1、 1 Investigation of AASHTO Live Load Reduction in Reinforced Concrete Slab Bridges F. El Me sk i 1； M. Mabsout 2 ； and K. Tarhini3 Journal of Bridge Engineering.Submitted July 30,2010；accepted Februray 24,2011；posted ahead of print March 2,2011；doi:10.1061/(ASCE)BE.1943-5592.0000237 1PhD Candidate,
2、Dept. of Civil and Environmental Engineering, American Univ. of Beirut； formerly, Project Engineer at Khatib and Alami, Beirut, Lebanon. Email: fme09aub.edu.lb 2Dept. of Civil and Environmental Engineering, American Univ. of Beirut, Lebanon. Email: mouniraub.edu.lb 3Dept. of Civil Engineering, US Co
3、ast Guard Academy, New London, CT 06320 Email: Kassim.M.Tarhiniuscga.edu Abstract: This paper presents the results of a 3D finite element study that investigated the effect of multi-presence factor of load reduction factors used in the AASHTO Bridge Design Specifications. Typical one-span, two-equal
4、-span continuous, simply supported, three- and four-lane reinforced concrete slab highway bridges were selected for this study. AASHTO HS20 design truck loads are first placed transversally in all lanes, positioned side-by-side and close to one edge of the bridge slab； this fully loaded condition se
5、rved as a reference case. Reduced loading patterns are then investigated using 3D FEA, with design loads in two out of three lanes (Reduced 2/3), three out of four lanes (Reduced 3/4), and two out of four lanes (Reduced 2/4). The longitudinal bending moments and deflection results obtained for the F
6、EA reduced and fully loaded bridges are directly compared. Furthermore, a correlation between the reduced load cases and AASHTO reduction factors or multiple presence factors is made for concrete slab bridges. For the three- and four-lane bridge cases, AASHTO Standard Specifications generally correl
7、ate well with or overestimate the FEA reduced maximum moments and edge beam moments by up to 15% and 30%, respectively. This over-estimation is more pronounced in short-span bridges. It is recommended that a reduction factor of 25% be applied only 2 for reinforced concrete slabs with span lengths gr
8、eater than 12 m (40 ft) and a reduction factor of 10% be applied for spans less than 12 m (40 ft). The AASHTO LRFD overestimated the maximum longitudinal moments and edge beam moments by up to 15% and 40%, respectively. This over-estimation by AASHTO LRFD, which is larger for longer spans, is increa
9、sed to 40% and 55% when compared to the FEA results due to reduced moments. This research supports the current AASHTO LRFD multiple-presence factors of 0.85 for three lanes and 0.65 for four lanes in estimating longitudinal bending moments in concrete slab bridges. The FEA results highlight the impo
10、rtance of considering span length in determining the multi-presence factors when designing three-lane or more concrete slab bridges. Thispaper will assist bridge engineers in quantifying the adjustment factors used in analyzing and designing multi-lane reinforced concrete slab bridges. Keywords: Con
11、crete slab bridges； Load reduction； Multi-lane multi-span bridges； Finite element analysis； AASHTO Standard Specifications and LRFD. Introduction According to the U.S. Federal Highway Administration.s (FHWA) National Bridge Inventory data, 23.7% of the nation.s 597,787 bridges are structurally defic
12、ient or functionally obsolete as reported in Better Roads Magazine 2009. Also, the Portland Cement Association (PCA) 2008 reported that out of the 139,031 reinforced concrete bridges, 29.3% are considered structurally deficient or functionally obsolete. The high number of deficient bridges means tha
13、t a considerable number of bridges are being recommended for weight limiting posting, rehabilitation, or decommissioning and replacement. Reinforced concrete slab bridges offer economic alternatives for short-span bridges in the United States and particularly in developing countries where cast-in-pl
14、ace concrete is common practice. The main advantage of cast-in-place concrete slab bridges is the ability to field adjustment of the roadway profile during construction. Typically, the design of highway bridges in the United States must conform to the 3 American Association of State Highway and Tran
15、sportation Officials (AASHTO) Standard Specifications for Highway Bridges (2002) or AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (2007). The analysis and design of any highway bridge must consider truck and lane loading. However, truck loading provisions govern for sh
16、ort-span structures when considering AASHTO Standard Specifications. AASHTO specifies a distribution width for highway loading to reduce the twoway bending problem into a beam or one-way bending problem. Alternately, an empirical expression for live-load bending moment is provided. Therefore, reinfo
17、rced concrete slab bridges are designed as a series of beam strips. AASHTO Standard Specifications design procedures were originally developed in the early to mid 1900s based on research work by Westergaard (1926, 1930), Jensen (1938, 1939), and Newmark (1948). The goals of AASHTO LRFD Bridge Design
18、 Specifications were to develop comprehensive specification and achieve more uniform margin of safety for all bridge structures. AASHTO LRFD procedures specify HL93 live load which is a combination of HS20 trucks or design tandem with lane loading. AASHTO permits a reduction in live-load intensity o
19、n a bridge deck due to the improbability of having all lanes of bridge superstructure loaded simultaneously. These live-load reduction factors are used to account for the probability of having all lanes loaded at the same time and at locations along the bridge deck producing the maximum bending mome
20、nt in an element of a bridge superstructure. AASHTO Standard Bridge Specifications and LRFD procedures specify that results obtained from analyses of three- and four-lane bridge decks where all lanes are loaded simultaneously are to be multiplied by reduction factors. Sanders (1984) summarized and d
21、ated the various changes in the AASHTO Standard Specifications over the years and noted that these reduction factors were originally introduced in 1941 in the third edition. However, Sanders (1984) also reported that the greatest confusion appears to be in the appropriateness of using the provisions of reduction in load intensity for determining the design bending moments in a girder. Some engineers permit the reduction of live load, while others do not. Taly (1996) reported that bridge designers
