High Strength Bolts
Use of Direct Tension Indicators (DTIs) in Bridge Structural Steel
May 14, 2003
DTIs are being used by many State DOTs to indicate the proper tensioning of structural fastener assemblies in steel bridgework and other structures. DTIs are washer-shaped devices with 'bumps' on one face which are intended to flatten in a predictable manner such that Engineers can visually inspect bolts for proper tension in slip-critical or fully pretensioned bolted connections.
DTIs used by State DOT's in steel bridgework are manufactured to ASTM Standards, and are currently available for Type 1 bolts either plain or mechanically galvanized and for Type 3 bolts either mechanically galvanized with an epoxy coating or from weathering steel. DTIs are also available in metric sizes in finishes and materials similar to those for inch-series.
DTI Installation and Inspection:
The Federal Highway Administration (FHWA) recommends that DTIs be placed under the bolt head of structural bolt assemblies to be tightened by the nut. This ensures that the DTI is indicating that the tension on the bolt is along its full length, and that the bolt has not been 'trapped' by premature removal of drift pins or inadequate snug-tightening. A 'trapped' bolt is one which is pinched between the steel plies such that it can't be tensioned along its full length.
DTIs can also be used under the 'nut' side of a structural bolt assembly, and tightening such assemblies from the bolt head side will similarly maximize the accuracy of the DTI and enable trapped bolts to be detected. DTIs placed under the turned element may give slightly less accurate tension indications due to the washer sliding across the bumps on the DTI. I have found through experience that the hardened washer will often stick to the turned element (nut or bolt head) during tensioning, thus sliding on the DTI.
Other configurations may be used in conformance with the Installation Instructions of the DTI Manufacturer and applicable State requirements. Regardless of the specific configuration of the structural bolt assembly and the DTI, it is very important that the pre-installation tests be done the same way the assemblies will be placed and tightened in the steelwork.
Quality control should be a part of any good installation method. During fastener tensioning with DTIs, a second ironworker is required to hold the unturned element, just as is done with any method involving heavy hex structural bolts. In other words, the use of DTIs does not relieve contractors of the need to hold the 'unturned' element (usually the bolt head) on a fastener assembly.
This ironworker at the unturned element should also check the gaps of the DTIs visually, assisted by use of a feeler gage to ensure they are properly compressed. Performing this quality control check may not be easy for the wrench operator.
After installation the project inspector provides the quality assurance check. This is done with a feeler gage in addition to visual inspection.
All DTIs must be installed and inspected following the above noted criteria.
DTIs and Through-hardened Steel Washers (F436):
DTIs are a load cell used to indicate that fastener assemblies have been properly tensioned. DTIs may be heat-treated to a hardness range, which depends primarily on the specific DTI manufacturer's design for the size and type of DTI. However, the hardness of DTIs will typically be considerably lower than the hardness of a standard through-hardened steel F436 washer. Thus, DTIs shall not be used as a substitute for hardened steel washers.
Hardened steel washers or plate washers are required to be used on 'over-sized' or slotted holes in steel bridgework, even when DTIs are used. In such cases, DTIs should not be placed directly on top of the 'over-sized' or slotted holes, but on top of the hardened washer covering the 'over-sized' or slotted hole.
Through-hardened washers for bolts 1 inch and larger have an inside diameter 1/8 inch larger than the bolt diameter. When using 1 inch or larger bolts with DTIs placed over a hardened washer, plate washers or special hardened washers with an inside diameter 1/16 inch larger than the nominal bolt diameter shall be used. When standard washers with holes 1/8 inch larger than the nominal diameter were used, the DTIs would not be adequately supported, leading to erroneous tension indications. ASTM is in currently in the process of amending F436 to address this issue of inside diameters that are too overly large.
DTIs for Special Applications or with Unique Features:
In addition to standard ASTM-recognized DTIs, there are also proprietary DTIs which are not covered by ASTM standards designed for anchoring, signage, and other applications. There are also DTIs with self indicating feature that can be used to verify bolt tightening. Use of all such devices should be undertaken with the full knowledge of the Engineer of Record and in accordance with properly prepared installation and inspection instructions and requirements, and according to AASHTO LRFD bridge construction specifications.
DTIs for anchors or signage are not interchangeable with DTIs for structural bolts, as the tensions are very different. Similarly, use of such devices may require the use of feeler-gages of a different size than those used to inspect structural DTIs.
DTIs with self indicating feature are another type. The proper use of these devises, and the visual appearance of the correctly installed bolt, should follow the manufacturer's written instructions. Verification and testing should be made according to AASHTO LRFD bridge construction specifications.