Traditional Design Method (Equivalent Strip Method)
The Traditional Design Method, typically referred to as the Equivalent Strip Method, is based on flexure of the deck in the transverse direction. The equivalent strip method applies to concrete deck slabs that are at least 7 inches thick, have sufficient concrete cover, and have four layers of steel reinforcement, with longitudinal and transverse layers at both the top and bottom of the deck slab. In a typical girder bridge the longitudinal direction of the deck is parallel to the main supporting girder, and the transverse direction is perpendicular to the main supporting girder. If the deck is only supported by the main supporting girders, then the deck is typically designed for primary reinforcement in the transverse direction, and that primary reinforcement is perpendicular to the direction of traffic.
The equivalent strip method assumes a transverse strip of deck supports the truck axle loads. The transverse strip is to be treated as a continuous beam, or simply supported beam as appropriate, assuming pinned supports at the centerline of each girder web. The deflection of the beam is assumed to be zero for this design procedure. The width of the strip is determined in accordance with AASHTO LRFD (7th Edition, 2014) Article 4.6.2.1. As shown in Table 4.6.2.1.3-1, a different equivalent width is used for the overhang, and for positive and negative moment regions of the deck.
To determine live load effects, the strip can be analyzed with classical beam theory, moving truck axle wheel loads laterally, along the transverse strip, to produce moment envelopes. Multiple presence factors and the dynamic load allowance (impact) should also be included. Article 4.6.2.1.6 of the AASHTO LRFD (7th Edition, 2014) allows the axle wheel loads to be considered as concentrated loads, or as patch loads whose length along the span is taken as the length of the tire contact area plus the depth of the deck. The tire contact area should be computed in accordance with AASHTO LRFD (7th Edition, 2014) Article 3.6.1.2.5.
The primary reinforcement, along the transverse strip is designed using conventional principles of reinforced concrete design, similar to a one-way slab. The design location for maximum positive moment is at the location of the maximum positive moment. However for negative moment design, the design location for a typical steel girder bridge can be taken at a point that is located at one-quarter of the flange width, measured from the centerline of the support, in accordance with Article 4.6.2.1.6. In bridges where the flange width varies, to be conservative, designers will typically use the smallest flange width to determine the negative moment design location.
In lieu of more precise calculations, unfactored design live load moments for many practical concrete deck slab spans can be found in Table A4-1 of the AASHTO LRFD (7th Edition, 2014). In this table, the design live load moments are provided as a function of girder spacing (S). Multiple presence factors and the dynamic load allowance (impact) are included in the tabulated values shown in Table A4-1. Interpolation is permitted between the girder spacings and design sections provided in the table. The tabulated values are not to be used for the design of the deck overhang.
The use of the equivalent strip method also requires that distribution reinforcement be placed in the secondary direction in the bottom of the slab, per Article 9.7.3.2. The amount of distribution reinforcement is based on a percentage of the primary reinforcement required to resist the positive moment in the primary direction, along the transverse strip. For primary reinforcement placed perpendicular to traffic, this secondary reinforcement in the bottom of the slab shall be taken as a percentage of the primary reinforcement equal to 220/S0.5, but does not need to be greater than 67%, where S is the effective span length and is equal to the effective length specified in Article 9.7.2.3.
The amount of reinforcement in the secondary direction in the top of the deck slab depends on whether the deck slab is in an area in which the main supporting girders are subjected to negative or positive flexure. If the deck slab is in an area of positive flexure, nominal reinforcement such as #4 bars spaced at 12 inches may be required. However, if the deck slab is in an area of negative flexure, additional steel reinforcement is required per Article 6.10.1.7, as discussed later within this section. This additional steel reinforcement may affect both the top and bottom reinforcement in the secondary direction.