An Introduction to Design of Concrete Slabs on Grade for Professional Engineers

De: J. Paul Guyer

Narrado por: Virtual Voice

There are several factors that have a considerable effect on determining the design thickness of a concrete slab-on-ground and can be divided into above and below slab-on-ground factors. The type, strength, and uniformity of the subgrade or subbase are the below slab-on-ground factors and are discussed. The type of loads, magnitude of loads, and frequency of moving loads travel are the above slab-on-ground factors and are addressed. The concrete mixture design and strength of the slab-on-ground is discussed. In this discussion, the key factor in the design of a slab-on-ground—its thickness, h. Three classical design methods and the finite element (FE) method are available to engineers for determining the slab-on-ground thickness. The classical design methods are: Corps of Engineers (COE), Portland Cement Association (PCA), and Wire Reinforcement Institute (WRI). They are based on continuous support to the slab-on-ground and uniform modulus of subgrade strength, k. The slab thickness calculation is insensitive to minor changes in k-values; therefore, it is not critical to obtain an accurate k-value. It is, however, directly related to the strength of concrete defined by the concrete flexural strength, fr, which is measured in accordance with ASTM C78. The COE and WRI methods use a value equal to 7.5, while PCA method uses a value equal to 9. For heavy loading, however, it is recommended to use the lower factor of 7.5 with a factor of safety equal to 2; refer to Section 3-10. The FE method, Section 5-12, unlike the classical methods, does allow for variable modulus of subgrade in the analysis. Slabs-on-ground are designed for the critical load or load combinations that generate the maximum stress and maintain surface cracking at an acceptable level. Typically, forklifts control the flexural design of slabs-on-ground, storage racks control punching shear and concrete bearing design, while distributed live loads control negative moment design in aisles between distributed loads, dowels at joints, and settlement of slabs-on-ground; refer to Figure 5-1 (Packard 1996). Accordingly, the SER will design the overall slab-on-ground thickness for the allowable moving load that controls flexural stresses. The SER will then check for punching shear of concentrated, line, and distributed loads at localized areas where constructed, applied, or both, verifies that excessive settlement does not occur below a stationary load, and ensures that cracks are not developed in aisles between distributed loads. The design procedure covers subgrade conditions, steel reinforcement, and various details such as jointing, dowels, and the inclusion of vapor retarder, if needed.