AISC


6.11. Other General Information

6.11.1. Why is the design strength of a bolt calculated in the AISC Specification on the basis of the nominal crosssectional area rather than the net tensile area that remains after threading?

The ratio of stress area to nominal bolt area ranges from 0.75 for ¾-in. diameter to 0.79 for 11⁄8-in. diameter (per the Guide to Design Criteria for Bolted and Riveted Joints). Accordingly, to simplify calculations, the lower bound reduction of 0.75 is incorporated in AISC-tabulated nominal strength values for use with nominal bolt areas.

6.11.2. When is it permissible to reuse high-strength bolts?

High-strength bolts that have not been previously subjected to significant pretension are suitable for reuse. However, highstrength bolts that have been pre-tensioned may or may not be suitable for reuse as follows.

As stated in the RCSC Specification Section 2.3.3, ASTM A490 bolts and galvanized ASTM A325 bolts are never suitable for reuse if they have once been pre-tensioned in accordance with the procedures in the RCSC Specification Section 8.2. Reuse of nongalvanized ASTM A325 bolts is acceptable if approved by the SER. As a simple rule of thumb, a black ASTM A325 bolt is suitable for reuse if the nut can be run up the threads by hand.

Note the qualification in the RCSC Specification that “Touching-up or re-tightening bolts that may have been loosened by the installation of adjacent bolts shall not be considered to be a reuse.” Similarly, fit-up bolts (which are snug-tight when initially installed) may be left in place and subsequently fullytensioned, if required, as permanent bolts in the connection.

A discussion of the performance of high-strength bolts repetitively tightened can be found in the Engineering Journal article “Reuse of A325 and A490 High-Strength Bolts” (3rd Quarter 1991; available at www.aisc.org/ej).

6.11.3. What minimum stick-through is required for high-strength bolts?

None. As defined in the RCSC Specification Section 2.3.2, “The bolt length used shall be such that the end of the bolt extends beyond or is at least flush with the outer face of the nut when properly installed.” Some contract documents include a stick-through requirement (minimum protrusion of the bolt point beyond the nut). However, because the threaded length for any given bolt diameter is constant regardless of the bolt length, a stick-through requirement (which may require a longer bolt) increases the risk of jamming the nut on the thread run-out. Because a stick-through requirement does not enhance the performance of the bolt, its specification is discouraged. Note that there is no specified maximum limitation on bolt stick-through. However, in order to properly tension highstrength bolts, sufficient thread must be available. The use of additional flat washers under the head and/or nut is a common solution when there is a risk of jamming the nut on the thread run-out. Multiple washers are permitted under either or both the head and the nut. Nut jamming is not a concern for fully threaded ASTM A325T bolts. (See 6.2.6.)

6.11.4. When an extended end-plate moment connection is specified as slip-critical, must the slip resistance of the bolts at the tension flange be reduced for the tension present?

No. Because the tensile and compressive flange forces are equal and opposite, any loss of slip resistance adjacent to the tension flange of the beam is compensated for by an increase in slip resistance adjacent to the compression flange.

6.11.5. As indicated in the AISC Specification Table J3.2, when the pattern of fasteners in a bolted joint exceeds 38 in. in length, tabulated design strengths should be reduced to 83.3 percent of the tabulated values. Why?

As indicated in the Guide to Design Criteria for Bolted and Riveted Joints, the average shear strength per bolt varies with the number of bolts in the joint due to the non-uniformity of force distribution; see Figure 5.28 herein. To simplify joint design, bolt shear strengths in the RCSC Specification (see Commentary Section 5.1) and 2010 AISC Specification incorporate a reduction to allow the use of consistent per-bolt design strength for joints up to 38 in. in length. However, if joint length exceeds 38 in., the designer must further reduce the design strength. This phenomenon is a by-product of shear lag in the connection.

6.11.6. How do hot-dip galvanizing and mechanical galvanizing processes differ?

In the hot-dip galvanizing process, the piece is first degreased and cleaned with a combination of caustic and acidic solutions. After rinsing, the piece is dipped into a tank of molten zinc for a specified period of time. The full process is described in ASTM A153. In the mechanical galvanizing process, the piece is similarly cleaned and rinsed. The piece is then tumbled in a mixture of various-sized glass beads and a predetermined amount of water, with small amounts of chemicals and powdered zinc added periodically. Collisions between the glass beads, zinc and the piece cause a cold-welding process that applies the zinc coating. Powdered zinc is added until the specified thickness is attained. The full process is described in ASTM B695.