Lateral Stability of Structures
Lateral Stability of Structures
(OP)
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
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






RE: Lateral Stability of Structures
RE: Lateral Stability of Structures
RE: Lateral Stability of Structures
DaveAtkins
RE: Lateral Stability of Structures
I had thought of this possible solution also, but thought of the timber floor as not being rigid enough. According to a few books, a timber floor is very flexible and weak. Would you ignore this and design the floor as a horizontal cantilever with a couple taking the torsion anyway?
damo74
RE: Lateral Stability of Structures
RE: Lateral Stability of Structures
"at the steps point transfer the loads to the adjacent (parallel) walls. You need to design the walls so that the point loads from the chords along the rim joists (tension & compression)".
I'm not sure I understand fully. Could you spell it out a little further for me?
RE: Lateral Stability of Structures
Hope this helps. It's a thought anyway. Depending on the loads and the number of steps (offset) it may be possibility.
RE: Lateral Stability of Structures
As opposed to a lateral brace or frame at the drop, if you could detail the drop in such a way as to make the diaphragm continuous, that would obviously be much preferred.
Assuming you have 2x framing and around 6 inches:
If the drop is something like 6 inches, it seems to me like adding a 2x12 or something simliar at the drop point would work. On the high side, the plywood is nailed to the 2x12. On the low side, the plywood is nailed to the lower 2x8 framing. The key point is that the 2x8 nailed directly to the 2x12, so that the shear is transferred. You'd have to design the nailing pattern and 2x8/2x12 for the diaphragm transfer shear. If the drop is larger than 8 or so inches, it may be easiest to use a deeper glulam at the edge of the drop, nailing your 2x framing on the low side directly to the glulam.
If it is TJI joist framing, it will be similar to above.
NOTE: I've never done this before, as I've never had a floor drop in a wood building. This is just brainstorming of how I would do it.
RE: Lateral Stability of Structures
RE: Lateral Stability of Structures
DaveAtkins
RE: Lateral Stability of Structures
Is the stepped floor at the first floor level only? If so would there be much wind load on this floor?
Even so, could you look at the front half and back half as two seperate diaphrams, each of half the depth?
The term rigid has specific meaning (I thought) when talking about diaphrams. Wood diaphrams are known as flexible diaphrams aren't they? That distinction has to do with how the loads are distributed to the shear panels (based on relative stiffness of the shear walls I thought). It doesn't mean that becasue they are not rigid they don't work as diaphrams.
If you're looking at a "simple" 2 story house, perhaps the 2 shallower diaphrams will be sufficient( compared to one deeper diaphram)
RE: Lateral Stability of Structures
Again, in the case of this diaphragm, I'd add a deep beam next to a shallower beam to transfer the shear, as I explained in the previous post.
RE: Lateral Stability of Structures
Here is a good refernce on diaphragms.
http://u
In my opinion, you could treat the two diapragms as separate. They would have to be either supported at the center, or designed for the extra torsion resulting from only being laterally supported on three sides. However, this doesn't pass my common sense test. do I really want a wood framed structure with two separate lateral resistance systems in it? What happens to the partitions or cieling at the point where the two diaphragms joint?
RE: Lateral Stability of Structures
I agree with the approach that DaveAtkins takes in his first post.
RE: Lateral Stability of Structures
Assuming as per the example, a change in level running across the building such that the gable walls intercept two levels of floor:
Horizontal (wind) force is transferred to the two parts of the floor where it forms a shear force in the diaphragm. Consider the diaphragm as a cantilever spanning from the front or back wall with the free end at the change in level. The bending moment is wl^2/2. Assuming that the step occurs near the centre of the building, this is actually the same bending moment as assuming simple supports at front and back walls with a continuous floor level as wl^2/8 (in this instance) has a value of l twice that of the cantilever.
Now in order to ensure rigidity of the cantilevers the gable walls provide a restoring moment by horizontal forces acting as a couple.
If the change in floor level is substantially away from the centre of the building, a shear force will be present at the change in level which should be resisted by a vertical truss formed at the change in level.
RE: Lateral Stability of Structures
RE: Lateral Stability of Structures
The best way to find a solution to a problem is the simplest way. To me, the simplest way is to add a deeper member (on the high side) at the floor drop attached to a shallower member (at the low side) with a connection that could develop the entire diaphragm capacity.
RE: Lateral Stability of Structures
RE: Lateral Stability of Structures
Sketch a square shape on plan. This is the floor. Draw north, south, east and west. Assume the front wall is due south (this is the cantilever support). The north side is the tip of the cantilever. The floor is fixed to walls on 3 sides, namely south, east and west.
If the force is applied to, say, the west gable, the floor will attempt to rotate about the south eastern corner.
Assuming the floor is fixed to the east and west walls, these will provide a moment resisting couple, as the east wall reaction is acting due north and the west wall reaction is acting due south. The southern wall can be designed to resist the total shear force applied to the west wall, as in a typical cantilever. We do not need to worry about any moment resistance at the support of the cantilever (i.e the southern wall) because the resisting moment is applied by the couple as stated earlier.
That's how I applied my logic. Does everyone agree?
RE: Lateral Stability of Structures
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?
RE: Lateral Stability of Structures
Your answer preceeded mine - I agree but see my comments on eccentric divisions of the diaphragm...
RE: Lateral Stability of Structures
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?
RE: Lateral Stability of Structures
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...
RE: Lateral Stability of Structures
Will talk again, for sure.
RE: Lateral Stability of Structures
Great website link you provided. Looks like that professor did a great job on getting down to the basics and explaining diaphragm behavior.
RE: Lateral Stability of Structures
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).