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Aspects of bolted connections of FRP structures

Researcher: Navroop Matharu
Project duration: 2010-2013
Supervisors: Prof. Toby Mottram
Funding body: EPSRC


Glass fibre reinforced polymers (FRPs) are increasingly becoming an accepted and competitive alternative to traditional building materials in providing structural solutions in civil engineering works. The use of FRP materials has been affluent in both the marine and aerospace industries for many years, with major benefits being high strength-to-weight ratio, corrosion resistance and low maintenance over a long life cycle.

By using the FRP processing method of pultrusion, structural shapes such as I-beams, wide-flange sections, channels and leg angles can compete with their steel equivalents when the FRP structure meets the client’s needs. The structural integrity and strength of framed structures using pultruded shapes and systems can largely be controlled by the strength of the connections. The predominant methods of connection are by bolting, bonding, interlocking, or a combination of these. Bolted joints (often using stainless steel bolts) are the most practical due to their ease of assembly, low maintenance and general cost effectiveness compared to other types of connections.

The full potential of a FRP joint is difficult to achieve due to the material being both orthotropic and heterogeneous, making the full set of material strength properties difficult to characterise. In addition, the design guidance available is provided by data relating to materials strengths derived from non-standardized testing procedures or in-house materials testing with little or no provenance. Therefore, the development of FRP composites in construction would be aided by unifying the various test procedures in order to provide sound data for reliable design guidance in detailing of connections.

Aims and Objectives

The aims and objectives of the research are:

  • to develop a test methodology for the determination of pin‐bearing strength, and to characterise this key strength property required for the design of bolted connections with FRP shapes,
  • to complete a series of plate‐to‐plate tests that characterise the resistance of bolted connections of Pultruded material, which shall lead to the proposal of new or improved design guidance,
  • to use a critical evaluation of the targeted test results to either revise or modify current design provisions so that the risk of structural failure is minimised.


The research project will involve extensive laboratory testing to scope known gaps in knowledge for the preparation of scientifically-founded provisions for the design of bolted connections for FRP structures. There are two work packages which address the determination of pin-bearing strength and uses strength test results from plate-to-plate bolted connections to evaluate and calibrate resistance formulae for codification.

Dissemination of the results and findings from an evaluation of the test series will impact structural engineering research by assisting the growing FRP industry by way of the preparation of recognised design rules, such as would be found in the new-build parts to the future structural Eurocode for FRP materials. Design standardisation for the structural material of FRP will provide the confidence for wealth creation and future innovation towards buildings and bridges that have an overall performance to satisfy the drivers for sustainability and a Green Economy.

This project forms a part of the larger research project for Connections and Joints for Buildings and Bridges of Fibre Reinforced Polymer.


Mottram, J.T. (2009) Determination of Pin-Bearing Strength for the Design of Bolted Connections with Standard Pultruded Profiles. Proceedings of the 4th International Conference on Advanced Composites in Construction (ACIC 2009), Edinburgh, 483-495.

Mottram, J.T. and Turvey, G.J. (2003) Physical Test Data for the Appraisal of Design Procedures for Bolted Joints in Pultruded FRP Structural Shapes and Systems. Progress in Structural Engineering and Materials 5 (4), 195-222.

Mostoller, J.D. and Troutman, D.L. (2010) An Investigation of Pin Bearing Strength on Composite Materials, Creative Pultrusions Inc., Alum Bank, PA. (Literature library)

Pre-Standard for Load & Resistance Factor Design (LRFD) of Pultruded Fiber Reinforced Polymer (FRP) Structures, submitted to American Composites Manufacturer Association (ACMA)), 9th November 2010, ACSE, p. 216. Now with the Standards Committee to pass through an ANSI standards process before being adopted as a national standard in the USA.

Wang, P. (2004) Structural Integrity of Bolted Joints for Pultruded GRP Profiles. PhD Thesis, University of Lancaster, UK.