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Standard pultruded structural sections, imitating those found in conventional steel construction, are often used in civil engineering for non-sway braced frames (Turvey, 2000). PFRP sections are often connected together by conventional (stainless) steel bolting. These mechanical connections joining either pultruded profiles, such as single leg-angle and channels, or pultruded plate material (such as bracing members) are often referred to as “pultruded connections” (Bank, 2006). It should be noted that here, and onwards, the term connection and joint are with respect to terminology used for steel construction (EC3) as described in BS EN 1993-1-8 (British Standards Institute, 2001). A connection is two or more elements meeting, and for design proposes is the assembly of the basic components transferring actions. A joint can contain several individual connections and is where two or more members are interconnected together.

Bolted connections in PFRP structures provide ease of assembly and maintenance, as well as being capable of transferring the actions experienced in primary load bearing structures. The types of pultruded connections accepted in the ASCE pre-standard (ASCE, 2012) are shown in

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Figure 2.1, with frame joints, such as web-cleated connections, in part (a) and plate-to-plate connections, such as the bracing members in part (b).

Figure 2.1: Typical Frame Connection Detailing adapted from Strongwell (2013) showing (a) Beam-to-Column and (b) Bracing Members

It is known that in PFRP structures the weakest part of a structure can be the connections (Turvey, 1998). Furthermore, the characterization of the strength and stiffness of FRP materials is complicated, mainly as a result of the number of material variables involved, with Godwin and Matthews (1980) going as far as stating:

“…in view of the very large number of variables involved, a complete characterisation of connection behaviour is impossible”.

These variables can include considering the directional mechanical properties in the three principle material axes and an elastic response with the material yielding (rupturing) without significant ductility. Therefore, design of joints with FRP materials must account for parameters related to the type of connection and its associated geometry as well as the directional strengths; the situation is more complex than traditional materials, like steel.

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Godwin and Matthews (1980) set out bolted connection parameters and subjectively divided them into the three groups of: material parameters; fastener parameters; design parameters. The mass of published information on mechanically fastened connections in glass FRP (including material manufactured by pultrusion) is largely related to experimental results. Although, there have been prominent computational and analytical models of PFRP bolted connections (McCathy et al., 2005; Hassan et al. 1996), stress analysis through finite element methods proves complex due to the various complications mentioned. Moreover, the prediction of local stress concentrations and verification with empirical testing is equally challenging both of which are paramount to understanding the structural behavior of the connections.

There are three conventional methods of connection adopted with PFRP shapes and systems. They are mechanical fastening; adhesive bonding and a combination of both mechanical fasteners (including the mechanical interlock of PFRP shapes) and bonded connections. The type of connection used requires careful deliberation of all the relevant parameters affecting the performance of the connection. For example, if dismantling is required this excludes bonded connections, whereas if thin section members (not commonly found in civil engineering structures) are to be joined it would be implausible to use mechanical fasteners. Mechanical fasteners can be found of FRP or stainless steel, with the latter being more commonly used due to the financial economy achieved, but in corrosive environments or the need for electromagnetic transparency is required the former has clear advantages. Some of the main advantages and disadvantages (Broughton et al., 2002) of bolted and bonded connections are summarised in Table 2.1.

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Table 2.1: Review of Bolted Connections in FRP

Advantages Disadvantages

 No surface preparation is required

 Easily disassembled with component damage

 Straightforward manufacture and inspection

 Full strength developed immediately

 Insensitive to peel forces

 Simple connection configuration

 Residual stress generally not a problem

 High stress concentrations due to drilled holes

 Added weight of mechanical fastener

 Metallic parts have poor fatigue resistance

 Special sealants needed for weather tightness

 Possible corrosion of the metallic fasteners

 Difficult to achieve high connection strength

Table 2.2: Review of Bonded Connections in FRP

Advantages Disadvantages

 Lightweight

 Good fatigue life

 Smooth surface contours of connection

 Relatively high stiffness

 Damage tolerant to certain degree

 Cannot be disassembled

 Weakened by environmental degradation

 Require surface preparation

 Difficult to inspect for connection integrity

 Takes time to develop full strength

 High quality control required

 Sensitive to peel and cleavage stresses

 Difficult in bonding thick sections

Although, adhesively bonded connections are known to provide a higher efficiency than mechanically fastened connections; preference is given to the latter for its simplicity and hence most pragmatic solution. The key disadvantage of mechanical fastening (Eriksson, 1998) is the inability of the connection capacity to be more than 50% of the material. In bolted lap- connections of FRP material, with current factors of safety (Strongwell, 2013 and Creative Pultrusions, 2010) maximum working loads is <15% of the material strength available. Therefore, the need arises for the engineering uncertainties to be diminished through further targeted research of these connections (Turvey, 1998). Despite, some challenges for wider exploitation of bolted connections, the use of adhesive bonding is rarely used in pultruded structures due to durability issues (Bank, 2006). Considering civil engineering structures have life spans of tens of years, up to say 50 years, the long-term properties of pultruded adhesively bonded connections are still unknown and subject to research (Cadei and Stratford, 2002).

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