Assessment of the residual load-carrying capacity of large span members in wood

• Forschungsstelle: COST
• Projektnummer: C12.0049
• Projekttitel Englisch: Assessment of the residual load-carrying capacity of large span members in wood

Schlüsselwörter (Englisch)

load-carrying capacity; glulam beams; numerical simulation; assessment; estimation; cracks

Forschungsprogramme (Englisch)

COST-Action FP1101 - Assessment, Reinforcement and Monitoring of Timber Structures

Kurzbeschreibung (Englisch)

Impressive large span constructions are more often realized in timber or engineered wood products over the past years. For example, glulam beams provide the possibility for the use of timber in halls, bridges and multi storey buildings. The limitation in span of solid timber members was lifted due to glulam production. But the environmental conditions are different for these constructions. A quite high humidity with constant temperature is prevailing e.g. in roof constructions for swimming pools whereas low temperature and dry conditions are prevailing in ice sport arenas. The climate changes have an influence on the long term behaviour or can even lead to cracks and thus to a weakening of the timber members. Therefore the objective of the project is to develop reliable methods for assessing the residual load-carrying capacity of cracked/damaged timber members. The delamination or the existence of cracks respectively is mostly one of the major visible indicators for a first assessment of timber constructions. But how to respect these cracks observed when estimating the residual load-carrying capacity of timber structural elements with cracks? The residual load-carrying capacity of such members will be investigated and evaluated by means of numerical simulations based on existing crack situations gathered from real structures. By investigating different distributions and parameters of cracks, methods for the estimation of the residual load-carrying capacity of glulam beams with cracks will be developed.

Partner und Internationale Organisationen (Englisch)

AT; BE; HR; CZ; DK; EE; FR; DE; EL; IE; IT; NL; NO; PL; PT; SI; ES; SE; CH; MK; UK

Abstract (Englisch)

Impressive large span constructions are more often realized in timber or engineered wood products over the past years. For example, glulam beams provide the possibility for the use of timber in halls, bridges and multi storey buildings. The limitation in span of solid timber members was lifted due to glulam production. But the environmental conditions are different for these constructions. A quite high humidity with constant temperature is prevailing e.g. in roof constructions for swimming pools whereas low temperature and dry conditions are prevailing in ice sport arenas. The climate changes have an influence on the long term behaviour or can even lead to cracks and thus to a weakening of the timber members. Therefore, the objective of the project is to develop reliable methods for assessing the residual load-carrying capacity of cracked/damaged timber members. The delamination or the existence of cracks respectively is mostly one of the major visible indicators for a first assessment of timber constructions. But how to respect these cracks observed when estimating the residual loadcarrying capacity of timber structural elements with cracks? The residual load-carrying capacity of such members was investigated by numerical simulations based on existing crack situations gathered from real structures. Therefore an investigation was conducted about the most frequent characteristics of timber structures including the elements and encountered crack distributions. The parameter studies carried out include different distributions and parameters of cracks and support the compilation of guidelines of the practitioners. In addition, experimental test series were carried out to investigate the influence of cracks on the stiffness and the load-carrying capacity and to validate the numerical model and analytical approaches. Main characteristics could be found and different models could be provided for the estimation of the residual stiffness and load-carrying capacity of cracked timber members showing one main crack. Norway spruce glulam beams with artificial horizontal slits of different length and depth were reinforced using self-tapping screws and threaded steel rods in order to restore their load-carrying capacity and stiffness. The study aimed at evaluating the effects of strength and stiffness of the applied reinforcing elements on the load-carrying capacity and stiffness of glulam beams after retrofitting. Self-tapping screws and threaded steel rods of different diameter have been evaluated in the study and different numbers of reinforcing elements have been applied. Shear failure of the beams with artificial slits of different depth was provoked in loading cycles with stepwise installation of the reinforcing elements in the beam parts failed in the preceding test. The reinforcing effect of the tested self-tapping screws and threaded steel rods reached and partly exceeded the estimated level calculated with selected analytical models. Unfavourable structural behaviour arose in some cases from crack opening during installation of the rods causing a very low initial stiffness. Comparison of test results to calculations of stiffness and loadcarrying capacity of the reinforced beams applying the ? method, the shear analogy method and a truss model revealed that all methods provided good estimates of strength / stiffness of the reinforced beams and can be applied to design the reinforcement. Further on, first orientation test series were setup to investigate the effect of climate changes/moisture content on the gradient over the cross-section. A differentiation in the three material axes as well as different sizes were respected for a controlled wetting process in a climate chamber. It could be evaluated that daily or weekly climate changes result in a change of moisture content only in the outer zone of the cross-section; therefore, a distinction can be made in active and passive zone where a lower service class could be applied. As first proposal, a differentiation of the service class over the cross-section could be used for the activation of existing load capacities (active zone = SC 3, passive zone = SC 2) and, therefore, increase the capacity.

Datenbankreferenzen (Englisch)

Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. [phone] Swiss Project-Number: C12.0049