By Alfred F. Wong, P.Eng., FCSCE
CISC provides this column as a part of its commitment to the education of those interested in the use of steel in construction. Neither CISC nor the author assumes responsibility for errors or oversights resulting from the use of the information contained herein. Suggested solutions may not necessarily apply to a particular structure or application, and are not intended to replace the expertise of a professional engineer, architect or other licensed professional.
Question 1: What is the difference between “snug tightness” and “snug-tightened condition”?
Answer: The term “snug tightness” is defined in CSA S16-14, whereas the term “snug-tightened condition” is used in the reference below. They share the same definition: 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 into firm contact.
Question 2: Firm contact is defined in CSA S6-14 as the condition that exists on a faying surface when the plies are solidly seated against each other, but not necessarily in continuous contact. Does the lack of continuous contact adversely affect the clamping force in the joint and the joint’s slip resistance?
Answer: Continuous contact throughout the faying surface area between thick plies may not be possible. Snug tightness is attained if the plies are solidly seated against each other. As explained in the reference below, “if the specified pretension is present in all bolts of the completed joint, the clamping force, which is equal to the total of the pretensions in all bolts, will be transferred at the locations that are in contact and the joint will be fully effective in resisting slip through friction”.
Question 3: CSA S6-14 permits burrs that are 2 mm in height or less to be left in contact surfaces of bolted joints. Do these burrs affect the clamping force in the joints and their slip resistance?
Answer: Research studies conducted in the 1980s and 1990s demonstrated that the presence of small burrs did not adversely affect the slip resistance. However, greater effort may be required to bring the contact surfaces into “firm contact” in order to attain the “snug tightness”, which is particularly important when the turn-of-nut method is used to pretension the bolts. If the small burrs are not flattened prior to the nut turn count, a small reduction in bolt tension for the specified nut turn is expected.
Question 4: Should ASTM F959 direct tension indicators be used as tension calibrators for pre-tensioned bolts installed using the turn-of-nut method?
Answer: CSA S16-14 recognizes both the turn-of-nut method and the use of ASTM F959 DTIs as two independent methods for installation of pre-tensioned bolts. In any case, if for whatever reason the bolt tension installed using the turn-of-nut method needs calibration, DTIs give unreliable results for two reasons:
- the deformation of the DTI consumes part of the turn provided, and
- the turn-of-nut method is expected to achieve significantly larger pretension than the DTI method, and the larger pretension and slip coefficients are recognized in the design in accordance with S16-14, but these larger pretension values are not explicitly defined in S16.
Question 5: CSA S16 does not specify the oversize hole diameter for 1-inch bolts. Where may I get such information?
Answer: CISC Handbook of Steel Construction – 11th Edition, in Table 3-47, includes oversize hole dimensions for the Imperial series of bolts. The hole diameter values for bolts not covered in S16-14 were converted from the values provided in the reference below and rounded to the nearest millimetre.
Reference: RCSC. 2014 Specification for Structural Joints Using High-Strength Bolts. Research Council on Structural Connections.