Steel column bracing within Timber framing - K-factor

[JackRob]

Hello,
Long time lurker first time poster.

I was having a discussion regarding steel column K-factor in the weak direction, within timber framing structures. (moment frames lateral support in weak direction)

Single-Family home, 2-Story, (30'x60'), flat roof. In the back of the house we have a steel moment frame, and a lot of shear-wall's within the house in each direction. Full plywood diaphragm tying into all the steel/members/walls. Axial loading primarily due to Snow/Live. I've attached a sketch for the back portion of the house. 1st floor and roof have similar framing.

Col 1 & 2 are not continuous, Col 3 is continuous.

The discussion was whether, in the non moment-frame direction, if all 3 steel columns are considered Pin-Pin (K=1) or if columns 2 & 3 are considered (K=2), as column 1 is the only column with a member tied back directly to a lateral restraint.

I argued that they will each behave closer to Pin-Pin (K=1), as the diaphragm is tied into the entire lateral restraint system which dies into the steel. Thus the diaphragm connection at the steel framing would have to fail in order for the column to be unsupported. at the roof and floor levels.

Who is right here?

 

[Earth314159]

It looks like it is pin-pin (K=1.0) to me unless I am missing something. The diaphragm restrains the frame. If the frame is droped significantly below the floor level where there is no restraint, then you may end up cantilevering the frame out of the foundation in which case you would consider a higher K value.

 

[JackRob]

Thank you for the input Earth314159.

The structure is all flush framing, beams/joists at the same elevation. Steel beams have a nailer at the top flange, carriage bolts, nailed sheathing.

I believe one of the arguments was with the wood diaphragm to steel. That issue has been discussed on this forum a few times, to which most people have argued that wood diaphragms to steel beams work for lateral support. So in theory if the diaphragm is accepted to work this way, this should also mean it restrains the column. Using the 2% rule, even with 30kips on the column, you're looking at 600lbs to restrain it.

If the joists ran parallel to the steel beam, but the diaphragm still carry's to the steel beam, does this make a significant difference?

 

[Earth314159]

With joist parallel to the beam, the connection to the diaphragm to the nailer on top of the beam is still usually adequate. I would say that the connection of perpendicular joists and the plywood together is better. If there is any doubt, you can also add blocking perpendicular to the beam if the joists are parallel. This will also help for wind loads.