Modelling the hygro-mechanical creep behaviour of FRP reinforced timber elements (original) (raw)

Mechano-sorptive Creep of FRP Reinforced Laminated Timber Beams

2014

The reinforcement of timber using fibre reinforced polymer (FRP) rods or plates is widely accepted as an effective method of increasing the strength and stiffness of members, while at the same time reducing the variability in properties. The short-term behaviour of these reinforced members is well understood. The long-term or creep behaviour has received less attention. Due to the hygroscopic nature of timber, creep is accelerated by moisture variations, resulting in the socalled mechano-sorptive effect. In reinforced timber beams, the influence of the reinforcement and adhesive on the long-term response must be taken into account. The objectives of the present work are to determine the durability of reinforced timber beams with respect to load duration (viscoelastic creep) and variable climate (mechano-sorptive creep), and to develop appropriate modification factors for design purposes. Sitka spruce is the most widely grown specie in Ireland and is the focus of this study. Glued La...

NUMERICAL STUDY OF FRP REINFORCED TIMBER MEMBERS SUBJECTED TO VARIABLE CLIMATES

SHATIS 2019, 2019

The use of FRP reinforcement, even in small percentages, has been shown to improve the shortterm flexural behaviour of timber members. This technology has been successfully used in new construction and in the repair and renovation of existing buildings across Europe. However, the enhancement of the long-term behaviour due to FRP reinforcement is often disregarded in design. In this study, a coupled finite element numerical model was developed to examine the influence of a variable climate on the long-term deflection of FRP reinforced members. The timedependent coupled hygro-mechanical model utilises a thermo-hygro analogy to define the movement of moisture through the member depending on the relative humidity of the surrounding environment. The model considers the elastic and viscoelastic behaviour of timber, in addition to the moisture dependent, mechano-sorptive creep and swelling/shrinkage behaviour. The model has been validated against experimental results from long-term variable climate tests on unreinforced and reinforced timber beams under four-point bending. A parametric study was carried out to examine the influence of reinforcement material on the long-term behaviour of reinforced timber members over a ten-year period under a sinusoidal relative humidity cycle. The materials considered were glass fibre reinforced polymer (GFRP), basalt fibre reinforced polymer (BFRP), aramid fibre reinforced polymer (AFRP) and carbon fibre reinforced polymer (CFRP). Results have shown that unreinforced members experience the largest deflection over the ten-year period, as expected. The deflection behaviour of the FRP reinforced beams was found to be dependent on the stiffness of the FRP material with the least stiff GFRP reinforcement experiencing a greater deflection than the stiffer BFRP, AFRP and CFRP materials. By considering the relative creep deflection results, it has been shown that a single creep design factor kdef may be used to predict the long-term performance of reinforced beams regardless of FRP type.

Behaviour of creep of timber beams under natural environmental conditions

WIT Transactions on the Built Environment, 2014

The current research is devoted to the investigation of relationships in creep behaviour of timber beams under natural environmental conditions with the purpose of detecting the factors significantly affecting such parameters as the variable moisture content of wood, temperature, stress level, span-depth ratio, macrostructure of wood and other factors, and to establish a mathematical model applicable for determining parameters related to prognosis of deflection of timber beams under variable loads. This study includes the analysis of experimental test results in static bending of 17 softwood (Pinus Sylvestris L.) beams with different span and span/depth ratios regarding a variation of moisture content, relative humidity and air temperature. The mathematical model examined in this study for the description of creep has been developed on the basis of the Burger body concept. A sufficiently good compatibility of average experimental and modelled strain values of timber beams was observed in the results. These results testify that the developed creep model may be used for predicting deformation of timber beams, and constants involved are applicable for natural environmental conditions. This study is part of extensive research that is aimed at contributing to the determination of accurate model parameters and to establishing an adequate and practically applicable mathematical model for more accurate predictions of final deflection of timber beams for design purposes. Keywords: timber beams, deflection in bending, development of creep under natural environmental conditions, mathematical model of creep.

