HLR Sections 6.4 - 6.7
Analysis
CONTENTS
| 6.4 K-Factor Analysis 6.5 Vehicle Load Reduction Factors for NT Structures 6.6 Dynamic Load Allowance (DLA Factors) 6.7 Other Checks
| Other Links: | Main Index | Analysis Options | Analysis Methodology | Ratio Method |
6.4 K-Factor Analysis
| An alternative facility has been provided to perform the analysis of non-standard structures, such as culverts and portal frames. The basic maximum moment due to the heavy load vehicle, Mhl, is calculated assuming the structure is a simply supported beam. The factored moments Ml, M2, M3 are then multiplied by a factor, K, and the resulting values compared to the actual capacity, Mc. Two K-factors may be specified for each span group in the structure to allow for checking of both positive and negative moments. In the case of a culvert, for example, this would permit a check to be made of the positive top-slab moment as well as the end corner moment. K is, in effect, a moment multiplier, converting a simply-supported mid-span sagging moment to an equivalent fixed-end moment. It cannot, therefore, be used to analyse structures for which moment cpacities are not given.
6.5 Vehicle Load Reduction Factors for NT Structures
| The Northern Territory, Australia, has a special class of single lane bridges for which the design moment is derived by applying a vehicle reduction factor to the maximum calculated heavy load moment Mhl. The following assumptions and restrictions apply: A single lane NT structure is identified as having a Girder Type 9 The reduction factor is only applied to simply supported bridges The widest axle in the vehicle is used when determining the factor The Axle Width - Moment Reduction Factor relationship can be viewed and edited using the NT-Factors form (refer to Section 8.6 for further details). Any number of points can be used to define this relationship. Assuming a cartesian axis system, the axle width is defined along the X-axis and the corresponding moment reduction factor is given along the Y-axis. During the analysis, if HLR detects a Type 9 structure, it refers to NT-Factors form and calculates a vehicle reduction factor based on the largest value of axle width in the heavy load vehicle. If an exact value does not exist in the table, it interpolates one. It then applies this factor to each axle load in the heavy load vehicle prior to performing the beam analysis and calculating M1, M2 and M3. Note that the above moment reduction factor-axle width relationship can be applied to any structure, providing it is defined as a structure Type 9.
6.6 Dynamic Load Allowance (DLA Factors)
| A separate dynamic load allowance factor (DLA1, DLA2, DLA3) can be applied to each of the three notional travel conditions checked by HLR (refer to Section 6.2 for a description of these conditions). DLA2 and DLA3, the two DLA factors applicable to travel at restricted speeds and/or location on the bridge deck (and corresponding to M2 and M3), are specified in the Options / Load Effects form and will be applied by HLR to all structures on the route being assessed. DLA1, representing the dynamic load allowance factor for vehicle travel at unrestricted speed, is determined differently. If a default DLA1 has been entered in the Options / Load Effects form this value will be used in calculating the maximum moment (M1) for all structures in the current route i.e., it will over-ride any structure-specific DLA value that may exist in the database. If this value is left zero, HLR will check whether a dynamic load allowance factor has been specified in the database for each individual structure. If a DLA value is found against a particular structure, that value will be used to modify the calculated HL vehicle moments for the effects of dynamic impact on that one structure only. Note that for multi-span bridges HLR allows a DLA factor to be specified for each span in the structure. If it is present, this value will be used to determine the heavy load vehicle effects described in Sections 6.2.2 and 6.2.3. If a DLA factor is not available, HLR will calculate one using an empirical method based on an assumed DLA-frequency spectrum. The lower bound frequency value is set at 0.9*120/Span and the upper limit is taken as 1.1*120/Span. DLA factors corresponding to each limit are derived from the frequency-DLA relationship given in the Options table (by default based on a modified form of the 1998 Austroads Bridge Design Specification). The largest of the two values is taken as the final DLA factor. The default relationship can be edited to suit local requirements. When performing a ratio check, the DLA used for the design vehicle (DLAdesign) is based on either the NAASRA relationship DLA = 16/(L+40) for pre-1992 structures, or the 1998 Austroads Bridge Design Specification for post-1991 bridges. The DLA factor applied to the heavy load vehicle, DLAaust, is determined in the same way as that described in the preceeding paragraphs. The default frequency-DLA curve can be modified via the Options/DLA-Frequency Curve form (refer to Section 8.4 for further details).
6.7 Other Checks
| During the analysis process for any given structure a number of checks are first performed by HLR to detect major non-structural restrictions. These include height or width restrictions, absolute restrictions imposed by the asset manager and any other condition that may preclude the structure from undergoing further analysis or from the heavy vehicle travelling over it. Warnings are also issued if link or structure data does not exist or if vital structure data is incompatible or missing. All route and structure data, together with the corresponding calculated results and restrictions, is continually saved to a log file during the analysis. Warnings and error messages are displayed on screen and appropriate entries to this effect are also made in the log file. The log is an ASCII text file that forms the basis of all output reports. If required it may be viewed, edited and printed (via the Edit/Log File option on the main menu bar).