Residential diaphragm design or lack there of 1

[jeffhed]

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.

 

[AELLC]

Ok mine is similar but notably different in how dead loads are entered to reduce hold downs - see attached

I can input 3 different uniform loads, and 4 different conc. loads (beams, girder trusses)

I do have hat you call drag diagram

 

[jeffhed]

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.

 

[AELLC]

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.

 

[jeffhed]

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.

 

[AELLC]

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).

 

[jeffhed]

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.

 

[AELLC]

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.

 

[jeffhed]

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

 

[AELLC]

I already did...

http://eng-tips.com/viewthread.cfm?qid=361178

 

[jeffhed]

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.

 

[AELLC]

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.

 

[MileyDiley]

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.

 

[AELLC]

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)

 

[AELLC]

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.

 

[boo1]

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

LOL

 

[MileyDiley]

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.

 

[AELLC]

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.

 

[MileyDiley]

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.

 

[AELLC]

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.

 

[jeffhed]

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.