Speaking in Code
Speaking in Code
Structural Steel Bolted Connections
The Nuts and Bolts of Nuts and Bolts
The story of Bolted Connections is an epic tale. It is a story that includes intrigue (fastener component selection), drama (design of the connection), mystery (pre-installation verification), action (installation), thrills (inspection), horror (arbitration), history (references) and the occasional miracle.
Below, F&R's code professionals have prepared a brief overview of some of the major elements associated with the selection, installation, and testing of bolted connections.
Depending upon issues such as the application and loading conditions that the joint will experience in the structure, the designer must carefully select which type of joint is required for any given project. The three types of joints that define bolted connections are...
Snug-tight bolt installation. The Research Council on Structural Connections (RCSC) defines a snug-tightened joint as one in which the bolts have been installed in accordance with RCSC section 8.1. The plies should be in firm contact (at bolt locations) but not necessarily in continuous contact. No specific level of installed tension is required to achieve this condition.
Pretensioned joint installation. A pretensioned joint is one that transmits shear and/or tensile loads in which the bolts have been installed to provide a pretension of 70% of the bolt's tensile strength in the installed bolt. They shall be designed in accordance with the applicable provisions of RCSC sections 5.1 and 5.3, installed according to section 8.2, and inspected in accordance with section 9.2. Requirements for faying surface (the plane of contact between two plies of a joint) condition inspection shall not apply to pretensioned joints.
Slip-critical joint installation. Slip-critical joints are technically a special case of pretensioned joints in which the faying surfaces are specifically prepared to meet a specified level of slip resistance.
Pre-installation verification is the testing of as-received bolts, nuts, and other components at the job site prior to installation in order to verify that they meet minimum code requirements. It also clarifies the proper implementation of the selected pretensioning method and adequacy of the installation equipment. For calibration wrench pretensioning, this testing shall be performed daily for the calibration of the installation wrench. The installation load determined in the pre-installation verification shall be applied to all bolts in the joint, progressing systematically from the most rigid part of the joint in a manner that will minimize relaxation of previously pretensioned bolts.
Snug-tight Bolt Installation. When dealing with snug-tight bolt installations, the code states that “the snug-tightened condition is the tightness that is attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the plies (faying surfaces) into firm contact.” The inspection of snug-tight joints requires that the inspector ensure that all fastener components comply with RCSC, section 2, visually determine that the plies of the connected elements have been brought into firm contact, and ensure that washers have been used as required in RCSC, section 6. No further evidence of conformity is required for snug-tight joints. The magnitude of the clamping force that exists in a snug-tightened joint is not a consideration.
For those of you who are unfamiliar with what exactly a “spud wrench” is, let us help you out a bit!
A spud wrench is an open ended wrench used in steel erection and steel inspection procedures. The opening in the open end can vary from a 7/8” opening up to a 2 ½” opening depending on the size of the bolts. The length of a spud wrench can range from about 12” up to about 24”. One end of the wrench has a continuous taper which makes lining-up bolt holes easier in the field. The wrench is forged from select alloy steel to withstand heavy loads and high-leverage.
Pretensined & Slip-critical Joint Installation. Regardless of which of the pretensioned bolted connection is being dealt with, these types of joints require more exacting inspection and testing as required by RCSC, section 9.2 through section 9.3. The degree and types of inspection procedures vary based on the individual types of connections. Turn-of-nut pretensioning methods may be required, or calibrated wrench pretensioning methods may mandate that the inspector observe pre-installation verification testing. The use of twist-off-type control bolts and direct tension indicators require additional inspection procedures in the field.
Arbitration is a testing/inspection procedure performed only after it is suspected or unknown that bolts in pretensioned or slip-critical joints do not have the proper pretension (RCSC, section 10). There are three distinct elements involved in the arbitration process:
- A representative sample of five bolt and nut assemblies of each combination of diameter, length, grade and lot in question shall be installed in a tension calibrator (i.e., Skidmore Wilhelm).
- A manual torque wrench with dial, or one that may be adjusted to indicate defined torque has been reached, shall be applied to the pretensioned bolt.
- Bolts represented by the sample shall be tested by applying, in the tightening direction, the arbitration torque to 10% of the bolts but no fewer than two bolts selected at random in each joint. If no nut or bolt head is turned relative to its mating component (by arbitration torque), the joint shall be accepted as properly pretensioned.
It would truly be a miracle if an erection contractor actually was able to comply with the dozens of specifications laid out in the code requirements the first time through. That is why inspectors like F&R are here – to help builders meet their requirements as quickly and efficiently as possible.
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Written by:Alan TuckFroehling & Robertson