Tightening of Structural Bolts: Report on the Torquing Requirements of Bolts in Structural Connections

by Albert Schepers GS Engineering Consultants Inc.
Prepared April 1993 Updated April 2009

Scope

This report is limited to the torquing requirements of high tensile bolts in structural connections. Research is limited to standards and information published by the Canadian Institute of Steel Construction (CISC). Technical aspects or studies are beyond the scope of this report. For detailed information, reference should be made to the CISC publications listed at the end of the report and to associated research and publications.

At one time torque values were specified. Current practice, however, demonstrates that achieving a specific torque is not necessary. In addition to changes in torquing requirements, the need for flat washers has changed over the years. Previous requirements stated that washers were needed under either the head or the nut, depending on which was being tightened, and in some cases were needed under both the head and the nut. Currently, washers are not mandatory except under specific conditions such as A490 bolts and oversized holes.

Findings

Historical Study

This is a brief overview of the code requirements for the torquing of bolts in structural connections. The specific codes referenced are the CSA Standards S16 and S16-01, Allowable Stress Design and Limit States Design. Currently, CSA S16-01 is the standard used for the design of Steel Structures for Buildings. Until 1969, CSA S16 was the design standard used for the design of Steel Structures for Buildings. The change was made due to a change in design philosophy, from allowable stress to limit states design.

In CSA S16-1961, a value for pretensioning high strength bolts was specified. No reference is made to the torquing of the bolts to achieve this pretension. The method described in the standard is the Turn of the Nut Method. This method assumes that the fasteners are snug tight and then tightened through either a half or three quarter turn. Specific requirements are given in paragraph 35.5.4 of CSA S16-1961.

Over the intervening years, specific torque values were included in the design standard. The earliest version in which they were included is found in CSA S16-1969. Torque values may have been included in other releases of the standard. They were not available, however, for review at the time this report was prepared.

The 1969 release of CSA S16 was the last for the allowable stress design standard. The CSA S16-01, Limit States Design, standard replaced CSA S16, Allowable Stress Design, in the early 1970's. Torquing requirements were included in CSA S16-01 until 1978 but were not specified in the 1984 or 1989 editions. All editions since 1984 refer to the Turn of the Nut Method for the pretensioning of high strength bolts in structural connections.

Over the same time period, the requirement for hardened washers also changed. The practice had been to install a hardened washer under either the nut or the head, depending on which part was being turned, with some situations requiring washers under both parts. The current standard requires hardened washers under either the nut or the head of A325 bolts, depending on which part is being turned, for bolts fitted in oversized holes only. Connections using A490 bolts require that hardened washers be installed.

Discussion

Over the years, the requirements for the torquing of bolts has changed. The requirements of the early 1960's (that no specific level of torque is necessary when pretensioning high strength bolts) were incorporated into the requirements for the 1990's. The only real change in the standards over the years has been in design philosophy, shifting from allowable stress to limit states.

Research has shown that the required pretension in bolts may not be achieved when a specified torque was applied. This is due to several factors including: whether the bolt is galvanized or coated, the tolerance between the nut and bolt and the amount of latent cutting oil on the threads. In some cases, research has shown that bolts may fail in tension before achieving the specified torque.

Another factor affecting the decision to use the Turn of the Nut Method for pretensioning bolts is the fact that the amount of pretension on the bolt does not affect the shear capacity of the joint. The finish on the faying surfaces and burrs have also been found to have little effect on the shear capacity of the connections.

Conclusions

Inspections are currently carried out using a torque wrench as there is no other practical method for measuring how tight bolts are. This does not mean that a minimum torque must be achieved but that some torque has been applied; the bolt is more than finger tight. In addition, connections are not checked to determine whether washers have been installed except for A490 bolts or when the connection is made with oversized holes.

References

"Connections for Steel Structures", Canadian Institute of Steel Construction, September 1989
Diliger, W., Beauchamp, J. C., Cheung, M.S., and Gali, A., 1981, "Field Measurements of Muska River Bridge", J. of Struct. Eng., ASCE, Vol. 107, No. 11 Nov.
"Handbook of Steel Construction", Canadian Institute of Steel Construction, second edition 1974
"Handbook of Steel Construction", Canadian Institute of Steel Construction, fourth edition 1986
Kulak, G.L., Fisher, J.W., and Struik, J., "Guide to Design Criteria for Bolted and Riveted Joints", J. Wiley
Mann, A. P., and Morris, L.J., 1984, "Lack of Fit in High Strength Bolted Connections", J. of Struct. Eng., ASCE, Vol. 110, No. 6, June
"Structural Steel for Buildings", Canadian Institute of Steel Construction, first edition 1962