NCHRP-RPT-592-AppendixM.pdf
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1、APPENDIX M LANE SPACING STUDY M-1 When performing a rigorous method of analysis on a bridge superstructure, there are a number of ways that live loads can be positioned. The AASHTO Standard Specifications (AASHTO 1996) and the AASHTO LRFD Specifications (AASHTO 1998) specify a traffic lane width of
2、3.66 m (12 ft). This means that, when the minimum distance from the curb is considered, the first two trucks will be spaced 4 feet apart, and each subsequent truck will be at a spacing of 6 feet. When using an automated load positioning procedure, it is simpler to assume that the trucks have a const
3、ant spacing. For this project, a four foot spacing is used. The issue of whether or not the variation in truck spacing beyond the first two trucks has a significant effect on live load distribution factors must be considered. For any given bridge, many options exist for live load placement. For exam
4、ple, a bridge with a clear roadway width of 50 ft, there are 8 possible loading cases. For four lanes loaded, the loading cases are as follows: 2-6-4-6-6-6-6-6 ft, 3-6-4-6-6-6-6-6 ft, etc. When the outermost line of wheels of the first vehicle is 4 ft from the barrier, the fourth vehicle can be spac
5、ed either 4 ft or 6 ft from the third vehicle. This many loading cases lengthen the analysis process. One possible simplified procedure for load placement is to place all vehicles 4 ft apart. The load placement is then 2-6-4-6-4-6-4-6 ft, 3-6-4-6-4-6-4-6 ft, etc. The objective of this study is to de
6、monstrate that use of a constant four foot vehicle spacing will have a negligible effect on live load distribution factors. Method of Analysis SAP2000 was used to model the bridge superstructures. The models were analyzed using the grillage analogy method. Frame elements were used to define the supp
7、orting members as well as the transverse deck slab members. The section properties M-2 for the supporting members were based on a composite section. The presence of end and intermediate diaphragms was not considered in this study. Geometric and Structural Properties Two straight (no skew) bridges we
8、re investigated in this study. The first bridge, shown in Figure M-1, is a four span precast concrete bulb-tee beam bridge that is symmetric about the centerline of the second pier. The second bridge, shown in Figure M-2, is a two span steel I-beam bridge. The cross section of the steel I-beam varie
9、s along the span length. The same varied section is used for both the exterior and interior beams as well as both spans. M-3 Vehicle Loading All vehicles used in this study were an HL93 truck as specified in the AASHTO LRFD Specifications (1998). The automated load positioning capability of SAP2000
10、was utilized for maximum bending moment. However, point loads were applied near the support to obtain the maximum shear. The distance between the interior face of the barrier to the outer most line of wheels of the first vehicle was varied from 2 ft to 6 ft with 1 ft intervals transversely across th
11、e bridge. In the standard load configuration, all vehicles were spaced 4 ft apart, while in the varied load configuration, vehicles were spaced either 4 ft or 6 ft apart. The live load placement configurations are illustrated in Figure M-3 through Figure M-6. 2.67 m1.37 m2.67 m1.37 m2.67 m2.67 m 0.5
12、33 m 12.34 m 0.533 m 203 mm (a) 37.90 m37.80 m Symm. C Bearing L Beginning of Bridge (b) Figure M-1: Precast Bulb-Tee Beam Bridge: (a) Typical Cross Section; and (b) Plan View 13.79 m 3.35 m 229 mm 0.91 m 0.533 m 3.35 m3.35 m3.35 m C Bridge L (a) Beginning of Bridge End of Bridge 45.72 m45.72 m (b)
13、Figure M-2: Steel I-Beam Bridge: (a) Typical Cross Section; and (b) Plan View M-4 18301830122012201830610 Figure M-3: Standard Vehicle Spacing (Precast Bulb-Tee Beam) 18306101830122018301830 Figure M-4: Varied Vehicle Spacing (Precast Bulb-Tee Beam) 1220610183012201830183018301220 L C Bridge Figure
14、M-5: Standard Vehicle Spacing (Steel I-Beam) M-5 1830610122018301830183018301830 C Bridge L Figure M-6: Varied Vehicle Spacing (Steel I-Beam) Results Distribution factors are calculated by dividing the maximum response value (shear or moment) at a critical location by the maximum response value from
15、 a single beam line loaded with one vehicle. The critical location is determined from the beam line analysis. For the precast bulb-tee beam bridge, the maximum shear and moment from a beam line analysis is 290 kN and 2128 kN-m, respectively. For the steel I-beam bridge, the values are 294 kN and 253
16、1 kN-m, respectively. The maximum shear and moment values from the grillage analysis are shown in Table M-1 and Table M-2 for the precast bulb-tee beam bridge and the steel I-beam bridge, respectively. Table M-3 and Table M-4 show the differences in distribution factor when comparing the standard an
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- NCHRP RPT 592 AppendixM
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