Glulam Columns Research Articles

Experimental Investigation of Rotational Behavior of Glulam Column-Beam Connection Reinforced with Carbon, Glass, Basalt and Aramid FRP Fabric

In the domain of modern timber structural systems, timber frame constructions distinguish themselves as preferred and commonly used building methods. Their appeal arises from their architectural adaptability and their distinctive attributes, which enable rapid assembly. In this type of structural system, the effectiveness of the connections between beams and columns plays a pivotal role in determining how forces are distributed, ensuring lateral stiffness, and upholding structural safety. Various methods have been developed to ensure column-beam connections in wooden structures. Column-beam connection points deteriorate and get damaged over time. These critical areas need to be strengthened over time. In this study, glulam columns (140 mm × 140 mm) and beams(140 mm × 280 mm), which are often used as load-bearing elements in wooden structures, were used. Columns and beams are connected to each other according to the wooden notching method. Column-beam connection areas are reinforced with carbon, glass, basalt and aramid fiber reinforced polymer fabrics. After the strengthening process, bending tests of the column-beam connection samples were carried out and the load carrying capacity, total amount of energy consumed, and maximum stiffness values were determined. Additionally, FRP damages occurring in the column-beam connection areas were observed during the experiments. The optimal outcomes for encasing column-beam connections have been identified with carbon-based fiber reinforced polymers. Glassbased fiber reinforced polymers yielded the least favorable results. Aramid-based fiber-reinforced polymers demonstrated similar outcomes to those wrapped with carbon-based counterparts. Consequently, it can be deduced that reinforcing column-beam connections with FRP fabrics, be they carbon, aramid, basalt, or glass-based, can markedly enhance their strength and durability, thereby extending their operational lifespan.

Enhancing fire resistance of glulam columns with modified laminas via resin impregnation and compression

Wood's vulnerability to combustion compromises its structural integrity during fire incidents, primarily due to a decrease in effective cross-section area. This study investigates the efficacy of impregnation Chinese fir lumbers with a 30 % concentration of borate-containing phenol–formaldehyde resin, coupled with a 30 % compression treatment, employed as exposed side laminas for glulam columns. Full-scale glulam columns underwent one-sided fire exposure to assess the impact of the modified laminas. Results reveal a significant increase in column ignition time by 55–195 s, due to the combined treatment. The charring rate of columns containing a single modified lamina in the fire-side zone decreased from 0.733 to 0.552 mm/min after 60 min of fire exposure and further reduced from 0.503 to 0.351 mm/min after 120 min fire exposure for double modified laminas. Compared to the control, glulam columns containing a single modified lamina showcased a 31 % increase in residual bearing capacity at 60 min fire exposure duration and a 62.1 % increase at 120 min with double modified laminas. ABAQUS simulation results corroborated experimental findings, highlighting substantial enhancement in fire resistance achieved due to the modified laminas in the fire-side zone.

Axial compressive performance of cruciform timber-encased steel composite columns: Experimental investigation and buckling analysis through 3D laser scanning

Timber-encased steel composite (TESC) columns have garnered increasing attention owing to their high mechanical performance. However, limited studies have focused on the compressive performance of cruciform TESC columns and accurate evaluations of the full-field deformation of embedded steel components. In this paper, a novel cruciform TESC column embedded with a thin-walled cruciform steel component was proposed. 22 specimens including TESC columns, bare steel columns and bare glulam columns were tested under axial compression. Parameters such as the width and thickness of the cruciform steel component were considered. 3D laser scanning technologies were applied to measure the full-field deformation of 20 steel members, thus quantitatively assessing the reduction in the deformation of steel components resulting from the lateral restraint of timber. Finally, a nonlinear fitting equation was established to predict the load-carrying capacity. The results showed that the TESC columns exhibited significant enhancements in stiffness and load-carrying capacity. The lateral restraint of timber caused the buckling modes of the steel to change from torsional buckling to mainly local buckling. Moreover, the high post-yield stiffness of the steel components before reaching the maximum load indicated that the timber offered sufficient lateral restraint to make full use of the steel strength. The 3D laser scanning method accurately measured the full-field deformation of the steel at the failure state. Compared to bare steel columns, the torsional rotation of the steel components decreased by up to 83.14 %, while the maximum absolute lateral deflection decreased by up to 80.89 %. The suggested strength formula offered sufficient accuracy in predicting the load-carrying capacity of short TESC columns, with ratios between the calculated and experimental values ranging from 0.908 to 1.120.

