glulam. Analyses were conducted on pine timber harvested from three different locations in Poland. Two beam variants were manufactured, differing in the timber arrangement, horizontal vs. vertical. It was shown that the static bending strength of beams manufactured in the vertical timber arrangement variant is slightly higher than that of beams produced from horizontally arranged layers, with the latter beams characterised by a smaller confidence interval for this strength. Moreover, it was found that the difference in the value of the 5th percentile for both beam types is slight and both beam types are considered to exhibit a high bending strength of over 40 N/mm2.

Calculation of large-span glulam structures as a soil base-foundation-above-ground structure system

The object of research. Long-span glulam arches are widely used as coatings for public and sports buildings. The studies carried out concern double-hinged segmental arches with spans of 60 m and more.
 Description of the problem. The study of the peculiarities of the work of arches as a system "soil base - foundation - above-ground structure" is associated with the significant influence of uneven deformations of supports on the stress-strain state of the above-ground structure. A change in the stress-strain state in the structure itself, associated with uneven deformations of the foundations, can lead to a significant decrease up to the complete exhaustion of the bearing capacity
 Main scientific results. This article provides an analysis of the change in the stress-strain state of glulam arches when calculating the system "soil base-foundation-above-ground structure". It is noted that non-uniform deformations of supports have the greatest influence on the stress in the support zones of the structure. The range of critical non-uniform subsidence has been determined, which should be limited when calculating and designing the foundations of large-span arches. It has been confirmed that double-hinged glulam arches work well in conditions of uneven subsidence due to the peculiarities of the design scheme.
 The results of the studies carried out and the recommendations provided will significantly improve the reliability of large-span structures and require mandatory inclusion in the current practice of calculation and design. In addition, additional requirements for the distance between mine workings for large-span structures should be introduced when performing engineering and geological surveys. This distance should be reduced for a clearer account of the deformability of the foundations for columnar foundations, and the number of workings should be increased.
 The area of practical use of the research results. The cross-section of the glulam arches is selected taking into account additional deformations of the supports, making it possible to avoid unpredictable damage to the structure during operation.
 Innovative technological product. Glulam arches, the cross-section of which is determined taking into account additional deformations from uneven subsidence of the supports
 Scope of application. Glulam arches calculated in this way can be used as covering for public and sports buildings for various purposes.

Strength Properties of Structural Glulam Elements from Pine (Pinus sylvestris L.) Timber Reinforced in the Tensile Zone with Steel and Basalt Rods.

The study analyzed potential applicability for asymmetric reinforcement of glulam beams using materials with a higher modulus of elasticity. Reinforcement elements included smooth and ribbed steel rods as well as basalt rods. These rods were placed only in the tensile zone, assuming that they will not only impart increased rigidity but first of all will reduce the scatter of bending strength values. What is significant, tests were conducted on timber with defects, as it is most commonly used in industrial practice. Analyses showed that this provides an increase in rigidity close to the assumed level. A significant increase in strength was observed. The manufactured beams reinforced with steel and basalt rods were characterized by mean bending strength amounting to 54 and 47 N/mm2, respectively. However, no significant improvement was found in the scatter of the observed variable. Beams reinforced with steel exhibit a 20% higher strength than unreinforced beams. The lower strength of beams reinforced with basalt bars may be related to the lower modulus of elasticity of the basalt itself.

Mechanical performance of glulam beam-to-column connections with coach screws as fasteners

The mechanical performance of beam-to-column connections plays an essential role in the design of post-and-beam glulam structures. This paper presents an investigation into the mechanical performance of glulam beam-to-column connections with coach screws as fasteners. A series of monotonic and reversed cyclic loading tests were conducted on the beam-to-column connections with coach screws, and the failure modes, moment-resisting capacity, stiffness, ductility and energy dissipation of the connections were analyzed. Results showed that the use of coach screws was an effective way to increase the initial stiffness, ductility and energy dissipation of the glulam beam-to-column connections. The strength of coach screws and glulam members was fully developed, and the moment-resisting capacity of the beam-to-column connections was improved due to the adoption of the coach screws. It was noted that the rotational deformation and energy dissipation of the beam-to-column connections was mainly governed by the mechanical performance of the screwed connections. Moreover, a separating analytical method and a finite element model were established for the tested glulam beam-to-column connections, and results indicated that the stress distribution, deformation and moment–rotation relationships of the connections can be predicted efficaciously versus the test results.

Laboratory and In-Situ Testing of Integrated FBG Sensors for SHM for Concrete and Timber Structures.

Long-term structural health monitoring (SHM) plays an important role in the safety of public transport infrastructure such as bridges or tunnels and warns in the event of any emerging problem. This article describes development and testing of system based on fiber Bragg grating (FBG) sensors that can detect changes in strain and temperature. The first phase of the research has been focused on the development of new fiber optic sensors for the monitoring of concrete structures and their investigation in laboratory conditions. The work also shows novel applicability of the same FBG technology for glulam structures. Mechanical loading tests of the concrete beam as well as glulam beam with embedded sensors were carried out. Data measured by developed fiber optic sensors were compared with the readings from reference sensors as well as with the analytically calculated values. The achieved results proved good agreement between the measured data, analytical data and reference methods. In second phase of the research, the pilot installation of the sensors was carried out on the newly constructed prestressed-concrete bridge. The bridge was monitored throughout pre-stressing phase and monitoring continued after the completion of the construction works. Problems with the fragility of the sensors occurred during the measurements, but the obtained results provide a good basis for further improvement of the system.

Evaluating the durability of structural glulam bonded with aqueous polymer-isocyanate adhesive by two kinds of accelerated aging treatments

Glued laminated timber (glulam) is widely used in building construction and is required to have high durability as a structural member. A decrease in adhesion properties induces the delamination of glulam and declines its mechanical properties. Aqueous polymer-isocyanate adhesive (API) is relatively new in terms of structural applications, and there is little information available regarding the durability of glulam laminated by the glue. In this study, two different cyclic accelerated aging treatments were used to assess the durability of API bonded glulam consisting of sugi, hinoki and spruce. The durability was estimated by a change in shear strength. In both accelerated aging treatments, the wood failure percentage of glulam did not decrease although the shear strength declined. Thus, it was assumed that the decrease in shear strength was affected by wood cracks rather than deterioration of adhesives. Additionally, the durability of API bonded glulam, resorcinol formaldehyde resin (RF) bonded glulam, and solid wood was compared using a deterioration model. Eventually it was presumed that API bonded glulam had high durability equal to or greater than solid wood and RF bonded glulam when exposed to a cyclic wet-dry treatment.