Modeling of timber beams strengthened with various CFRP composites
Introduction
Wood is a widely used structural material for lightweight buildings and short-span bridges. Glued laminated (glulam) timber and sawn lumber are commonly used for structural engineering applications. Deterioration of existing timber structures may result from increased service loads, ageing, and biological attack [1], [2], [3], [4]. Biological deterioration of timber elements due to the diffusion of enzymes, for instance, is a phenomenon analogous to, though different in mechanism from, corrosion-induced deterioration of steel or concrete elements. Rather than costly replacement of deteriorated timber elements, repair or strengthening of the elements may be recommended [3], [4], [5]. Conventional strengthening/reinforcing methods for timber structures use steel plates or bars, aluminum plates, or simply timber patches [6], [7], [8], [9]. Such methods, however, may increase dead loads, transportation expenses, and installation costs. Conventional repair methods generally require mechanical connection (e.g., bolts and nails) which may not be effective in deteriorated timber. Steel components are susceptible to corrosion and aluminum plates may buckle when thermal loads are applied [10]. The application of fiber reinforced polymer (FRP) composites is a promising solution to upgrade/strengthen timber systems, particularly where increased load-carrying capacity or stiffness is desired. FRP composites provide favorable strength and stiffness-to-weight ratios; are non-corrosive; and offer excellent tailorability, reduced long-term maintenance costs, and prompt installation on site [11], [12], [13]. Although FRP composites are intensively used for strengthening concrete structures [14] and more recently have been introduced as viable methods of retrofitting steel structures [15], relatively little information is available for timber applications such as the interaction between FRP composites and damaged timber element, the effect of various types of wooden species on the performance of FRP-strengthening systems, and the failure mode of strengthened members. This paper presents a modeling approach to predict the flexural behavior of timber beams strengthened with carbon FRP (CFRP) composite materials to address the identified research gap mentioned above. The developed model is further used for a parametric study of CFRP-strengthening for timber applications, including various CFRP properties and wood species.
Section snippets
Strengthening of timbers with FRP composites
FRP composites may be bonded on the tensile soffit of timber beams to enhance load-carrying capacity and stiffness. Such strengthening work is conducted with externally bonded FRP strips/sheets or near-surface mounted (NSM) FRP bars [4], [16], [17]. The NSM application is a relatively new strengthening technique in which a narrow groove is cut into the member and FRP composites are inserted with a bonding agent [18], [19]. Prestressed FRP laminates have also been shown to improve the
Experimental program
This section provides a summary of the experimental work on which the modeling effort is based; further details are available in Kim et al. [38]. The experimental program included six Douglas Fir beams having dimensions of 38 mm wide × 138 mm deep × 2690 mm long. The beams were recovered from a 38-year old Quonset and used to evaluate the effects of CFRP-strengthening. The moisture content of the beams was not measured prior to testing, given that the members have been inside the Quonset for
Numerical modeling approach
To predict the behavior of timber beams strengthened with CFRP composites, a three-dimensional computational model was developed using the general-purpose FEA program ANSYS. The following summarizes detailed modeling approaches.
Load–deflection response
Fig. 3 shows the load–displacement relationship at midspan of selected beam specimens. The load-carrying capacity and corresponding deflection of all timber beams are available in Table 1. The predicted response of the model agreed well with that of the experiment. An average error of 8.2% in the ultimate load was obtained, as shown in Table 1. The response of the beams was essentially linear until failure occurred (Fig. 3). It should, however, be noted that limited damage propagation was
Parametric study
A parametric study was conducted to further examine the response of timber beams of various species and CFRP having different properties, based on the FEA modeling approach (i.e., the beams marked with ‘FEA’ in Table 1). To provide a consistent evaluation of the effect of specific parameters, the beam dimensions used for this study were those of Beam DF2 unless otherwise stated. The beams were simply supported and subjected to four point bending (Fig. 1(b)).
Summary and conclusions
This paper has presented a modeling approach to predict the flexure of timber beams strengthened with CFRP composites, including a comparison to experimental behavior. The model included orthogonal properties of the wood species and was able to simulate two distinct failure modes of the CFRP-strengthened timber beams, namely, the grain fracture at a flexure-critical region (near midspan) and at the end of the bonded CFRP composites where stress concentrations occurred. A parametric study was
Acknowledgement
The writers would like to acknowledge Mr. Mozahid Hossain at North Dakota State University for the experimental contribution.
References (54)
- et al.
Composite repair of timber structures
Constr Build Mater
(2002) - et al.
Study of glulam beams pre-stressed with pultruded GRP
Comput & Structures
(2005) - et al.
Feasibility investigation of the shear repair of timber stringers with horizontal splits
Constr Build Mater
(2007) - et al.
A method for flexural reinforcement of old wood beams with CFRP materials
Composites
(2005) - et al.
Fir and chestnut timber beams reinforced with GFRP pultruded elements
Composites
(2007) - et al.
Application of the modified damage index method to timber beams
Eng Struct
(2008) - et al.
Fracture behavior of damaged wood beams repaired with an adhesively-bonded composite patch
Composites
(2009) - et al.
Finite element modelling of anisotropic elasto-plastic timber composite beams with openings
Eng Struct
(2009) Limit states design of wood structures
(1986)- et al.
Composite reinforcement of timber in bending
Canadian J Civ Eng
(2000)
Strengthening timber bridge beams using carbon fiber
J Struct Eng, ASCE
Wood-aluminum beams within and beyond the elastic range
Forest Prod J
Reinforced wood laminated beams
Forest Prod J
Steel-reinforced wood beam design
Forest Prod J
Steel-reinforced glued laminated timber
J Struct Eng, ASCE
Timber beams strengthened with GFRP bars: development and applications
J Compos Constr, ASCE
Fiber-reinforced polymer composites for construction-state-of-the-art review
J Compos Constr
Deterioration of FRP-to-concrete bond under fatigue loading
Adv Struct Eng
Repair of bridge girder damaged by impact loads with prestressed CFRP sheets
J Bridge Eng ASCE
Behavior and strength of FRP-strengthened RC structures: a state-of-the-art review
Struct Build, ICE
The use of FRP composites in enhancing the structural behavior of timber beams
J Reinf Plast Compos
Strengthening of dapped timber beams using glass fibre reinforced polymer bars
Canad J Civ Eng
Comparison of three flexural retrofit systems under monotonic and fatigue loads
J Bridge Eng ASCE
Fatigue behavior of structures strengthened with fiber reinforced polymers: state-of-the-art
J Compos Constr, ASCE
Prestressed FRP sheets as external reinforcement of wood members
J Struct Eng, ASCE
Cited by (145)
Analytical model to simulate the behavior of notched wood beams (temperate species) under creep behavior in outdoor conditions
2024, Procedia Structural IntegrityAnalysis of behavior and failure modes of timber beams prestressed with CFRP strips
2022, Composite StructuresExperimental research on wood beams strengthened with engineered bamboo laminates attached with self-tapping screws
2022, Journal of Building EngineeringCitation Excerpt :In recent decades, research efforts have developed innovative retrofitting and strengthening methods for existing timber members and connections using various advanced materials. Fibre reinforced polymer (FRP) composites, for example, have proven a promising means of retrofitting existing timber beams due to their favourable strength and stiffness-to-weight ratios and overall behaviour compatible with that of timber [1–6]. FRP materials, in the form of strips, sheets or bars, are externally-bonded or near-surface mounted on the soffit and sides of timber beams.