The railing crash resistance was determined using the provisions of SA13.3.1. The dimensions of the parapet are shown in Figure 4-2. Design Step 4.4 Concrete ParapetĪ Type-F concrete parapet is assumed. sacrificial layer is assumed in the design. Using a deck overhang thickness of approximately ¾" to 1" thicker than the deck thickness has proven to be beneficial in past designs.įor this example, an overhang thickness of 9 in., including the ½ in. (S13.7.3.1.2), unless a lesser thickness is proven satisfactory through crash testing of the railing system. Design Step 4.3 - Overhang Thicknessįor decks supporting concrete parapets, the minimum overhang thickness is 8 in. However, for resistance calculations, the integral wearing surface is assumed to not contribute to the section resistance, i.e., the section thickness for resistance calculations is assumed to be 7.5 in. The integral wearing surface is considered in the weight calculations. Applying these provisions to the design of deck slabs rarely controls deck slab design.įor this example, a slab thickness of 8 in., including the ½ inch integral wearing surface, is assumed. The provisions in this article are meant for slab-type bridges and their purpose is to limit deflections under live loads. In addition to the minimum deck thickness requirements of S9.7.1.1, some jurisdictions check the slab thickness using the provisions of S2.5.2.6.3. Most jurisdictions require a minimum deck thickness of 8 in., including the ½ inch integral wearing surface. For slabs with depths less than 1/20 of the design span, consideration should be given to prestressing in the direction of that span in order to control cracking. Thinner decks are acceptable if approved by the bridge owner. The specifications require that the minimum thickness of a concrete deck, excluding any provisions for grinding, grooving and sacrificial surface, should not be less than 7 in. The Empirical Design Method requires less reinforcement in the interior portions of the deck than the Approximate Method.įor this example, the Approximate Method (Strip Width Method) is used. The overhang region is then designed for vehicular collision with the railing system and for dead and live loads acting on the deck. No other design calculations are required for the interior portions of the deck. Once these limitations are satisfied, the specifications give reinforcement ratios for both the longitudinal and transverse reinforcement for both layers of deck reinforcement. Certain limitations on the geometry of the deck are listed in S9.7.2. This testing indicates that the loads on the deck are transmitted to the supporting components mainly through arching action in the deck, not through shears and moments as assumed by traditional design. The Empirical Design Method is based on laboratory testing of deck slabs. Shear and fatigue of the reinforcement need not be investigated. The reinforcement is designed to resist the applied loads using conventional principles of reinforced concrete design. The loads transmitted to the bridge deck during vehicular collision with the railing system are determined.ĭesign factored moments are then determined using the appropriate load factors for different limit states. The total moment is divided by the strip distribution width to determine the live load per unit width. Multiple presence factors and the dynamic load allowance are included. The truck axle loads are moved laterally to produce the moment envelopes. The width of the strip for different load effects is determined using the equations in S4.6.2.1. The strip is assumed to be supported on rigid supports at the center of the girders. The equivalent strip method is based on the following:Ī transverse strip of the deck is assumed to support the truck axle loads. The second is called the Empirical Design Method (S9.7.2). The first method is called the approximate method of deck design (S4.6.2.1) and is typically referred to as the equivalent strip method. The AASHTO-LRFD Specifications include two methods of deck design. This signifies that, at this level of loading, damage to the structural components is allowed and the goal is to prevent the collapse of any structural components. The resistance factor at the extreme event limit state is taken as 1.0. In addition to designing the deck for dead and live loads at the strength limit state, the AASHTO-LRFD specifications require checking the deck for vehicular collision with the railing system at the extreme event limit state. Comprehensive Design Example for Prestressed Concrete (PSC) Girder Superstructure Bridge Design Step 4 Deck Slab Design
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