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Re: pretensioned HS bolts

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Karen,

The first thing is to make sure you understand that in ALL seismic connections the bolts are required to be high strength and to be fully tensioned.  However, it is your choice whether to design them as bearing bolts or slip critical bolts.  There are a few cases where SC bolts are required as you mentioned but in AISC "speak" significant load reversals are NOT seismic loads.  They are primarily talking about major and many load reversals such as in a crane beam.

It probably sounds crazy to use bearing bolts in seismic connections but the testing showed that in moment frames and bracing the bolts would nearly always slip for the demand required when using the higher R factors.  That is, your "slip critical" bolts will eventually end up as bearing bolts.

This is why the AISC Seismic Provision, 7.2a, requires fully tensioned bolts but allows the bolts to be designed as bearing.  The AISC commentary gives a good explanation also.  Looking at it another way, AISC is covering themselves both ways.

Thomas Hunt, S.E.
ABS Consulting



"Casano, Karen" <Karen.Casano(--nospam--at)dgs.ca.gov>

08/14/2002 10:51 AM
Please respond to seaint

       
        To:        "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
        cc:        
        Subject:        pretensioned HS bolts



Hello,

I am hoping to get some input or guidance from all of you structural steel
experts about the use of fully-tensioned or slip critical, high
strength(A325SC or A490SC)bolts. I have always specified them for use on all
seismic connections including drag lines and any joints subject to direct
tension, but I'm noticing that other engineers don't do this.  
According to the RCSC Specification for Structural Joints Using A325 or A490
Bolts, SC bolts should be used for joints "subject to significant load
reversal", and "joints in which, in the judgement of the Engineer, any slip
would be critical to the performance of the joint or structure..".  Do these
apply to seismic resisting systems (or wind)?  In FEMA 350 and AISC Chapter
4, the bolted MF connections show use of "pretensioned" bolts, however, in
the HSS manual Moment connections, I don't see mention of tensioning the
bolts.  
In regard to the direct tension connections, Chapter J in the AISC ASD
manual (Table J3.2)indicates that the allowable tension for a high strength
bolt is independent of the intial tightening force (listed as 44 ksi for
A325 and 54 ksi for A490); however, the RCSC specification (Table 2)
indicates that applied static tension allowables are only for pretensioned
bolts (footnote b).  ??


karen casano, PE
san diego