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In this interview, Stephen Browning, Structural Engineer from Strongwell talks to AZoM about the current industry methods for making these connections focusing on the mechanical connections and a combination of mechanical and bonded connections.

The primary method for pultruded composite connection to other composites, as well as other materials, is mechanical connections such as bolts, screws, and rivets. Bonding alone using adhesives is not typical, but bonding with adhesives and installing bolts, screws or rivets is common.

Mechanical connections are preferred because they were established as the standard for pultruded structural shapes at the onset of manufacturing pultruded composites. Connection methods were adopted from the steel industry, tested for strength, and then published – there is historical precedence and proven performance using mechanical connections.

Bearing-type connections are connections where the bolt, screw, or rivet “bear” on the inside of the pultruded composite drilled hole. Forces in the connection are transferred through this contact between the mechanical connector and pultruded composite.

Strength is determined from the pin-bearing strength of the pultruded composite, and this strength is determined by following an ASTM Test Standard. Known strength is important since engineers need the pultruded composite strength to design connections.

This type of connection has yet to be developed by the pultruded composites industry. It can be achieved, but design methods need to be developed. Braced frames using x-bracing or knee-braces are the current, typical construction type for pultruded composite structures.  

Methods for rigid, or semi-rigid connections, as seen in the steel industry, would be beneficial for conditions where the structure cannot have x-bracing or knee-braces, and this condition does occasionally occur. It’s uncommon simply because a design method has yet to be standardized and published.

With respect to pultruded composites, adhesives alone in connections are often the exception due to the long-term reliability of this type of connection being in question, and the requirements for achieving a reliable bonded connection. Other factors include loading, the type of structure in question, the life expectancy of the structure, etc.

Additionally, consideration of environmental conditions such as humidity and temperature are critical for bonded connections, and often require these connections to be performed indoors and allowed to cure over time. Mechanical connections can be completed quickly, compared to bonding alone, with little or no regard for environmental conditions.

A common method for increasing strength and durability of a traditional mechanical method is to include bonding of the pultruded composites with the mechanical connection. This allows for an increase in strength, and structural redundancy in a bonded and bolted connection, for example.

From what I have observed, epoxy resins are a commonly chosen adhesive; however, other adhesives are available such as urethanes and methyl methacrylates, which often are chosen due to limitations of epoxy adhesives.

Brittle behaviour, chemical exposure, in-service temperature exposure, and strength are typical subjects of concern when choosing an adhesive.

The two main contributing factors are environmental conditions during assembly of pultruded composites and the service conditions while the pultruded composite structure is in use. As mentioned previously, humidity, temperature, and preparation of the connection are critical factors to be considered for achieving a successful bonded connection.

Mechanical connections can be completed quickly, compared to bonding alone, with little or no regard to environmental conditions.

More information can be found on the American Composites Manufacturing Association website and participating company websites, such as the Strongwell website.

Stephen Browning is a licensed professional engineer with approximately 17-years of experience in structural analysis and design, including wood, steel, concrete and pultruded composites.

Currently, he is the structural engineer located at the Strongwell Corporate Offices in Bristol, Virginia.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

Originally from Ireland, Mychealla graduated from Northumbria University in Newcastle with Bachelor's degree in Journalism with English Literature. After spending a year traveling around Asia and Australia she moved to Manchester. In her spare time, Mychealla can be found spending time with family and friends, hiking, going to the gym/doing yoga and like everyone else getting stuck into the latest Netflix series.

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