RE: ...no subject...[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: ...no subject...
- From: Charlie Carter <carter(--nospam--at)aiscmail.com>
- Date: Mon, 10 Jul 2000 15:38:54 -0500
>AISC, ASD Ninth Edition/LRFD 1St Edition, Volume II - Connections :
>"The occurrence of full design wind or earthquake loads is too
>infrequent to warrant consideration in fatigue design.", goes on
>to say -(p 1-11) "Consequently, slip critical connections are not
>normally required or used by engineers for wind or seismic loads
This publication was written well before the 1997 AISC Seismic Provisions required that bolts for high-seismic applications (generally, those with R taken as greater than 3) be pretensioned with faying surfaces prepared to achieve a class A slip resistance. I think the recommendation is still valid for wind applications and low-seismic applications (generally, those with R taken as 3 or less). All it really says is that wind and earthquake do not create fatigue like you would find in a mill building or vibratory load application with 20,000 or more load cycles.
>However - AISC's Seismic Provisions, dated April, 1997, section 7.2.a says:
>"Connections may use bearing values, but nevertheless - the faying
>surfaces are to be a minimum of Class A friction condition." This seems
>somewhat of a contradiction to me, but the requirements are
The idea here is that we know we can't prevent slip under high-seismic loading because we don't really know what the upper bound on the force is. So we select the number of bolts based upon bearing values but install them with pretension and prepare the faying surfaces for Class A slip resistance. The resulting joint is a bearing joint that has a reduced initial slip resistance, but is practical for design and construction. Otherwise, bolted flange plates might lap each other at the mid-span of the beam. (-:
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