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The development of standard methods and criteria for the stress grading of structural timber elements by non-destructive, fast and cheap bending tests, will allow producers, distributors and customers of such elements, have a better control of the marketed product. This paper analyzes the most suitable length of free span for stress grading of structural timbers, number of points to be assayed in each element, data precision and evaluation of cost of these tests for small business.

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Although geometrical data describing structures are generally represented by their nominal values, practice has revealed that this is not the more appropriate for prestressed laminated wood plates, once the lamellae present significant size variability. The importance of this variability increases when it affects the height of plate cross sections, they also affect their moments of inertia which depends on the value of the height raised to the third power. Preliminary studies showed standard deviations of almost 10%. As far as stress-laminated plates are regarded, the variability of dimensions along the length of the length of the pieces introduces a new source of variability for the cross section moment of inertia. The obtained surfaces aren't completely flat, presenting variability due the size variability along the length of the pieces. Consequently, the moment of inertia of the cross section of a stress-laminated plate is not obtained by summing up the individual moment of inertia of each piece. On the other hand, modulus of elasticity also presents a strong variability not only among the height of pieces, but also along their length. Preliminary experiments reveal a standard deviation of 23% for plates from Pinus taeda. The variability of moment of inertia of pieces and of the plate itself was determined, as well as the variability of the modulus of elasticity. The results show that rigidity just can be estimated probabilistically; that softwoods are suited for system of high redundancy; that the calculation of deformations and stress distribution along the structures are dependent on this variability and thus the referred calculations are nothing more than estimations with a certain level of confidence to be established according to the control procedures adopted; and finally that further research must be developed on probabilistic numerical factors to be incorporated to the calculation expressions of rigidity of the cross section of stress-laminated plates.

<p>Today, all over the world, large scale urbanization is forcing more and more the importance of developing new technologies related to the rational use of wood, contributing to solving the problems of urban traffic. There is also an aesthetical need of showing the melting of up-to-date high technology with beauty. The cable-stayed stress-laminated timber footbridge, with curved modules meets all these needs. The building of a prototype, enabled static and dynamic tests to be carried out. The experimental results revealed the technical and economical feasibility of the following items: cable stayed footbridges with timber from reforestation; decks only made of stress laminated timber; curved plates of stress laminated timber. It also revealed that the human induced vibrations leads to the most critical design condition. The loss of deck curvature study indicated the need of further research considering the time effect, to confirm the initial success achieved. Based on results of these testing, the numerical model calibration was done. In order to investigate which variables are responsible for the footbridge response, several simulations were performed with the calibrated model.</p>