RE: Design Moment For Semi-Rigid Connections[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Design Moment For Semi-Rigid Connections
- From: Charlie Carter <carter(--nospam--at)aiscmail.com>
- Date: Tue, 24 Apr 2001 14:04:40 -0500
> In the Design of
Semi-Rigid Connections in Steel
> Frame buildings I have typically used moment
> magnitudes generated by Wind or Seismic
> Loading [per AISC's] Engineering for Steel
> Construction Chapter 4.
> In "Design Tables for Top and Seat Angle w/ Double Web
> Connections" by Yosuk Kim and Wai-Fah Chen
> in the Engineering Journal 2nd Quarter ...
> they used 1/2 the Plastic Moment of the
> beam as the design moment for connection
> ( "6" times my wind moments).
> Is this a Limit State
Criteria? Should I redesign
> my connections based on this? Is this approach
> overkill for buidings less then 35' high and in
> low seismic regions?
Your approach is consistent with usual practice for flexible wind connection design. The Kim and Chen paper just used 50 percent of the fixed end moment as a way to get an end moment for their design example. There's no requirement to use that as a minimum, though.
The design approach for flexible wind connection systems is the same in LRFD and ASD, so it's not a limit-states (or any other) criteria either -- just a convenience used by the authors in writing their paper. Given that wind moments may be much lower than 50 percent of the fixed-end moment, it may be overkill as you mentioned. Personally, I'd use the analysis value as is customary in historic practice.
Flexible wind connections are a "poor-man's" approach to PR connections and were popularized by AISC's Bob Disque in the early 60s: Disque recommended that the beams and beam end shear connections be designed for gravity as simple beams, the flange connections be designed for the wind moments from a portal (or similar) analysis, the columns be designed for gravity plus wind moments, and the details be such that there is enough inelastic deformation capacity to avoid overstressing fasteners or welds. There are two papers he wrote on it in the AISC Engineering Journal (3rd qtr 1964 and 1st qtr 1975). The overall frame stability of flexible wind connection system was addressed by Gertsle and Ackroyd, also in the AISC Engineering Journal (1st qtr 1990). Their main conclusion was that the system was OK up to 10 stories, within other limits as described in that paper. It may be interesting to note that the steel frame of the Empire State Building was designed this way. However, there is also significant masonry infill and core bracing in that building, so it's not really just a flexible wind connection system. Bob Disque told me he thinks flexible wind connections are best suited for two stories or less.
Although these kinds of systems have always been talked about as flexible WIND connection systems, I personally think they could be applied for seismic as well, provided the R factor is not taken greater than 3. Perhaps we should call them flexible moment connections to be more generally applicable.
P.S. AISC's Engineering for Steel Construction ("The Big Red Book") was integrated into the AISC Manual of Steel Construction (as Volume II) and no longer exists. It's information (mostly on connection design) is now found here for LRFD:
and here for ASD:
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