Joining techniques for fiber reinforced polymer composite bridge deck systems

Joining techniques for fiber reinforced polymer composite bridge deck systems

Composite Structures 69 (2005) 336–345 www.elsevier.com/locate/compstruct Joining techniques for fiber reinforced polymer composite bridge deck system...

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Composite Structures 69 (2005) 336–345 www.elsevier.com/locate/compstruct

Joining techniques for fiber reinforced polymer composite bridge deck systems Aixi Zhou *, Thomas Keller Composite Construction Laboratory, Swiss Federal Institute of Technology—Lausanne, EPFL-CCLab, BAT. BP, CH-1015 Lausanne, Switzerland Available online 27 August 2004

Abstract Bridge decks made from fiber reinforced polymer (FRP) composites have been increasingly used in rehabilitation and new construction of pedestrian and highway bridges. For each application, connections are inevitable due to limitations on shape size and the requirements of transportation. Connections for FRP bridge decks include primary and secondary load-carrying joints and non-structural joints. Primary and secondary load-carrying connections are most concerned in construction, which include component–component connection, panel–panel connection, and deck-to-support connection. Unfortunately, the technical background, development and design guides of FRP bridge deck connections have not been documented adequately in literature. This paper attempts to provide technical background, developed joining techniques, and design principles concerning the joining of FRP decks. Design requirements, characteristics, performances, advantages and disadvantages of developed FRP deck connection techniques are discussed. Design principles for adhesively bonded joints and mechanical fixing and hybrid joints involving cutouts are also provided.  2004 Elsevier Ltd. All rights reserved. Keywords: Fiber reinforced polymer; Composites; Bridge deck; Superstructure; Connections; Design; Construction

1. Introduction Structural shapes made from Fiber Reinforced Polymer (FRP) composites have been increasingly used in structural systems for rehabilitation and new construction of pedestrian and highway bridge decks [1–12]. FRP bridge decks are usually provided in modular panel forms. In construction, deck panels are usually connected to their supports to transfer loads transversely to the supports that bear on abutments. Current commercially available FRP decks for rehabilitation and new construction can be classified into two categories according to the types of assembly and construction: sandwich panels and multi-cellular type panels. Sandwich panels have two basic forms: foam core sandwich panel and honey*

Corresponding author. Tel.: +41 21 693 3225; fax: +41 21 693 3240. E-mail address: [email protected]fl.ch (A. Zhou). 0263-8223/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2004.07.016

comb sandwich panel. Multi-cellular panels have varied geometrical forms and can be used without or with foam core materials. Two basic cellular FRP deck panels have been developed according to the techniques of processing and assembly: panels from adhesively bonded pultruded shapes and panels from adhesively bonded filamentwound shapes. For each modular FRP deck panel form, connections are inevitable due to limitations on shape size imposed by the manufacturing process and the requirements of transportation. Three classes of connections involving composites are identified in the Eurocomp Design Code and Handbook [3]: (1) primary joints, which carry major strength and stiffness to an assembly for the whole-life of the structure; (2) secondary structural joints, whose failure would be only local failure without compromising the entire structure; (3) non-structural connections, whose main purpose is to exclude the external environment. Connections for FRP bridge decks include all these categories. However,

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this research focuses on primary and secondary loadcarrying connections for FRP bridge decks. These loadcarrying connections include component–component connections to form modular FRP bridge deck panels (henceforth referred as component level connection, or CLC), panel–panel connections to form FRP bridge deck systems (henceforth referred as panel level connection, or PLC), and FRP deck-to-supports connections to form bridge superstructures (including deck-girder, deck-abutment and deck-barrier connections, etc., henceforth referred as system level connection, or SLC). In general, bridge systems shall be designed for specified limit states to achieve the objectives of safety, serviceability and constructability with regards to the issues of durability, inspectability, economy and aesthetics. FRP deck connections shall be designed under the guideline of this philosophy to achieve the specified limit states for each construction. However, specific design requirements for FRP deck connections vary with the levels of connection. In component level connections, the main objective is to ensure the integrity of the deck panel and the load transfer efficiency between the jointed components. In the panel level, major concerns are the deck systemÕs load transferring and carrying capability (bending moment and shear force, and resistance to dynamic loads, etc.), panel–panel compatibility to deformations imposed by thermal or moisture effects, and the constructability of connections. In the system level, shear transfer and connection constructability are major concerns. The advantages of advanced FRP composites would be lost if the characteristics of the associated joints were not properly understood and the connections were not properly designed. Key manners for joining FRP composites are mechanical fastening and adhesive bonding. The combination of bonding and fastening can be used to take the advantages of both methods when the connection is properly designed and constructed. Due to its advantages of the simplification of processing and thus a saving of production cost and possible refined joint geometry, adhesive bonding is generally used for connecting permanent FRP deck components to form

