Sustainable Pedestrian Bridges Using Advanced Materials

Pedestrian bridges made of fiberglass-reinforced polymer (FRP) are becoming a competitive alternative to conventional concrete and steel pedestrian bridges for spans between 5 and 30 m. FRPs are lightweight, high-performance, durable materials that have proven to perform well in a wide variety of corrosive environments. Multiple life cycle cost analyzes have shown that FRP bridge construction is an economical and competitive option when considering the costs associated with design, construction, and maintenance operations. One way to achieve cost competitiveness is through the production of standardized systems to create a catalog of versatile pre-engineered solutions for small and medium-span pedestrian bridges.

Pedata has led a research project to develop SUPERBAM, a new standardized and affordable pedestrian bridge. SUPERBAM has the advantages of being high strength and lightweight with the excellent durability of FRP materials. The emphasis of this research project has been to develop an efficient, competitive, and aesthetically attractive structural system for pedestrian bridges with small and medium spans. Research has focused on conceptual studies and numerical models to validate and optimize concepts. The constructed prototype has been tested in a research laboratory.

Detailed structural analysis and optimization of various cross-sections and laminated solutions have been carried out using the Finite Element Method (FEM) under static and dynamic loads to achieve the most cost-effective design. Specialized Finite Element software has been developed with features including a database of predefined laminates, automatic geometric construction, and definition of standard load cases. By using these tools, it is possible to make highly reusable molds for laminates, which in turn reduces the total cost of FRP bridges.

Introduction

The development and improvement of sustainable mobility around the world are urging the construction of pedestrian bridges in urban, rural, and remote areas. Meanwhile, the most developed countries are coping with the aging of their transport infrastructures. A significant number of bridges require periodic maintenance, even after major repair or replacement, to safely perform their functions. The construction of public infrastructure has a significant impact on sustainability, and the use of advanced materials such as FRP and stainless steel contributes to its sustainable development.

The objective of this research project is to use high-strength, lightweight, and reusable materials that require minimal maintenance and speed up construction. Most of the existing FRP pedestrian bridges are trusses made with standard pultruded profiles. There are technical limits to the use of these systems, mainly due to their manufacturing process and poor aesthetics. In addition, the joints are usually bolted and their design is one of the most critical challenges.

Adhesive connections can also be used in combination with bolts. In the bridge sector, there is a need to develop new structural concepts using FRP or hybrid structures.

As a result, the SUPERBAM research project was initiated, which has developed specific geometries and aesthetically pleasing structural concepts to make the most of advanced materials (i.e. high-strength, lightweight, durable, and sustainable) through the use of modular systems.

Research Objectives

The objectives of the SUPERBAM research project include the following tasks:

Technological steps and how they have been addressed

The purpose is to develop specific structural concepts for FRP or hybrids for pedestrian bridges instead of repeating the same solutions that are already being used for concrete and/or steel structures. This conceptual design includes the design of a modular construction structural system, the analysis of the dynamic behavior of ultralight structures to mitigate vibration and the design of joints and assembly methods.

A feasibility study of collapsible and unfoldable structures has been carried out to assess the possibility of obtaining a structure with the ability to change its geometry from a compact configuration to a functional form. A deployable structure requires active elements during installation. New concepts are validated using specific numerical software or by adapting advanced numerical codes used in other sectors (for example, the aerospace and naval industry).

An analysis of the manufacturing process was also carried out to select

• the most suitable materials (e.g. GRP, CFRP, stainless steel, hybrid structures)
• the manufacturing process (for example, extrusion, vacuum infusion, manual design, etc.), and iii) the structural forms.

Manufacturing Technique

Traditional techniques used in the manufacture of FRP components include:

Mold

The main structure is U-shaped and the profiles will be built with separate molds. The molds must allow the creation of several identical sections with a minimum of effort. They must be cost-effective and reusable, and therefore continuous experimentation is underway. Molds are typically built using a blueprint.

To begin, a prototype of the section is prepared and refined to remove imperfections. Successive layers of fabrics pre-impregnated with liquid resin are then laid down to create an inverted, self-supporting structure (i.e. the mold).

There are single-use molds and also permanent molds. The molds can also be divided into male and female molds. A male mold is used to form the interior of the frame, and a female mold is used to form the exterior of the frame.

External laminate

The mold surface is stabilized at a high temperature and vacuum integrity tested. A release agent is applied to the stabilized surface of the mold. Next, a thick layer of gel coat is typically applied, which is designed to provide a uniform finish with the desired color. A liquid resin is then applied and the first layer of reinforcing fabric is placed on top. Rollers are used to apply pressure to the fibers to infuse and de-air the layer. These steps are repeated by applying layers of fabric and resin until the desired thickness is reached, ensuring during the process that the layers perfectly fit the complex shape of the mold. Vinylester is placed as a filler between consecutive thin layers of fabric to absorb the resin and achieve thickness. An evolution in structural design and resin matrices has allowed for a reduction in filler use and a substantial increase in the proportion of direct fiber in the laminate. This reduces the number of layers needed to obtain the design strength.

Vacuum technique

The incorporation of vacuum techniques in traditional contact lamination leads to significant advances in the physical and mechanical properties of composite materials. A closed soft container shrinks and eventually collapses if the external pressure increases. The same effect occurs if the internal pressure decreases. By enclosing the laminate in a sufficiently strong bag and removing the air inside, a uniform pressure of about 1 atmosphere (101.325 kPa) can be obtained at any point, even if the shape is complex. The results obtained by vacuum lamination present significant advantages over traditional contact lamination. The benefits of this technique include the reduction of thickness, air content, and final weight. In addition, consistent and consistent quality is being achieved along with fewer imperfections.

Sandwich infusion

The most basic and common process of all infusion techniques is to first place layers of fibers (the core) and other inserts on the outer surface of the mold without using resin. This can be done slowly to ensure a clean shape, which is an important factor in the final quality of the part and the entire project. Once this first step is complete, the vacuum bag and other infusion-specific items are placed on top of the group. When the assembly is sealed with the help of a vacuum, the first compaction is carried out to stabilize the piece, increase the fiber content by volume and reduce voids. After reaching the desired level of compression, holes are opened to saturate the part with liquid resin while all the air inside is expelled with vacuum tubes.

Assembly and finishing

The various pieces that make up the pedestrian bridge are joined with mechanical connectors and structural adhesives. Final assembly will require final finishing, painting, and polishing, until the desired finish is achieved.