Sway of a building

[cve60069]

Dear Engineers

I have been asked by a local builder to design a post to support his house.

The builder had built a single-storey extension on the rear of the house and then taken all the walls out from inside the ground-floor except for a couple of partitions. To support the upper-floor, the builder had two 203x203uc welded together which he supported off the end-walls and a brick-pier approximately mid-span. The ends of the beam sat upon concrete-padstones built into a cavity-wall with a 140-mm inner-leaf: no piers!

Building-control has 'failed' his design and asked for a remedy, namely a moment-resisting 'goal-post' frame but the builder has asked if I could design a wind-post; I said I would give it a go and I am in the process of calculating the moments. The plan is to cut a steel-post into the existing cavity-wall and to support the beam off the post and to make the foot of the post moment resisting. The top of the post will be pinned and the load is directly over the post: no eccentricity. Hopefully, the post will sit within the wall leaving the surface flush.

My question is concerning the sway, particularly the mass being supported. I attach a sketch of the problem. How do I 'factor in' the mass, please? I apply my wind-load to the side of the house and I restrict the deflection of the top of the wind-post to 1/300 (8-mm). I am assuming the deflection is small enough so as not to allow the mass being supported from moving beyond the point of no return. Do I need to design the post for moment caused through the wind-force combined with the axial-force to support the upper-floor only or is there an 'Inertia' I should be taking into account?

Hope I made sense.

Regards

 

[msquared48]

I take it you have very little wind there.

Your contractor client is a dreamer, and very unrealistic with restricting the lateral resisting component of the structure to one central column if I understand this correctly. The exterior walls should be taking the lateral.

Mike McCann
MMC Engineering

 

[SteelPE]

Well, I'm not sure where you are located but basically what you are asking for is how do you figure p-delta effects of your cantilever column system. The AISC has some methods to handle this problem. Your local jurisdiction probably references a code that takes into account similar effects. If they don't, you could always iterate the problem until you get equilibrium. Pretty simple statics problem, just may take a little bit to do.

 

[SteelPE]

Oh, and I agree with Msquared that using a single cantilever column to support an entire structure seems unrealistic.

 

[paddingtongreen]

I suppose, if you apportioned most of the wind load to the shear walls, you could design a moment foundation although I'm not sure of its stiffness, but a pinned foundation and moment connection to the beam looks to be more sensible. It is hard to imagine the shear walls being less stiff than this column.

As to how to figure in the mass. Apportion a central zone to the column and a peripheral zone to the exterior walls. The resulting load can be used for a P-delta effect. I don't see any other effect.

BTW, a "wind column" is a peripheral column, pinned top and bottom, carrying wind load to the foundation and to a separately braced floor or roof system. They tend to be used in the exterior walls of buildings with a large interior column spacing grid, to keep the girt size manageable.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin