Live Load Distribution for Slab bridges
One of the common types of deck sections in bridge structures is the solid concrete slab bridges, These sections are frequently used for short spans , usually less than 50ft,The slab bridge does not have any girders, and therefore, the load must be carried principally by flexure in the longitudinal direction.
The analysis and design of any highway bridge must consider truck and lane loading. However, truck-loading provisions govern for short-span structures when considering AASHTO standard specifications.
According to both AASHTO LRFD and AASHTO STD specifications, one of the methods for slab bridge analysis is the “Equivalent Strip Widths for Slab-Type bridges”
AASHTO specifies a distribution width for high- way loading or an empirical formula to reduce the two-way bending problem into a beam (one-way) bending problem, Therefore, reinforced concrete slab bridges are typically designed as a series of beam strips.
AASHTO Standard Specification:
concrete slab bridge is designed according to the provisions for main reinforcement parallel to traffic. The AASHTO design procedure was originally developed in the 1940s, based on the re- search work of Westergaard (1926, 1930) and Jensen (1938 193) For simply supported slab bridges, AASHTO standard specifications suggest three approaches to determine the live-load bending moment for HS20 loading:
1.AASHTO section 3.24.3.2 provides empirical equations:
Where :
S: Span length [ft in Eqs. (1a) and (1b) or m in Eqs. (2a) and (2b)]
M: Longitudinal bending moment per unit width [lb-ft/ft in Eqs. (1a) and (1b) or N.m/m in Eqs. (2a) and (2b)]
2.AASHTO Appendix A gives the live-load bending moment per lane for a span length up to 90 m (300 ft). The live-load bending moment per foot of width is obtained by dividing this value by twice the distribution width E:
Analysis and design of a unit wide strip using the appropriate wheel loads. For HS20 loading, the wheel loads are 18 kN (4 kips), 72 kN(16 kips), and 72 kN (16 kips) with axle spacing of 4.2 m (14 ft). The appropriate wheel loads are then divided by the distribution width E [Eqs. (3a) or (3b)]. This approach is generally used for continuous spans and is currently adopted in the AASHTO LRFD design specifications
AASHTO Load and Resistance Factor Design Specifications
AASHTO LRFD section 4.6.2.3 provides an equivalent strip width to design slab bridges similar to the previous bridge specifications. This simplistic approach is to divide the total statical moment by the bridge width to achieve a moment per unit width for design. The moments are determined by establishing the structural width per design lane. The equivalent width E of longitudinal strips per lane for both shear and moment is determined using the following formulas:
The width for one lane loaded is:
where
E is in millimeters in Eqs. (4a) and (5a)[inches in Eqs (4b) and (5b)]
L1: span length in millimeters (feet) taken to be the lesser of the actual span or 18,000 mm (60 ft);
W1:edge-to-edge width in millimeters (feet) of bridge taken to be the lesser of the actual width or 18,000 mm (60 ft) for multi- lane loading, or 9,000 mm (30 ft) for single-lane loading
AASHTO LRFD (3.6.1.2) live load HL93 requires the consideration of lane loading plus design truck or lane loading plus tandem. The bending moment is determined for the design lane divided by the width (E) to determine the moment per unit length for design
References:
American Association of State Highway and Transportation Officials [AASHTO]2002. Standard specifications for highway bridges,17th Ed., Washington, D.C.
American Association of State Highway and Transportation Officials[AASHTO ] (2007). LRFD design specifications, 4th Ed., Washington,D.C.
M. Mabsout and et .al (2004) ,”Wheel Load Distribution in Simply Supported Concrete Slab Bridges” JOURNAL OF BRIDGE ENGINEERING © ASCE /MARCH/APRIL 2004
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