Design and qualification of timber buckling restrained brace for mass timber-braced frames

A ductile lateral force resisting system (LFRS) for mass timber frames is proposed utilizing Mass Timber Buckling-Restrained Braces (MT-BRBs), with mass timber as the primary casing material. Compressive tests were conducted on Mass Plywood Panels (MPP) with and without horizontal self-tapping screws inserted perpendicular to laminations to assess differences in stiffness, peak load, and maximum displacement. Screws perpendicular to lamination enhanced performance, increasing the peak load by nearly four times and peak displacement by more than three times. Withdrawal tests were also performed on timber blocks with screws installed parallel or perpendicular to laminations. The withdrawal force when screws were placed parallel to lamination was 0.68 times that of the force when screws were placed perpendicular to lamination. Seven MT-BRBs 5.5 m (18 ft) long with a targeted tensile yield strength of 293.5 kN (66 kip) were designed and tested using two loading protocols; a fatigue protocol and a high strain protocol. Six braces were built using steel bolts to assemble the mass timber casing and screws were utilized perpendicular to lamination; the seventh brace used screws to assemble the mass timber casing and screws were utilized perpendicular to lamination. For the fatigue loading protocol, the cumulative inelastic demand (CID) achieved by the TBRBs was 2.9–6.9 times the value required by AISC 341. The maximum strain in the steel core for all six tests exceeded 13.3 times the yield strain, indicating excellent ductility. The MT-BRB assembled with screws did not achieve stable hysteresis curves and failed prematurely. The MT-BRBs assembled with bolts exhibited stable hysteretic behavior with a maximum strain of 3.1 %, which corresponds to an inter-story drift of 4.9 % for the prototype mass timber braced frame. The novelty introduced in this paper is the elimination of external steel plates or carbon fiber wraps used to postpone splitting of the timber casing; instead, screws perpendicular to lamination were utilized that were successful in postponing premature splitting failure of the MPP casing. In addition, the MPP casing was built without any hardwood spacers, or internal steel plates by incorporating a profile in the MPP casing thus simplifying construction. The fastener design of the MT-BRB was developed for both bolts and screws specifically for an MPP casing, and the overall MPP casing dimensions were checked against the NDS specifications for laminated built-up columns. • Screws perpendicular to lamination postponed splitting of mass plywood panel (MPP). • Mass timber buckling restrained braces (MT-BRB) achieved a maximum strain of 3.1 %. • MT-BRB built with an MPP casing and steel bolts had stable and ductile performance. • MT-BRB built with an MPP casing and screws did not perform well.