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Engineering With FRP Using The ACMA Pre-Standard
Pultruded FRP sections are orthotropic, so strength and stiffness differ along the longitudinal, transverse and through-thickness directions. The strongest direction is along the pultrusion line, where continuous glass fibres carry most of the stress. Transverse strength is typically a fraction of the longitudinal capacity and is dominated by the resin matrix.
Because of this directional behaviour, the ACMA pre-standard uses a load and resistance factor design approach similar to steel and concrete LRFD, but with additional adjustment factors for:
- Resin type
- Service temperature
- Moisture exposure and chemical environment
- Time effects and creep
- Governing failure mode (rupture, buckling, bearing and so on)
The webinar uses a typical FRP walkway layout as an example:
- Post spacing about 3 m
- Walkway width is about 2.4 m
- Post heights 1.5 m to 7 m
- Design live load around 3 to 4 kPa plus wind in Region A1
Representative Treadwell pultruded profiles are selected, such as:
- Square hollow posts: 102 x 102 x 8 mm
- Outer joists: C section 152 mm x 9.5 mm
- Inner joists: I beam 152 mm x 9.5 mm
- Bearers: double C channels
- Bracing: equal angle 102 x 102 x 9.5 mm
With a typical elastic modulus of 18-21 GPa for pultruded glass FRP, serviceability deflection often governs beam design.
The workflow presented is:
- Build a space frame model in a structural analysis program such as SpaceGass, using Treadwell’s FRP section library.
- Assume pinned joints for FRP members, since they are bolted rather than welded.
- Run serviceability combinations to check deflection, including both flexural and shear deformation.
- Run ultimate combinations to obtain design bending moments, shear forces and axial forces.
- Apply ACMA adjustment factors to derive design capacities.
- Design and verify bolted connections and bracing using ACMA connection rules.
Key Benefits
Design Aligned To A Dedicated FRP Standard
Using the ACMA LRFD pre-standard means:
- Capacities are based on test data and FRP-specific behaviour rather than simply reusing steel rules.
- Multiple adjustment factors account for resin type, temperature, moisture, and chemical exposure, ensuring designs match the real environment.
- Time effects and creep are explicitly covered through long-term factors, giving more realistic deflection and strength predictions over the service life.
Better Control Of Deflection And Long-Term Performance
The webinar emphasises that:
- Serviceability often governs FRP beams due to their lower modulus than steel.
- Shear deformation is significant for deeper FRP profiles and should be accounted for in the flexural deflection.
- The ACMA long-term deflection factor lets designers estimate creep effects under sustained loads, similar to methods used in concrete and timber design.
This reduces the risk of bouncy walkways or excessively long-term sag.
Safer And More Robust Connection Design
Connection design is a critical part of FRP structures, and the session shows how the ACMA rules help:
- Edge distance, end distance, and bolt spacing are expressed as simple multiples of the bolt diameter, making them easy to apply in detailing.
- Main failure modes are checked explicitly, particularly bearing and net tension in the FRP around the bolt.
- The proportional load taken by each bolt row is adjusted depending on whether FRP to FRP or FRP to steel is being connected.
By tracking fibre direction for each member and bracket, the designer can use the correct longitudinal or transverse material properties at every bolt group, avoiding overestimation of capacity.
Efficient Member Sizing With Realistic Bracing Rules
The example highlights how lateral torsional buckling controls the design of joists and beams:
- Unbraced lengths of around 3 m may provide limited bending capacity.
- Adding a midspan lateral bracing point can significantly increase the moment capacity, often enough to satisfy the design moment without increasing section size.
Similar principles apply to compression members:
- Post and brace capacities are checked for material crushing, global buckling and local buckling.
- The webinar notes that very slender sections might pass axial capacity checks but fail in the connection due to limited thickness at bolt lines, reinforcing the need to design members and connections together.
