1. Introduction
1.1 Introduction
This report was derived from ongoing research conducted by researchers at Colorado State University (CSU). The project focuses on the innovative use of layered wood-concrete members for bridge structures. Current research at CSU involves composite wood-concrete beams of various depths. These beams consist of a bottom layer of dimension lumber and a top layer of reinforced concrete. The two layers are interconnected by a shear key / anchor detail that provides inter-layer force transfer. A primary focus of the research is to quantify the degree of composite action achieved by the layered beam system with notch type connections. Figure 1.1 shows the common definitions of complete, partial, and zero composite action.
Figure 1.1 Composite Action Definitions
In this report, the degree of composite action is calculated for several composite wood-concrete beam specimens. A high degree of composite action is desired in layered wood-concrete beams because it leads to both reduced deflections and increased load-carrying capacity. In a simple bending member as shown in Figure 1.1, the bottom outermost fibers at mid-span are stressed in tension, whereas the top outermost fibers at mid-span are stressed in compression. The wood-concrete beam member is an effort to combine the compressive strength behavior of concrete with the tensile strength behavior of timber to provide an improved composite beam.
When complete composite action is realized the layered beam acts as a one-layer beam with mixed material properties. In this case the beam is stressed such that all or most of the concrete is in compression and all or most of the timber is in tension, depending on the depth of each material. Also there is complete transfer of stresses between the two layers on the layer interface, and no interlayer slip (relative horizontal movement) occurs (Figure1.1 a). Complete composite action is the most efficient combination of the two materials in a layered beam configuration. Conversely, when the beam has no composite action, the behavior of the wood-concrete beam is that of an individual concrete beam deflecting on top of an individual timber beam. In this case, the concrete beam and the timber beam are both stressed in a combination of tension and compression. Furthermore in beams with no composite action there is no transfer of stresses between the two layers, and there is large relative movement of the concrete layer with respect to the wood layer, i.e. inter-layer slip occurs (Figure 1.1 c).
When connections are made between the concrete layer and the timber layer, partial composite action is developed. Although the different layers are stressed both in tension and compression, the situation is better than that for the case where there is no composite action. More of the concrete is stressed in compression, and more of the wood is stressed in tension. Interlayer slip does occur but it is smaller in magnitude than the slip developed in the beam with no composite action. Thus the case of partial composite action falls between the limits of no composite action (worst performance) and complete composite action (best performance). Throughout the remainder of this report the performance of the wood-concrete beams tested at CSU are discussed in terms of composite action and a closely related measure of composite efficiency.
The method of concrete-wood interconnection used in the CSU research consists of a notched shear key. The notched shear key transfers forces between the two layers via bearing contact on one side of the notch. The notch shape is trapezoidal, with inclined bearing surfaces to mitigate stress concentrations at the corners of the notch. As a result of the inclined bearing surfaces a non-zero vertical component of the bearing force develops. To equilibrate this vertical force component, and to close any gaps between the two layers in the notch, an anchor connection is used. Many types of anchor connections exist, but in this report only two types (glued-dowel, and mechanical) are examined.
On average, the glued dowel connectors were 3/8" diameter threaded rod with a length equal to the concrete depth plus the notch depth plus roughly half the wood layer depth plus roughly 1/2". The mechanical connectors were made of the same threaded rod with a rough length equal to the beam depth plus 1/2". The modulus of elasticity of the Hilti dowel is specified by the fabricators as 2.1 x 105 N/mm2 or 30.46 x 103 kip/in2.
Figure 1.2 Notched Shear Key Connection Free Body Diagram
Some prior CSU research studies concentrated on the application of wood-concrete composites to floors in commercial buildings. These research studies included laboratory tests of a large number of small dimension beam specimens consisting of nominal 2" x 4" (3.81 x 8.89 cm dressed size) or 2" x 6" (3.81 x 13.98 cm dressed size) wood layers with a 5.08 cm - 7.62 cm (2"- 3") thick concrete layer. Tests of several thin-layered deck specimens of the same dimensions have also been completed at CSU. For bridge applications, higher flexural and shear resistance are required and deeper layered systems and stronger interlayer connections are needed. Originally a test of a full width bridge deck was planned, but it was determined that insufficient load carrying capacity was available in the existing test frame. Consequently, as a compromise a study was conducted of the behavior of wide beam specimens. These specimen dimensions were configured to be consistent with those that would be needed in a full bridge deck. Also, two much deeper beam specimens were prepared and tested for potential use in timber trestle bridge chords. These deep beam specimens involved nominal 8" x 8" (18.42 x 18.42 cm dressed size) solid sawn wood members and a 25.41 cm (10") thick concrete layer.
