Chapter 2: Limit States and Loads

Section 1: Limit States

Importance Factor

Classify all bridge designs as typical bridges, as defined in Article 1.3.5, when applying the operational importance factor, ηI
, to strength limit states.

Redundancy

Add the following to Article 1.3.4 as follows:
ηR 1.05
for nonredundant members and members requiring an analysis to establish system redundancy. Do not consider single-cell boxes and single-column bents nonredundant, unless approved by TxDOT Bridge Division.

Extreme Event Limit States

Revise the following definition in Article 1.2 as follows:
Extreme Event Limit States
—Limit states relating to events such as earthquakes; ice load; structural member or component failure; and vehicle or vessel collision, with return periods in excess of the design life of the bridge.
Revise Article 1.3.2.5 as follows:
Extreme Event Limit States - The extreme event limit state shall be taken to ensure the structural survival of a bridge during a major earthquake or flood, or when collided by a vessel, vehicle, or ice floe, possibly under scoured conditions, or after failure of a structural member or component.
Extreme Event I and II
Provisions under Extreme Event I need not be considered except for regions near Big Bend as noted in the subsequent section on Earthquake Effects.
Provisions under Extreme Event II must be considered only when vehicular collision or vessel collision evaluation is required. For dead load (DC and DW), use a 0.9 or 1.0 load factor, whichever generates the critical load case.
Extreme Event III
Supplement Article 3.4.1 with the following:
  • Extreme Event III - Load combination relating to a structural member or component failure as it relates to the System Redundancy Evaluation for Steel Twin Tub Girders as discussed in Chapter 3 – Superstructure Design, Section 17.
Supplement Table 3.4.1-1 with the following:
Load Combination Limit State
DC DD DW EH EV ES EL PS CR SH
LL IM CE BR PL LS
WA
WS
WL
FR
TU
TG
SE
Use One of These at a Time
EQ
BL
IC
CT
CV
Extreme Event III
γp
1.10
1.00
--
--
1.00
--
--
--
--
1.00
1.00
1.00
1.00
Supplement Table 3.4.1-2 with the following:
Type of Load, Foundation Type, and Method Used to Calculate Downdrag
Load Factor
Maximum
Minimum
DC: Components and Attachments for the evaluation of system redundancy as specified in the TxDOT Bridge Design Manual Chapter 3, Section 17, for Extreme Event III only
1.10
0.90
All load effects during an assumed fracture event due to both permanent and assumed transient loads shall be amplified by a factor of 1.20 to simulate the dynamic effects of a fracture on the twin tub girder span(s).

Foundations

For typical multi-column bridges, determine design loads for foundations at Service I Load Combination. Determine foundation loads for single column bents and other non-typical substructures using Service I and Service IV Load Combinations. For Service IV Load Combination, include the vertical wind pressure as specified in Article 3.8.2. For foundation loads on typical multi-column bents and abutments, distribute the live load equally to all supporting foundations, assuming all lanes are loaded. Use the multiple presence factor, m, per Article 3.6.1.1.2.

Live Load Deflection

Check live load deflection using Articles 2.5.2.6.2 and 3.6.1.3.2. Calculate deflection using a live load distribution factor equal to the number of lanes divided by the number of girders. Use the deflection limits given in Article 2.5.2.6.2.