HLR Section 6.9
Continuous structures with no capacities
CONTENTS
| 6.9.1 Assessment Methodology 6.9.2 Structures With Girders 6.9.3 Structures Without Girders 6.9.4 Design Vehicle DLA Factor 6.9.5 Design Vehicle Lane Reduction Factors 6.9.6 Paper describing the Assessment Methodology
| Other Links: | Main Index | Ratio Method | General Assessment Methodology |
6.9.1 Assessment Methodology
| This section describe the methodology used to assess continuous structures for which capacity data is not available. Although in principal it is possible to use the moment ratio method for continuous structures with no live load capacities, this method is not used by HLR. Instead, the program compares the moment envelopes produced by the heavy load vehicle with those of the original design vehicle and inserts the words "No capacities" in the right-hand comments field of all output reports. In effect this methodology assumes that the moments produced by the original design vehicle constitute a notional live load "capacity" for the structure. A paper describing the assessment process may be viewed by clicking here. It was conjointly written by officers within the bridge sections of the South Australian and Victorian State Road Authorities. As in the analysis of continuous bridges with capacities, a comparison is done for three notional vehicle travel conditions (in effect, three comparative HL vehicle moments - M1, M2 and M3). This is done for both conventional girder bridges (a girder spacing has been entered for the structure) and for slab/box girder bridges (zero girder spacing and/or the code [B] entered into the span comments field). Moments M 1, M 2, M 3 are based on the algorithms described in Sections 6.9.2 (structures with girders) and 6.9.3 (structures without girders). Finally, the ratios Ri of moments M i to the notional capacity Mc are calculate as:
| R1 = M1/Mc | Unrestricted travel speed
| R2 = M2/Mc | Restricted travel speed (e.g., 10 kph) but not position
| R3 = M3/Mc | Restricted travel speed (e.g. 5 kph) and position (e.g. centreline only)
These ratios are compared with the allowable working stress (serviceability) overload factor, R. Factor R is obtained from either the Analysis tab (from the field labelled Working Stress Overload Factor) or from the "*WSF=.." specification in the structure database. The "*WSF=.." specification is entered into the general comments field located on the "Structural Data" tab. Note that if negative moments govern then the corresponding R values will be shown in the Restricted Bridges Report as negative. Refer also to the General Assessment Methodology for a description of how vehicles are moved and modelled on continuous structures.
6.9.2 Structures With Girders (Gspacing > 0)
| Width-modified axle loads (if switched on in the Analysis tab) are used to determine the maximum heavy load moment, Mhl, at each section for each span group then factored to produce a set of equivalent girder moments, (M1, M2, M3) representing three possible travel conditions. (Refer to the document "Width-Modified Axle Loads.pdf" for further details). A moment envelope simulating the section capacity, Mc, is also produced by running the original Design Vehicle over the structure. The capacity is then calculated as:
| M c = Ra * M dv * (1 + DLA dv)
where Ra is the design vehicle reduction factor (from the second tab in the structure database) and DLA dv is the dynamic load allowance associated with that design vehicle (refer to Section 6.9.4 below). The HL effects are determined as follows:
| M l = M hl * (1 + DLA 1) * VRF | Unrestricted travel speed
| M 2 = M hl * (1 + DLA 2) | Restricted travel speed (e.g., 10 kph) but not position
| M 3 = M hl * (1 + DLA 3) * GF2 / GF 3 | Restricted travel speed (e.g., 5 kph) and position (e.g., centre)
VRF represents the HL Vehicle Reduction Factor as specified in the Analysis tab (emulates the provisions of the Austroads 1992 Bridge Code Multiple Lane Modification Factor); GF i are girder distribution factors that reflect the various positions on the bridge deck that may be traversed by the heavy vehicle; and the DLAi factors represent dynamic load allowances for the three possible vehicle travel conditions. DLAi and GFi factors are specified on the Options / Load Effects form and may be changed to suit local conditions. The raw moment & shear envelopes produced by the Design Vehicle are saved as text files into the ..\TempFiles subfolder and can be viewed through the Detailed Results tab. They are designated by the suffix dv in the structure number. (For example, if the raw HL Vehicle envelope for structure number 65030 is saved as 65030.OUT the envelope produced by the design vehicle will be stored as 65030dv.OUT).
