As Fibre Reinforced Polymer (FRP) becomes increasingly popular and demands deeper design understanding, the focus is shifting toward smarter connection selection and design optimisation that streamlines decision-making and delivers simpler, faster and more effective solutions.
Wagners CFT Project Delivery Lead, Mitch Kelly explains this approach is reshaping how FRP connection design is undertaken in practice.
“Reducing unnecessary crush blocks can improve constructibility, reduce installation time and deliver huge cost efficiencies, provided the structural performance is still verified,” Mitch Kelly said.
“In any structure, the member or structural component itself can be strong, durable and well designed but if the connection does not transfer loads correctly, the system will underperform,” Mr Kelly said.
“For FRP, this is especially important because the material is directional and connection loads can introduce localised failure modes, so when we talk about connection design, we’re really talking about facilitating the load path,” he said.
“There is not going to be a one-size-fits-all connection type and a number of things need to be considered when selecting and designing connections.”
Pictured – An FRP bridge being preassembled at the Wagners CFT manufacturing facility.
Getting the most from FRP connections means using crush blocks where they add value and avoiding them where they do not.
“The aim is not to remove crush blocks. Crush blocks are important and necessary in the right locations. The aim is to stop using them where the connection selection framework does not justify them,” Mitch Kelly said.
“If we add a crush block where it is not necessary, we add direct supply cost and installation time and complexity without a proportional structural benefit,” Mr Kelly said.
“If we completely exclude crush blocks, then we risk crushing, pull out, delamination, fatigue or poor long-term performance,” he said.
“Optimisation sits between those two extremes and it means matching the connection detail to the load type, orientation and the installation methodology.”
Pictured – A handrail post connection design from the Wagners CFT Drafting Guide.
Understanding the directional properties of FRP is essential for effective connection design.
“FRP is very strong, lightweight and corrosion resistant, but it is not isotropic, it is anisotropic. This means it does not behave the same in every direction,” Mitch Kelly.
“As a result, the directional behaviour of FRP becomes a critical consideration in the design and detailing of connections,” Mr Kelly said.
“Pultruded FRP has a very strong longitudinal performance because the fibres are predominantly aligned along the length of the member,” he said.
“If you can imagine trying to pull apart a platted rope, it is very strong. The transverse is different. It is not as strong due to having less fibre percentage in the transverse direction.”
“Loads acting across the fibre direction through the wall or around a fastener hole can create local effects that do need to be managed.”
“FRP also has a lower shear modulus than steel so local defamation and load transfer need to be considered carefully.”
Pictured – The fibre layout of the FRP profiles.
Several factors influence whether FRP connections require additional local support.
“Because many structural FRP profiles are thin hollow walled sections such as SHS, RHS and CHS sections, the wall of the section may need local support at fastener locations. That’s where crush blocks and crush tubes come into it,” Mitch Kelly said.
“The same external load can require a different connection depending on the fibre direction and the profile you’re using,” Mr Kelly said.
“Section geometry is often the difference between a necessary crush block and an unnecessary one,” he said.
“Wall thickness affects local bearing and crushing resistance. A thin wall will obviously behave differently to a thicker wall under the same load.”
“Hollow sections can also be vulnerable to local wall defamation, particularly when a bolt is overtightened or when you introduce a load through a small contact area.”
“Loads acting along the fibre direction are not the same as going across the laminate so as counterintuitive as it might sound, using a larger profile to reduce connection complexity may end up being the better option for a project.”
“As a rule of thumb, thin walled hollow sections with transverse load demand tend to require crush blocks. Thicker wall sections or connections dominated by longitudinal force may be able to use bushes or riveted brackets.”
Pictured – The mechanical and physical properties of square hollow sections (SHS).
Understanding potential failure modes is a critical part of designing reliable connections.
“Connection design, especially in FRP, is really about managing local failure risks,” Mitch Kelly said.
“Fastener holes can create stress concentrations. That means the area around the hole can become a starting point for failure if the load is not properly distributed,” Mr Kelly said.
“FRP can also be more vulnerable where loads act through the thickness of the laminate because that’s not where the material is typically strongest,” he said.
“If the load is poorly distributed, the laminate can delaminate which means the layers of the FRP can actually pull apart rather than acting together.”
“Some other common failure modes need to be considered in FRP and probably generally in other materials as well.”
