Standard DTI FAQs
Tension in Bolts
- Where are correctly tensioned bolts needed?
- You break some bolts before their DTI's have been compressed sufficiently.
See breaking bolts
- Your DTI's have been completely flattened so that no even a .005" thick feeler gage can be inserted. Is this OK? See failure or stripping of bolts
Where are correctly tensioned bolts needed?
The Research Council on Structural Connections states that bolts must be correctly tensioned (see what bolt tension needed) where the connection is designed as "slip-critical" (that is connections where slip would be detrimental to the behavior of the joint, where the joint will experience fatigue or significant load reversal, joints with oversized or slotted holes, where welds and bolts are used together), where the bolts are used in direct axial tension, where the bolts are considered to be in the category of "pre-tensioned shear/bearing". Where the faying surfaces are coated and the design calls for transfer of loads by friction, the joint surface must be qualified and the bolts must be fully pretensioned. Most engineers require correctly tensioned bolts in connections supporting moving machinery, connections governed by seismic loads, connections between the members of a wind resisting system, moment connections, connections in steel which shares load with concrete members, in roof truss splices, in roof truss bracing and connections of roof trusses to their columns, where the prying load on arrays of bolts is high or indeterminate, where the fit between members is or is suspected to be poor, and in connections of frames having a high slenderness ratio.
Correctly tensioned bolts never make the strength of the connection less than if the bolts are not tensioned, and the discipline of making the field crews tighten ALL of the bolts in a structure or in a connection to a predetermined level promulgates a level of quality control which states that the designer intends to see that a quality project is achieved. In certain countries where the skill level of bolt installers is likely to be low, insisting that all bolts be correctly pretensioned is generally believed to make the average achieved bolt tension higher than if no tightening specification is enforced.