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bridge deck panels. In panel and system levels, adhesive bonding, mechanical fastening and the combination of bonding and fastening have been used. However, the development of connecting techniques for FRP bridge deck systems, especially in panel and system levels, has not come up with the growing demands of the FRP deck rehabilitation and new construction. In some demonstration FRP bridge deck projects, cracks and damages appeared in deck connection regions after field exposure to real traffic loadings and environmental conditions, as shown in Fig. 1. The need for efficient and reliable loadcarrying joints that can endure long-term fatigue and environmental attacks has become apparent. Unfortunately, the technical background and the development of FRP bridge deck connections have not been documented adequately in literature. In a previous paper by Zhou and Keller [13], only joining techniques for FRP decks were briefly reviewed. This paper provides more details for the technical background, developed joining techniques, and design principles concerning the joining of FRP decks in component level, panel level and system level. In this paper, the development of connection techniques for FRP bridge decks is presented. Joining techniques for component–component connections, panel–panel connections and deck-to-support connections that have been developed in past decade will be reviewed. These techniques include mechanical fastening, adhesive bonding, and hybrid joining. Characteristics and performance of each connection technique will be discussed. Design guides for adhesively bonded FRP deck joints are also presented. Finally, guides for the design of mechanically fixed or hybrid FRP deck connections involving cutouts will be provided.

2. Joining techniques for FRP bridge decks 2.1. Component level connections for FRP bridge deck panels Component level connections are usually permanent. The main objective for CLC joining is to ensure the

Fig. 1. Cracking at FRP deck connection regions. (a) At panel connection. (b) At component connection.

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integrity and the loading-transfer efficiency of the formed deck panels. The deck panels will be evaluated, usually via laboratory testing, to make sure that the assembled deck panels satisfy the specified limit states. Primary design criteria are deflection criterion and strength criterion. The deflection criterion is usually represented by a deflection index, L/xxx, where L is the deck span length and ‘‘xxx’’ is a value to be specified (e.g. 500) to satisfy the stiffness requirement for a specific design. The strength criterion is often represented by specifying a limit value for strain or stress in critical directions and locations where maximum strain or stress may occur in the deck. When these conditions are satisfied, the component level connections should be designed with the additional considerations of ease of processing and the resistance to long-term fatigue loadings and environmental attacks. Because of the advantages that adhesive bonding can offer over mechanical fastening, adhesive bonding has been widely used in component level connections to form FRP bridge deck panels, as shown in Fig. 2(a) and (b). However, in some cases [12], mechanical fastening has been used to provide necessary binding forces when the components are in curing (see Fig. 2(c)). Bolted joints have also been analyzed for joining FRP components [11]. However, the comparison study showed that a bonded joint is easier to design and provides a larger safety margin than the bolted connection [11]. Laboratory experiments and field applications showed that there was no clear relationship between the load-carrying capacity of the modular FRP deck panels and the ultimate strength of the adhesively bonded connections. In some laboratory tests [12], local debonding noises were observed at some loading cycles before the ultimate deck failure. The deck continued to resist loads till the deck panel failed in the FRP profiles while no visible permanent damage in the bonded connections was observed. Fig. 3(a) shows the punching failure of a deck surface using a steel loading patch. Fig. 3(b) shows the internal FRP tube failure of a bonded cellular deck under a rubber-reinforced tire loading patch. In other laboratory tests [5], deck panels failed within the FRP components near the bonded line (Fig. 3(c)). No failure was observed in the adhesive layer and the joint interface. Delamination and buckling of FRP components were also observed in laboratory tests, as shown in Fig. 3(d). In all these cases, the adhesive layer and the adhesive–substrate interface were stronger than the FRP components. Therefore, no pre-matured adhesion or cohesion failure occurred. These observations showed that adhesive bonding is a viable technique for joining component level connections in FRP bridge deck panels. Therefore, this technique has been widely used for component level joints in FRP deck construction projects. A criterion for designing bonded component level connections shall be that an adhesively bonded joint