- Bearing failure – local damage around the contact point by a concentrated load
- Net tension – the remaining section left around a hole is not sufficient enough to carry the tensile force
- Shear out – occurs when the fastener tears a path towards the edge of the member
- Cleavage – a splitting type failure
- Delamination – the separation of the laminate layers
- Bolt pull through – occurs when the fastener or washer zone pulls through the section.
“The important point is that these failure modes are not random. They are influenced by load direction, magnitude, edge distance, bolt hole sizes, bolt spacing and wall thickness.”
Pictured – The different failure modes.
Different FRP connection systems offer different advantages depending on the application.
“The three connection options are crush blocks or inserts, bushes or crush tubes and rivets. Each has its place and the challenge is deciding on the best option for each connection,” Mitch Kelly said.
“At a high level, crush blocks provide the highest local reinforcement for high demand bolted joints,” Mr Kelly said.
“Bushes provide a middle ground solution. They support the bolt hole and help distribute local stress and improve bolt performance where full transverse reinforcement is not required as well as eliminating the crush of the hollow sections when the bolts are actually tightened.” he said.
“Riveted connections are usually in conjunction with a bracket and are permanent, efficient, very fast and for lower demand joints particularly where access or alignment makes bolting more difficult.”
“The right system depends on loading, FRP properties as mentioned earlier and the installation method so access, sequencing and site limitations.”
“We should not think of these as good or bad options. We should think of them as different tools for different connection conditions.”
Pictured – The three connection types being crush block, bush and rivet.
In high-demand structural connections, crush blocks can provide the reinforcement needed to maintain performance and reliability.
“Crush blocks are required where the connection needs the highest level of local reinforcement,” Mitch Kelly said.
“Typical triggers for these are high compression, high bearing, high shear or high pull out loads,” Mr Kelly said.
“They’re also important where long-term bulk preload is critical to be maintained and where the dominant stresses act transverse to the fibre orientation or when the wall buckling is at risk for a thin wall profile,” he said.
“Therefore, they are generally appropriate for high consequence primary structural joints where we need the most robust connection behaviour.”
“More often than not in those situations the crush block is not overkill, it is the correct engineering response for the demand.”
Pictured – Crush blocks being pushed into profiles and end caps being glued in.
Over-specifying crush blocks can add unnecessary complexity, cost and installation challenges to a project.
“The use of crush blocks isn’t the issue. The issue is using them everywhere by default,” Mitch Kelly said.
“Often crush blocks can be an extreme installation burden. To install a crush block or insert, the member needs to be pre-drilled in the precise bolt hole location then the crush block is pushed down the member and aligned with the hole,” Mr Kelly said.
“That takes time, especially when the detail is repeated many times across a structure,” he said.
“It is also often not feasible when setting out a structure as the bolt hole locations are sometimes not known until the members are actually installed in which case it’s often too late to be able to push the inserts in and they also generally allow for no tolerance at all during installation.”
“They also add additional costs, direction components, bolts, brackets, additional drilling, especially when you can potentially substitute with something like a riveted bracket.”
Pictured – Profiles ready for delivery with crush blocks inside.
Where connection demands are moderate, bushes can improve performance without the complexity of crush blocks.
“Bushes or sleeves or crush tubes are a practical middle ground. They are useful where the dominant force is longitudinal or moderate, where pullout forces are low to moderate and where crushing demand is lower,” Mitch Kelly said.
“When compared to a bolt alone, they improve bolt torque and tightening capacities and they help distribute the local stress around the bolt hole,” Mr Kelly said.
“A bush is not the same as a crush tube. It does not provide the same level of transverse reinforcement but where the demand is moderate, it can protect the laminate and improve constructibility without the burden of a full internal insert,” he said.
“They can be drilled and installed while the member sits in the final position and are useful when the whole position cannot be central to the member, so they create more tolerance and speed for installation as opposed to crush blocks.”
“That makes bushes particularly useful when you need the performance of a bolted connection but are also wanting practicality for install.”
Pictured – Bushes on a preassembled FRP bridge.
For lower-demand connections, rivets can provide a fast and efficient alternative to bolted systems.
“Rivets are the efficiency option. They suit permanent, lower demand, non-critical connections where the loads are low to moderate,” Mitch Kelly said.
“They eliminate all the alignment issues that are present with bolted connections as they are drilled and installed in their final position so similar to how a nail or screw would be used in timber with an L bracket or plate, that’s basically the same principle for rivets,” Mr Kelly said.