Relationships between wood properties and fire performance of glulam columns made from six wood species commonly used in China

Glued laminated timber stands as an increasingly favored engineered wood product in low-rise modern timber structures due to its commendable structural performance and carbon sequestration potential. Chinese standards presently fail to differentiate the effects of distinct wood species on the fire resistance of glulam. This study examined the charring rate and residual bearing capacity of glulam columns made from six commonly used wood species in China: poplar, Chinese fir, Douglas fir, hemlock, larch, and spruce. These materials exhibited densities ranging between 390 and 645 kg·m−³. ISO standard fire tests were conducted employing one- and two-adjacent-side fire exposures. Notably, the findings unveiled that high-density glulam columns demonstrated lower charring rates and slower rates of heat penetration. The two-adjacent-side fire configuration induced a corner effect, consequently amplifying the effective charring depth. The results showed a linear relationship between the ratio of diagonal to horizontal charring rate of the columns exposed to the two-adjacent-side fire and wood density. To enhance practical application, a formula was developed, employing the residual section method, to calculate the column's residual bearing capacity post two-adjacent-side fire exposure. This formula incorporates an adjustment depth to accommodate the corner effect, effectively mitigating the reduced wood strength within the pyrolysis zone.

Mechanical properties of fast-growing poplar glulam columns reinforced with steel plate

Fast-growing poplar with characteristics of fast growth, convenient processing, and strong adaptability is widely planted all over the world, but it is difficult to be directly used as structural timber due to its loose fibers and low strength. In order to enhance mechanical properties of fast-growing poplar, a new type of steel plate reinforced glulam fast-growing poplar is developed. Nine columns are tested including one fast-growing poplar column, two glued-laminated timber (glulam) columns and six steel plate reinforced glulam columns. The influencing factors include the steel plate thickness and eccentricity. Based on test results, axial stiffness, failure mode, load-displacement and load-strain relationships were investigated. Test results indicate that ultimate load and axial stiffness of steel plate reinforced glulam columns were respectively increased by 134.5–177.5% and 168.5–244.1% compared to fast-growing poplar column. With the increase of steel plate thickness and decrease of eccentricity, ultimate load increased by 10.9–18.3% and 27.2–92.3%, respectively. It was found that steel plate thickness and eccentricity have great impacts on bearing capacity of steel plate reinforced glulam columns. In addition, bearing capacity equation for steel plate reinforced glulam columns was established and test results coincide quite well with calculated results with difference of less than 5%.

Scenario analysis of embodied energy and CO2 emissions for multistory apartments in Indonesia

Contribution in the building sector to the global warming can be tackled by diminishing greenhouse gas (GHG) emissions (mainly CO2) not only from operational energy but also from the embodied energy (EE) of construction materials. Harvested Wood Products (HWP) such as Cross Laminated Timber (CLT), Glued Laminated (Glulam) timber, among others, make multistorey wooden buildings possible. These wooden buildings could help to reduce EE and CO2 emissions significantly. A material flow analysis (MFA) using an I-O (Input-Output) table was used to compare three scenarios for an 8-story apartment building in Indonesia (total floor area: 9140 m2): First, the building had a reinforced concrete structure. Second, the building had a “hybrid” structure with reinforced concrete cores and first-floor elements, consisting of CLT floor panels, and Glulam columns and beams. Third, the building used only CLT panels besides reinforced concrete cores. The results showed that the last scenario achieved the largest CO2 emissions and embodied energy reductions (58 t-CO2 and 905 GJ), compared with the first scenario (81 t-CO2 and 1110 GJ). Furthermore, we compare two methods to apply displacement factors (DF) to assess the CO2 emissions savings for each CO2 ton in wood products substituted in place of non-wooden products between the three building scenarios.

