Fiber reinforced polymer (FRP) is a composite material made up of a polymer matrix reinforced with fibers. The fibers are usually composed of glass, carbon, or aramid. The polymer is usually composed of an epoxy, vinylester, or polyester thermosetting plastic. Pultrusion is a manufacturing process for producing continuous lengths of the reinforced polymer with constant cross sectional properties. The pultrusion process involves pulling raw materials through a heated steel forming die using a continuous pulling device.

While FRP has been utilized heavily for decades in aerospace, water and wastewater treatment and storage, and marine vessels, its implementation into civil infrastructure at scale has been comparatively gradual. Due to its non-corrosive material properties, pultruded FRP framing solutions may be ideal for infrastructure exposed to harsh outdoor environmental conditions (e.g., salts from sea spray or from roadway de-icing treatments) or from caustic industrial conditions (e.g., chlorine manufacturing). Due to its light-weight and domestic production, pultruded FRP framing can be manufactured, fabricated, delivered, and installed in rapid fashion compared to heavier traditional infrastructure materials. Thus, pultruded FRP may be a key tool as engineers, facility asset managers, policy-makers, and other stakeholders struggle to finance repairs and maintenance for degrading infrastructure systems in a time-efficient manner for users.

Engineers from Frost Engineering & Consulting along with researchers working in the Structural Systems Laboratory at the University of Notre Dame (ND) Department of Civil & Environmental Engineering & Earth Sciences (CEEES) tested three innovative types of rotationally semi-rigid connections with pultruded FRP frame members in collaboration with researchers from New Mexico State University and University of Texas at Tyler. To laterally stabilize a structural frame against wind, earthquake, and service loads, designers must either specify diagonal bracing between column bays or specify joints (e.g., where beams and columns meet) to be rotationally stiff enough to limit deflections in the frame.

The industry partners in this project have identified that the preferred applications of pultruded FRP framing in use today are often in services where the height of the frames are low and the quantity and size of mechanical, electrical, and plumbing (MEP) utilities conflict with the presence of diagonal braces. Thus, the use of rotationally semi-rigid beam-column joint connections is preferred. Previously tested semi-rigid connections in FRP have often been based on designs where connection elements conflict with walking platforms, as well as other beams and braces. In consultation with its industry partners, the researchers designed and tested semi-rigid FRP connections specifically for implementation in prominently identified service conditions around the world.

Potential applications for pultruded FRP framing include short-span bridges, indoor and outdoor agricultural and industrial framing, telecommunications facilities, coastal facilities, mission-critical or space-limited projects, and egress, access, and catwalk platforms.

The engineers and researchers tested three unique connection types representing a variety of configuration options and expected performance characteristics. Testing was configured in the ND CEEES Satec universal materials testing machine (which has a 600,000 lb ultimate capacity) in the Structural Systems Laboratory. Displacements and rotations of the specimens were measured in all three dimensions using a system of string potentiometers, linear variable differential transformers (LVDTs), tilt-meters, and state-of-the-art digital image correlation (DIC). The tested connections types and their associated measured performance characteristics were then simulated as part of a prototypical in-service structural frame using finite element modelling software, and the early results are promising enough to conduct additional testing with samples meeting minimal sample size requirements to ensure statistical significance and design reliability. Furthermore, the identified failure modes allow researchers to improve the design for better expected performance in future testing and field applications.

The long-term goals of this experimental testing and future associated testing are to help the construction industry save time and money in initial construction, but even more importantly, to ensure that infrastructure being constructed today will require minimal life-cycle costs over time by reducing the amount of corrosion and thus regular maintenance and repair costs. Potential applications for pultruded FRP framing include short-span bridges (pedestrian and vehicular), indoor and outdoor agricultural and industrial framing, telecommunications facilities, coastal facilities, mission-critical or space-limited projects, and egress, access, and catwalk platforms.

The experimental research team was comprised of Kevin Walsh, PhD, PE (ND CEEES ’09) and Brent Bach of the University of Notre Dame; Brad Weldon, PhD (ND CEEES ’06 and ’10) and Grace McMurry of New Mexico State University; and Mike McGinnis, PhD of the University of Texas at Tyler. Special thanks also to Yahya Kurama, PhD, PE, Rob Devine, and Steve Barbachyn, PhD of the University of Notre Dame for their technical guidance with instrumentation.