1.2 Overview of the Research
The study described was conducted from May 2001 to August 2003. Two different types of beams were constructed. The first beam type, referred to as a deep beam (DB), had a solid sawn Douglas-Fir wood layer. The second beam type, referred to as a wide beam (WB), had a bottom wood layer made from either 16 or 24, 5.08 cm x 25.41 cm (2" x 10") Hem-Fir No. 2 dimension lumber boards laterally nailed together. In all specimens the concrete layer was cast from premixed commercial concrete having a specified compressive strength of 2.41 kN/cm2 (3500 psi). The concrete layer was reinforced according to the shrinkage and temperature effects given in ACI 318 99 concrete building code (ACI 1999). In all specimens the layers were interconnected using shear-key notches grooved out of the wood layer (Figure 1.3). For vertical anchorage either a dowel or mechanical connector was used (Figure 1.3). One type of connector (Glued-Dowel type) consisted of a short threaded rod glued into the bottom of the notch in the wood layer. The other type of connector (mechanical type) was a long threaded rod extending through a hole drilled in the wood layer and attached to a steel bearing plate nailed to the bottom surface of the beam. In both cases a plastic sleeve was placed around the portion of the connector surrounded by the concrete layer. This was done to prevent bonding of the concrete with the steel connector, as it is necessary to tighten the connections after the concrete cures. The two different connector types are shown in Figure 1.3 with the notch dimensions of the wide beam specimens. The notch configurations for the deep beam specimens are identical to those shown in Figure 1.3 except that the depths of the wood and concrete layers are different.
Figure 1.3 Notch Configuration (cross-section detail)
1.3 Description of the Deep Beam Specimens
Figure 1.4 shows the main longitudinal view, end view, and notch placement of the deep beam specimens. The beams had a span of 12.0 ft on simple supports. For each of the two deep beams the wood layer was comprised of a single surfaced dry 18.42 cm x 18.42 cm (7.25" x 7.25", i.e. nominal size 8"x 8") solid sawn Douglas Fir No. 1 timber. Each of the deep beams used a different type of notch anchor connection. Specimen 1, utilized the glued dowel type connection, and specimen 2 utilized a mechanical type connection. The moisture content of the timber was measured using a Delmhorst R-2000 electrical resistance moisture meter at a penetration depth of 3.81 cm (1.5"). Measured values were found to be less than 6 percent.
Figure 1.4 Deep Beam Dimensions (cross-section and end-view)
Two notches were made at each end of the beam, placed as shown in Figure 1.4. The glued-dowel notch connections utilized a 12.7 mm (1/2") diameter hole pre drilled to a depth of 101 mm (4") into the wood layer of the notch. The hole was then tapped with a 15.98 mm tap and the dowel connectors were set in place after applying a HIT HY 150 epoxy adhesive into the tapped hole.
The mechanical dowel connection used a 12.7 mm (1/2") diameter hole pre-drilled through the entire depth of the wood layer. The dowel consisted of a threaded rod welded to a steel bottom plate. The dowel was put in place by inserting the threaded rod up through the bottom of the hole. Then, the dowel was fixed in place by screwing the steel plate to the bottom side of the beam using wood screws.
The construction of the deep beams was completed by casting a 15.88 mm (6.25") layer of reinforced concrete over each of the wood layers. The concrete was properly consolidated using a mechanical vibrator, with special attention focused at consolidating the concrete in the notches. Shrinkage and temperature steel reinforcement was placed to satisfy the requirements of the ACI 318 99 concrete building code (ACI 1999). Placement of the reinforcement for the notches of the Deep Beams is shown in Figure 1.3. Placement of the reinforcement for the Deep Beams is illustrated in Figure 1.4. After allowing the concrete to cure for 28 days a 70 N m (50 lb ft) torque was applied to all the connector nuts to re-tighten the notch connections.
1.4 Description of the Wide Beam Specimens
The wide beams were similar to the deep beams, but had a longer span and thus larger dimensions. A drawing of the wide beam specimens is shown in Figure 1.5. The beams were 6.17 m (20 ft.) long with a 15.25 cm (6") deep concrete top layer and a 22.87 cm (9") wood bottom layer. The wood layer was constructed of 16 to 24 3.81 cm x 23.50 cm (nominal 2"x10") grade No. 2 Hem-Fir dimension lumber boards laterally nailed together. Two beams (wide beam No. 1 and wide beam No. 2) were 91.48 cm (3 ft.) wide, while the other two beams (wide beam No. 3, and wide beam No. 4) were 60.98 cm (2 ft.) wide. Each beam had six shear notches spaced evenly along the length of the beam. Notches were cut through the entire beam width, and notch dimensions were varied with respect to the type of inter-layer connection used in each notch (See Figure 1.3). Beams No. 1 and No. 4 had the glued dowel-type connections at the shear notch locations. Beams No. 2 and No. 3 had mechanical connections. The notches corresponding to the dowel connections were each 5.08 cm (2") deep with a 11.43 cm (4.5") bottom width and a 15.25 cm (6") top width. The notches corresponding to the mechanical connections were 3.81 cm (1.5") deep with a 12.71 cm (5") bottom width and a 15.25 (6") top width. A detail of the notch configurations for the wide beams was previously shown in Figure 1.3. The placement of the notched shear keys in the wide beam specimens is shown in Figure 1.5. Note that two different widths were used for the wide beam specimens, namely 60.98 cm (2 ft.) and 91.48 cm (3 ft.). Figure 1.5 is representative of only the 60.98 cm (2 ft.) wide specimens. Except for their width, the 91.48 cm (3 ft.) wide specimens have all other dimensions identical to those shown in Figure 1.5. The lateral spacing of the notch anchor connections was 33.03 cm (13") for the 60.98 cm (2 ft.) wide specimens, and 49.55 cm (19.5") for the 91.48 cm (3 ft.) wide specimens.
Figure 1.5 Wide Beam Dimensions (cross-sections and end-views)