Experimental research on axial compression performance of glulam columns reinforced by steel strips

ABSTRACT Currently, the traditional glulam reinforcement has the problems of stress concentration, long processing cycle and high cost. Therefore, it is of great significance to study the new reinforcement method of glulam columns. To study the influence of steel reinforcement strips on the axial compressive performance of glulam columns, 15 reinforced and 3 conventional glulams columns were tested under monotonic loading until failure. Test parameters included the spacing between reinforcement strips and the number of layers of steel strip at each reinforcement location. Observations were made on the influence of key mechanical properties including: failure modes, bearing capacity, stiffness, and ductility. The bearing capacity of the steel strip-reinforced glulam columns increased by approx. 5–22% and the average ductility coefficient of steel strip-reinforced glulam columns increased by approx. 11–41%. Additionaly, a calculation formula for the bearing capacity of the steel strip-reinforced glulam column was developed and found to have good with the test results.

A Comparison of Life Cycle Impact of Mass Timber and Concrete in Building Construction

Life cycle assessment, LCA is one of the tools that is used to measure the environmental impacts of a process or an operation. Various studies have mentioned the benefits of mass timber in building construction. This study presents an evaluation of the LCA of certain mass timber in relation to concrete-based materials. Using Athena impact estimator for buildings, the study compared the results of an LCA study for a house that is designed with concrete beams, concrete columns, and concrete walls with brick in the envelope category (Material group 1) with those that are made with glulam beams, glulam columns, CLT walls with spruce wood bevel siding (Material group 2), and another building with LVL columns, LVL beams, CLT walls with spruce wood bevel siding (Material group 3). The results are in line with those that were reported by the majority of previous researchers. For the location that is being reviewed (Calgary, Alberta), the designs showed that construction with wood materials having mass timber components will have a better environmental performance than that for a building design with more concrete-based materials. The building design with more concrete-based material (group 1) showed 242% and 60% higher global warming and acidification potential respectively than the building with glulam beams and columns (material group 2). Except for ozone depletion potential, material group 2 (with glulam beams and columns) has a lower impact than material group 3 (with LVL/PSL beams and columns). The differences in impacts are more pronounced when the comparison is with design with more concrete-based products. This report further shows that LCA can be helpful during the preliminary design to evaluate the expected environmental impacts of the choice of different materials. This study recommends that material manufacturers and building contractors pay attention to LCA results to evaluate areas for continuous improvement.

Impact of ventilation on the fire dynamics of an open‐plan compartment with exposed timber ceiling and columns: CodeRed #02

AbstractThe desire by developers and architects to build mass timber buildings using cross laminated timber (CLT) and glulam has significantly increased globally in the last decade due to its benefits with regards to sustainability as well as other architectural and commercial drivers. This paper presents novel experimental evidence from CodeRed #02, the second in a series of large scale fire experiments carried out inside a purpose‐built, open‐plan compartment to capture fire dynamics in large compartments with exposed timber. The experiment used a continuous wood crib (6 × 29 m) as a controlled movable fuel load. The aim of the CodeRed #02 experiment was to study the impact of reduced ventilation on fire dynamics by keeping all other parameters the same as CodeRed #01 (Kotsovinos et al., 2022), with the exception of ventilation area which was reduced by almost half. The reduction in ventilation was found to significantly impact the fire dynamics by slowing the fire spread and burning rate. The reduced ventilation led to an increased fire duration by 4 min 30 s, which corresponds to 20% longer duration compared to CodeRed #01. The reduced ventilation had a greater overall impact on the rate of flame spread across the CLT (−23%) than the crib (−8%) compared to CodeRed #01. While the maximum temperature and incident heat fluxes inside the compartment were approximately the same as in CodeRed #01, their evolution in time and space were significantly different. The external flames were higher than in CodeRed #01 (3–3.5 m compared to 2.5–3 m) and protruded further laterally (up to ~4–5 m compared to ~1 m) outside of the compartment from the large end openings. Unlike CodeRed #01, the external flaming from CodeRed #02 pulsated between visible flames and dark soot with significantly greater frequency. This was caused by the limited ventilation resulting in incomplete combustion in CodeRed #02. The peak heat release rate of CodeRed #02 was estimated to be 16.5% lower than CodeRed #01, despite having the same fuel load. The average char depth in the ceiling, as measured at the end of the experiment, was 28 mm. This was 11% greater than in CodeRed #01 and was likely due to the increased duration of the fire. After the extinction of the flaming, smouldering occurred for CodeRed #02 as it did for CodeRed #01. One of the glulam columns lost its restraint and fell to the ground because smouldering spread through the thickness of the member. The findings from this work can help engineers quantify the hazard and therefore identify appropriate solutions for the fire safety design of exposed mass timber buildings.

