Could you get this to work?

[Prestressed Guy]

This is an interesting one. Yesterday I was asked to look at a house built in the 1970's and have attached the basic frame section. The frames are constructed of 4x6 lumber @ 42" oc and 2x6 T&G decking for floors and roof. The loft floor stops about 10' from the front wall of the structure and there are interior walls in the plane of the frame at 10'-12' spacing. The longitudinal beams shown at the edge of the loft are (3) ply 2x10's with offset butt joints. I could not find any locations with 2 plies butted at the same location so I assume they are 16' long with 5'-4" offset.

 

[Prestressed Guy]

I take the evening off and look what happens. 8 posts since I last checked. I will address comments below in order of post. I am uploading a photo and see that the roof pitch is about 12:12 which just makes the thrust greater. Note that it appears that only one side wall is tilted but that is just because the fence is hiding it.

Msquared48. You state that it is unstable but that is only if there were no shear walls. With walls parallel to the frame at 12’-14’ on center it will not deflect laterally as stated by cal91.

Msquared48. There is a large window wall at one end and this is the end that has the cutback about 14’ for the loft. The lateral mechanism for these last two bays would be the roof diaphragms back to the 1st interior wall and to a pair of 6’ wall sections at the eaves. Given the short eave height these piers should be sufficiently stiff.

OldBldgGuy. I think you are heading in the right direction but I do not believe that the rafters have sufficient capacity in bending to support gravity loads. I.e. if you eliminate the walls and used a three-point support (fix center, roller at each tail) I think that it is likely that the tails would fail.

Kootk. I completely agree that it is unstable save for interior shear walls. My involvement is to listen to floor squeaks. The couple that currently owns the structure just bought it a month ago and after moving in noticed that there are a lot of squeaks in the loft floor. They tell me that everyone who looked at the building with them commented on how unusual the framing was to they were starting to get nervous. Their first contact with me was a two days ago. The item that they called me out to look at (floor squeaks) was easily resolved. The 2x6 T&G deck is nailed to the beams and over time a few have experienced enough shrinkage that they have slight gaps. When someone steps on these planks, the gap closes and the nail squeaks against the board. Running deck screws through the board into the beam below should solve that problem.

My spidey senses were on edge when I first walked in as I said in post #4 of this thread but I didn’t run as my first inclination wanted. I started moving around (to be sure, cautiously at first) and using my body mass and inertia to load and deflection test the structure. ;>) I was very surprised to find it quite stiff. At that point and given that I knew that the building was about 50 years old, I had to admit to myself that the system worked even though I could not see how. I do agree that construction stability would be a major challenge because without the roof and floor plank diaphragms and the interior shear wall it is very unstable at all phases.

Tomfh. Lateral stability I agree but gravity stability by bending of the tails, I do not think so. More on that in the next post.

Kootk. I can assure you that there are no moment connections anywhere in this structure (other than the treads of the spiral staircase but that is not relative to this discussion.)
I have a problem with our diagram because it shows racking and the numerous and closely spaced interior walls prevent lateral movement. I doubt that they are plywood sheathed but even as double sided GWB walls they add up to a lot of lateral restraint. I do not disagree with the mechanics of the racked diagram or your conclusions, just that it is not apropos to the existing conditions. I would contend that from an analytical standpoint, the bare frame would be no more or less unstable if the stub walls were vertical. The structure is symmetrical about the ridge so the tipping force on the walls is opposite and EAQUAL. They offset. A fully pinned symmetric frame under gravity only will always have 0 net lateral force.

 

[KootK]

Kootk. I can assure you that there are no moment connections anywhere in this structure

Yeah, I know. Here, I find that you need to carefully qualify your opinions in order to keep them from getting challenged needlessly on points of little consequence. A bit like lawyering I'm afraid. That's the only reason that I mentioned moment connections at all.

I have a problem with our diagram because it shows racking and the numerous and closely spaced interior walls prevent lateral movement.

There is no discrepancy. My sketch was meant to address the case where in plane shear walls did not exist. I posted it in an attempt to dispel the notion that this thing would somehow be inherently stable without such help. As I've said a couple of times now, I'm in the shear wall camp, even if those shear walls would have been unintentional originally.

The structure is symmetrical about the ridge so the tipping force on the walls is opposite and EAQUAL.