Direct Compatibility With Treadwell Tools And Profiles
The workflow presented aligns with the Treadwell ecosystem:
- Treadwell’s ArchitEX™ pultruded FRP profiles and access systems are defined with consistent section properties suitable for structural modelling.
- Treadwell provides SpaceGass libraries containing FRP materials and section definitions, so engineers do not need to build every profile from scratch.
- Internal design tools are based on a factor-of-safety approach that is intentionally conservative relative to ACMA LRFD capacities in most common environments, providing an additional margin when conditions are not extreme.
Applications
Industrial Walkways And Platforms
The example geometry shown in the webinar is directly relevant to:
- Access platforms around industrial plant and equipment
- Boardwalk-style walkways in corrosive or coastal environments
- Maintenance access structures where lightweight materials and easy installation are key
Using the ACMA methodology with Treadwell’s profiles allows engineers to size joists, bearers and posts for typical live loads and wind regions, while respecting deflection limits and connection capacities.
Water And Wastewater Facilities
FRP is widely used in water and wastewater plants for its corrosion resistance and non-metallic behaviour.
The design approach covered in the webinar applies to:
- Tank roof walkways and covers
- Clarifier access bridges and platforms
- Odour control covers and support framing
Adjustment factors for chemical exposure and moisture in the ACMA pre-standard are especially relevant in these environments.
Coastal And Marine Infrastructure
The combination of:
- Corrosion resistance against salt water
- Good slip resistance when paired with suitable anti-slip surfaces
- Electrical insulation near marine electrical equipment
Makes pultruded FRP a strong candidate for:
- Boardwalks and viewing platforms
- Jetty and marina access structures
- Coastal lookouts and stairways
The ACMA-based process helps verify both short-term and long-term performance under pedestrian and environmental loads.
Electrically Sensitive Sites
Because FRP is non-conductive, it is suitable for:
- Substations and switch yards
- Rail corridors and other sites with earthing and bonding constraints
The design framework presented in the webinar provides structural engineers with a clear method for combining electrical safety benefits with certified structural performance for platforms, ladders, and fencing supports.
Key Moments
- 01:02 🎓 Introduction to Treadwell Group and the focus on FRP engineering for the webinar.
- 02:07 🚧 Treadwell Group has diversified into seven main product categories, including access systems and environmental solutions.
- 04:12 🏗️ FRP is an anisotropic material, meaning its strength varies in different directions, unlike isotropic materials like steel.
- 05:30 📊 Using ACMA standards allows for multiple load and resistance factors, enhancing safety in FRP design.
- 06:25 🌡️ Temperature and moisture significantly affect the strength of FRP materials, and different resin types impact structural durability.
- 09:10 ⚖️ Design calculations prioritize deflection and failure modes, particularly for internal joints and beams in FRP structures.
- 13:39 🔗 The connection design in FRP is critical; pin bearing and late tension are often the most significant failure modes.
- 17:02 📏 Understanding load direction and fiber orientation is crucial for ensuring structural integrity in FRP connections and members.
- 19:09 📧 Questions can be directed to Treadwell Group for further clarification or project inquiries after the presentation.
Why Treadwell?
The session concludes by reinforcing how Treadwell supports engineers who want to design with FRP using the ACMA methodology:
- Treadwell supplies a broad range of pultruded structural members, gratings, handrails and fencing systems that are purpose-built for corrosive and electrically sensitive environments.
- SpaceGass material and section libraries from Treadwell simplify analysis setup so design teams can focus on member checks rather than data entry.
- Treadwell’s engineering team uses the ACMA pre-standard internally and can assist with member selection, deflection and strength checks, and connection detailing for real projects.
- Conservative in-house tools and experience with long-term FRP performance help bridge the gap between theory and site conditions, improving confidence in FRP solutions.
By combining ACMA-based design, practical examples and ready-to-use FRP profiles, Treadwell makes it straightforward for engineers to adopt pultruded FRP in demanding industrial and infrastructure projects.
