Massive timber floor system evaluated at Clemson University
Research at Clemson University is being conducted on an innovative, hollow massive timber floor system derived from a modification of solid Cross Laminated Timber that will be able to meet the stiffness requirement of longer spans. This floor system consists of two smaller 3-ply solid CLT panels that act as the top and bottom surface (or floor and ceiling) of the hollow panel. These two panels are separated by a glulam beam on an approximate 36″ o.c spacing. All three of these elements have to be connected together in order for them to act as one floor panel that resists out-of-plane bending loads. By making the cross-section deeper and removing extra wood in the middle that does not give much additional strength to the panel, a much more efficient cross-section is made in terms of wood use.
During this past Summer, Clemson researchers developed and ran computer models of the hollow panel in order to learn more about the mechanics of the panel and to determine what influences the strength and stiffness of the floor system the most. A parametric study was done by varying many variables in order to see the effects of geometry of the cross-section, quality of the boards in each layer and the connection method used to connect the two outer panels. It was found that the strength and stiffness of the connector was the most influential value. Therefore, we decided to run shear tests on many different connectors in order to find the best one for our panel.
This Fall, the Clemson team manufactured many CLT panels out of Southern Pine on a 4’x4′ Newman press. The boards for each panel were planed to 1.375″ and each panel was pressed at 150 psi to ensure a good glue bond was achieved. These panels were then made into shear test specimens to test the different shear connectors. Various shear connectors were used including large screws which were placed at an angle, glue bonds with different board layups, and steel mesh which is slotted and glued in slits in the wood. When ready for testing the specimens are placed under an actuator that presses on the glulam in the middle which is offset upward. This causes shear to go through the connectors on both sides. All specimens are tested to failure and the displacement at the connection is monitored so that the connectors’ stiffness can be obtained. You can see a video summary of tests here.
Overall, the glue bonds have proven to be very stiff and strong, but the strength can vary with the quality of the glue bond. The screws have been very predictable and hardly varied any, but while they are very stiff, they are less stiff than the glue connection.
This project officially began on October 1, 2012. Immediate need is to find a large press that will allow us to cold press a CLT panel, a minimum of 4′ x 8′. Both Clemson and NC State are looking for graduate students to begin working on the project starting January 2013.