“Rivets are not going to work in a critical, high sheer transverse load case, but can often make up the majority of the connections in a structure like a boardwalk,” he said.
“They are the right answer when the load case allows speed, simplicity and permanence to become the priority.”
Pictured – A riveted bracket on an FRP bridge.
The most effective connection is not always the strongest, but the one that best meets the project’s requirements.
“Crush blocks provide the highest security for high demand joints, bushes provide a good balance where performance and easy installation are needed and rivets are efficient and fast,” Mitch Kelly said.
“Optimised design is about enough strength, safe installation and cost effectiveness and the best whole of project outcome,” Mr Kelly said.
“The question shouldn’t really be what is the strongest possible connection,” he said.
“The better question is what is the simplest connection that safely satisfies the engineering demand and can be easily installed.”
Pictured – A comparison of connection performance.
A practical connection selection framework begins by assessing load type, direction and magnitude.
“The next step is to turn that thinking into a practical selection framework. This is not a replacement for engineering design checks or capacity tables, it is a way to structure the decision so we start in the right place every time,” Mitch Kelly said.
“The first filter to consider is load. So you start by asking what are the types of loads the connection is carrying? Is it tension, shear, bearing or crushing?,” Mr Kelly said.
“Then you ask about direction. Is the load acting along the fibre direction or across it?,” he said.
“Then you consider the magnitude. If it’s high pull out, high shear, high bearing demand, you usually have to go towards the crush block. If the demand is moderate, a bush may be appropriate. If demand is low, a riveted bracket may be sufficient.”
“The first decision point is just what is the connection actually doing.”
Pictured – Understanding load type, direction and magnitude is the first step in selecting the right FRP connection solution.
Once the connection demands are understood, attention turns to the FRP profile itself.
“The second filter is actually the FRP section. If you know the load type, direction and magnitude, now you have to work out which profile to use,” Mitch Kelly said.
“Check the wall thickness and the published connection capacities for the likely FRP profiles you think you’ll use and then ask whether the wall could potentially crush, buckle, delaminate,” Mr Kelly said.
“Then lock in some potential connection options as there’s always generally more than one connection option even when you get to this stage,” he said.
“It’s key to assess each connection point individually. Don’t assume that the one connection detail needs to fit the whole structure because that can often lead to oversights and oversights are potential opportunities for improvement.”
Pictured – The Wagners CFT Design Guide.
Installation considerations are the final step in selecting the most appropriate connection solution.
“The third filter is installation. A structurally sound detail still needs to be practical and safe to install,” Mitch Kelly said.
“Can the installer physically position, align and complete the connection? Does the detail allow for slopes, bolt hole eccentricity, site drilling, alignment? Does it create clashes, slow assembly or require extra handling? How often is the connection repeated across the structure?,” Mr Kelly said.
“Access, tolerance, sequencing, drilling and potential clashes can all determine the best detail and this is where design and construction need to meet,” he said.
“This is our last step to really lock in the connection design.”
Pictured – Crews installing an FRP collapsible handrail system in Queensland.
Connection optimisation begins when multiple solutions can satisfy the same engineering requirement.
“The same requirement applies to multiple solutions, which is where optimisation becomes possible,” Mitch Kelly said.
“A connection can satisfy the same requirement in more than one way,” Mr Kelly said.
“A joist to bearer connection for example can be detailed with a through bolt with crush blocks or tubes, a bolted bracket again with crush blocks or tubes or with riveted brackets,” he said.
“The best detail is the one that satisfies the capacity while improving constructibility and reducing overall project costs.”
Pictured – Graph showing selection conditions for the use of crush blocks, bushes and riveted connections.
A Western Australian stair project demonstrates how connection optimisation can improve constructability without compromising performance.
“Optimisation is not about reducing performance. It is about removing unnecessary complexity where the engineering review shows a simpler detail is sufficient,” Mitch Kelly said.
“An example of design optimisation is the Jindalee Beach stairs in Western Australia. The project involved repeated pile, joist, stringer, bearer and handrail post connections, as you would expect from a set of stairs,” Mr Kelly said.
“It was also a very tricky site with footings only able to be placed in certain locations in the sandstone rock, so tolerance and flexibility in the connections was a must,” he said.
“The original design issue was that multiple bolted bracket connections and anti-crush inserts, along with those site tolerances, created a design that was almost impossible to install without on-site modifications.”