Fig. 2. Deck panels through various joining techniques. (a) Bonded pultrusion shapes [5]. (b) Bonded sandwich [4]. (c) Bonding with fastening [12].

should be designed and fabricated to allow failure in the FRP substrate before failure in the adhesive or the interface. This criterion has been achieved for FRP bridge deck panels as results from laboratory tests have shown [5,12]. In Section 3.1, a design criterion for adhesively bonded FRP joints will be discussed in details. However, problems remain when bridge deck panels are exposed under long-term traffic loadings and environmental attacks, which is shown in Fig. 1(b). 2.2. Panel level connections of FRP bridge decks Panel level connections shall be designed to efficiently transfer bending moment and shear forces between

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Fig. 3. Failure modes of bonded cellular deck panels. (a) Surface failure [12]. (b) Internal failure [12]. (c) Delamination [5]. (d) Delamination and buckling [5].

jointed panels, and ensure deformation compatibility due to thermal effect, as well as the ease of on-site installation. In some special cases, the panel–panel connections may be designed and constructed to provide the possibility of disassembly for repair. Several techniques have been developed for panel level connections: the splicing tongue–groove connection (Fig. 4(a)) and the clip–joint connection via mechanical fixing [3]. Splicing connections require quality control on the dimensions of deck panels. It was shown that panels from pultruded shapes have better dimension uniformity than decks from sandwiched panels through VARTM and sandwiched panels from hand lay-up [10]. For easy on-site installation, adequate tolerance in panel dimensions must be ensured for splicing–bonding connections. However, a disadvantage of bonded connections is the

difficulty of disassembly for repair. Mechanical fixing has the advantage of easy disassembly. However, load transfer and failure resistant capability of mechanical fixing (such as the shear key and the clip-joint) is not as efficient as bonded joining. Results from constructed projects show that splicing– bonding connections have potential for application. For the shear key connection, cracks appeared after a period of exposure to highway vehicle loadings, which is shown in Fig. 1(a). The cracking at the shear key connection region shows that mechanically fixed connections are not reliable to resist dynamic vehicle loadings. In panel level connections, the joints shall be designed to provide enough connecting force to resist repeated loadings to maintain the integrity of the panel-connection-panel system.

Fig. 4. Typical panel connections for FRP deck systems. (a) Adhesive-bonding [10]. (b) Mechanical shear key [2].

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2.3. System level connections for FRP composite bridge superstructures The design of efficient deck-to-support connections is the most challenging topic in the development of FRP bridge deck connections. System level joints deal with connecting large-scale structures, and usually between different materials. Since some CLC and PLC techniques have been well developed to ensure the functions of the deck panels, the efficiency of deck-to-support joint governs the overall behavior of the formed superstructure. Among all deck-to-support connections, the deck-girder (or stringer) connection is the most prominent. Depending on the requirements of a specific project, the deck-girder connection could be a permanent joint with composite action or a joint with ease of disassembly without the necessity of composite action between the deck and its supports. In connections with composite action, the efficiency of shear transfer and constructability are major factors directing the connection design. Mechanical fixing, adhesive bonding and hybrid joints have been explored in connecting FRP bridge deck panels to their underneath supports, as shown in Fig. 5. The connection designs in Fig. 5(a) are approximated as simply supported conditions and not intended

to develop composite action between the deck and the supporting girders. A large portion of impact loads is taken by the FRP deck; therefore, this design is suitable for rehabilitation or repair to reduce the loading impacts on the existing supports. While in most cases, a composite action between the deck and its support is desirable. Composite action via bonding and hybrid joining offers several advantages for FRP decks: (1) the overall stiffness and load resistance capability of the connected system can be significantly increased (for short spans) compared to their individual components [6]; (2) the overall superstructure can have ductile characteristic when the girder is made from ductile material (steel or concrete) due to the loads transfer efficiency between the brittle FRP deck and its ductile support [6]; (3) when appropriately designed, the redundancy of the connected system can be achieved after unexpected failure of the joints [8]; (4) a ductile bonding layer in the connection can provide potential ductile behaviour of the formed superstructure system although the deck and its supports are brittle. Research is under way to confirm this concept [8]. Composite action, structural redundancy and system ductile characteristic are main objectives when designing hybrid or adhesive-bonded FRP deck-support connections for bridge superstructures. So far, the majority of

Fig. 5. Various deck-to-support connections for FRP bridge superstructures. (a) Fastened connection [12]. (b) Bonded connection [6]. (c) Hybrid connection [2]. (d) Detailed deck-girder connection [9].