Significance of Factors Affecting Creep Development in Timber Beams

2016

This study is a part of an extensive research of creep development in softwood (Pinus sylvestris) timber beams under natural environmental conditions. Large size test data sample obtained during long-term (approximately one and half year) static loading of timber beams in a four-point bending simulating the real service conditions of roof structures for winter and non-snow period has been processed and results presented. The correlation between creep deformation and its affecting factors, such as span to height ratio of beam, percent of latewood and width of year ring (a growth ring formed during a single year), orientation of year ring segments against main axis of cross section, number of year rings in 1 cm of wood, and density of wood have been analyzed and corresponding coefficients of correlation presented. It is concluded that the most significant creep development affecting factor is density of wood. Strong relationship between creep development and width of year ring, number...

Case-Study on the Creep Behavior of Interconnected Timber Elements using Wood-Wood Connections

2021

With the increasing use of automation and computer-aided manufacturing in timber construction, a standardized timber construction system using digitally produced wood-wood connections has recently been developed for basic building components. Its structural performance has already been characterized with static bending tests in a previous case-study. Based on this work, the creep behavior of such a construction system is investigated in this paper. An experimental test was conducted in outdoor conditions to perform a simple quantitative and comparative study with the creep-reduction factor k de f for wood-wood connections described in the Eurocode 5 standard. A large-scale specimen was placed on two supports under a ventilated shelter and exposed to natural variations of humidity and temperature over a total period of approximately 400 days. The results were reassuring for the long-term performance of this type of construction system. Creep due to the connections accounted for 25% of the final displacement. In addition, the existing guidelines concerning the factor k de f for wood-wood connections were conservative for this specific configuration and could be optimized with future investigations on this topic.

Creep functions for wood composite materials

Wood and Fiber Science, 2007

Two related functions used to describe the creep of non-wood materials were fitted to data for wood composite materials. The first function, which is linear in its constants, was unsatisfactory for representing the creep of plywood, oriented strandboard, laminated timber, and dry-process hardboard under the given loading and environmental conditions. The second function is nonlinear in its constants. Creep of plywood and oriented strandboard were moderately well represented by this function, while creep of laminated timber and hardboard were exceptionally well represented. Experimental creep data and estimates of empirical constants are presented.

Review of creep modelling for predicting of long-term behavior of glued-laminated bamboo structures

MATEC Web of Conferences, 2019

Glued-laminated bamboo has been widely used to substitute timber as a building material. This material classified as a viscoelastic material because it exhibiting properties that are common to both solid and liquid. Under long-term constant loading, the glued-laminated bamboo structures will experience creep deformation. The mechanical, power law and finite element models are common methods that used to predict the creep for viscoelastic material, some of them have advantages and disadvantages. In this manuscript, modelling of long-term creep is reviewed. The fundamental concepts of creep modelling, the influence of variable load level, and humidity were discussed to develop for computational applications. By using FEA program, a subroutine has been developed by previous researchers to accommodate the effect of orthotropic properties. In the future, the subroutine will be used and developed for numerical creep analysis of glued-laminated bamboo.

Mechano-Sorptive Creep in Reinforced Glulam

2018

An investigation was carried out to examine the effect of reinforcement on the creep behaviour of FRP reinforced timber elements in a controlled variable climate. Creep is accelerated by moisture variations due to a variable climate. This is termed the mechano-sorptive effect. In this paper, both unreinforced and reinforced beams are subjected to long-term creep tests loaded to a common maximum compressive stress of 8 MPa. The relative humidity of the variable climate was cycled between 65 ± 5% and 90 ± 5% every four weeks while the temperature remained constant at 20 ± 2°C. After 75 weeks, the total deflection and creep deflection of the reinforced beams was reduced by 16.46% and 8.37%, respectively, compared to the unreinforced beams. The results have shown that the creep behaviour of the unreinforced and reinforced beams is heavily influenced by the variable climate and it was found that the reduction in total deflection and creep deflection is statistically significant after jus...