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Residential diaphragm design or lack there of

Residential diaphragm design or lack there of

Residential diaphragm design or lack there of

(OP)
Our office (only two of us) are having a discussion on residential roof diaphragm design. I have attached a sample drawing of what I am talking about. Say you have a typical house with a nice long wall at the back of the garage you want to use for shear. However, the trusses run perpendicular to this wall so to provide a drag strut would involve blocking and coil strapping. My colleague says you can use the shear wall without a drag strut by limiting the diaphragm length to the length of the wall at the back of the garage as long as the unit shear on the shear wall is lower than the unit shear on the diaphragm. He also says that the collector for the transverse shear wall will also work as the chord for the longitudinal diaphragm. All this makes sense and could be proven with calculations and I have no doubt that the demand on the diaphragm would be well within the ability of a standard residential diaphragm. I agree the diaphragm he proposes will be strong enough to support the load, but I am unsure of how the remaining portion of the home ties into the diaphragm without a collector. My colleague says that the roof sheathing is all staggered and nailed and the rest of the home just goes along for the ride. This is the part I'm having a hard time accepting, especially for tension drag loads, which is why I say if you use the shear wall at the back of the garage for the longitudinal direction, you need a drag strut or at the very minimum blocking and edge nailing with coil strap out into the diaphragm a sufficient amount to develop your drag force into the adjacent diaphragm. What are others thoughts on this? Can you just rely on the sheathing and the nailing along the left hand side to go along for the ride? Or should a collector be provided. I realize residential loading typically is very low, however, this also can be a problem on large residential projects as well as commercial where loads are higher, but providing a drag strut the full length of the building would cause additional costs that may not be required.

RE: Residential diaphragm design or lack there of

Jeff,

I now do almost all residential, from tract homes to custom, up to 8000 s.f.

Most are in Phoenix area, very low seismic. We never do drag struts, collectors, etc, in this area for residential.

RE: Residential diaphragm design or lack there of

A couple of things here:

1. I normally, at the top story of a trussed roof residence, only use the exterior walls for shear to avoid the internal blocking required between the trusses.

2. The wall top plate of the exterior walls will function as a drag strut if properly detailed for connections between the top plates members.

As for the situation you have shown, I would try to eliminate the interior shear wall and just use the exterior walls. If that cannot be done, then a strap tie similar to a CMST would definitely be in order here, nailed off to the shear wall top plate and the blocking between the other roof trusses.

Mike McCann
MMC Engineering

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
The area I am in is typically a Seismic Design Category D, once in a while a C. Most times our wind and seismic loads are almost the same. Longitudinal direction is often controlled by seismic while transverse loads can be wind controlled but seismic will control sometimes. If you only have that shear wall behind the garage, how do you justify it bringing the rest of the home along for the ride without a collector? What happens if your unit shear on the wall is greater than the unit shear in the diaphragm? You would have to have a collector or the shear force could never be delivered to the wall.

msquared48,
1. I would rather not use the wall at the back of the garage for shear, however, many times it is needed to reduce the lateral load at the front of the garage, especially if the garage sticks out a ways. If the exterior walls jog back and forth quite a bit how do you justify it working all together without collectors? Judgement?

2. The same as above, if the exterior walls jog back and forth a bit, how do you justify the collector working if it is not a continuous straight line?

3. If you could not eliminate the interior shear wall, would you just run blocking and coil strap our long enough to develop the drag force? Maybe my colleagues idea of designing the diaphragm on one side and the rest of the home going along for the ride isn't too far off?

RE: Residential diaphragm design or lack there of

1. Well, remember and use your allowable 4' offset for the shear walls in one line. As for the drag forces, usually, unless this has a lot of brick on the walls or tile roof, seismic will not control (which would have a greater effect on any need for drag connectors) and wind will be your main consideration. You need to take a look at the magnitude of the wind pressure/suction forces and see if you feel the drag connectors are needed.

2. If the offset is 4 feet or less (a requirement for them to be considered in one line), I generally would not worry about it - the plywood should be able to do this. With some higher end houses, it could be a problem though.

3. Yes, I would use the strapping to create the drag lines rather than create sub-diaphragms.

Mike McCann
MMC Engineering

RE: Residential diaphragm design or lack there of

jeff,

I have done custom houses in California where all the engineering comes into play as you are describing.

