Abstract: A mortise and tenon joint for I-beam members. The joint includes an I-beam post and an I-beam beam that interlocks with the I-beam post. The I-beam post and the I-beam beam are each made of fiber reinforced pultruded polymer composite. The web of the I-beam post extends past the flanges of the I-beam post so as to form a tenon. The flanges of the I-beam beam have a pair of aligned throughslots so as to form a mortise that snugly receives, and is filled by, the tenon. The tenon has a plurality of throughbores through the pair of opposing faces thereof. The web of the I-beam beam has a plurality of throughbores that are aligned with the plurality of throughbores in the tenon when the tenon is snugly received in the mortise. Bolts extend through first washers, through the plurality of throughbores in the web of the I-beam beam, through the plurality of throughbores in the tenon, through second washers, and threadably engage in nuts so as to maintain the tenon in the mortise.