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- To: <seaint(--nospam--at)seaint.org>
- Subject: steel moment connections
- From: "Andrew Kester" <akester(--nospam--at)cfl.rr.com>
- Date: Thu, 2 Apr 2009 20:35:21 -0400
Listers, sorry this is a little long.
Maybe I am missing something very obvious here, but anyone who details steel connections maybe can shed some light on this.
I normally would design moment connections for the actual moment, but we are the specialty engineer in a situation where the EOR has called out to design the connections for the full moment capacity of the beams (when they don't provide the moment on the plans). They provide on their details that flange force shall be computed by T=0.95 X Mom / depth , standard AISC stuff. The structure is a mechanical floor (floor framing) and there is an upper level of penthouse, from what we can gather there may be some wind lateral in the structure. They have supplied all of the reactions and SOME of the moments.
We are trying to use flange plates rather than direct welding of the WF to the columns using full pen welds to make things easier for our client, the fabricator. The plate sizes we are getting seem unreasonably large in area, probably because I have always designed for the actual moment, not the moment capacity of the beam. Now we just used the beam span, type, and unbraced length to get the design moment from AISC charts, which is likely MUCH larger than the actual moment at the ends of the beams.
Here is what we cannot figure out, why if we calculate the Tu of the flange area, it is much less than the Tu we are calculating from the above discussion?? Now what I anticipated is the Tu of the flange area would be much greater than the value we were designing the flange plate for, because there is no way flange tension yielding controls the bending moment , right?
To put it another way, if we were to design the flange plate to fully develop the FLANGE AREA in tension, with the beam at 50ksi and the plate at 36 ksi, we get a much smaller plate than the other method. Why the difference if you are fully developing the flange in tension just as if you were doing a full pen weld directly to the column (which is also allowed per the EOR details)?
I reviewed my mechanics of materials book and everything to see what I was missing, but it did not get specific enough in this situation. I know in theory the web carries some tension, but I thought it was neglected and moment connections are always only flange-couples anyways....
Please CC me directly also:
Andrew Kester, P.E.
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