Roof Diaphragm Dispute

[UnneutralAxis]

I have a job where the shear flow in a wood roof diaphragm is a little over 400 plf. Blocking is required. The framing sub doesn't want to supply the blocking for whatever reasons and they have the truss supplier (also a PE) saying that the blocking is not necessary. I've never been in this situation before, and blocking is clearly necessary. He says that he's never even seen blocking on a roof diaphragm before. I say he needs to get out more...

I'm trying to resolve this as painlessly as possible. I'm self employed and am always afraid of being sued.

I thought about sending their engineer my calcs and saying he can review, refute, and sign and stamp a letter saying it is not required and then I would consider eliminating the blocking. At the same time, it's required and I'm the engineer on the job so I'm still not comfortable with that.

Can't imagine getting drug into court over some blocking, but stranger things have happened.

 

[msquared48]

They are the ones wanting a change to your design. It is their responsibility to provide you with calcs and details to support their position so you can say yes or no. You are the engineer of record and it is your call.

Don't be intimidated here, just look at their logic and see if it makes sense. There have been several threads here on diaphragm blocking and whether or not to do it. I, personally, am one to put it in, but others here would disagree, depending on the location. That does not mean that I do not consider their argument though.

If the framing contractor has never installed blocking on a roof diaphragm, either he does not have much experience, or does not read plans well. It's done all the time, but more so in retail or commercial than residential.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[UnneutralAxis]

The diaphragm spans 180 feet and has a tributary width of 24'. The shear walls connected to it are 60' long. I am just shaking my head how someone could think it shouldn't be blocked.

 

[msquared48]

So this is a commercial project...

The term "tributary width" is a little confusing to me here. Are you saying that the diaphragm is 180 feet by 24 feet? If so, then the aspect ratio is over the allowed ratio by quite a bit. Perhaps you could clarify...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

I agree with Mike. If they want to change the design, they should submit calcs showing it works, not try to twist your arm into changing your mind (I see contractors and subs do this all the time, though). As if you can magically make the diaphragm forces less than they are.

We've had blocking on roofs with smaller diaphragms than what you're talking about.

Maybe you only need blocking for some specific number of joist bays and not the entire roof.

Here is a way you might be able to make the problem go away. Ask him exactly why he doesn't want to provide the blocking. If it is shown on the drawings, then he bought it - he knew it was supposed to be provided. Once you get him to say it's a labor and/or money issue, ask him how much of a credit the owner will receive if he is able to eliminate or reduce the blocking requirements. That could cause him to just put in the blocking, because he's not going to give any money back to the owner.

 

[ash060]

I would think that you would need eave blocking at the minimum.

 

[UnneutralAxis]

Sorry, the tributary width thing is a vertical height. As in, total height that catches wind and distributes to the diaphragm. Yes, very much a commercial job here.

I checked my calcs and it's actually over 500 plf shear flow. I said 400 earlier as just a "gut feeling" thing. About 32,000 lbs to each shearwall. Shearwall length = around 60'. That's about 530 plf.

I considered looking at the shear diagram and ending the blocking when the shear becomes low enough, at a calculated distance from the shear walls. I've actually never done this and don't know it to be a standard practice. Has anyone else ever ended the blocking requirement a distance from the shear wall in this fashion?

 

[msquared48]

It appears to me that you may be talking here about the diaphragm edge shear trickeling into a drag link that leads to the shearwall. The shear force from the diaphragm along that drag link should be constant, but the force gradually accumulating as one meves toward the shearwall.

That's different than the blocking required by the shear diagram where I will vary the nailing and blocking requirements as needed based on the max shear seen in a region of the diaphragm.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

Do the shearwalls take up the entire wall? If you have 100' of building face, but only 60' of shearwall, you can provide a collector and design the diaphragm for 32000#/100' = 320plf.

 

[ron9876]

If you are sure that your calcs are correct and if you have it clearly called out on plan then I would handle it just as Lion06 suggested. I would politely listen to someone that had intelligent input.

On steel deck diaphragms I have often reduced the amount of sidelaps as the shear reduces.

 

[msquared48]

ron9876:

"On steel deck diaphragms I have often reduced the amount of sidelaps as the shear reduces."

I assume you are only talking about side laps in the interior of any diaphragm, or sub-diaphragm, where the shear varies, not at the diaphragm edges where the shear is constant.


Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[UnneutralAxis]

Man! Being able to look at a set of plans is worth so much! Sorry, but I just don't want to post them here.

This part of the building is a box. 140' (note typo earlier with 180) x 60'. Shear walls located under the diaphragm ends. Can't get much simpler than this situation. I am speaking of wind load perpendicular to the long face of the building.

Anyway, after speaking with the owner, it sounds like he is going to pay them additional $ for the blocking. It is only $1200, or about four 100ths of a percent of the total cost. Best resolution I can think of.

 

[DaveAtkins]

I hate to open another can of worms, but have you checked the diaphragm chords? 180' is an incredibly long diaphragm.

DaveAtkins

 

[msquared48]

Apparently only 140 feet Dave, but it's still long.

And the aspect ratio is OK too...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

Can you use any interior walls to act as shear walls and "break" the building up???

But from what I have read you are on spot - or at least pretty close!!

 

[UnneutralAxis]

Yes, I checked the chords, of course. Had to make a nice splice detail that when they turn to that page of the plans they'll probably want another $500 or so. It'll go something like this:

"We're just going to do a 6' splice on the top plate, OK? What do you mean, No? You're crazy as hell!"

It's what happens in commercial construction in rural towns. If you've not had to deal with it before, count yourself lucky. Nothing but homebuilders that are used to being under ZERO inspection requirements and have decided to take up commercial work during lean times. You can't convince them the situation I have described is different from building a 60x30 house. It's the same materials they are familiar with, but the forces are so much higher and you can't convince them of it. Everything you draw on the plans is crazy.

 

[BAretired]

24' seems like a large tributary width. Do you have a very high building or high parapets?

BA

 

[UnneutralAxis]

Yes, it is a large tributary height. But according to the framers, that actually doesn't matter. Who knew?

 

[Lion06]

Why in the world is the owner paying extra for something that was documented? That's ridiculous!!

 

[bigmig]

Why is a truss engineer giving you advice about building structure? They have no liability in it, other than their trusses. In other words, their opinion doesn't mean a damn thing because they aren't the ones stamping it.

If you are responsible for the design and will be stamping it, put the blocking on the plans. If they want to leave it out, good for them. They won't hold up 2 minutes in a courtroom.

If the owner is complaining, point out the % of cost for installing the blocking, and tell him he can hire another engineer to verify your conclusion if he wants.

If the truss engineer will offer to stamp your plans (which he won't) then give him an ear. Otherwise stop wasting your (and the owner's) time and money by entertaining the opinions of people who refuse to be held liable for their mouths.