6.9.3 Structures Without Girders (Gspacing = 0)
| This case normally applies to structures that cannot be modelled with beam or girder elements (such as slab structures or wide box girders with only a few cells). The analytical procedure is essentially the same as that described in Section 6.9.2 above for girder structures but with the difference that: The full vehicle axle loads, (not the reduced loads), are used in calculating the maximum HL moment (Mhl) The number of design lanes is taken into consideration when determining the capacity of the deck The effect of concurrent legal vehicles on the structure when the HL vehicle is present is taken into consideration
| The number of design vehicle lanes, Ldv, is given by: Ldv = Wk/3.1 where Wk represents the carriageway width between kerbs. The number of adjacent lanes, Lhl, available for concurrent legal vehicles when the HL vehicle is also present on the deck is: Lhl = (Wk - W - 0.8)/3.5 where W represents the width of the HL vehicle. Assuming that Mdv represents the moment envelope due to the Design Vehicle and LRFdv the Load Reduction Factor for the Design Vehicle, the relationship for capacity, Mc, is given by the expression:
| M c = Ldv * Ra * M dv * (1 + DLA dv) * LRFdv
Ra is the design vehicle reduction factor (sourced from the second tab in the structure database) and DLA dv is the dynamic load allowance associated with that design vehicle (refer to Section 6.9.4 below). LRFdv is dependent on the number of Design Vehicle lanes and the design year and is obtained from a table called "Design Vehicle Lane Reduction Factors.txt" stored in the /HLR4/Database folder. Refer to Section 6.9.5 below for details. Assuming that Mlegal represents the moment envelope due to the Legal Vehicle and DLAi represents dynamic load allowance for the three possible vehicle travel conditions, the HL effects Mi, are given by:
| M l = [M hl + (0.4 * Lhl * Mlegal)] * (1 + DLA 1) | Unrestricted travel speed
| M 2 = M1 - M hl * (DLA1 - DLA 2) | Restricted travel speed (e.g., 10 kph) but not position
| M 3 = M hl * (1 + DLA 3) | Restricted travel speed (e.g., 5 kph) and position (e.g., centre)
DLAi factors are specified on the Options / Load Effects form and may be changed to suit local conditions. The raw moment & shear envelopes produced by the Design Vehicle and Legal Vehicle are saved as text files into the ..\TempFiles subfolder and can be viewed through the Detailed Results tab. They are designated by the suffix dv and lv in the structure number. (For example, if the raw HL Vehicle envelope for structure number 65030 is saved as 65030.OUT the envelope produced by the design vehicle will be stored as 65030dv.OUT and the legal vehicle as 65030lv.OUT).
6.9.4 Design Vehicle Dynamic Allowance Factor
| The Design Vehicle Dynamic load allowance factor (DLAdv) will be calculated and applied in one of two ways: (a) If a Design Code DLA type has not been specified for the Design Vehicle then one of two default design codes will be assumed:- either NAASRA (if the structure was designed prior to 1992); or AUSTROADS. (b) If a design code DLA type has been specified for the Design Vehicle then the DLA appropriate to the specified design vehicle type will be used to calculate the DLA factor. A drop-down list of permitted Design Codes has been included on the Design Vehicle dialog box (accessed via the menu options: Vehicle / Edit an existing Design Vehicle). The current design codes supported by HLR are: NAASRA, AUSTROADS and AS5100 The analysis will be performed in the (+)ve sense (L-R) if the forward direction has been selected for the HL assessment. This is irrespective of the sense in which the HL route itself has been specified.
6.9.5 Design Vehicle Lane Reduction Factors
| A full description of the Design Vehicle Lane Reduction Factor, LRFdv, is given in a paper conjointly written by officers within the bridge sections of the South Australian and Victorian State Road Authorities (click here to view the General Assessment Methodology). The factors are stored in the /HLR4/Database folder in a text file called "Design Vehicle Lane Reduction Factors.txt" (see below).
| DESIGN VEHICLE LANE REDUCTION FACTORS Lane Reduction Factors (LRFdv) are based on the year of construction. If the year is unknown (Date = "0000") HLR will use the last row of factors. Number of design lanes, 6 ------------------------------------------------------------------------------------ Row, Date, Date Description, LRFdv per no. design lanes (1 - 6) ------------------------------------------------------------------------------------ 1, 1991, "Pre 1992 bridges", 1.00, 1.00, 0.90, 0.75, 0.75, 0.75 2, 1992, "1992 thru to 2003", 1.00, 0.90, 0.80, 0.70, 0.60, 0.55 3, 2004, "2004 and beyond", 1.00, 0.90, 0.73, 0.65, 0.60, 0.56 4, 0000, "Design yr unknown", 1.00, 0.90, 0.73, 0.65, 0.60, 0.55
| A unique factor has been derived for each date range and number of design lanes. If the design year is unknown the factors shown in the last row are used.