This paper presents a simple method based on modal response spectrum analysis to compute internal forces in structural elements belonging to gravity framing not part of the seismic force resisting system (SFRS). It is required that demands on these gravity load resisting system (GLRS) be determined according to the design displacement profile of the SFRS. The proposed new method uses the fact that if the linear stiffness properties of the GLRS not part of the SFRS have negligible values compared to those of the SFRS, only the latter will provide lateral resistance. Displacements of the GLRS then correspond to those of the SFRS alone. The new method is illustrated by computing the seismic responses of a symmetric and an asymmetric multi-storey reinforced concrete building. These results are compared to those obtained from the application of the simplified analysis method proposed in the Canadian standard for the design of concrete structures. Nonlinear time history analyses are also performed to provide a benchmark for comparison. Results show that the new method can predict shear and bending moment in all members at once with ease. Therefore, this new simplified method can effectively be used to predict seismic forces in elements not considered part of the SFRS.

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The Yukon River Bridge, a two-span truss bridge on the Alaska Highway, was damaged by a traffic accident in January 1982. This paper describes the actions taken to repair and strengthen the damaged structure. Highlighted in the paper is the repair of a fractured bottom chord, which opened up 2 in. (50.8 mm) after the accident. Field measurements of member forces taken during the repair operation and at proof load tests carried out after the repairs are presented. Structural responses obtained from theoretical computer analyses are compared with the field-measured values. Results of physical and chemical laboratory tests performed on samples of the bridge steel are reported, and lessons learned from the project are discussed.Le Font sur la rivihre Yukon, pont double travke de I'autoroute de ]'Alaska, fut endommagk par un accident de la circulation en janvier 1982. Cet article dtcrit les actions prises afin de rkparer et renforcer la structure endornmagke. L'article insiste particulihrement sur la rkparation de la rupture de la membrure infkrieure qui, aprks l'accident, ttait ouverte de plus de 2 pouces (50,8 mm). L'article prtsente les valeurs des forces agissant dans les barres du treillis mesurkes durant la rkparation et durant les essais de mise en charge. Les rksultats obtenus d'une analyse thtorique par ordinateur sont comparks aux valeurs mesurtes sur le site. Les rtsultats des essais mkcaniques et chimiques realists sur des kchantillons d'acier du pont sont donnCs, et les conclusions a en tirer sont discutkes.