Serviceability is the name of the game with floor slabs that will have lift truck traffic. The most vulnerable places on such a floor slab are the joints. The joints break down when a lift truck moves toward the joint, deflecting down the edge of the slab panel it is on, then bumping against the joint face of the adjacent, slightly higher, panel.
Relying on aggregate interlock for long-term load transfer at the contraction joints of such slabs is impractical, as we have previously noted (see Ref. 1 and 2). The American Concrete Institute (ACI) publications have been recommending dowels at joints for a number of years. ACI 360R-06 “Design of Slabs on Ground” (see a summary of this new document) states that “Doweled joints are recommended when positive load transfer is required,” and ACI 302.1R-96 and ACI 302.1R-04 “Guide for Concrete Floor and Slab Construction” have similar recommendations.
Most slab thickness design procedures assume that load is transferred between adjacent slab panels. Our experience is that to protect the joints proper load transfer is especially important when significant lift truck traffic is anticipated. Thus, doweled contraction joints should be used to minimize joint spalling due to lift truck traffic, minimize lift truck maintenance cost, and share the load to prevent the higher stresses resulting from the loading of free edges. But when dowels are used, the slab designer should consider the properties of the dowel system specified, which include its geometry, installation tolerances, and bond-breaking material, along with the cost of the dowel system. If only one of these properties are compromised, then severe and costly problems could occur.
This article is a continuation of an article we wrote in 1998 (see Ref. 3), where we discussed the many benefits of plate dowels. Tapered plate dowels have been in use for over four years on a number of projects. In this article, we will discuss the benefits of using tapered plate dowels in contraction joints and provide design recommendations for the size and spacing of these dowels for industrial floors to accommodate lift truck loadings. These design recommendations are based on both strength and serviceability criteria for lift truck loadings and are more rational than the traditional method of selecting the dowel size and spacing based on slab thickness.
Historical dowel design
Most of the significant dowel research and corresponding recommendations (such as in References 4 and 5) were done in the 1940s and 1950s. These recommendations were for round dowels and for highway traffic loadings with wheels spaced 5 to 9 feet apart. The dowel recommendations in ACI 302.1R-04 are based on these highway types of loads and may not be conservative enough for some lift truck loadings, while being too conservative for some other types of loads. For industrial floor slabs where lift trucks will be used, the wheel loads can be higher than on a highway—the tires are a hard solid material (as opposed to the large, soft pneumatic tires used for highway traffic), the load contact area is over a smaller area (due to the hard solid tire material), and the wheels are at a much closer spacing (18 to 42 inches).
The recommendations for round dowels for highway traffic loadings were developed with the objective of limiting the bearing pressure of the dowel on the concrete. But there are other dowel design requirements that are important for industrial floors slabs with lift trucks, such as the relative deflection between the slab panels, the effect of curling on the deflection of slab panels with dowels, and how curling affects the distribution of the force in the dowels due to the wheel loads. None of these were considered.