J. Toby Mottram

Dynamic Response of a Sheet Pile of Fiber-Reinforced Polymer for Waterfront Barriers

This paper presents the results from a combined experimental and advanced computational study to understand the dynamic response of a pultruded fiber-reinforced polymer (FRP) sheet pile of 9 m length that is installed into the ground near Venice, Italy. The peak embedment force of 10 kN is applied at the top as a sinusoidal compression force having a maximum frequency of circa 760 Hz. Physical measurements from accelerometers are reported for the lateral deformation response of a single sheet pile and of a unit restrained by an installed waterfront barrier. A finite-element modeling methodology for the two test configurations is developed by using the Strand7 code, so that advanced computational results can be compared against the field application measurements. Closed-form equations for the fundamental frequency are developed, with one accounting for the presence of rotary inertia and shear deformation. Dynamic responses at different embedment lengths (1–7 m) are examined, and a very good correlation is found between theory and practice. Numerically, the performance of the FRP sheet pile is compared with the response of a fictitious sheet pile of steel and with two new FRP geometries that increase stiffness to minimize flexure about the minor axis of bending. By increasing the mass by 10%, the maximum lateral displacement can be the same as the steel unit and 1/20 of the tested FRP unit. Findings of the research demonstrate that the FRP unit can be installed by using the same pile driving rig and procedure for steel sheet piling.

Structural Properties of a Pultruded E-Glass Fibre-Reinforced Polymeric I-Beam

This paper reports experimental data from a number of short term laboratory test studies. The data have been used to establish and evaluate the structural properties of a small commercial pultruded E-glass/vinylester I-beam (102 by 51 by 6-4 mm). Material properties for the flange and web material were measured in tension and compression. The coupon test methods for strength and elastic modulus properties were simplified from the recommendations in standard test methods without serious loss of performance. Three-point bend tests on full sections of the I-beam demonstrated that conventional linear elastic theory formulae may be used to predict the beam deflections and critical loading for lateral-torsional buckling, provided the moduli in the formulae are those of the section. Experimental data from test studies have been used to show the measured tensile strength of the flange material may provide a good estimate for the resistance of the beam in three-point bending, providing no other failure mechanism has a lower load.

Buckling of Built-Up Columns of Pultruded Fiber-Reinforced Polymer C-Sections

This paper presents the test results of an experimental investigation to evaluate the buckling behavior of built-up columns of pultruded profiles, subjected to axial compression. Specimens are assembled by using four (off the shelf) channel shaped profiles of E-glass fiber-reinforced polymer (FRP), having similar detailing to strut members in a large FRP structure that was executed in 2009 to start the restoration of the Santa Maria Paganica church in L’Aquila, Italy. This church had partially collapsed walls and no roof after the April 6, 2009, earthquake of 6.3 magnitude. A total of six columns are characterized with two different configurations for the bolted connections joining the channel sections into a built-up strut. Test results are discussed and a comparison is made with closed-form equation predictions for flexural buckling resistance, with buckling resistance values established from both eigenvalue and geometric nonlinear finite element analyses. Results show that there is a significant role played by the end loading condition, the composite action, and imperfections. Simple closed-form equations overestimate the flexural buckling strength, whereas the resistance provided by the nonlinear analysis provides a reasonably reliable numerical approach to establishing the actual buckling behavior.

Response of Beam-To-Column Web Cleated Joints For FRP Pultruded Members

Physical testing is used to characterize the structural properties of beam-to-column joints, comprising pultruded fiber-reinforced polymer (FRP) H-shapes of depth 203 mm, connected by 128 mm-long web cleats and two M16 bolts per leg. Testing is performed on two batches of nominally identical specimens. One batch had web cleats of pultruded FRP and the other had structural steel. The structural behavior of the joints is based on their moment-rotation responses, failure modes, and serviceability vertical deflection limits. Joints with FRP cleats failed by delamination cracking at the top of the cleats, and when the cleats were of steel, the FRP failure occurred inside the column members. Neither failure mode is reported in the design manuals from pultruders. At the onset of the FRP damage, it was found that the steel joints were twice as stiff as the FRP joints. On the basis of a characteristic (damage) rotation, calculated in accordance with Eurocode 0, the serviceability deflection limits are established to be span/300 and span/650 for the joints with FRP and steel cleats, respectively. This finding suggests that appropriate deflection limits, in relation to cleated connections, should be proposed in manufactures’ design manuals and relative design standards and design codes. Failure to address the serviceability, by the engineer of record, could lead to unreliable designs.

