Section 7: Nonredundant Steel Tension Member Inspections

Overview

Non-Redundant Steel Tension Member (NSTM) Inspections are limited to the areas under tension and/or areas subject to stress reversals in steel non-load path redundant members . These inspections are typically performed every 24 months but can be performed more frequently if conditions warrant. An NSTM Inspection is a hands-on (within arm's length of the component) inspection of a fracture critical member or member components. It may include visual and other nondestructive evaluation (NDE). NDE Methods may include dye penetrant, magnetic particle, or ultrasonic techniques. With the transition from the Coding Guide to the Specification for the National Bridge Inventory (SNBI) in accordance with 23 CFR 650.311, the collection and use of certain SNBI data is required to establish inspection intervals and to update and implement the reduced inspection interval policies by June 6, 2024. Prior to SNBI data collection, TxDOT will use equivalent criteria per the Coding Guide to determine the reduced interval inspection criteria. The equivalent criteria is provided later in this section.

Nonredundant Steel Tension Members

The most common types of NSTM members are tension flanges and parts of webs of flexural members such as beams and girders. Tension members of trusses, particularly eyebars, which commonly make up the lower chords of old trusses, can also be NSTM. Members subject to stress reversals are considered NSTM also.
Steel girders in two-girder bridges not evaluated for system redundancy in accordance with the TxDOT Bridge Design Manual are NSTM. Fracture of lower flanges in positive moment areas (midspan) and upper flanges in negative moment areas (over supports) can be expected to lead to full or partial collapse of the structure.
The majority of single member steel caps are NSTM. The exceptions are those where support columns or multiple cap members provide load path redundancy.

NSTM Inspection Techniques

NSTM inspection techniques may include non-destructive testing to determine the condition of a structural member. There are several types available, including radiographic, ultrasonic, dye penetrant, and magnetic particle inspection. All are acceptable methods, but each has limitations and may not be suitable for a particular situation. One single technique may not be sufficient to assess damage and a combination of more than one may be needed. Perform these types of inspection only if you have undergone the proper training.
The selection of the type of non-destructive testing method for a particular location is a function of the detail. For instance, potential cracks at the ends of welded cover plates are often inspected by radiographic methods. Cracks in pins are best inspected by ultrasonic techniques. Subsurface defects such as inclusions may be found by magnetic field irregularities, and cracks adjacent to fillet welds at tee-joints are usually inspected by dye penetrant. These methods are all described in more detail in the
Bridge Inspector's Reference Manual (BIRM)
.

Reduced Inspection Interval

The following outlines TxDOT policy and requirements for risk-based inspection intervals outlined in 23 Code of Federal Regulations (CFR) 650.311 using Method 1 inspection interval determination for changing the interval of NSTM inspections. The level of inspection for NSTM inspections with decreased intervals will be the same as that required for those scheduled on a 24-month interval.
Bridges meeting the criteria below require reduced inspection interval of 6 or 12 months:
  • NSTM Inspection Condition (Item B.C.14) ≤ 4 [Condition Rating for NSTM Superstructure (Item 59) or NSTM Substructure (Item 60) ≤ 4]
An inspection interval of 6 or 12 months is assigned based on engineering judgement considering the severity or recent activity of the deficiency. An active crack or identification of a new crack may be considered recent activity. The basis of either a 6-month or a 12- month interval must be clearly documented in the inspection report.

Extended Inspection Interval

Steel bridges where fatigue details have been verified may be considered for extended inspection intervals. Inspection intervals up to 48 months are allowed provided all the following criteria are met:
  • Year Built (Item B.W.01) ≥ 1979 [Year Built (Item 27) ≥ 1979] and fabricated in accordance with a fracture control plan.
  • NSTMs have no fatigue details with finite life, history of fatigue cracks, nor pin and hanger assemblies.
  • NSTM Inspection Condition (Item B.C.14) ≥ 6 [Superstructure Condition Rating (Item 59) ≥ 6, and Substructure Condition Rating (Item 60) ≥ 6]
  • Inventory Load Rating Factor (Item B.LR.05) ≥ 1.0 [Inventory Rating (Item 66) ≥ 1.0, when expressed as a rating factor or Item 66 must be greater than or equal to 36 when expressed in tons.]
  • Routine Permit Loads (Item B.LR.08) = A or N [Item 41 must be “A”]

Bridge Inspection Interval Tolerance

The requirements and acceptable tolerances for inspection intervals for NSTM inspections is the same as that for Routine Inspections. See Section 4 for Routine Inspection Interval Tolerance requirements not repeated here.