Bolts as they relate to steel frame construction, are commonly categorized into two primary types:
High Strength Bolts – Heat treated during manufacturing to develop greater strength.
Carbon Steel Bolts – Unfinished or Common Bolts
In modern day steel frame construction, bolted structural connections almost exclusively are bound by engineering that requires high strength bolts. Below are the several types of bolt connections currently found:
Bearing Type Connection – are connections stressed primarily in shear and need only be installed to a snug type connection. Movement between members is resisted by the bolts bearing and pressing against the sides of the bolt holes. Bearing connections don’t fully ‘seat’ themselves until the system is loaded.
Slip-Critical Connection (Friction Type) – A connection stressed primarily in tension where the bolts are tightened during installation (preloaded) so friction between the adjoining faces of the steel members resist movement. Slip critical connections are fully seated when tightened to specified strength. For this reason, connections like column splices and beam to column connections in tall buildings must be slip-critical.
Most openings in steel members for connections are approximately 1/16″ larger than the diameter of the bolt itself. Washers may be specified to insure that if the openings are oversized, the bolt head and nut have adequate member contact. Also if bolts are preloaded, galling (tearing) of the surface can be minimized by the use of washers. Structural bolts are typically tightened with an impact wrench. Bolt tension is not critical in bearing type connections, but in slip-critical connections, bolts are typically tightened to at least 70% of tensile strength.
There are many ways to determine the tension in a given bolt connection, below are some methods:
Turn-of-Nut – Bolt is tightened snug, then turned an additional specified fraction of a turn. This specified turn amount depends on factors such as bolt length, bolt alloy, etc; but commonly falls between 1/3 and 1 full revolution.
Load-Indicating-Washer – The bolt will come equipped with a Direct Tension Indicator (DTI) washer which goes under the head or nut. When the bolt tightens, the ‘corrugated’ DTI washer flattens letting the installer know the bolt is properly tightened.
Torque Wrench – Use of a powered wrench which notifies the installer the bolt has been tightened to a preset torque setting. Lite duty and heavy duty Pneumatic Impact Wrenches are also widely used.
Torsion Control Bolts – These have a protruding splined end opposite the head or nut and are utilized with a power shear wrench. These bolts can be installed with only one worker. The wrench grabs the nut and the splined end during tightening, and when the spline piece breaks off, the bolts has proper tension.