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Fitting Up Bolts

When faced with an application that requires larger scale connections and intense loads, there are a few options to look at when choosing fasteners. Lag screws are a common choice because they are some of the toughest fasteners and differ greatly from normal wood, self-drilling or sheet metal screws.

The fitting up bolt is the cousin of the lag screw. Both types of fasteners are used in the assembly of large connections and both must have a through hole drilled in the desired assembly. Fitting up bolts are a “quick assembly, temporary” fastener primarily used in structural steel connections and building construction applications.

Like its cousin the lag screw, fitting up bolts have hex heads and a unique thread. These fasteners were designed with a hex head design to allow iron workers to apply an open end wrench from any direction, allowing a quicker and safer assembly when faced with connecting “I” beams several hundred feet in the air. The hex head also provides a larger bearing surface around the hold which provides more friction between the interfaces of the connection. This is required in this type of static connected joint.

The unique thread present on fitting up bolts is called Acme thread form or trapezoidal thread. Trapezoidal threads offer speed during assembly and high clamp-load strength. This type of thread design has a flat or square crest on top of each thread, and the distance between the crest of each thread is greater than normal course thread. This feature allows the mating nut to be assembled and tightened quickly from various positions in the field.

Fitting up bolts always require a nut, which is why they bear the name bolt, not screw. Unlike lag screws, fitting up bolts are only used as temporary connections. They are designed to act as an alignment aid to hold the beams and structure in place until the structural bolts of a higher grade can be installed and tightened for the final assembly.

To learn more about fitting up bolts, their uses or the difference between lag screws and fitting up bolts, contact Bossard at ProvenProductivity@bossard.com.

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August 29, 2014

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