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FRP deck-girder connections have utilized hybrid joining in which conventional shear studs or stirrups (see Fig. 5(c) and (d)) are used to provide composite action. These types of connections have a proven history in the civil engineering domain, therefore are generally accepted by bridge engineers. However, these connections are developed for steel or concrete deck-girder connections. In connections shown in Fig. 5(c) and (d), holes are required at some desired spacing in the deck. The cutouts bring concerns about the stress concentrations and possible vulnerability to fatigue loads and environmental attacks in the deck panelÕs cutout regions. Efforts must be taken to protect the cutout regions from environmental attacks. Some FRP deck-girder, deck-railing and deck-barrier connections used in current construction also utilize well-developed connection techniques for steel and concrete bridge systems [9]. However, since FRP decks are different from steel decks and concrete decks in constitutive materials and structural forms, efforts shall be taken to develop adapted connection techniques for FRP deck-girder and FRP deck-railing connections. When the deck supports are wide and flat, it is possible to use all-adhesive connection (Fig. 5(b)). In this case, adhesive bonding is preferable to hybrid connecting mainly because of fewer steps involved in the bonding process and more evenly distributed stresses in the joint. However, the difficulty of quality control of adhesive bonding during on-site installation and a lack of confidence in the fatigue and durability of the adhesive layer have hindered its application. Research is currently underway to tackle these problems [6].

3. Connection design for FRP decks and superstructures 3.1. Design of adhesively bonded joints The development of high strength synthetic adhesives has made possible the bonding of large timber structures and expanded the adhesive bonding of metals, alloys

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and composite materials. Most constructed FRP bridge decks are made from pultruded glass fiber reinforced polymer (GFRP) composite shapes (plates, tubes, etc.) [1,3,5–8,10,12]. Therefore in this section, we focus on design of adhesively bonded joints for pultruded GFRP profiles. Usually, pultruded GFRP profiles have E-glass fibers in form of roving bundles used as reinforcements in the middle. The direction of the roving fibers runs in the pultrusion direction. Additional glass fiber mats are used as the outside layers for shear resistance. The mats can be of chopped strand mats (CSM), woven mats, or their combinations. The cross-section of a 10 mm thick pultruded GFRP laminate is shown in Fig. 6. A polyester surface veil is often added for protection and surface finishing. The average fiber volume fraction of the GFRP profiles ranges from 35% to 50% for many available deck systems [5,6,12,15]. Resins of pultruded GFRP shapes for bridge deck applications are usually thermosetting resins, such as polyester and vinyl ester resins [14]. The joining adhesives are usually epoxy and polyurethane. Research showed that the stressstrain curves of GFRP laminates are linear-elastic; while the epoxy adhesive shows a slight non-linear elastic behavior [15]. The basic design rule for adhesive bonding of fiber reinforced composites is that the bonding layer must always be stronger than the adherends [16]. This rule can generally be satisfied for the adhesively bonded GFRP pultrusions. Studies showed that bonded GFRP pultrusions with good surface treatment always fail in the adherends rather than in the adhesive layer or the adhesive–adherend interface [15]; hence, the strength of the adhesively bonded joint is determined by the adherends—the pultruded GFRP profiles (or laminates). Therefore, the strength prediction of the joint and the associated failure criterion should focus on the pultruded GFRP laminates. Delamination in the mat region in the through-thickness plane is the dominant failure of adhesively bonded GFRP laminates. For strength prediction of bonded joints from pultruded

Fig. 6. Cross-section of a 10 mm laminate (50·).

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GFRP laminates, the following quadratic stress failure criterion can be used [15]:  2  2 rz sxz þ ¼1 ð1Þ k r  rz;u k s  sxz;u

3.2. Design of fastening and hybrid joints with cutouts Mechanical fixing technologies (such as bolting and riveting) have been well developed and widely accepted in many engineering disciplines. When easy disassembly is required for a design, mechanical fixing through bolting is an efficient and economical way of joining. However, when mechanical fastening (or hybrid bonding/ fastening) connection technique is used for FRP bridge decks, cutouts are usually required. Cutouts break reinforcing fibers in the FRP component and bring local stress concentrations. Therefore, calculations must be made to estimate the possible stress concentrations when joining FRP with mechanical fastening involving cutouts. Since most cutouts in applications are in circular forms, this part will concentrate on the calculation of stress concentrations for circular cutouts.