On the other hand, here in Phoenix, builders would object violently to long CMST straps, tight nailing and blocking in roof plywood, etc, and the plans checkers don't require them. The only notable failures here have been garages racked by windstorms when the concrete roof tiles have not yet been stacked on the roof combined with deficiencies in garage door jamb shear and hold downs.

That entire roof acts much better than you think w/o collectors etc - the way I explain it to architects, etc - imagine you have cardboard sheet about 3 feet long x 3 feet wide folded to the shape of that entire roof and you try to distort the cardboard in the sense that lateral shear would - the cardboard actually has substantial stiffness against buckling and racking as long as it was properly attached to little trusses a few inches apart.

But you can't use this analogy for seismic, why do I think it is allowable for wind? Perhaps the shaking and repetitive cycling accelerations with seismic - that is progressively making nails bend and pop out. At any rate, if I insisted on the collectors and straps, all my clients would probably leave me.

Look at the attached detail- about the highest shear I calculate typically is 400 plf, more than the basic roof sheathing is rated for, but with wind forces here probably very conservative.

RE: Residential diaphragm design or lack there of

ps - I forgot -

If I was designing this house for seismic, I would calculate reqd length of roof deck for shear and then subtract out the length of the shear wall - the remainder would be the reqd length of drag collector.

RE: Residential diaphragm design or lack there of

(OP)
msquared,
1 & 2. Right, I did forget about that 4'-0" offset. As I said before, seismic is always a concern as it is usually about the same magnitude of the wind loads (almost every house is tile roof also). As far as the magnitude of drag forces, that has often been my question as well. How much force can the diaphragm transfer where there is no edge nailing? In the past I have figured if the allowable diaphragm shear is 230 plf with 6" o.c. edge nailing, without blocking the nailing would be at 24" o.c. (or whatever the truss/joist spacing is). So in the case of 230 plf and 24" o.c. truss spacing leaves me 230/4 = 57.5 plf. Not much, but definitely within the realm of a majority of single story wood framed structures. We also have set the drag force at 2000 lbs before we worry about it. So if my unit shear in my shear wall is less than the unit shear in my diaphragm, the diaphragm shear is less than 57.5 plf and my drag force is less than 2000 lbs, then in the past I have not worried about collectors. Then I have a way to rationalize why no collector is required. I don't know if this is over conservative or not. a 2000 lb drag force isn't all that hard to get. But I have a hard time getting a handle on when I "feel" a drag strut should be provided without coming up with some kind of numbers. Too conservative?

3. OK. So you would consider what my colleague has suggested as creating one large subdiaphragm? Do you think that his feeling that the sheathing and nails is enough to bring the rest of the home along for the ride?

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
Here framers aren't all that adverse to providing coil strap to tie drag trusses to shear walls. We usually only make them coil strap when the truss ends at the wall. If it runs next to the wall we will use a detail similar to what you have shown that specifies some sort of shear transfer between the wall and the truss. If the trusses are perpendicular, in the past we have run the coil strap and blocking out far enough just to develop the drag force into the diaprhagm. If we had a 2300 lbs drag force and a 230 plf diaphragm, we would need to run straps and blocking out 10'-0".

I also agree that the diaphragm is much stiffer than what we would imagine. However, I like to have a concrete reason why I am ignoring something, rather than just saying we don't do it that way just because. If I have a rational reason for doing something, I feel like I am at least giving my clients something with some numerical backing rather than a guess that may not be correct. Perhaps I am just anal, paranoid (or a robot), but my mind doesn't like to accept feelings without numbers. I have only been licensed for 6 years, so maybe that is part of it too.

RE: Residential diaphragm design or lack there of

jeff,

If a custom home builder sees all that (what I call advanced structural engineering) detailing, they add it to their estimate and the homeowner pays for it. Frequently no problem.

However I deal with a lot of tract and semi-custom, and to them, it would be like telling the Chevrolet corporation that all the their seats need to be covered in hand-stitched leather instead of vinyl and cloth.

We all need to have numbers and calcs to justify what we do, it is something instilled during University and helps keeps us honest. But houses have so many secondary load paths etc, we get into the non-engineered, well that's the way it is always done around here mentality.

It seems only in California, houses do fall down due to earhquake, and in tornado areas, there's no way to keep a house from being destroyed.

RE: Residential diaphragm design or lack there of

AELLC:

Actually there is a way. You just do not have a view. bigsmile

Mike McCann
MMC Engineering

RE: Residential diaphragm design or lack there of

Mike,
Are all new tract homes in tornado alley now designed to withstand the worst tornado?

