Research on the viability of snug-tightened bolts, conducted over the past 15 years by the Metal Building Manufacturers Association (MBMA), Research Council on Structural Connections (RCSC) and the American Institute of Steel Construction (AISC), has resulted in time savings and simplified inspections for contractors and erectors.
In essence, connections using A325 high-strength bolts are permitted to be snug-tightened in most applications. Adopted by the 2003 International Building Code (IBC), the new snug-tight guidelines are being adopted by local code authorities across the nation.
Research has shown that snug-tightened high strength bolts, commonly used in the primary frames of metal buildings, provide connections just as strong as those that are fully tightened without requiring the time, effort and tools required to install the latter.
RCSC defines the snug-tightened condition as “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.” Firm contact is further defined as “the condition when the planes of contact between two plies are solidly seated against each other, but not necessarily in continuous contact.”
Since snug-tightened bolts do not require a specified pre-tension, the installation and inspection requirements are much less demanding and can be done through simple visual verification.
Snug-tightened bolts are not a new idea – in fact, they have been permitted in connections since 1985, but only where the load is transferred by shear in the bolts and bearing stress in the connected material.
The recent change, as adopted by RCSC, AISC, and IBC, extends snug-tightened A325 bolts to connections where they are in tension or combined shear and tension, such as bolted end-plates, as long as the application involves static loading. It is important to note that static loading includes environmental loads such as wind and snow.
The research sponsored by MBMA, RCSC and AISC compared both fully tightened and snug-tightened bolts through many cycles of loading. As it turns out, after a building is subjected to a number of wind cycles, the pretension of fully tightened bolts is reduced and is comparable to the tension of snug-tightened bolts.
Installation and Inspection
The bolt holes of snug-tightened joints are required to be aligned to permit insertion of the bolts without undue damage to the threads. Tightening of the bolts needs to progress systematically starting at the most rigid part of the joint. More than one cycle through the bolt pattern may be required to achieve the required snug-tight condition.
The only inspection required for snug-tightened bolts is to verify that:
· the proper fastener components have been used, including washers where required;
· the connected plies are clean steel;
· the bolt holes meet the diameter; and
· quality requirements and the plies of the connected elements have been brought into firm contact.
According to both AISC and RCSC, as long as the bolt holes are not oversized or slotted, snug-tightened A325 bolts are permitted in all low-rise buildings except for the supporting structure for cranes over five-ton capacity or other machinery or equipment where live loads produce impact or reversal of stresses. If A490 bolts are used, they must be fully tightened unless they are in shear/bearing with no tension present.
The use of snug-tightened bolts in seismic regions will require the judgment of the metal building manufacturer, taking into account the seismic design category of the particular application, or any specific requirements of the local authority having jurisdiction.
In terms of how snug-tightening is regarded by local authorities, acceptance is growing. Most building officials accept these new guidelines because of their adoption by the current IBC, even if they have not yet adopted the latest version.
MBMA continues to sponsor and participate in research, such as that related to snug-tightened bolts, which advances building performance and maximizes the value of every construction dollar invested in metal building and roofing systems. For more information on metal building and roofing systems, visit the MBMA website at www.mbma.com.