Strengthening of Curved-Chord Wooden Lattice Trusses with Post-Tensioned Steel Rods

摘要

This paper discusses strengthening and remediation of a roof structure at the Massachusetts Institute of Technology’s (MIT) W33 Cage Building. Twenty-four, curved-chord wooden lattice roof trusses comprise the roof structure of this athletic facility that has been in service for over 50 years. The trusses, which span 100 ft and are predominantly constructed of ‘two-by’ (2x) dimension lumber, had become a concern due to multiple, local failures of the bottom chord. To avoid future safety concerns and the continuation of costly maintenance associated with repairing the fractures locally, Simpson Gumpertz and Heger (SGH), on behalf of MIT, designed a global remediation and strengthening scheme for the trusses. The main feature of this repair was the installation of post-tensioned steel rods designed to relieve the truss bottom chords of overstress due to code-prescribed snow loads. Custom fabricated, steel brackets were installed at the heel joints to transfer the top-chord loads into the new steel tension rods. Strain gages were employed to monitor the transfer of dead load into the steel rods during initial post-tensioning. The post-tensioned strengthening scheme provided an economic solution for prolonging the service life of the truss structure and efficiently utilizing other existing portions of the roof structure.