Lateral Stability 2

[damo74]

Hi everyone.

I have a query about simple lateral stability of buildings. If you take a simple cube and apply lateral forces to it (say east to west), I normally assume the roof and any floors act as deep horizontal beams and transmit all the forces back to supporting shaer walls (north and south).

But, what happens when one of the supporting shear walls (say south) is completely missing. Say for example an aircraft hangar with a completely open entrance on one side or a steel frame structure with a full wall of glazing on one side (again the south wall)? Is it safe to assume that the twisting will be taken by the east and west walls? The structure would then be working like a 3 sided box.

What have other engineers assumed?

 

[111213]

in general we have to design an non-kinematic system.

in your case there are several ways to satisfy this:

1. if the roof has shear-stiffness, then it distributes
the forces to the 3 remaining wall.

2. if the roof has not enough shear-stiffness (as usual in steel-structures) you may substitute your eastern wall by
a moment-frame or by columns with fixed bases

3. if the roof has a fair amount of shear-stiffness, you may consider a mixed system, maybe not the best choice..

 

[Lion06]

I think this can only work if you have a rigid diaphragm.
The shear in the south wall will cause the torsion you are talking about which will be resisted by all three walls, not just the east and west walls.
This will act as a three sided torsion box, as you have stated. I think you will actually find that the direct shear in the south wall will be counteracted by the torsional shear and you will get a smaller net shear in the south wall than from the direct shear alone.

 

[damo74]

"if the roof has shear-stiffness, then it distributes
the forces to the 3 remaining wall."

I assume you mean a concrete roof which would be quiet stiff. What about a timber roof with say a plywood membrane fixed to the top or bottom of the rafters? In the absense of structural calculations and using experienced engineering judgement, would this be a possible solution that you have undertaken in the past?

 

[csd72]

This has been covered recently.

thread726-208022

 

[111213]

damo74, it depends on the special case. have you an sketch ? in general it is possible to judge the stiffness
of an timber roof. it may be not the best solution, considering the in plane deflections.
i sure did it in steel structures with braced roof planes.
to better understanding, do a sketch

 

[apsix]

Its effectiveness also depends on the relative lengths of the walls. If the open side is the shorter side it will be more difficult to make it work.

 

[hokie66]

You can do this for small buildings, but aircraft hangars and the like are always done as portal frames.

 

[csd72]

Or prestressed steel arches such as strarch (another good Australian invention).

http://www.airport-technology.com/contractors/design/strarch/

Incorporating the columns into portal frames also has the added benefit of reducing the size of the roof beams required.

 

[hokie66]

Yes, the Strarch method of prestressed steel arches has been used for many hangars. They had a tough time several years ago when one collapsed while under construction, but I think they have fought through that.

 

[csd72]

I didnt know about that one.

A friend of mine used to work for them in their sydney office.

 

[hokie66]

It was at RAAF Fairbairn, Canberra, in about 2003. A number of people injured, and at least one executive and an engineer of the company charged. I don't know the outcome, but if you are interested, Google probably knows. I think the erection procedure was at fault rather than the design.

 

[damo74]

Excellent advice (especially that previous discussion listed above by csd72).

Thanks guys