Effect of Elevated Temperatures on the Mechanical Performance of Pultruded FRP Joints with a Single Ordinary or Blind Bolt

Presented in this paper is a combined experimental and analytical modeling study of the strength of pultruded fiber-reinforced polymer (FRP) single-bolted double-lap joints subjected to tensile loading and elevated temperatures. Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) are conducted on the polymeric composite material to determine the glass transition temperature and decomposition temperature, respectively. Based on the DMA and TGA results, and to cover glass transition without any material decomposition, the six temperatures selected for the test program are +23+23, +60+60, +100+100, +140+140, +180+180, and +220°C+220°C. Three nominally identical joints are tensioned to failure at each temperature. A total of 36 double-lap joints are tested, comprising 18 joints fabricated with ordinary steel bolting and the other 18 with novel blind bolting. A comparison is made based on load-displacement curves, failure modes, and maximum (ultimate) loads. It is found that both methods of mechanical fastening experience a reduction of 85% in maximum load as the test temperature increases from +23+23 to +220°C+220°C. Three proposed empirical or mechanism-based models for characterizing strength under elevated temperatures are shown to provide good predictions for the maximum loads obtained in the test program.

Numerical evaluation of pin-bearing strength for the design of bolted connections of pultruded FRP material

This paper presents finite-element predictions for the strength of a pultruded fiber-reinforced polymer (FRP) material subjected to pin-bearing loading with hole clearance. One of the distinct modes of failure in steel bolted connections is bearing. It is caused by the compression action from the shaft pressing into the laminate, and when there is no lateral restraint the mechanism observed at maximum load shows brooming for delamination failure. Each lamina in the glass fiber polyester matrix material is modeled as a homogeneous, anisotropic continuum and a relatively very thin resin layer is assumed to contain any delamination cracking between stacked layers. A cohesive zone model is implemented to predict the size and location of the initial delamination, as well as the load-carrying capacity in a pin-bearing specimen. Finite-element simulations (as virtual tests) are performed at the mesoscale level to validate the modeling methodology against experimental strength test results with delamination failure, and to show how pin-bearing strength varies with parameter changes. For an example of the knowledge to be gained for the design of bolted connections, the parameteric study in which the mat reinforcement is either continuous strand or triaxial (+45°/90°/−45°/chopped+45°/90°/−45°/chopped strand) shows the latter does not provide an increase in pin-bearing strength.

Effects of Pedestrian Excitation on Two Short-Span FRP Footbridges in Delft

Reported in this paper is an evaluation of the vibration behaviour of two footbridges in The Netherlands having main spans of about 15 m. Short-span footbridges over canals and rivers are embedded in the landscape of Delft and elsewhere. Increasingly, these bridges are made of Fibre-Reinforced Polymer (FRP) composites, utilising the high-strength and light-weight nature of the material, and taking advantage of fast installation and low maintenance costs. Due to low mass, these FRP bridges might be sensitive to dynamic excitation by human actions. The two footbridges investigated in this paper have the main bearing structure which consists of either two or four longitudinal beams made of vacuum infused FRPs with foam cores connected by an FRP deck. Modal testing revealed that fundamental vertical natural frequency of the two structures at 4.8 and 6.1 Hz is in the range typical of similar structures made of concrete and steel, whilst the corresponding damping ratio for the wider, slightly cambered, bridge was exceptionally high at 7.9%. The vibration response to dynamic force by people walking was typically up to 1 m/s2. While both of these light-weight structures performed satisfactorily under the regular pedestrian loading, the higher frequency – higher damping structure represents an example of successful control of pedestrian-induced vibrations by means of longitudinal restraints at the end supports and slightly curved shape of the main structure.

Experimental Investigation of the Dynamic Characteristics of a Glass-FRP Suspension Footbridge

Due to high strength- and stiffness-to-weight ratios, good durability performance in a variety of environments and quick installation, fibre reinforced polymers have increasingly been utilised for construction of highway and pedestrian bridges. Their relatively low mass and stiffness make these bridges potentially susceptible to vibration serviceability problems, which are increasingly governing the design. Currently, a lack of experimental data on the dynamic characteristics of polymeric composite structures is hindering their wider application and the development of design guidance. To fully exploit the benefits of using these structural materials in bridge engineering requires a better understanding of their dynamic behaviour. The aim of this paper is to utilise ambient vibration measurements to experimentally identify the dynamic characteristics (i.e., natural frequency, damping ratio and mode shape) of a glass fibre reinforced polymer deck suspension footbridge in the UK. It is found that the Wilcott footbridge possesses a relatively high density of vibration modes in the low frequency range up to 5 Hz and has damping ratios of most of these modes >1%.