Formwork for Concrete Slabs
Conventional wood systems for horizontal concrete work are made of plywood or lumber sheathing for decking. As it will be discussed in Chapter 3, the thickness of plywood or lumber is determined by structural analysis and is a function of the applied loads, type of wood or plywood, and the spacing between sheathing supporting elements. Plywood is preferred over lumber sheathing because it provides a smooth concrete surface that requires minimum finishing effort. The use of plywood for decking is also productive because of its large panel size (4 X 8 in.) (1.22 X 2.44 m).
Sheathing is supported by horizontal members called joists or runners. Joists are made from dimension lumber spaced at constant intervals that are a function of applied loads and the type of lumber. It is a recommended practice to round down the calculated joist spacing to the lower modular value.
Joists are supported by another set of horizontal members perpendicular to the joists, called stringers. The stringers are supported by vertical members called shores. In all-wood conventional formwork systems, shores are made of dimension lumber that have square cross sections [i. e., 4 X 4 in. (101.6 X 101.6 mm) or 6 X 6 in. (152.4 X 152.4 mm)]. Shores are rested on heavy timbers, called mudsills, to transfer the vertical loads to the ground. In the case where a slab-on-grade exists, shores are rested directly on them. Figure 2.1 shows a typical all-wood conventional formwork system for concrete slabs.
Vertical timber shores can be replaced by the scaffold type, which has been proven to be more efficient because of its high number of reuses and its height, which means that no splicing is typically required. The scaffold-type shoring system consists of two vertical steel posts with horizontal pipe between them at regular intervals. Adjustable screw jacks are fitted into the steel posts at both ends. The top jacks are fitted into steel caps called tee – heads. The bottom jacks are fastened into rectangular steel plates. Adjacent vertical steel posts are braced together by steel X braces. Figure 2.2 shows a typical scaffold-type shoring system.