I take exception to this. What about unbalanced snow and live loads? What about wind and seismic acting concurrently those loads? Even if the gravity load is balanced somehow, it's still a problem in a stability sense. Remember, every stability problem is not about where the ideal structure starts off. Rather, it's about where it'll end up given imperfections etc. There're plenty of Euler columns that are loaded symmetrically on paper but are still altogether unstable.

A fully pinned symmetric frame under gravity only will always have 0 net lateral force.

This is only true of structures that are stable. That stability cannot be presupposed in this case as that's the issue at hand. You're right, you wouldn't develop net lateral reactions here. But that implies lateral movement as I suggested in my last post.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Prestressed Guy]

Kootk I agree with all of your points. I should have added more qualifiers to my statement about the analytical model statement. I was only referring to the uniform gravity loading case with no outside lateral forces. Certainly an unbalanced dead and or live load would induce lateral loads in this structure that would not be induced in a structure that had vertical bearing elements. My statement should have been "A theoretical fully pinned symmetric frame under uniform symmetric gravity loads will always have 0 net lateral force." I was getting tied of typing.

 

[KootK]

A theoretical fully pinned symmetric frame under uniform symmetric gravity loads will always have 0 net lateral force

This doesn't change anything in my opinion. Consider a structure that is:

1) Fully symmetric in terms of it's internal composition.
2) Fully symmetric in terms of it's external loading loading.
3) Fully pinned.

Such a structure will have a zero net lateral force but may still very much have a stability problem, as your structure here would have in the absence of the shear walls etc. This, because the imperfection that initiates instability neuters the symmetry, pretty much by definition.

With loading and geometry 100% symmetric, I would agree that the situation would be no worse off for having the sloped knee walls as compared to plumb framing.

A theoretical fully pinned symmetric frame under uniform symmetric gravity loads will always have 0 net lateral force.

Excepting stability issues, I actually believe this to be true for structure with asymmetrical loading.

Rest up your fingers and get back to work. We can debate further on the weekend if you're still interested.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Prestressed Guy]

I think that I might know how this building is working. Given my assumption that it is not possible to make a moment connection in wood framing I started by visualizing the free body of the wall / rafter joint. In doing this I realized that in order for that joints forces to resolve, the rafter must be in tension, otherwise the wall is going to fall over.

Well, that realization put everything into a different light! Once I came to grips with that I started looking to see what could be holding this thing up. I think that the basic gravity load path is 3 bearing points, center wall and the 3-ply 2x10 beams that are place above the loft floor joist/rafter joint. This beam runs the full length of the building down both sides and passes through all of the partition walls. My theory is that these beams are bearing at the partition walls down to the floor structure. When looking at it this way the rafter tails have a little bit of axial tension and relatively small amount of bending with support at the end provided by the inclined wall.

 

[KootK]

Bingo! The owner is gonna love you. At least they should.

- There isn't a stability issue because the sloped walls aren't really load bearing.
- This nicely addresses the construction issue that tripped my spidey senses earlier.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Prestressed Guy]

Thanks KootK. The owners were happy when I left because they saw my reaction when the load path resolved itself in my mind. They were decidedly nervous for the first 20 minutes or so as I walked around with a big scowl on my face saying thinks like HMM and HUH while scratching my head and glancing longingly towards the exit door! They were watching me intently when I stopped and stared at the incline wall, then slowly up the rafter, ran my eyes both ways along the 3-ply and suddenly said a happy OH! I think that I heard them both exhale about then.

 

[BAretired]

Coming into this discussion a little late, but it appears the structure is not entirely symmetrical as the vertical wall appears to be slightly off center. Uniform load on the second floor would tend to cause the structure to move to the right.

I disagree that the rafter tails are in tension. They are primarily in bending, but slightly in compression. The short sloping walls are also in compression.

Without the benefit of shear walls, this structure is unstable.



BA

 

[Prestressed Guy]

BAretired. here is the free body. With only two members both of which are inclined the one must be in tension and the other in compression or the node will displace. Here is the simple diagram. If the tail is in compression (pushing towards the node) the wall will fall over.

 

[BAretired]

Haydenwse,

You are assuming a hinge in the rafter at the junction to second floor. Actually, the rafter is continuous. If the sloping wall is removed, the rafter would be a cantilever which would deflect downward. The sloping wall prevents that, so the sloping wall is in compression. That compression can be resolved into components parallel and perpendicular to the rafter. One causes axial compression while the other causes bending in the rafter tail.