A comparative study between glulam and concrete columns in view of design, economy and environment

In this paper, it is attempted to study possible sustainability solutions for building structures. In this context, comparisons are made between two load-bearing columns with different building materials – glued laminated timber and concrete – with regard to structural design, economic consequences and the emission of greenhouse gases. In terms of structural design, the results show that with small axial forces, glulam columns will result in smaller cross-sectional areas compared to concrete columns. However, at larger axial forces, concrete columns will result in smaller cross-sectional areas than glulam columns. An increased column length also means larger dimensions for glulam columns, but this does not always apply to concrete columns. With respect to environmental impact, it is shown that using glulam columns is the more environmentally friendly option. From an economic point of view, the cost estimates for glulam and concrete columns may vary depending on the country and the abundance of the construction material. In Sweden, a forest-rich country, it is shown that the costs for both column types are quite similar considering small axial loads. At higher axial loading, concrete is generally the cheaper alternative.

Fire dynamics inside a large and open‐plan compartment with exposed timber ceiling and columns: CodeRed #01

SummaryThere is an increasing global demand to build from timber as it is a sustainable and attractive material. One of the key challenges associated with timber buildings is their performance in a fire, in particular, for medium‐ and high‐rise buildings and when timber is exposed. Research on this topic to date has been performed in compartments smaller than 84 m2 which does not capture the fire dynamics of large compartments. This paper presents the first in a series of experiments carried out inside a large, purpose‐built, open‐plan compartment with a floor area of 352 m2. The large‐scale compartment had a fully exposed, unloaded, cross‐laminated timber (CLT) ceiling and glued laminated timber (glulam) columns, made with adhesives that have been tested to not exhibit char fall‐off in fire. At 352 m2 floor area, this is currently the largest compartment fire experiment carried out globally. The compartment characteristics and the arrangement of the wood crib mimicked the previous series of experiments in Poland in a non‐combustible compartment by Rackauskaite et al. (2021). The experiment was instrumented with thermocouples measuring the gas temperature in the compartment, and above two of the openings, as well as the temperature within the timber members. Plate thermometers were also included to measure the heat flux to the ceiling and of the external flaming. Additionally, fixed and drone‐mounted cameras captured the fire. A protected steel column was added to calculate the equivalent fire exposure when compared to a standard fire resistance furnace test. The experiment was allowed to burn out without fire‐fighting intervention. Continuous measurements were taken for 48 h. The wood crib was ignited at one end of the compartment. Flames rapidly spread across the timber ceiling to the other end of the compartment with the rest of the wood crib being involved in the fire at 5 min 36 s from ignition. The total duration of the fire was 22 min 30 s, at which time visible flaming ceased. Significant external flaming was observed through the openings in the order of 3 m in height but varied with position as the fire travelled across the compartment. Analysis of the experimental results and camera footage showed that the presence of the timber structure approximately doubled the heat release rate (HRR) compared to the value expected from the wood crib alone. Smouldering combustion of the timber members continued for hours after the cessation of flaming, burning in several hotspots, resulting in holes through the CLT slab. This paper improves the understanding fire dynamics for open‐plan, exposed timber compartments and has shown that issues such as external flaming, smouldering and the increase in HRR need to be considered when deriving practical design solutions.

Compressive performance of steel-timber composite L-shaped columns under concentric loading

Steel-timber composites (STCs) have emerged in the construction of large-span structures and multistory buildings. In this paper, a novel L-shaped column fabricated using STC (L-STC) was proposed. The L-STC column consisted of equal-leg angle steel and glulam. The angle steel was bonded inside the glulam. Twenty-two L-STC columns were tested under axial compression and compared with four L-shaped glulam columns as control groups. Parameters including wood species, steel ratios and column lengths were considered. The failure modes, stiffness, load-carrying capacity and ductility of the L-STC columns were analyzed. The test results demonstrated that L-STC columns displayed a (near) full composite action. The short L-STC columns failed due to crushing failure while the long L-STC columns failed due to global buckling. The stiffness and load-carrying capacity of the L-STC columns were enhanced by more than 56% and 33%, respectively. Increasing the width of the angle legs was recommended to achieve a higher load-carrying capacity. Furthermore, a finite element analysis was developed and validated. The load distribution on components of the L-STC columns was presented, and restraining effects of the timber on steel were analyzed and discussed. It was found that enhancing the timber grades, column lengths or reducing the steel ratios offered preferable lateral restraint of timber on steel.

