American Institute of Steel Construction
Live Webinar: High Strength Bolts - April 14, 2011
AISC Live Webinar: High Strength Bolts
Date: April 14, 2011
Additional Questions and Answers from the Presentation
No.
Question
Answer
We are not quite sure we understand the question. Pretensioning anchor rods
is outside of the scope of AISC and RCSC Specifications. It can be done, but
the RCSC pretensioning methods do not apply. The EOR will have to
determine installation requirements that will achieve their desired pretension
and account for any items that may contribute to relaxation of the rod
elongation (loss of pretension). A couple examples of those items are concrete
creep and debonding of the rod from concrete.
There are anchor rods with strengths equivalent to A325. F1554 Gr. 105 is
close, A449 and A193 Gr. B7 also have similar ultimate tensile strengths to
A325. See AISC Specification Section A3.4 for approved rod Specifications
and 13th Edition AISC Steel Construction Manual Table 2-5 for a summary of
ultimate tensile strengths.
1
Anchor rods can effectively be pretensioned against
steel-it takes a little bit of concrete creep/shrinkage to
relieve all the pretension - from AISC design guide #1.
Can we then rely on pretensioning against concrete?
2
Is there an A325 Anchor rod(bolt)?
3
Do we have to be concerned of nuts turning loose after
being pretensioned due to fatigue or repeated use?
It depends on the application. A good summary is provided at the following link:
http://www.larrymuir.building.officelive.com/Loose.aspx.
4
Can double nuts be a substitue for pretensioning? All
nuts are to be snug tightened- this something I want to
suggest to reduce cost and speed up the process.
No.
5
How many threads need to be engaged to develop the
tension capacity of the bolt?
See FAQ 6.11.3: http://www.aisc.org/dynamicmain.aspx?id=2100.
Our spec requires 3 threads in the grip of the
connection. How important is this requirement in the
performance of the connection?
There are not any AISC or RCSC requirements similar to this. However the
bolt must be short enough to allow full engagement of the nut and tolerance for
tightening. Structural bolts typically have a constant thread length that is
independent of bolt length. It seems like this requirement may be aimed at
providing sufficient thread length so the nut won't hit the thread run-out. This
can be accomplished by good detailing practices without requiring a specific #
of threads within the grip. Bolt length selection tables are published (see
Engineering Journal article by Carter on this subject).
6
There's always a solution in steel.
1
American Institute of Steel Construction
Live Webinar: High Strength Bolts - April 14, 2011
Additional Questions and Answers from the Presentation
No.
7
8
9
10
Question
Answer
Tighening method is based on that specified in the contract documents. Per
Section 4 of the RCSC Specification, the EOR shall specify the type of joint as
In order to tighten A325 regular connections, should we
snug tightened, pretensioned or slip-critical. RCSC Specification Section 8
use turn of nut method or is snug tight good enough?
contains the installation requirements. Snug tight is not an appropriate
substitution for joints specified to be pretensioned or slip critical.
No. Pretensioning requirements in AISC and RCSC Specifications are not
guarantees that nuts will not loosen. Typically preventing nut loosening is a
In order to guarantee nuts do not get loose, do we have
consideration separate from pretensioning and based on anticipated loading
to use turn of nut method?
and type of service. See FAQ 6.5.1:
http://www.aisc.org/dynamicmain.aspx?id=2100.
Eccentricity is always considered. However, testing has indicated that for
certain configurations, the AISC design procedures account for the eccentric
When do you consider eccentricty in a simple shear
effects without explicitly adressing them in the design. Eccentricity is discussed
connection?
for each connection type discussed in AISC Steel Construction Manual Part
10.
13th Edition AISC Steel Construction Manual Table 2-5 lists Fu for the
fasteners approved for use with the AISC Specification. SAE grades are not
approved for use with the AISC Specification, so we are not familiar with a
Is there source for Fu of all bolts including SAE, etc.?
source with relevant design information. Maybe the Industrial Fasteners
Institute would have an appropriate reference. See also FAQ 6.2.5:
http://www.aisc.org/dynamicmain.aspx?id=2100.
High strength steel bolts above a certain critical strength level are susceptible
to cracking as a result of hydrogen embrittlement. This susceptibility applies to
ASTM A490 bolts. There is only one protective coating approved for use by the
A490 Specification, and it is a Zinc/Aluminum compound. Cadmium plating of
A490 bolts is not permitted.
11
Can you comment on enbrittlement of A490 bolts when
they are cadmium plated?