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
You are right with homes having multiple load paths. All those partition walls with sheetrock on both sides are definitely contributing, as well as the ceiling diaprhagm. I am in southern Utah (St. George) and the Santa Clara river flooded and washed out some homes on the banks. But many homes were left standing with one corner or the entire half of the home cantilevering over the edge with no foundation. They stood there for a week while the flood was occuring then they were underpinned, back filled and they are still there. Imagine what the engineering would be like to make a home do that. I guess the we have always done it that way is my stumbling block since I haven't been at it all that long. So having numbers helps me develop some engineering judgement. I was trained from the get go on commercial buildings and there is peer review on all commercial projects here, even lattice structures. So I have been trained from the beginning to have calculations for everything so that it stands up to scrutiny in peer review. Many of my clients do large custom homes, so the rules still apply, but on the smaller homes I would like to eliminate some of this stuff but be able to do it with piece of mind. Which can be hard to when you have been doing it one way for a long while. So I am guessing that you probably have no problem with my colleagues method of using a large subdiaphragm and the rest goes for the ride. Looking at our typical roof diaphragm which has 8d nails and 12" o.c. in the field and using increases of 1.6 for wind and seismic and 1.1 for diaphragm nailing I get an allowable load per nail of 133 plf. Which is higher than the diaphragm loads I get on many of the smaller homes. I have no doubt the sheathing can resist a tension/compression force of that magnitude when nailed off every 24" o.c.

RE: Residential diaphragm design or lack there of

jeff,

Yes I have a more relaxed attitude about residential design after being an employee at 3 different all-residential firms.

The important thing is to calculate shearwalls and hold downs accurately, or else you slide down and get sloppy/negligent after a lot of years of doing this.

The other thing is showing accurate details that match the calcs.

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
Woking at multiple firms definitely is a help. I have worked at the same firm for 15 years starting out as a drafter while I was going to school and now as an engineer. Over that time I have had one mentor who is 65 and has pretty much turned eveything over to me. His comments to me most times is that's how we have always done it. He laughs when he sees the look on my face trying to accept it, without numbers and tells me he remembers feeling the same way. He proved much of this stuff to himself years ago, so long ago (probably 30+ years)that he can't tell me how he proved it to himself, just that he applied numbers to something years ago and isn't worried about it. I think doing all commercial design for my EIT years was great and helped me learn how to design all kinds of commercial buildings and with all kinds of materials. Then you move to residential stuff and it is hard to leave that commercial design mentality behind. But now that I am licensed and he is going to be gone in less than a year, I am trying to cement my engineering judgement as far as residential engineering goes. And asking questions and seeing how others are doing residential design also helps in developing my engineering judgement.

RE: Residential diaphragm design or lack there of

jeff

I am 66 but work as a one-man shop. I can see how your mentor's attitude is typical, I am that way also. A lot of things that get established as standards and then you forget exactly how the numbers are run.

The important thing is to stay sharp, even though you may forget how to use classical hand-calculation methods that were taught at University. For example, I see a lot of questions on these forums about results of continuous beam and space frame analyzers - basic engineering principle answer a lot of those questions.

Myself, I can't even remember most calculus anymore (but you have to always remember how load-shear-moment-deflection diagrams generally work), but I think I engineer stuff better than ever. If something looks fishy on those diagrams, I can recognize it is not correct.

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
Thanks for all your input, it helps to hear others mentalities as far as residential goes. I just want to do things right, but I don't want to make people do stuff that they don't need to. I have also been taught by my mentor that it is important to have calculations in a lawsuit. So where I am just on the cusp of being on my own, I want to be able to have some sort of numbers for what I am doing. Even if it just a one time calculation I scan and keep on file for later reference. I think I am going to start looking into this subdiaphragm method more since it may be the easiest way to show some sort of calculation. Another thing is msquared48 has said to look at the magnitude of the drag force to determine if a collector has to be detailed or not. He didn't say what kind of magnitude he uses for that sort of judgement. I feel 2000 lbs or less is a fairly small drag load, what level would drag forces need to get to before you started to think maybe you needed a collector?

RE: Residential diaphragm design or lack there of

Also, do you guys calculate actual tension or compression in the wall top plate along shear wall lines...that was always a big deal when I was doing houses for CA? I got some interesting straps and nailings for those.

It was so tedious I have an Excel that does all that now.

