A cellulose-based structural building panel assembly includes a cross-laminated timber (CLT) core which is reinforced with one or more post-tensioned tendons stressed to a pre-selected tensioning force, following the placement as part of the panel assembly. The tendons are provided within a sleeve which is grouted with a channel formed in an underside of the core and which after post-tensioning of tendons is infilled with a binder securing the tendons in a fully bonded configuration.