MODERN STEEL CONSTRUCTION september 2013
Field Verification
Is the field verification of dimensions for connections to an
existing structure the responsibility of the steel fabricator?
Typically, field verification of existing conditions is the
responsibility of the contractor, not the steel fabricator or
erector. The AISC Code of Standard Practice Section 1.7.3
states:
“… field dimensioning of an existing structure is not within
the scope of work that is provided by either the fabricator
or the erector.”
However, it is always a possibility that the contract documents
could state otherwise. I suggest you check what the contract
documents say, as this could be different for each project.
Keith Landwehr
First Order Analysis Method
In the calculation of N
i
in AISC 360-10 Equation C2-8 for
a portal frame where the columns have different lengths,
which length should be chosen to represent the story
height (L)? Should it be the shorter one, the longer one,
the average of the two or something else entirely?
The equation that you are referencing was developed assuming
columns of equal height. However, it can be adapted to your
situation. There are two options that I think would be applicable.
➤ Conservatively use shorter column height.
➤ Discretize your notional loads to be on a per-column basis;
this option uses a notional load on each lateral column
based on its height and tributary load.
Heath Mitchell, S.E., P.E.
Reuse of High-Strength Bolts
Can bolts that have been pretensioned be reused?
RCSC Specification Section 2.3.3 prohibits the reuse of
“ASTM A490 bolts, ASTM F1852 and F2280 twist-off-type
tension-control bolt assemblies, and galvanized or Zn/Al Inor-
ganic coated ASTM A325.” It allows the reuse of black A325
bolts, at the discretion of the engineer of record. The Com-
mentary provides further information:
“Pretensioned installation involves the inelastic elongation
of the portion of the threaded length between the nut
and the thread run-out. ASTM A490 bolts and galvanized
ASTM A325 bolts possess sufficient ductility to undergo one
pretensioned installation, but are not consistently ductile
enough to undergo a second pretensioned installation. Black
ASTM A325 bolts, however, possess sufficient ductility to
undergo more than one pretensioned installation as suggested
in the Guide (Kulak et al., 1987).”
Section 4.5 of the Guide to Design Criteria for Bolted and
Riveted Joints provides more detailed references and research
on this topic.
Erin Criste
Slip-Critical Surface Preparation
In a slip-critical connection, are the surfaces under the
bolt head and/or washer subject to the same of surface
preparation requirements as the inside plies?
No. The area under the nut and head are not subject to the
same requirements as the faying surfaces. The Commentary
to the RCSC Specification (a free download from www.
boltcouncil.org) does, however, caution against large coating
thicknesses (including under the head and the nut). It states:
“Tests have indicated that significant bolt pretension may
be lost when the total coating thickness within the joint
approaches 15 mils per surface, and that surface coatings
beneath the bolt head and nut can contribute to additional
reduction in pretension.”
Larry S. Muir, P.E.
Coating Thickness Measurements
When painting multiple beams on a paint rack is it accept-
able to check the coating thickness on one piece per rack or
do I have to check the coating thickness on all of the beams?
The requirements of SSPC-PA2 (May 2012) address your ques-
tion. Section 8.2 requires that five spot measurements (three read-
ings per spot) be taken for each 100 sq. ft of coating surface. PA2
simply says “random” spots. If each of your beams consists of 100
sq. ft of coating per beam, then you’ll be checking each beam.
Assuming less than 100 sq. ft for each beam, then it is typi-
cal to find direction in your in-house painting and inspection
procedure. The contractor has the option of defining “ran-
dom” so as to provide direction to the painter and/or coating
inspector. I have experienced five spots on one beam and five
spots split between multiple members depending on the con-
tractor’s experience and consistency of the coating process.
Keith Landwehr
steel interchange
Heath Mitchell is director of technical assistance and Erin Criste is staff engineer, technical
assistance at AISC. Keith Landwehr and Larry Muir are consultant to AISC.
Steel Interchange is a forum to exchange useful and practical professional ideas and
information on all phases of steel building and bridge construction. Opinions and
suggestions are welcome on any subject covered in this magazine.
The opinions expressed in Steel Interchange do not necessarily represent an official position of
the American Institute of Steel Construction and have not been reviewed. It is recognized that the
design of structures is within the scope and expertise of a competent licensed structural engineer,
architect or other licensed professional for the application of principles to a particular structure.
If you have a question or problem that your fellow readers might help you solve, please
forward it to us. At the same time, feel free to respond to any of the questions that you
have read here. Contact Steel Interchange via AISC’s Steel Solutions Center:
1 E Wacker Dr., Ste. 700, Chicago, IL 60601
tel: 866.ASK.AISC • fax: 312.803.4709
The complete collection of Steel Interchange questions and answers is available online.
Find questions and answers related to just about any topic by using our full-text search
capability. Visit Steel Interchange online at www.modernsteel.com.