Concrete Girder Strength Design Check Calculations for Task Order 14

09, 12, 13, 15, 23, 30, 42, 45, and 44, Kabul-Kandahar Highway, prepared by The Louis Berger Group Inc. / Black & Veatch Special Projects Joint Venture. The review aimed to verify the accuracy of the calculations and the validity of the Joint Venture's conclusion that the as-built bridges have adequate strength. The original girder design of these bridges was based on concrete with a compressive strength of 45 MPa, but as-built concrete strengths resulted in minimum values ranging from 25 MPa to 30 MPa. The Joint Venture updated their original design calculations for the lower concrete strengths and determined that the as-built bridges have adequate strength. However, Tetra Tech's review noted specific comments on the attached spreadsheet, which generally either increase the load acting on the girders or reduce the girder capacity. The Joint Venture used a combination of home-grown design templates (MathCAD, Excel) to analyze and design the bridge girders, rather than standard bridge design software. Tetra Tech's review included a detailed check of the input parameters, spot checking the analysis calculations, and a detailed check of the design calculations. The evaluation was limited to the review of a representative structure from each of the two distinct bridge types, and a review of the presented calculations from the remaining structures confirmed that they are similar to those reviewed and that the Tetra Tech comments also apply to them. The design check calculations included a summary chart noting the required flexural strength and provided flexural strength for the girders of each bridge. The chart is summarized below: Strength, Mu (kN-m) % Capacity Section Required Provided Used 09A 3188 9859 32% 09B 9712 33% 12A 2182 7636 29% 12B 7363 30% 13 6207 6615 94% 15A 2963 10005 15B 9964 23 787 1157 68% 6278 95% 42 6159 93% 44A 3153 9228 34% 44B 9090 35% 45A 45B Since the reinforced-concrete bridges were over-designed, Tetra Tech anticipates that those bridges may have adequate capacity after Tetra Tech's comments are addressed. The three post-tensioned bridges, however, do not appear to have adequate capacity. Tetra Tech recommends that the Joint Venture review the attached comments, update the design calculations, and resubmit to USAID. The Joint Venture should consider including the mild steel reinforcement in the post-tensioned girder design to achieve additional capacity. In addition, Tetra Tech recommends that USAID request that the Joint Venture provide recommendations for bridges which do not have adequate capacity based on the as-built concrete compressive strengths. These recommendations should include rating calculations to identify what vehicular live load can be supported by the bridges, so that USAID can post bridges for reduced truck loading, as required. Tetra Tech's review also noted specific comments on the design calculations, including issues with resistance factors, distance factors, and load combinations. The comments generally either increase the load acting on the girders or reduce the girder capacity. Tetra Tech recommends that the Joint Venture address these comments and update the design calculations to ensure that the bridges have adequate capacity.