“The original design was very sound and every connection was adequate.”
“The aim in optimisation was to keep structural performance where required and maintain the good design work that had been done, but simplify the lower demand connections and using riveted brackets and through bolt details where possible.”
Pictured – Jindalee Beach stairs in Western Australia.
Several targeted design changes helped improve constructability and reduce installation complexity on the project.
“For this optimisation, there were four main changes. Firstly the inner joist to bearer bolted bracket connections were replaced with 3 millimetre riveted brackets,” Mitch Kelly said.
“This avoided pre-drilling and pushing more than 10 anti-crush inserts per bearer,” Mr Kelly said.
“Obviously there are a lot of bearers in the structure. If they’re not perpendicular to the joist due to site constraints and pile positions, then there’s no way to pre-drill all the bearers beforehand,” he said.
“They need to be done based on joist locations which are unknown until the bearers are actually sitting next to the piles. And once the bearers are attached to the piles, there’s actually no way to push those inserts into the bearer for the joist connection because they can’t push past the crush blocks that are in there for the pile connection.”
“The second change is the outer joist to bearer bolted bracket was replaced with a heavier gusseted riveted bracket.”
“Similar to previous, we avoided the inserts and simplified the connection, but the heavier gusseted bracket was able to overcome the loading from the handrail post.”
Pictured – One of the design optimisations used in the Jindalee Beach stairs.
The final design adjustments helped minimise components, simplify installation and improve project outcomes.
“The third change was the stainless handrail offset bracket at the top of the post was changed from a through bolt to a standard riveted offset bracket,” Mitch Kelly said.
“This again avoided pushing the insert in the top of the post which was potentially creating clashes and removing tolerances in that connection,” Mr Kelly said.
“The fourth and final change is the side bracket connecting the pile to the bearer was replaced with a through bolt,” he said.
“This reduces the chance of a clash with the joist. It removed additional bolts and brackets from supply and reduced the number of holes to be drilled in general.”
“The end result from these changes was fewer inserts, fewer brackets, less drilling, more site tolerance and lower clash risk while still having an adequate connection type for each connection.”
“The supply cost reduction after the design optimisation was actually passed through to the client as a discount from the quoted price on this job.”
Pictured – Sandstone rock encountered on site created challenges for footing installation.
Connection selection should be guided by the demands of the application rather than a one-size-fits-all approach.
“The goal is to not make every connection lighter or remove all crush blocks, the goal is to make every connection appropriate,” Mitch Kelly said.
“Use a crush block or insert where there is high load, high consequence, high pull out, high bearing, transverse demand or preload retention,” Mr Kelly said.
“Use a bush or crush tube where there is moderate demand and the goal is to improve performance, sheer capacity and laminate protection,” he said.
“Use a rivet or riveted brackets where loads are low to moderate, joint is permanent, access is limited and installation speed and efficiency matters.”
“In short, the simplest connection that safely satisfies the engineering demand while taking into consideration the supply and installation costs.”
Pictured – A riveted bracket installed on a preassembled FRP bridge structure.
The key to effective connection design is choosing the simplest solution that safely meets the engineering requirements.
“Connection selection starts with understanding the load type, direction and magnitude,” Mitch Kelly said.
“FRP section geometry, wall thickness and fibre orientation directly affect connection performance,” Mr Kelly said.
“Crush blocks are required where the demand is high, transverse, high consequence or preload critical,” he said.
“Bushes and rivets are sufficient and beneficial where the load requirements allow.”
“Installation access, tolerances, sequencing, drilling, clashes and maintenance can determine and often do determine the best detail.”
“Spending time to optimise and make the right connection selection improves constructability, reduces installation time and delivers cost efficiencies without compromising structural performance.”
Watch the full webinar ‘The Right Connection: Design (Part 1)’ featuring guest speakers, Wagners CFT Project Delivery Lead, Mitch Kelly and Structural Engineer, Sabin Raut – https://www.youtube.com/watch?v=BoJaXFmyfsY&t=2207s
Read or download Wagners CFT Design Guide – https://www.wagnerscft.com.au/app/uploads/2024/07/J7998_WAG_WCFT-Design-Guide-2024_WEB3.pdf
Have a question about connections for an FRP structure? Get in touch with the Wagners FRP experts – https://www.wagnerscft.com.au/contact-us/
