Steel Moment Frame Connection 3

[roscoe]

I am designing and building a residence less than 2 km from a type A fault (The Hayward Fault). This will be a home constructed in the hills on an upslope lot. The front of the house requires a steel moment frame with two columns W12x31 and two beams W12x87 (one at the 1st story and one at 2nd story; each story is approx 10' high) The calculated seismic lateral loads are 22.5 kips at the 2nd story, and 31.5 kips at the first story. I've been reading a lot on the post-Northridge FEMA reports and am wondering which of the pre-qualified moment connections to use. I think we will use either the welded flange plate connection (WFP) or bolted, unstiffened end-plate (BUEP) connection. My question is: 1) does anyone have any comments on the which is the best moment connection for this type of residential application in terms of strucural integrity, ease of installation, cost, etc and 2) I noticed that the FEMA report only pre-qualifies the connections for steel grade 50 and above; what about using A36 steel? Thanks in advance

 

[Taro]

1. All of the prequalified connection types listed in FEMA 350 have been tested and shown to deliver acceptable strength and ductility for seismic resistance. The most economical connections for relatively low forces (residential applications) are usually shop welded/field bolted connections such as the BUEP because expensive field welding and inspection are minimized.

2. A992 or similar steels are recommended for moment frames because there are more stringent material property requirements that produce more reliable performance.

Also, I would caution you that using W12x87 beams with W12x31 columns will likely result in a "strong-beam/weak-column" situation in which the columns may form a collapse mechanism. This configuration is not preferred, because the columns are usually required to support significant gravity load in addition to providing lateral resistance. If it is absolutely necessary to use weaker columns than beams, I would recommend designing for amplified forces in order to reduce the amount of inelastic action required of the columns.

 

I would add to Taro's response that the problem with A36 steel is that it does not have a defined upper bound for the yield point. Many rolled shapes that were "A36" had a yield point above 50 ksi. This is typically not a problem with normal loading, but in seismic design, when you are designing the connections to be stronger than the members, member overstrength is a big concern. ASTM A992 gives you a definite upper bound on the yield. It is my understanding that for small to medium size rolled shapes (all but jumbo columns), A992 is the standard now anyway, and I typically specify it for all rolled shapes.

The strong column/weak beam is not necessarily required if you are designing the frames as Ordinary Moment Frames and using the R value of 4.5, but it is a very good idea! This is only critical at the floor. It doesn't matter whether the upper (roof) beam is stronger than the column, since a plastic hinge at the very top of the column is acceptable.