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
Our lateral spreadsheet has a drag force diagram that we just enter in the walls and diaphragm length and it calculates the drag force for us. We then run blocking and strapping out far enough into the diaphragm to develop that drag force. The numbers will come out the same as yours. And yes, we do check the chord forces in the top plates. We also have a spreadsheet that we just enter in the diapraghm that is the worst case and see what the chord forces would be. Residential is almost always just the minimum nailing because I almost always shear the interior garage wall and tie it through so that keeps all of my diaphragm ratios to a point where diaphragm shear and chord forces are pretty low. Typical construction method here is to sheath the entire exterior, so that interior transverse garage wall is sheathed most times. I have attached a sample of our spreadsheet for a single wall line.

RE: Residential diaphragm design or lack there of

(OP)
Your program definitely makes it easier when dealing with point loads. On our program we just underestimate the dead load tributary width or figure out an equivalent uniform tributary width for the direction that results in the maximum uplift.

RE: Residential diaphragm design or lack there of

Instead of tasking the time to underestimate, just modify your excel to multiply the actual dead load plf by the wind or seismic reduction factors - there shouldn't have to be any manual calculations or decision-making, estimating etc...the Excel should do all that.

All my beam, header , lateral analysis sheets are set up with this premise for loading (see attached), but if anything has more complex loading, I have "atypical" sheets that handle it - pretty much eliminate hand calcs and estimating.

RE: Residential diaphragm design or lack there of

(OP)
We have similar setups on our beam programs. The reason we have to do a little estimating is because our lateral spreadsheet it much older (15+) than many of the others so it tends to be less efficient. I haven't tried to add much as far as overturning resistance goes because with the 0.6 dead load reduction, it takes a lot of dead load to provide a small amount of overturning resistance, especially with short walls. An extra sheet is a good idea that would be easy to implement, I could have it programmed to provide an equivalent uniform load tributary width as well to make it easy to input into our spreadsheet.

RE: Residential diaphragm design or lack there of

Yes the very short walls don't seem to care how much dead load is on them, they have high uplift but most walls can benefit from a lesser hold down or none at all. I see this as a great time-saver for me because atypical extremely high uplifts which take a lot of time for me to detail, etc can be reduced to something routine in my details and hold down schedules.

The main thing is value engineering which is common in multifamily residence buildings and tract homes.

My worksheets are actually one huge workbook which is about 18 yrs old and is constantly being improved for more functionality - for example I am almost done with the ability to input for bearing walls and posts under multispan trusses, girder trusses or beams without having to "detour" to a separate continuous beam analyzer - and those reactions are input as dead loads correctly reduced by 0.60 etc in my shearwall design sheets easily by inputting a code for the reaction location, such as GT2BM, which means the interior support on the right hand side of the second span of girder truss GT2. (I don't have to look up any specific numbers and then input them).

RE: Residential diaphragm design or lack there of

(OP)
When we have shearwalls that exceed our standard holdowns, we will get more into what the uplift resistance loading is. Most times when I run into that though is on large custom homes, not so much the smaller homes where margins are smaller. It sounds like your lateral workbook is where I am trying to get to. Our lateral spreadsheet includes everything for lateral loads, wall studs, diaphragms, shear walls and holdowns. I would like to add some sheets for some of the gravity loading that you have mentioned. What sort of I do you use for continuous girder trusses? Do you assume a 3 ply girder and use the I for that? We ususally just run a generic beam calculation to determine the reactions.

RE: Residential diaphragm design or lack there of

jeff

We are getting off topic, want to continue along this topic in Wood Design?

I can give you some ideas in general, show you pdf or values only of my worksheets, but can't give you the actual Excel because that is proprietary to me of course.

The whole idea of my workbook it is all linked together so I can do calcs for most any wood structure in one workbook, no hand calcs at all, and it prints out in one pass. Input is streamlined too.

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
Yes. I'll put it in the wood design under "Wood design spreadsheets"

RE: Residential diaphragm design or lack there of

(OP)
AELLC,
Whoops. Can I delete my post or do I just have to leave it there? Back to the original topic, do you feel the subdiaphragm method my colleague has proposed would be a satisfactory method? Do you see any problems with that? And also, to finish this post, what would you feel was a minimum drag force level that you would not worry about drag forces until they reached that force level? I have used 2000 based on the fact that on a typical roof truss in this area, drag forces do not change the trusses until they exceed about 2000 lbs.

