lateral stability of structures

huangyhg

lateral stability of structures
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
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i think your logic is correct.  you have to detail in such a way as to transfer the diaphragm load across the step in the floor.
i agree.  any suggestions as to how i might do this?
i disagree.  i recommend considering each diaphragm as a rigid, three sided, open on one side, diaphragm.  lateral load is transferred to the shear walls, in typical fashion, but the torsion in each diaphragm is resisted by a couple in the perpendicular walls.
daveatkins
thanks daveatkins,
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
if i understand this correctly, you want to have the two-level or stepped floor to act as one. in this case, figure out the chord loads for as if it were a one level floor and 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) are safely transferred to rest of the structure.
kxa,
"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?
i do a lot better on paper but will try. if you are analyzing the floor as a deep beam, you will have, let's say, compression in the front and tension in the back. the side walls take the shear. now that the floor is split in two levels by the steps (how many?), the tension and compression forces in the front and rear chords (rim joists) need to still be resisted by something. since the chords are no longer continuous along the front and rear, you might be able to resist the chord loads by placing solid blocking in the walls together with metal straps since the loads can be either tension or compression. the wall will also need to resist the resulting shear.
hope this helps. it's a thought anyway. depending on the loads and the number of steps (offset) it may be possibility.
kxa, i have to disagree with your creative solution.  i agree that it will work for the bending resulting from the diaphragm action, but how will it resist the shear?  imagine a 30 foot long steel beam.  at the very center, a 1 foot section of the web is cut out.  now, the flanges could still handle the compression / tension, but there is no way to resist the shear, meaning it will fail.
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.
aggieyank, this is a two-storey house and appears to be of wood frame construction. the steps are located in mid span and should not see much shear loads (along the steps). anyway, the idea is to follow the load path and provide enough structural support that would safely withstand the design loads.
yes, i know that authors have said that a wood diaphragm cannot be rigid.  for this situation, i would definitely be ok with assuming the half diaphragms are rigid, and analyzing them as i stated earlier.
daveatkins