12
Engineering judgment is all we can invoke here because I am not aware of any
physical testing. So, treat each direction as a separate load case and be
How do you consider block shear for connections loaded conservative in assessing the results. The Seismic Design Manual collector
in two directions? SRSS?
connection design example illustrates on such way to tackle this problem.
There is another method shown in the IBC Design Examples for the SCBF
system.
There's always a solution in steel.
2
American Institute of Steel Construction
Live Webinar: High Strength Bolts - April 14, 2011
Additional Questions and Answers from the Presentation
No.
13
14
Question
Answer
Yes, bolt preloads in excess of the specified target value (70% of the bolt
ultimate strength) are acceptable. As the bolt yields with the application of
Re. pretensioning, as long as not we're not breaking the
more and more applied deformation (twist, if you like) we find that even on the
bolt, exceeding the specified pretention value is
verge of failure, the strength of the bolt will still be above the specified
acceptable? What if the pretension turns out to be less
minimum value of 70% UTS. There are many physical tests that support this
than 70%?
observation. In response to the second part of the question, if the pretension is
less than 70%, then the installation is not acceptable.
Dr. Kulak stated that friction force is independent of
friction area, so why is ovs hole SC value less than
standard hole SC capacity?
15
Can you discuss the issue of bolt banging? We've had
some structures evacuated when slip occurs because
the slip sounds like a gun shot.
16
When would you use the values for slip-critical
connections as a strength limit state vs. a serviceability
limit state?
There's always a solution in steel.
I hope that what I said was that the friction force is independent of the area of
the contact surfaces. Yes, connections with oversize (ovs) holes and with
standard holes have different values. This is intended to recognize that the
consequences of slip in these two cases obviously will be different.
I know that this phenomenon has been reported, although I hope that it hasn’t
occurred very often! The main thing to know is that this is not a safety issue.
The topic is a lengthy one, however, and I suggest that you look at the AISC
Frequently Asked Questions and also see what the Engineering Journal says. I
believe that there is at least one paper in EJ written on the topic. You can also
find MSC articles on the subject here:
www.aisc.org/bookstore/itemRedirector.aspx?id=14854 and
www.aisc.org/bookstore/itemRedirector.aspx?id=14858
The most illustrative case (for those designing buildings) is to require the
condition of “no slip permitted” at ultimate when the designer sees that
ponding, or something similar, can take place as the loads come on. Generally
speaking however, the joints in a building structure do not have to be designed
as slip-critical. An example of a serviceability limit state requirement is for the
joints in a bridge structure. Clearly, we cannot allow slip under the design loads
because in this case slip (or even polishing under repetitive loads) can result in
fatigue cracks.
3
American Institute of Steel Construction
Live Webinar: High Strength Bolts - April 14, 2011
Additional Questions and Answers from the Presentation
No.
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19
Question
Answer
For TC bolts, how can you "calibrate" when the spline
will twist off the bolt? Or should it really say "check" to
make sure there is proper tension in the bolt when the
spline shears off?
If the assembly has been properly calibrated, then by definition the spline
shears off at the intended pretension. Using something like the SkidmoreWilhelm hydraulic calibrator, a representative sample of the fasteners to be
used (same geometry and friction conditions as will be used in the real
structure) is tested to ensure that the proper pretension is attained at the time
that the twist-off of the spline takes place. If any of the field conditions change,
e.g., the bolt length changes from, say, 4 in. to 6 in., then a new calibration is
required. This somewhat lengthy answer is really the reverse of your question:
there is proper pretension when the tip shears off.
Readers of this assembly of questions can look at the figure in the original
handout. This is the proverbial “good question.” Either there was enough
On the diagram on page 51(slide 101), how were the TC clearance on the backside of the connection to be able to use the powered
bolts installed with the spline pointing into the web of the wrench (the likely case) or a few bolts were installed manually. For the bolt
column?
sizes illustrated, a manual installation is not a problem. The connection shown
looks to me like a laboratory test specimen and perhaps that is reflected in
your question.
Neither of these approximations is very good! A little later on in the
presentation (look at slide 32) we noted that, based mainly on physical tests,
Isn't shear stress in a circular section calculated as
the relationship for the shear strength (at ultimate) as a function of ultimate
4V/3A (instead of V/A)?
tensile strength of the bolt is (see eqn. below). As an LRFD expression, just
throw in a resistance factor. If you want it in ASD, it can be worked out as
required.
τ  0.62 σ u bolt
There's always a solution in steel.
4