RE: Residential diaphragm design or lack there of

just leave it there.

I think the subdiaphragm is overkill, not needed for residential.

Agreed for the 2000. Most trusses can take much more without effect.

RE: Residential diaphragm design or lack there of

Quote (aellc)

Look at the attached detail- about the highest shear I calculate typically is 400 plf, more than the basic roof sheathing is rated for, but with wind forces here probably very conservative.
Looks like you could do without the sheathing above the top plates, which seems to cause a construction sequencing problem. The sheathing has to go up after the perp trusses go up but before the parallel trusses? Just have the truss supplier design the drag truss for all the load.

I'd also swap the top flat 2x, Simpson connector, and vertical 2x for a single vertical 2x block that accepts edge nailing from the roof sheathing and is deep enough to nail to the drag truss top chord.

I also took one of AELLC's sketches and added a little to it. This is how my office (in Southern CA) would address a condition similar to the one in the original post - but basically we try very hard to avoid such scenarios.

RE: Residential diaphragm design or lack there of

I use the plywood to satisfy our local plans checkers. Apparently they have been trained but are not University-degreed engineers.

During construction, the city inspectors rarely go on a ladder, so anything not easily visible from the ground goes unnoticed (the builders here use the detail method they have always used, and tend to ignore the current drawing detail)

RE: Residential diaphragm design or lack there of

ps

I should point out that most Plans Checkers here make vague comments such as "Details must show blocking at all shear transfers from one shear element to another", instead of supplying us with a copy of the standard detail they are looking for.

It is impossible to even discuss issues with them, so we routinely throw in all the blocking and plywood to get the project approved.

Now if 10 different engineer companies show that same detail in 10 different versions, chances are it is going to be built "the way the Rough Framer always does this detail, and he has 28 years of experience", and the Inspector will never critique it because he can't see it clearly from the ground.

I hope it isn't that way in California or any other high-seismic area.

RE: Residential diaphragm design or lack there of

How can a non engineer checker, check structural designs?
How can the building inspector inspect from his truck?

LOL

RE: Residential diaphragm design or lack there of

Framing quality / adherence to plans depends on so many things. Quality goes down w/ population density and sale price of the homes for the most part (so most tracts are fairly meh). That's not to say that a nice custom home can't be completely butchered, it's just not as common.

Any detail that isn't fairly close to typical is generally not followed well.

One example would be the tall truss blocking I attached in my last post. Our detail shows A35s to a truss vertical along the vertical legs of each block (in lieu of toe nails in shear). I would say I've seen those connectors installed less than 5% of the time. The truss supplier usually provides the vertical in the truss near the blocks (or we get it added during review), but the framer just does not install the A35s. If we do observation, we ask that they install them, but I believe that after a phase or two, they are no longer installed.

Inspectors here do regularly inspect the roof sheathing, at least.

RE: Residential diaphragm design or lack there of

Here in the Phoenix area (we have a lot of towns surrounding Phoenix proper), many towns' residential plans checkers are non-degreed. It is only the commercial project checkers that are engineers.

The inspectors walk the job properly but for some reason the rarely go up on a ladder.

RE: Residential diaphragm design or lack there of

In CA in my experience, most 1/2-story residential plan checkers are "plans examiners" and not licensed/degreed engineers. They usually look at the calcs and point out anything that does not exactly match on the plans.

So if we have a 12' shear wall in the calcs but it's 14' long on the plans, we sometimes are told that the plans don't match the calcs.

RE: Residential diaphragm design or lack there of

Miley,

Exact same here. Check call out for beam or header size, and span.

Check for matching shear panel type, length of wall, hold down call out.

Any trained monkey can do that.

We even occasionally get : beam actually installed is larger - we have to write a letter.

Shear wall is longer - we have to write a letter.

Installed hold down is a STHD14 instead of LSTH8 - we have to write a letter.

Please shoot me and put me out of misery.

RE: Residential diaphragm design or lack there of

(OP)
AELLC and Miley,
It is the same here as well. Although the majority of the residential I do are larger more expensive custom homes, so it gets built closer to plan than the smaller stuff. The last few homes I have done have been very complex with steel moment frames and large glulam and steel beams. The inspector looked at it and told the contractor I had to be present for the inspection. I showed up and he handed me the drawings and said "Where do you want to start?". Essentially I did the inspectors job. Kind of feels a little like double jeopardy, now I get a chance to make a mistake on the design phase and on the inspection phase.

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