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Design: BRG

Date: 7/2010

SLAB DESIGN EXAMPLE
Design example is in accordance with the AASHTO LRFD Bridge Design Specifications, 5th Ed. (2010) as prescribed by TxDOT Bridge Design Manual - LRFD (May 2009). Design: 8" Slab with standard reinforcing Type Tx40 Prestressed Precast Concrete Beams (36" top flange) 9'-0" Beam Spacing, 3'-0" Overhang with a T551 rail"AASHTO LRFD" refers to the AASHTO LRFD Bridge Design Specification, 5th Ed. (2010) "BDM-LRFD" refers to the TxDOT Bridge Design Manual - LRFD (May 2009) "DM" refers to the TxDOT Bridge Detailing Manual (August 2001) "TxSP" refers to TxDOT guidance, recommendations, and standard practice.

TYPICAL SECTION
(DM, Ch. 5, Sect. 9, Typical Transverse Section Reinforcing)

8 in Slab thickness, 2 intop clear cover, 1.25 in bottom clear cover Bars A ~ # 5's @ 6" Bars B ~ # 5's @ 6" Bars T ~ # 4's @ 9" Bars D ~ # 5's @ 9"

(BDM-LRFD, Ch. 3, Sect. 2, Geometric Constraints) (BDM-LRFD, Ch. 3, Sect. 2, Design Criteria) (DM, Ch. 5, Sect. 9, Typical Reinforcing)

Deck Design
Criteria)

(AASHTO LRFD 9.7.1)

Use the Traditional Method in AASHTO LRFD 9.7.3 to design the slab. (BDM-LRFD, Ch.3,Sect. 2, Design

Use approximate analysis method of AASHTO LRFD 4.6.2.1. (AASHTO LRFD 9.6.1) For interior bays, use the unfactored live load moments in AASHTO LRFD Table A4-1. (AASHTO LRFD C4.6.2.1.6) For overhangs place one wheel load one foot from the rail. (AASHTO LRFD 3.6.1.3.1) Check the Service Limit State and the Strength Limit State. (AASHTO LRFD 9.5.2 & AASHTO LRFD 9.5.4) The ServiceLimit State is checked by the crack control limits. The live load deflection of the slab is satisfactory by inspection. The Strength Limit State is checked by checking the Ultimate Moment Capacity and the Minimum Steel Requirement (AASHTO LRFD 5.7.3.3.2). Fatigue need not be checked for concrete decks. (AASHTO LRFD 9.5.3) The Extreme Event Limit State (AASHTO LRFD 9.5.5) is satisfied through railcrash testing. (BDM-LRFD, C. 3, Sect. 2, Design Criteria) Check the distribution reinforcement in the secondary direction. (AASHTO LRFD 9.7.3.2)
LRFD Slab Design Example 1 July 2010

Effective Strip Widths:
The effective width of the strip is the width over which one axle of the design truck or tandem acts. To get the load per unit width, divide the live load by the effective strip width. (AASHTOLRFD
C4.6.2.1.3)

Alternately, the live load moments for the positive and negative regions in interior bays from AASHTO LRFD Table A4-1 can be used for design. (AASHTO LRFD C4.6.2.1.6) Therefore, we will use the live load moments from AASHTO LRFD Table A4-1 for the positive and negative regions in interior bays, and place one axle on the effective strip for the overhang.

Depiction ofTransverse Strip Widths
In the equations for strip widths, the values for "X" and "S" are in feet but the strip width resulting from the equations is in inches.

Overhang Region: Primary Strip Width: PrimaryStripOH Edge Strip Width: = 45  10 X

(AASHTO LRFD Table 4.6.2.1.3-1)

AASHTO LRFD 4.6.2.1.4c defines a transverse edge as a transverse strip along the beam that is located at the edge of theslab as shown above. The Article states, "The effective width of a strip, with or without an edge beam, may be taken as the sum of the distance between the transverse edge of the deck and the centerline of the first line of support for the deck, usually taken as a girder web, plus one-half of the width of strip as specified in Article 4.6.2.1.3." The intention of this article is to take aneffective strip width as half of the transverse strip width plus the additional slab width past the beam end. When the beams are parallel to traffic, the centerline of the first line of support for the deck is the line that intersects the ends of the beams at the center of the web. The additional slab width is the distance from the beam end to the center of the joint minus half of the joint width....
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