The "turn of nut" method is visible in this bolted connection on the Canadian Museum for Human Rights in Winnipeg, MB, Canada by Antoine Predock.
Most bolts can be simply installed to a snug-tight condition, i.e. to the maximum of a worker’s strength. They do not have to be pre-tensioned. Bolts only need pretensioning under special conditions: when slippage cannot be tolerated, for seismically stable connections, when subjected to impact or cyclic loading, when they are in pure tension or when oversized holes are used. Otherwise, the snug-tight condition is adequate for the normal end connections of beams. Deciding to pretension a bolt is a question of the application rather than how large a load it needs to transfer. If bolts do have to be pretensioned, “turn-of-nut” is the preferred method. After the bolts are snug-tightened, an additional fraction of a turn is applied to the nut to achieve the desired tension in the bolt. Usually, a worker will draw a chalk mark across the diameter of the bolt before applying the extra turn. Hence, an inspector can check if the fraction of turn was observed. In many conditions, only an additional third of a turn is needed to achieve the desirable pretension in the bolt. TC bolts are another way of achieving the desired tension in the bolt, but many feel that the conventional “turn-of-nut” method is the most reliable. It is actually very difficult to determine the tension in a bolt based on a torque value because friction plays an important role. For calculating the tension in the bolt it has to be derived from the torque value. Once converted, the value is often not representative of the real tension in the bolt. This is especially true for galvanized bolts. Left: The head of the Tension Control bolt is quite distinct from the regular Hex Head bolt. The washer and nut for tightening are on the backside of the connection, so connection design must provide access to the rear for tightening. TC bolts are used where slip prevention is important. On the Bow Encana erection they are being used to secure the temporary column to column connections prior to finish welding. Right: This beam is ready to ship, its splice plates attached with high-strength Hex Head bolts. Structural bolts like these will normally place the nut side where access is easiest.
In Architecturally Exposed Structural Steel design (see Chapter 6: AESS: Design and Detailing) the choice of bolt head, pattern of attachment and preference for the side of the connection on which the bolt heads are located will be important to the visual architectural appearance. Much of the required construction tolerance for erection will be a function of the degree of precision in the alignment and drilling of the holes for the bolts. It is a common misconception that bolt holes are routinely oversized to make it easier to align members during erection. Imprecision will result in accumulated errors that actually make erection more difficult. Bolt holes within a steel framing system have tight tolerances – tighter even in AESS design where “fit” is important. Slotted holes are only used where secondary systems, such as curtain wall, are attached to the steel framing, in order to adjust for deviations between the alignments of the systems used.
Hex Head versus Tension Control Bolts Left: Assembly of a Hex Head bolt. A standard washer, sitting on either side of the connection between the steel and the head/nut, assists in distributing the load. These types of bolts are usually installed to a snug-tight condition and they normally do not need to be pretensioned. Right: Assembly of a Tension Control bolt. The special compressible washer is placed only at the rear side of the connection. There are some proprietary types of washers that contain small pockets of brightly colored material that will squeeze out when the desired tension is achieved.
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