Section 2: Loads
Anchor: #BABCAAIELive Loads
Use HL-93 design live load as described in Article 3.6.1.2 of the AASHTO LRFD Bridge Design Specifications unless design for a special vehicle is specified or warranted.
Design widenings for existing structures using HL-93. Rate existing structures using AASHTO Standard Specifications and HS20 loading. Show load rating and design loads on the bridge plan, for example, HS 20 (Existing) HL-93 (New).
Do not use the reduction in the multiple presence factor (m) based on Average Daily Truck Traffic (ADTT) on the bridge as suggested in the AASHTO LRFD Bridge Design Specifications under commentary on Article 3.6.1.1.2, Multiple Presence of Live Load.
For simple-span bridges, do not apply the provisions for two design trucks as described in Article 3.6.1.3.1 of the AASHTO LRFD Bridge Design Specifications.
Disregard recommendations to investigate negative moment and reactions at interior supports for pairs of the design tandem provided in the commentary provided in the AASHTO LRFD Bridge Design Specifications under Article 3.6.1.3.1, Application of Design Vehicular Live Loads.
Anchor: #i1350595Braking Force
Take the braking force, BR, as 5% of the design truck plus lane load or 5% of the design tandem plus lane load. See Article 3.6.4 in the AASHTO LRFD Bridge Design Specifications.
Anchor: #i1350628Vehicular Collision Force
See Article 3.6.5 in the AASHTO LRFD Bridge Design Specifications. Ongoing TxDOT-sponsored research supports the following policy:Abutments and retaining walls—Due to the soil behind abutments and retaining walls, the collision force in Article 3.6.5 need not be considered.Bents—Bents adjacent to roadways with design speeds of 50 mph or less need not meet the requirements of Article 3.6.5. Bents adjacent to roadways with design speeds greater than 50 mph and located within 30 feet of the edge of roadway (defined as edge of lane nearest the column) must meet at least one of the following requirements:Protect with an approved barrier. The Bridge Division can provide details of acceptable barriers.Design for 400-kip load. Use a high collision strut between the columns if necessary.Validate that the structure will not collapse by analyzing the structure considering removal of any single column. Analyze using Extreme Event II Limit State. Use 1.25 load factor for all dead loads and 0.5 load factor for live load. Consider live load only on the permanent travel lanes, not the shoulder lanes.
Special considerations include the following:Single-column bents—Generally single-column bents have sufficient mass and will meet the requirements of Article 3.6.5. No further analysis is required for columns with a gross cross-sectional area in excess of 20 sq. ft. and a least dimension of no less than 3 ft.For structures within 50 ft. of the center line of a railway track, meet the requirements of AREMA or the governing railroad company.
Anchor: #i1350704Earthquake Effects
Except for regions near Big Bend that are susceptible to seismic ground motion, disregard provisions such as Article 3.10 in the AASHTO LRFD Bridge Design Specifications pertaining to earthquake loads unless specified otherwise.
Anchor: #i1350734Vessel Collision
TxDOT requires that all bridges crossing waterways with documented commercial vessel traffic comply with AASHTO LRFD Bridge Design Specifications, Article 3.14. For widening of existing structures, at least maintain the current strength of the structure relative to possible vessel impact, and increase the resistance of the structure where indicated if possible. Consult the TxDOT Bridge Division for assistance interpreting and applying these design requirements.