Lateral Stability of Structures 7

[damo74]

Folks,

I have a lateral stability problem that keeps appearing in front of me.

When looking at simple 2 storey housing of typical construction I would normally assume that any wind load on the front/rear walls is transmitted to the gables via diaphragm action of the timber floor. Also, any wind load on the gables will be transferrred to the front and rear walls by the same mechanism.

However, when steps are introduced in the floor panel (say from gable to gable along the centre of the dwelling parallel to the front wall), my logic of transferring all of the wind load down to foundation breaks down. The floor is then split up into 2 seperate diaphragms. When behaving as horizontal deep beams, the front diaphragm spans from the front shear wall to the step in the floor. The rear diaphragm spans from the rear shear wall to the same step in the floor.

By my reckoning, I require some form of lateral stability at the step in the floor. This could be provided by an additional wall or possibly a steel/concrete frame. The wall option doesn't always suit the layout and the frame option appears to be an overkill for simple 2 storey house construction.

Could anybody confirm that my logic is correct and, if so, provide me with any other solutions that they have used for this case of the stepped floor as I'm sure it's quite common.

Thanks

 

[pba]

Restraint forces at a cantilever support are a shear force - That is the load reaction and a moment force equal and opposite to the applied moment. As I understand it that is what we have in each part of the building. We have a diaphragm floor with walls around three sides. Assume that the side without a wall (the change in floor level) lies to the east: The west wall takes a longitudinal force equal to the applied wind force (shear force or load reaction as described above). The north and south walls take longitudinal forces equal and opposite to each other and this provides the moment resistance (in reality a torsion). This system is stable and does not need any structure at the open face. The other half of the building would similarly also be stable...
BUT
If the change in floor level is not at the middle, a shear force would be present at the interface and this would need to be dealt with at the change in floor level.

The gable walls or north and south as described above are actually in tension or compression as there would not be a physical break between the two halves.

Does this help?

 

[pba]

damo74

Your answer preceeded mine - I agree but see my comments on eccentric divisions of the diaphragm...

 

[damo74]

pba,

I agree with almost everything said in your last comment.

If the step occurred at a quarter of the distance between the front and rear walls, surely the shorter diaphragm would simply be more stiff than the longer one? The shear reaction would still find it's way back to the cantilever support.

The shear along the gable walls would differ in magnitude in each floor section, but this should not pose any real problems. The shear would act in opposite directions and the difference between the 2 values should be supported by the gable walls.

Do you agree?

 

[pba]

Yes a shorter diaphragm would be stiffer but it would be supported on shorter north and south wall sections. The longer diaphragm would be more flexible but would be supported on longer north and south wall sections! I don't know which would be more flexible but it is unlikely they would be the same.

To prevent movement occuring at the change in floor level a method of shear transfer is needed. I would think that studs with ply sheathing or diagonal braces would deal with this.

The problem is now possibly more analagous to a steel beam splice! Cover plates on the flanges are the north and south walls. Shear transfer at the web is the change in floor level. Of course you wouldn't offset the web in a steel beam splice...

 

[damo74]

pba I agree. That's a good point. Thanks a lot for your ideas. That goes to everyone else too.

Will talk again, for sure.

 

[astructurale]

AggieYank,
Great website link you provided. Looks like that professor did a great job on getting down to the basics and explaining diaphragm behavior.

 

[HouseBoy]

Perhaps for a "simple" house you don't need for the floor diaphram to be full depth of the house and two diaphrams (each of half the depth) would work fine. I'd explore that before I tried to transfer the shear across such a step.

Just a thought.

I'd also be just as concerned about the diaphram behavior at the first floor relating to the bracing effect that the first floor has on the tops of the foundation walls (from lateral earth pressure).