Evaluation of the Seismic Response of an Innovative Hybrid Steel‐Timber Structure

AbstractThis study evaluates the seismic response of an advanced hybrid steel‐timber structure using a nonlinear static and dynamic analyses. A three‐story prototype building located in Vancouver, BC, Canada was designed first. The gravity load resisting system (GLRS) consists of cross‐laminated timber (CLT) floor slabs, glulam beams, and glulam columns. The lateral load resisting system (LLRS) includes a chevron‐type steel Concentrically Braced Frame. The numerical model of the structure was then developed in the OpenSees program. Nonlinear hysteresis response of steel braces and timber joints were explicitly simulated. The results of the analysis revealed that the hybrid structure possess an acceptable lateral stiffness while providing sufficient ductility to dissipate seismic energy. No unsatisfactory response was observed in the timber system.

Experimental static and seismic behaviour of glulam beam-to-column connection with screwed-in threaded rod joints

To evaluate the static and seismic behaviour of glulam beam-to-column connections with screwed-in threaded rods, nine specimens grouped in three were tested under both monotonic and reversed cyclic loads. The failure modes, moment resistance, initial rotation stiffness, ductility, and energy dissipation capacity of the developed connections were investigated. The results indicated that the developed beam-to-column connections showed superior structural performance. Furthermore, with the introduction of a steel bracket, the hybrid screwed-in threaded rod connection features larger stiffness, higher load-carrying capacity, remarkable ductility, and better energy dissipation capacity. The main failure modes included the yielding of steel brackets, as well as the yielding or rupture of the threaded rods, which indicated a ductile behaviour. The connection specimens with steel columns showed larger stiffness than those with glulam columns, which is reasonable for the bigger compressive deformation of glulam columns.

Fire resistance and burnout resistance of timber columns

For any construction material, failure during or after the decay phase of a fire is possible, notably due to delayed temperature increase and material properties degradation. As mass timber construction is increasingly proposed for large buildings, there is a need to better understand the susceptibility of timber members to fire decay phases. This paper investigates the resistance to full burnout of timber columns. First, a dataset of glulam columns tested under standard fire is analyzed using finite element modeling, showing conservative agreement. Then, the numerical model is used to analyze the columns under ‘standardized’ natural fires comprising a cooling phase. Adopting a systematic procedure based on the minimum Duration of Heating Phase (DHP) that leads to failure, the burnout resistance of the columns is found to range from 20 to 50% of their standard fire resistance. These results indicate significant susceptibility to failure during the cooling phase, which is due to the combined effect of delayed heating and loss of mechanical properties at relatively low temperatures for timber. Based on heat transfer and structural mechanics, it is thus shown that guaranteeing resistance to full burnout with timber structures presents important challenges, even when setting aside the question of combustibility and auto-extinction.

Experimental investigation of the rotational behaviour of glulam column-beam joints reinforced with fiber reinforced polymer composites

Glulam columns and beams are structural wood engineering products, which are formed by gluing the timbers together in parallel with the help of glue and are widely used in the construction of wood structures. Steel plates, aluminum plates or simply wooden parts are commonly used at the joints of wood structures. Steel elements that are often used as joints can corrode and also aluminum plates are not resistant to heat. For this reason, the use of non-metallic fasteners is one of the topics that have been emphasized recently. In this study, it is aimed to strengthen the joints of glulam column-beam connection with carbon fiber reinforced polymer. For this purpose, 2 column-beam joint samples were produced from each connection type using WV90080 and ALUMIDI 160, WV90110 and ALUMIDI 200 connection elements. One of the samples produced in each connection element was wrapped, and the other was not. The column-beam joint samples produced were subjected to the load–displacement test within the framework of the experiment. It was determined that the load bearing capacity, energy dissipation capacity and stiffness values of the samples whose column-beam connection area was strengthened. It has been specified that reinforcement with carbon-based FRP fabric increases the strength and durability of the column-beam connection area.

