Empirical Design Method
The Empirical Design Method is based on experimental research of reinforced concrete deck slabs, and employs the notion that the deck behaves more like a membrane as opposed to a series of continuous beams (transverse strips). Experimental research indicates that the primary structural action by which concrete slabs resist concentrated wheel loads is not flexure, but a complex internal membrane stress state referred to as internal arching that distributes the live loads from the deck to the supporting girders. This internal arching occurs due to the cracking of the concrete in the bottom of the slab, in the positive moment region of the design slab, and the resulting upward shift of the neutral axis in that section of the slab. Membrane compressive stresses develop which transmit the vertical live load from the deck to the girders, relying on the lateral confinement at the girder that occurs with the use of a composite design and ties between girders, such as those provided by cross frames with top struts or top flange lateral bracing.
The internal arching can be thought of as an internal compressive dome. Failure will usually only occur when there is overstraining around the perimeter of the wheel footprint, and will be in the form of a punching shear. The internal arching action of the concrete alone cannot resist the full wheel load, but a small amount of isotropic reinforcement is more than adequate to resist this small flexural component. The isotropic reinforcement also creates a global confinement, which is required to produce the internal arching effects.
Per Article 9.7.2.4 of the AASHTO LRFD (7th Edition, 2014), the empirical design method can only be used if several limitations related to the geometric configuration of the concrete deck slab are satisfied. The empirical design method does not necessarily employ any design procedures, as the minimum reinforcement required is specified. The minimum amount of reinforcement is 0.27 in.2/ft of steel for the bottom layer in each direction and 0.18 in.2/ft of steel for the top layer in each direction. The steel reinforcement ratios correspond to a 7.5 in. thick deck slab, and may need to be adjusted if a thicker deck slab is used. Also, spacing of the steel reinforcement can not exceed 18 inches, and the steel reinforcement must have a yield strength of 60 ksi or greater. The empirical method can not be applied to cantilever portions of the deck slab.