where rz is the actual local through-thickness tensile stress at the adherends; sxz is the actual local throughthickness shear stress.rz,u is the through-thickness tensile strength of the adherends; sxz,u is the shear strength in through-thickness plane. rz,u and sxz,u are material properties which can be determined from tests with a special device [15]. When conducting tests using the special device to obtain rz,u and sxz,u values, the applied through-thickness tensile and shear stresses were evenly distributed along the specimens [15]. However, in a lap joint, the through-thickness tensile and shear stresses have highly concentrated stress peaks at the edges. Therefore, factors kr and ks were introduced to consider the differences. Further studies [15] show that kr is mainly related to the adherend tapering level; and ks is mainly related to the overlap length. Fig.7 shows the values of kr and ks as the function of relative tapering ratio and overlap length respectively [17]. The tapering ratio is obtained by dividing the tapered length by the overlap length. This criterion in Eq.(1) takes the same form as the quadratic criterion in [18] and similar to the experimentally validated interaction equation in [19]. The criterion proposed in [18] was mainly for the general case of delamination initiation of laminated composites staring at the free edges. The interaction equation in [19] was for adhesively bonded ARALL laminate joints whose adherends consisted of thin aluminum alloy sheets alternating with aramid fiber/epoxy prepreg layers, while the accurate interaction equation in [19] took the form of (rz/rz,u) + (sxz/sxz,u)2 = 1. The predictions of jointsÕ ultimate capacities based on failure criterion of Eq.(1) showed good agreement with the experimental values [15].

3.2.1. Stress concentrations around cutouts in a flat profile The stress distribution near a circular hole in composite orthotropic plates under biaxial in-plane loading has been examined analytically and is based on the complex variable mapping approach [20,21]. However, this solution is cumbersome to apply. Some simple polynomial expression for the stress distribution has been derived for the uniaxial loading case and a widely varying range of orthotropic open-hole plates under any biaxial loading [22,23]. When the dimension of the cutout is very small compared to the plate, the following equations can be used for calculating the stress concentration factors [23]: 1 K orth ¼ ½1 þ c  ða þ kÞ B a ð2Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where a ¼ Exx =Eyy , c = q/p, k ¼ 2ða  mxy Þ þ Exx =Eyy . The coordinate system is shown in Fig. 8. When the effects of finite dimensions are significant, the calculations K orth ¼ 1 þ k  c  a and A

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4.0 3.5





3.5 3.0

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0.8

1

0

50

100

150

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Overlap Length (mm)

Fig. 7. kr and ks factors for double lap joints [17].

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Fig. 8. Coordinates for a circular cutout in an infinite orthotropic flat plate.

need to be modified by appropriate correction factors to account for finite width effects [24]. 3.2.2. Stress concentrations around cutouts in a cylindrical profile In many cases, cylindrical profiles are used for FRP bridge deck design and construction [25]. Fig. 9 shows the local failure around a circular cutout in a cylinder made from carbon fiber reinforced polymer composites from laboratory tests under four-point bending load. It is seen that the failure first occurred around the cutout section and locally at the cutout edges [25], and the cutout region was always the weakest part in the whole system. The calculation of stress concentrations for an orthotropic cylindrical profile is more complicated than a flat one. Analytical solutions have been developed for designing of curved aerospace FRP shell structures with circular cutouts [26]. In these solutions, the classical shell theory, in which the Kirchhoff hypothesis holds, can be used for thin shells. With the hypothesis, the problem of the stress concentration around a circular cutout in a cylindrical orthotropic shell can be solved in the form of the sum of a general solution from the uniform part (as the same in an isotropic cylinder with

