Prestressing can be defined as the preloading of a structure before application of the required design loads in such a way as to improve its overall performance. In traditional materials the most common application of prestressing is found in concrete since the strength of concrete is much higher in compression than in tension. The prestressing of concrete involves application of a compressive load to the structural member prior to the design loads so that tensile stresses can be reduced or eliminated.
Fiberglass reinforced plastic (FRP) is a high strength material in tension and compression but with a low modulus of elasticity (E). The low modulus makes it difficult to use FRP’s high strength characteristics in most structural applications. Fiberglass Trusses, Inc. has solved this problem with FRP by prestressing a type of structural form known as beam-trusses.
Historically, this type of structure, which includes the kingpost, queenpost and bowstring truss, used low strength and stiffness wood for the top member and post(s), and high tensile strength and stiffness iron or steel for the bottom tension member. Iron or steel, which were relatively expensive at that time, were used in small quantities to optimize the structural characteristic of the form. This proved to be a very economical approach to strengthen and stiffen wood structures to carry heavier loads and span larger distances with smaller deflections.
Fiberglass Trusses, Inc. uses a similar material approach in that it uses the high strength and low stiffness characteristics of FRP for the main member and posts of the system. For the tension member depending on the structural requirements there are variety of material options that can be used to provide high tensile strength and stiffness. These include the following:
The major innovation of the system is that it recognizes the increased strength and stiffness characteristics derived from the “prestressed” beam-truss form combined with low modulus FRP. Prestressing the FRP beam-truss allows for the following unique characteristics:
- Increased strength
- Minimize dead and live load deflections by precambering
The system can also be easily modified to provide additional strength and stiffness at a future time.