It is possible that the 3-2x10 beams carry some of the second floor load into the partitions as you have suggested but I don't know how much roof load they could carry. That would depend on their connection to the rafters which is not specified.

But whether or not the beams carry roof load into the partitions, the structure relies on the shear walls for lateral stability.

BA

 

[WARose]

This is one of those where the owner goes: "It's been there for 50 years!" And I say: "I couldn't care less.....if I can't prove it works on paper, I'm not messing with it."

 

[KootK]

I think that I heard them both exhale about then.

I bloody bet. Just imagine how concerned you'd be about the security of the investment that you'd made as the owner. Frankly, when I look at your initial sketch with fresh eyes, I feel a bit foolish for failing to see this in the first place. Darn that hindsight.

I agree with BA on the rafter tails not being in tension. I left that alone as it would seem to be of little consequence at this point, other than as an aid to clarifying general understanding (which always has some merit).

While there's little doubt that the shear walls are the lateral resisting elements in the conventional sense (W/EQ/P-Delta), I think that it's still a pretty big win to have some assurance that unbalanced gravity loading is no longer prone to making substantial contributions to lateral instability. Although you may have expressed it a bit differently, I'm sure that was what was nagging at you originally as well.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Prestressed Guy]

Here is a quick run of the rafter / wall.

 

[Tomfh]

Below, I've drawn a racking mechanism

Without the benefit of shear walls, this structure is unstable.

Yes it can fall over if there are no cross walls, the same as any frame/beam supported on parallel walls can fall over.

I don't think it's fair to criticise this system on the grounds that it's not stable without the shear walls when we don't make such criticisms of other systems.

Harder to erect, and more unstable during erection, no doubt...

 

[BAretired]

Tomfh.

I believe we do make a similar criticism of other buildings. A rectangular building which relies on diaphragm action of the roof and end walls for lateral support is unstable without end walls whereas a building with frames may not need end walls. This building has frames, so it needs to be stated that shear walls are required for stability.

BA

 

[Prestressed Guy]

I think that some people have gotten hung up on my referring to the "basic frame" in my initial post. I stated that it was constructed of dimensional lumber and had numerous interior walls parallel to the frame. Not all frames are moment frames and I did not say that it was a moment frame. For a structural system I would say that this structure is a type B.24 per table 12.2-1 Building Frame with Light-Frame walls with shear panels of all other materiials with an R of 2.5.

 

[KootK]

I don't think it's fair to criticise this system on the grounds that it's not stable without the shear walls when we don't make such criticisms of other systems.

Speaking only for myself, I wasn't concerned for this system because it's not stable without shear walls. I was concerned for it because its nature is such that unbalanced gravity loads would induce lateral displacements. And that's not generally true of typical systems. In fact, it's a situation that requires a greater than typical amount of designer attention to lateral load paths and P-delta effects. Of course, this is moot given recent revelations about beam support.

I think that some people have gotten hung up on my referring to the "basic frame" in my initial post.

And rightly so. Initially there was speculation that the frames themselves might provide the lateral stability. And, of course, now we're debating issues that are somewhat independent of your original question.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

One of the more unusual aspects of this structure is that the beams are on top of the second floor joists instead of the more usual situation where the joists bear on top of the beams. The attachment of beams to floor joists or rafters is not specified. The photograph showing the rafter connection does not show the beam because the floor stops ten feet from the end wall. If the beams are distributing significant load to the partitions, the connection of beam to structure would need to be evaluated.



BA

 

[Prestressed Guy]

In my initial post I tried to give all of the most important info. [pre]"The frames are constructed of 4x6 lumber @ 42" oc and 2x6 T&G decking for floors and roof."[/pre] Wood construction pretty much precludes moment connections.

[pre]"... there are interior walls in the plane of the frame at 10'-12' spacing."[/pre] Any wall that goes from floor to floor or floor to roof can be used as a shear wall.

[pre]"The longitudinal beams shown at the edge of the loft are (3) ply 2x10's with offset butt joints. I could not find any locations with 2 plies butted at the same location so I assume they are 16' long with 5'-4" offset."[/pre] I made a point to call out the 3-ply beams so that they were not missed.

 

[BAretired]

All of a sudden, the lines of text in this file are too long to read. This has happened before, and I believe it has something to do with pictures or sketches
which have been introduced above.

BA