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a circular cutout) and a particular solution of the nonuniform equation [26,27]. The solution can be obtained with the help of HankelÕs first function and the Krylov function. For an orthotropic cylindrical shell whose axes of orthotropy (1, 2, 3) coincide with the cylindrical coordinate lines (r, h, z), and with a free small circular cutout under uniform in-plane loading (subject to p and q, which are uniform pressures in r and h directions respectively), considering the far field axial stress p as reference stress, the stress concentration factors at two critical locations (h = 0, 90) can be approximately obtained as:    pb2 pb2 ð1  kÞ K h¼90 3 þ c 1þ þ 2 2 2   2  pb ð1  cÞ 1  2   2 pb ð1  kÞ ð5c  1Þ þ K h¼0 ð3c  1Þ þ 2 2   2  pb ð1  cÞ 1  ð3Þ 2 where c = q/p, the ratio of the far-field bi-axial uniform pressure loads; k = E2ffi /E1, the orthotropy ratio; ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi p b2 ¼ 3a2 kð1  v12 v21 Þ=4Rh, a is the radius of the hole, h is the shell thickness, and R is the radius (or curvature) of the cylinder. The above approximation was derived for infinite cylindrical shells. However, in application, it can be used for estimating the stress concentration factors in the initial design stage. This estimation can be further validated by numerical calculation or experimental verification. Fig. 10 shows the comparisons of calculations from Eq. (3) and finite element analysis using first-order shear deformable shell element (SHELL93) and 3 dimensional solid element (SOLID95). In this calculation, the average median radius of the cylinder is 462 mm, the thickness is 10 mm, E1=68.8 GPa, E2=51.9 GPa, G12=4.6 GPa, m12=0.31. It is shown that, for small cutout size (a/R 6 0.07), the stress concentration factors (SCFs) predictions from the simplified formulation are closed to the SCFs obtained from the first-order shear

Fig. 9. Failure around a cutout in a cylinder made from carbon fiber reinforced polymer and filled with lightweight concrete. (a) Compression failure [25]. (b) Compression failure detail.

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a constant value of 30 mm. The SCFs on the edge r = a were obtained as SCF ¼ rh =r1 h , where stress rh is the stress at r = a and r1 h is the applied uniform pressure at infinity (for example, r1 h ¼ p for uniform axial pressure loading in the longitudinal direction). As shown in Fig. 11, the interaction effect is not significant when L/ a P 3. Therefore, in practice, a recommended distance between cutouts would be L/a P 3. When close distances cannot be eschewed, measures must be taken to arrange the cutout pattern in a reasonable manner to reduce local stress concentrations and interactions.

5

Stress Concentration Factor (K)

4

3

2

K_90° (Eq.3)

K_0°(Eq.3)

K_90° (1st Order Shear)

K_0°(1st Order Shear)

K_90° (3D Solid)

K_0° (3D Solid)

1 θ = 90°

0

θ = 0°

-1

4. Conclusion

-2 0.00

0.05

0.10

0.15

0.20

a/R Fig. 10. Effects of cutout size on stress concentration under uniaxial loading.

deformable shell theory and complete 3D solid modeling. Through parameter study, it was shown that the parameters that affect the magnitude of SCFs of FRP cylindrical shell with single circular cutout are: cutout size, material properties, shell thickness and loading conditions [25]. When multiple cutouts are required in a connection design, measures must be taken to reduce the negative effects of the interaction of cutouts. Fig. 11 shows the case of two similar circular cutouts aligned along the generatrix. The interaction region lies in 70 6 h 6 180. The stress states at the cutout edge with two cutouts approach the stress state with a single cutout as L/a increases, where a is the cutout radius and L is the distance between two cutouts. In the analysis, the cutout size a was kept as

Stress Concentration Factor (K)

4 L/a=3 L/a=4 L/a=6 L/a=10 Single Cutout

3

2

Joining techniques and design principles for FRP bridge decks are presented. The techniques for connecting FRP deck panels vary according to the levels of joining. In the component level, adhesive bonding is the most efficient way. The majority of current FRP bridge decks use adhesive bonding to assemble individual components to form deck panels. In some cases, the assembly process is assisted with mechanical means. In panel–panel connections, bonded splicing connections and mechanically fixed connections have been developed for installation. Improvement should be made to ensure mechanical fixingÕs ability to resist dynamic loadings. The system level deck-support connection is the most challenging issue in developing FRP deck connections. Because of their advantages, adhesive bonding and hybrid joining are promising connecting techniques for system level connections. In some situations, adhesive bonding possesses advantageous characteristics over hybrid joining because of its simple installation process. In each design and construction, efforts shall be taken to provide structural redundancy for the connected superstructure, i.e., the underneath girders can still take the loads in case of unexpected failure in the bonded or hybrid joints. In terms of adhesive-bonded bridge superstructures, research should be conducted to investigate the long-term fatigue loading and environmental effects on the adhesive-bonded system level connections. In terms of fastening or hybrid connections involving cutouts, care must be taken to reduce the local stress concentrations at the cut out regions.

1

L

References

θ = 90°

0

θ = 0°

-1 0

30

60

90

120

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θ (°) Fig. 11. Effect of relative distance on stress states at one cutout edge of two similar cutouts along the generatrix of a cylinder.

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