Time-dependent load performance of notched wood-concrete composite beams
Researchers at Colorado State University (CSU) are examining the feasibility of using wood-concrete composite floor/deck systems as an alternative to concrete floor slab systems. The primary aim is to show that a structurally effective, durable solid wood-concrete layer can replace the cracked portion of the concrete slab and its rebar. Concrete needs a companion material to account for its lack of tension carrying ability. Wood is good in tension, if tension defects such as knots do not exist. Since the wood layer deck can replace the formwork for a solid slab, the gain is to leave it in place and use it structurally to reduce the concrete slab thickness by about 50% by interconnecting the wood and concrete layers. Ceccotti (1995) has closely examined wood-concrete flooring systems and provided a summation of many of the benefits compared to light frame wood floors. These include: 1) reduction of the vibration problems associated with timber structures, 2) an improvement of the sound insulation, 3) better fire resistance, 4) better seismic behaviour, and 5) reduction in the likelihood of catastrophic failure. The competitive merit of such mixed construction is borne out by several examples of successful commercial projects in Europe (Natterer, 1998). The concept has also been tried in the reconstruction of timber floors and ceilings (Toratti and Kevarinmaki, 2001). Layered wood-concrete composites exhibit composite behavior that is it is bounded by two extreme limits. The upper limit (‘fully composite’) is when the interface between the layers is considered perfectly bonded and allows no relative motion (‘slip’). The lower limit (‘non-composite’) is when the layers are completely unbounded with neither mechanical bond nor friction taking place between the two layers. For non-composite behavior, no interlayer shear transfer takes place. The actual circumstance is that the layer beam exhibits partial composite behavior while also experiencing slip between the layers. Actual systems are stiffer than the non-composite limit state while less stiff than the fully composite state. Prior to conducting load tests of layered floor/deck specimens, preliminary load tests of layered wood-concrete beams where done by Fast et al. (2003). This included subjecting specimens to either creep tests and/or cyclic loading tests which simulate their typical service life. Results were used to evaluate the efficiency of the beam specimens as related to the degree of partial composite action achieved.