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We have been asked by our management to consider how barrier designs may change as a result of recently-proposed higher weight limits on our interstate highways.  Of particular interest is whether additional reinforcement will be required in our TL-5 barriers in order to withstand an impact from a tractor-trailer weighing up to 97,000 pounds.

In a prior analysis and design effort, MwRSF researchers determined a linear relationship for the lateral design impact load for tractor trailer vehicles striking rigid parapets. Of course, these results were determined using historical crash testing data for both the total IS values as well as the IS value for the tractor's rear tandems. From TRP-03-149-04, we reported design loading for PL-3 (50,000-lb vehicle) and TL-5 (79,366-lb vehicle) impact conditions was 153 to 153 kips and 243 to 248 kips, respectively. Based on this evidence, we prepared some preliminary TL-5 barrier designs using 217-kip design load based on the structural capacities of prior TL-5 barriers.

Later, MwRSF designed and crash tested a TL-5 open concrete bridge railing as well as a concrete median barrier using a reduced load condition similar to that provided above. Thus, it seemed reasonable to use existing yield-line analysis procedures (YLAP) in combination with the reduced lateral impact load.

You noted below that a vehicle weight of 97,000 lbs is being considered by the IaDOT. As such, you inquired as to the predicted design load with such a weight increase. Based on an increased IS of (97000/79366)=1.22, the new design load would be approximately 265 kips if using existing YLAP.

P.S. " Please note that the design of expansion joints are treated differently than interior sections since fewer yield lines can be developed due to the discontinuous rail.