10.034 Minimum Bolt – Nut Engagement

Q I’m working on a project where the bolts were mis-ordered short. When installed according to the plans and specs, the end of the bolt is flush with the face of the nut. How much of a bolt must project beyond the nut when properly installed?

— Special Inspector in San Francisco

A 10/12 – Updated response prepared by Terry Egland, a principal at Testing Engineers, Inc. and a registered engineer in California

Though a common rule-of-thumb is “three threads past the nut”, it is just that – a rule-of-thumb, with no basis in fact, In order to arrive at the right answer, we need to follow the reference document chain.

If we start with the 2012 IBC, Section 1705.2.1 states “… Special inspection for structural steel shall be in accordance with quality assurance inspection requirements of AISC 360”, which is a very broad statement. Chapter 35 is a little more specific by designating AISC 360-10 “Specification for Structural Steel Buildings”.

Section N5 (6) uses the provisions of RCSC Specification for Structural Joints Using Gigh Strength Bolts to confirm materials, procedures and workmanship. Thus, we have arrived at the collect specifying document, Specification for Structural Joints Using High Strength Bolts, hereafter referred to as the RCSC 2009.

Section 2.3.2 of the RCSC 2009 states, “The bolt length used shall be such that the end of the bolt extend beyond or is at least flush with the outer face of the nut when properly installed.” So while the old rule-of-thumb won’t hurt, the short bolts will work just fine,

Reference Documents

2009 RCSC “Specification for Structural Joints Using High Strength Bolts”

AISC 360-10 Specification for Structural Steel Buildings