Section 2: Loads

Dead Loads

Do not design for a future wearing surfaces unless specifically directed by the district. Dead load of wearing surfaces shall only be applied to cases where such a wearing surface is part of the current design or a known future application.

Live Loads

Use HL93 design live load as described in Article 3.6.1.2 unless design for a special vehicle is specified or warranted.

Design widenings for existing structures using HL93. Rate existing structures in accordance with the Bridge Inspection Manual. Show load rating and design loads on the bridge plan sheets, for example, HS20 (Existing) HL93 (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 Article C3.6.1.1.2, 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.

Disregard recommendations to investigate negative moment and reactions at interior supports for pairs of the design tandem provided in the commentary to Article 3.6.1.3.1, Application of Design Vehicular Live Loads.

The provisions of Article 3.6.1.1.2 shall not be applied to the extreme limit state when evaluating system redundancy as specified in Chapter 3 – Superstructure Design, Section 17 - System Redundancy Evaluation for Steel Twin Tub Girders.

Braking Force

Take the braking force, BR, as 5% of the design truck plus lane load or 5% of the design tandem plus lane load.

Vehicular Collision

Replace Article 3.6.5 with the following:

Abutments and retaining walls:

Due to the soil behind abutments and retaining walls, the collision force need not be considered.

Bents:

Investigate bents for collision when located within a distance of 30.0 ft. to the edge of roadway. A bridge deck adjacent to the column is considered an adjacent roadway.

Investigate the need for vehicular collision design for the final condition after all construction is completed, not during construction phases with temporary traffic conditions.

Investigate the need for vehicular collision design by determining the annual frequency for a bridge bent or pier to be hit by a heavy vehicle,, or optionally the annual frequency of bridge collapse,

A F_{B C}

Do not design bents and piers for collision when

{AF}_{H P B}

is less than 0.001. Use the following equations to determine

A F_{H P B}

The annual frequency for a bridge bent or pier to be hit by a heavy vehicle:

A F_{H B P} = 2 (A D T T) (P_{H B P}) 365

ADTT = the number of trucks per day in one direction

The annual probability for a bridge pier to be hit by a heavy vehicle:

P_{H B P} = \{\begin{matrix} 3.457 \times 10^{- 9}, \\ 1.090 \times 10^{- 9}, \\ 2.184 \times 10^{- 9,} \end{matrix} \begin{array}{l}  \end{array} \begin{array}{l} for undivided roadways in tangent and horizontally curved sections \\ for divided roadways in tangent sections \\ for divided roadways in horizontally curved sections \end{array}

{AF}_{H P B}

is greater than 0.001, the designer may optionally calculate

A F_{B C}

using Equation C3.6.5.1-1. If

A F_{B C}

is less than 0.001, the bents do not need to be designed for vehicular collision.

When designing for collision, there are two design choices: redirect the collision load or provide structural resistance.

When the design choice is to redirect the collision load, the protection must meet at least one of the following requirements:

Protect with a structurally independent, founded, 54 in. tall, MASH Test Level 5 approved concrete rail if the top edge of the traffic face of the rail is within 3.25 ft. from component. The back of rail should be offset from the column to allow dynamic displacement of the rail without the rail impacting the column.

Protect with a structurally independent, founded, 42 in. tall, MASH Test Level 5 approved concrete rail if the top edge of the traffic face of the rail is between 3.25 ft. and 10 ft. from component.

Protect with a 42 in. tall single slope concrete barrier (or 42 in tall, MASH Test Level 5 approved barrier equivalent) if more than 10 ft. from component.

When the design choice is to provide structural resistance, the design must meet the following requirements:

Design the pier for an equivalent static force of 600 kips. The force is acting in a direction of zero to 15 degrees with the edge of the pavement in a horizontal plane. Apply the force at a distance from 2.0 ft. to 5.0 ft. above ground, whichever produces the critical effect being analyzed

The load may be considered to be a point load or may be distributed over an area deemed suitable for the size of the structure and the anticipated impacting vehicle, but not greater than 5.0 ft. wide by 2.0 ft. high centered around the assumed impact point.

See Chapter 4 - Substructure Design, Section 7 - Columns for Multi-Column Bents and Section 8 - Columns for Single Column Bents or Piers for design information.

For structures with a clear distance of 25 ft. or less from the center line of a railway track, adhere to the requirements of American Railway Engineering and Maintenance-of-Way Association (AREMA), or the governing railroad company.

Earthquake Effects

Except as noted below, bridges and structures in Texas do not require analysis for seismic loading due to the low seismic hazard as shown in Article 3.10.2.

The TxDOT Bridge Standards and conventional bridge configurations have been evaluated for seismic effects and do not require further analysis.

For conventional structures with superstructure unit lengths or interior bent "H" heights outside of the limits stated in the TxDOT Bridge Standards and which are located in Brewster, Presidio, Jeff Davis, Culberson, Hudspeth and El Paso counties, check Minimum Support Length Requirements outlined in Article 4.7.4.4.

Non-conventional or exotic bridges do not require seismic evaluation, except those located in Brewster, Presidio, Jeff Davis, Culberson, Hudspeth and El Paso counties. In these locations, evaluate the structure for earthquake effects as required by Article 3.10. Contact the TxDOT Bridge Division for guidance.

Temperature Gradient

Simply supported prestressed beams

Cast-in-place slab and girder spans

Cast-in-place slab spans

Spliced I-shaped girders

Steel I-beams

Steel plate-girders

Steel tub-girders

Vessel Collision

TxDOT requires that all bridges crossing waterways with documented commercial vessel traffic comply with Article 3.14. For widening of existing structures, at a minimum 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.

Pedestrian

Do not apply a pedestrian load to sidewalks when evaluating system redundancy at the Extreme Event III limit state.