fema 350 bolted flange plate moment connection 1

[nitnelion]

I'm running into a problem with a moment connection and I was wondering if anybody could shed some light. I have a W18x119 beam (25'long) framing into a W18x143 column. It's designed to be an OMF. These sizes were picked to meet drift. I want to use a bolted connection to avoid alot of full pen welding in the field. I'm working through fema 350's design steps for a BFP connection (see fema 350 section 3.6.3.1). I calculate that Mpr is 16572 kip-in for the first step. This is basically the full plastic strength of the beam. So far so good. However, in the second step, I calculate Mf (moment at the face of the column) is 82458 kip-in! (see fema figure 3.3 - Mf = Mpr+ 2Mpr(x)/L') This is unbelievably large - way more than the actual capacity of the beam. The plate size and number of bolts required to make this work is enormous. What am I doing wrong? Thanks.

 

[JJMT]

nitnelion
i belive that Mf reflects the condition that you are 'forcing' the plastic hinge into the beam away from the connection. The prequalifed connection testing and design critera are to ensure that significant plastic deformation occurs away from the connection and in the beam section. I would suggest checking your 'x' distance and/ try reducing it (perhaps a different connection, i'm fond of extended end-plates). Also, if you only need a OMF, you do not need to use a prequlified connection design. I reccomend you look over AISC Seismic Provisions for Structural Steel Buildings, Part I, section 11.2a. I typically use the design and detail of the prequalifed connections for the reduced flextural strenth.

 

[tngolfer]

What if you went with a "dog bone" plastic region in teh beam to reduce the moment capacity of the beam and ultimately of the connection?