STEEL PORTAL FRAME STABILITY IN RESIDENTIAL FRAMING

[DSPE01]

I AM WORKING ON A DESIGN WHEREBY THE ARCHITECT IS REMOVING 18' OF INTERIOR BEARING WALL AND REPLACING IT WITH A STEEL BEAM. SUPPORTING 2ND FLOOR AND CEILING FRAMING. THE ARCHITECT WOULD LIKE TO USE A TS4X4 (APPROX. 16' TO BASEMENT FOOTING). A SIMPLY CONNECTED BEAM SITTING ON TOP OF A POST CAP PL IS BY ITSELF UNSTABLE. I WOULD HAVE TO RELY ON THE HOUSE FRAME STABILITY FOR STABILIZING THIS SIMPLE FRAME. IS IT APPROPRIATE TO USE 2% OF THE POST LOAD AS THE FORCE REQUIRED TO BRACE FOR STABILITY IN 2 DIRECTIONS?? WOULD IT BE MORE APPROPRIATE TO CONSTRUCT A MOMENT FRAME OR WOULD THAT BE TOO CONSERVATIVE.

 

[a2mfk]

If its just a gravity frame, and you properly tie into the diaphragm in both directions and the beam is properly braced, I don't see a glaring issue from your description.

 

[csd72]

dspe01,

you have not given enough information to give a complete answer but it sounds like you need an experienced structural engineer.

The answer depends on if the 18' of wall is required for stability. If it is then you need to also consider deflection of the frame under the lateral loads.It is common practice here to use a picture frame (4 sided frame) in these situations.

 

[DSPE01]

Thanks, the top flange of the beam will be braced to the wood floor joists at 4'oc. It is an old house with t&g flooring & wall sheathing on a diagonal so I am not sure if that will provide adaquate diaphragm and shear wall action. The load is just residential gravity load.

 

[DSPE01]

I am a Structural Engineer however I don't typically work with residential framing. I understand that bracing columns in both directions is always critical. I am just not sure if it is common practice to rely on the house floors and walls to provide stability support (no external lateral forces beyond stability).
thanks

 

[csd72]

You have to make that judgement call when you view the building. Personally I would only rely on diapragm action from Tand G floors for a limited span.

 

[apsix]

If you're removing 18' of wall, stability of the whole building will have to be assessed.
If the house is stable, your frame likely to be stable if tied to the floors.

 

[msquared48]

OI would consider abandoning the steel and going with an upset wood Glulam or Microlam (top would be flush with underside of the floor diaphragm. Just have to temporarily shore on either side of the beam to cut the joists to get the beam in, placing the cut joists on joist hangers. Gives more headroom this way. This solution will avoid mixing steel and wood beams, something I like to avoid if at all possible.

I assume from you post that the TS 4X4 is the end column, not the beam. If it is the beam, it probably will not work considering deflection limitations.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

If you are tying the steel beam into the floor diaphragm and the diaphragm is adequatley braced the yes, I would say the portal frame is sufficiently restrained.