Concrete Box Culvert Load Rating by Design Load Correlation

TxDOT carried out a project to determine the load rating of concrete box culverts reflective of in-service performance. The project included load testing and finite element analysis on a sample of culverts to correlate design loads to load ratings. Per the report, the following load ratings were determined for qualifying concrete box culverts. The report is available on the Bridge Division website.
Table 6-3 Concrete Box Culvert Load Rating Summary by Design Load Correlation
Load Ratings*
HS-20
SU4
SU5
SU6
SU7
EV2
EV3
INV
OPR
INV
OPR
INV
OPR
INV
OPR
INV
OPR
OPR
OPR
14.8
24.8
0.95
1.58
0.84
1.41
0.78
1.30
0.76
1.27
1.28
1.03
*HS-20 ratings are reported in HS format. All other ratings are reported as rating factors.
The above ratings are applicable to concrete box culverts that meet the following three criteria:
  1. Condition rating Item 62 (SNBI: B.C.04) must be greater than or equal to “5”. This requirement will ensure that this approach is only applied to structures in fair or better condition.
  2. Year built Item 27 (SNBI: B.W.01) is not later than 1999. This requirement is to capture culverts designed by Allowable Stress Design (ASD) method.
  3. Item 41 (SNBI: B.PS.01) is coded as “A” - open to unrestricted traffic (SNBI: Open (O)).
If Table 6.3 is utilized to determine load ratings of a culvert, the culvert does not need to be load posted, and Figure 6.4 or Figure 6.5 (On/Off System Load Posting Flowchart) is not applicable.
Concrete box culverts that do not meet the above three requirements, nor requirements for Assigned Load Rating must have either calculated load rating or assumed load rating. Use the guidelines in the section “Assumed Load Ratings” for concrete culverts without plans.
If a concrete box culvert bridge has another type of structure such as an expansion slab, the non-culvert part shall be evaluated by one of the three load rating methods: Assigned, Assumed, or Calculated ratings. Report the lower load rating of either the culvert or the nonculvert parts for the structure.
Items 63 and 65.1 (SNBI: B.LR.04), methods used to determine Operating and Inventory Ratings, will be coded as “1” – Calculated Load Rating Based on Load Factor (LF) (SNBI: LFR).
The following flow chart shows the culvert load rating procedure.
Sample alt text
Figure 6-2. Load Ratings for Concrete Box Culvert