On the Mechanical Behaviour of Innovative Moment Connections between Composite Floor Panels and Glulam Columns

ABSTRACT This experimental study investigates the response of six innovative moment connections between composite floor panels and glulam columns. Each composite panel consists of five glulam beams and ultra-high performance fibre reinforced concrete cladding. The dimensions of the individual elements of the floor panels and the columns are fixed. The present study focuses on: (i) the experimental investigation (strength and stiffness) of each type of connection, and (ii) the numerical investigation of the connection stiffness capable of reducing the deflection of the middle span. The design criteria of each connection from beam to columns must consider the maximum loading conditions (axial load N, shear load Q, bending moment M) and conform to the design limits. The experimental setup, connection configurations, and material properties are introduced, followed by a detailed account of the test results and observations. According to the results, the innovative connections with fibre reinforced polymer (FRP) bonded rebars (carbon or glass) can reduce the deflection of the floor panel by 45% compared with common bolted connections.

Glulam columns made of European beech timber: compressive strength and stiffness parallel to the grain, buckling resistance and adaptation of the effective-length method according to Eurocode 5

This article presents experimental and numerical investigations on the buckling behaviour of glulam columns made of European beech (Fagus sylvatica L.) timber and a design proposal. First, the compressive strength parallel to the grain (textit{f}_{mathrm{c,0}}) and the modulus of elasticity parallel to the grain (textit{E}_{mathrm{c,0}}) were experimentally determined in tests on stocky columns (slenderness ratio lambda = 12.5) of strength classes GL 40h, GL 48h, and GL 55h. Subsequent experimental buckling tests on slender columns with buckling lengths of 2.40 m (lambda = 41.5) and 3.60 m (lambda = 62.3) allowed investigating the buckling behaviour and quantifying the influence of the buckling length on the buckling resistance. Applicability of the effective-length method, which is the design method for columns in Eurocode 5 (EN 1995-1-1: Design of timber structures - Part 1-1: General - Common rules and rules for buildings. European Committee for Standardization, Brussels, 2010), was evaluated and a proposal for input parameters valid for columns made of European beech glulam is made. Numerical simulations revealed buckling resistances very close to the experimental results, confirming the proposed critical relative slenderness ratio lambda _{mathrm{rel,0}} = 0.25 and the fitted straightness factor beta _{mathrm{c}} = 0.25. In addition, the numerical simulations allowed for an extension of the scope of the investigations.

Full-scale construction and testing of stress-laminated columns made with low-grade wood

Stress lamination is a simple construction technique widely used in timber bridge decks. This study presents a new application of stress lamination for making timber columns. Using prestressing rods and friction forces instead of glue, numerous connections and twenty-five full-scale columns were built with low-grade wood and structurally tested. The results show how stress-laminated columns display comparable buckling performance to solid timber and glulam columns. A six-month testing period with novel connection details further confirms how prestress losses can be reliably controlled. This work highlights the broader potential and performance of stress lamination in modern timber engineering and building design.

Stovner Tower – accessible pedestrian walkway

ABSTRACTThe Stovner Tower forms part of an activity park in the Stovner district of Oslo. The tower, providing a serpentine walkway through the tree tops with a maximum height of 15m, is designed to be universally accessible. The route is based on a series of straight lengths connected with abrupt changes of direction based on a turning radius of 4m or 7.5m. The 2m wide walkway has an overall length of 270m with a constant 1:15 slope. There are four intermediate horizontal platforms intended as relaxation and viewing areas. Each platform is of variable size with minimum and maximum dimensions of 5.1m and 7.1m respectively. The principal deck structure is steel, formed from a central tube with a diameter of 750mm. The tube is made up of a series of 2m long segments which are faceted as the walkway changes direction. Steel cantilever ribs are located at 1.5m centres to support both the solid timber decking and the 1.3m high guardrail. The central steel tube is supported at multiple locations by tripod columns formed from three glulam timber sections. The GL32c glulam columns vary in size from 200×200mm to 350×350mm depending on their height. The timber columns are anchored directly to the bedrock located close to the surface in this area. Steel walkway sections were shop welded in transportable lengths with bolted connections for on site. Timber to steel connections are also bolted. Steel is painted for corrosion protection. All timber, both solid C24 for decking and glulam columns, is treated for external use. A 3D SAP2000 FE model was used for verifications including a buckling analysis for the slender timber columns and a modal analysis to check for frequencies that would make the structure susceptible to pedestrian‐induced vibrations.