Load Rating

Owned by Becca Alford (Unlicensed)
Last updated: Jun 02, 2022 by barngrover
3 min read

LRFR Analysis

When the user selects the 5th Edition of LRFD as the governing specification, the calculation of each load rating follows the procedure outlined in The Manual for Bridge Evaluation (MBE), First Edition/2008 by AASHTO. This section was primarily designed for load rating bridge superstructures, and gave no guidance on several conditions that apply to culverts. These conditions include vertical and horizontal soil loads, water pressure and horizontal live load surcharge. These loading conditions can create reversed effects to individual members of the culvert. For example, horizontal earth pressures produce opposite top slab moments compared to vertical live loads and gravity loads. Also, vertical loads on the top slab can reduce soil pressure effects on the exterior walls. Consequently all of these actions must be considered when calculating the ultimate resistance in the numerator of the load rating equation. Therefore Eriksson Culvert adopted a load rating equation that is identical to the Equation 6A4.2.1-1 given in the 2nd Edition of the MBE for culverts and is shown below.  In this equation, live load surcharge is considered a transient load derived from the presence of a vehicle axle positioned adjacent to the edge of the culvert that creates additional soil pressure and is therefore placed in the denominator.  Also note that in the first edition of the MBE, it was assumed that the max/min effects of the horizontal earth load is produced using the load factors for EH.  This is the method used when the user selects the 5th Edition of LRFD as the governing specification.

When the user selects the 6th Edition (or newer) of LRFD, the calculation of each load rating follows the procedure outlined in The Manual for Bridge Evaluation (MBE), Second Edition/2010, by AASHTO (including all interims up through 2014).  Support for culverts was added with the 2013 Interims in Article 6A.5.12.4.  The new load rating equation, 6A.5.12.4-1 (shown below), is equivalent to the procedure currently contained in Eriksson Culvert, with one exception.  The new MBE states that max/min effects for horizontal earth loads shall be produced via the ratio of the maximum and minimum equivalent fluid pressures (basically the method followed in STND).

In summary, when the governing specification is LRFD 5th Edition, the load factors for EH are used in the load combinations and rating equations.  When the governing specification is LRFD 6th Edition (or newer), the ratio of the maximum and minimum fluid pressures are used for EH instead.

A load rating is produced at the 3 critical sections of each member of a culvert. The controlling rating of either shear or moment is shown in critical sections table in the text report output. If the value of the live load shear or moment is zero for a member the load rating output is zero, and shown to the user as NC, meaning Not Calculated or Not Checked.  Also, in the walls at the mid-height, if there is no negative live load moment, then the negative critical section is not rated.  Similarly, if there is no positive live load moment at wall mid-height, then the positive critical section is not checked.  If the load rating exceeds 99.0, the rating output is shown as 99.00. If live load exists at one of the critical sections while the dead load action is in excess of the capacity of a member a negative factor will be shown.

 

Load Rating in Design Mode

The program initially calculates the reinforcing steel required and member thicknesses to satisfy the shear, axial and moment demand on each section.  Following this computation the requirements of minimum reinforcing steel and fatigue stress limits are checked for adequacy on the reinforcing steel and increased if necessary.  Once a final steel reinforcing area is calculated, a trial bar size and arrangement is checked against crack control requirements until satisfactory.   This finalized pattern is then displayed in the output and used for calculating the moment inventory and operating load ratings for each member and each members thickness is used to obtain the shear inventory and operating ratings. 

 

Load Rating in Analysis Mode

Here the reinforcing area and member thickness are defined to the program in the culvert properties dialog box.  The program performs checking of all appropriate limit states and a rating for both moment and shear is calculated.  The moment rating is based on the allowable moment capacity of each member while, the shear rating is calculated based on the total shear capacity of each member including both concrete and shear steel.