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Roof Diaphragm Dispute

Roof Diaphragm Dispute

Roof Diaphragm Dispute

(OP)
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.

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

(OP)
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.

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

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.

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

(OP)
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?

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

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.   

RE: Roof Diaphragm Dispute

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.

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

(OP)
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.

RE: Roof Diaphragm Dispute

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

DaveAtkins

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

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!!

RE: Roof Diaphragm Dispute

(OP)
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.

RE: Roof Diaphragm Dispute

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

BA

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

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

RE: Roof Diaphragm Dispute

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.

RE: Roof Diaphragm Dispute

unneutralaxis-
Where in the construction process is this project?

If the contract has already been awarded, then the contractor bought the blocking.  He owns it, plain and simple.  He should NOT be getting extra money for something that was on the drawings the way he bid them.  If he can't read the drawings to bid properly, then that's on him.  

It pisses me off and it's not even my project.  

There are often times where the guys that build the stuff have good ideas.  They know how things get put together.  

This is not one of those times, though.   

RE: Roof Diaphragm Dispute

It's not healthy to piss off a Lion.  

I'll have to agree with him.  bigsmile

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

RE: Roof Diaphragm Dispute

(OP)
Lion - all foundation work was completed today.  Framing should begin next week.  

I can't tell you how much it bothered me today.  I think the truss engineer is just that.  A truss engineer.  Not saying that all are one trick ponies, so please no one take offense at that.  I don't think he understood what the blocking was even for because they typically don't have to think about a MWFRS.  So (best I can tell) he likely thought that the blocking was to brace the top chord of the trusses in compression.  Hate to jump to that conclusion, but from our little discussion today that was the impression I was left with.  

I don't care anymore.  I decided that I'm not even entertaining suggestions on the blocking.  It's going in and that's that.  Don't like it... then don't put it in, owner won't get certificate of occupancy, and you won't get paid.  Then you can rip the entire roof off and do it again when you decide you do feel like putting in the blocking.

Also, like I said earlier, they say they are giving them money for the blocking.  The owner is closely related to the GC... if that explains the $ issue.

 

RE: Roof Diaphragm Dispute

Stick to your guns, UA, the diaphragm must be blocked for shears as high as that.  Don't get mad, don't get excited, just tell them that this is the way it is.  You are right!!!

BA

RE: Roof Diaphragm Dispute

msquared48 yes I am talking about interior zones of boxes, etc.

RE: Roof Diaphragm Dispute

Dear Mr. Engineer:
Why are your foundations so big, your uplift connections have so many nails, your this and your that....?

Dear Mr. Truss Engineer (wait, what? A truss ENGINEER? You talked to an actual truss engineer? Who knew they existed?):
You design the trusses, I will handle the rest.Thanks.That blocking has nothing to do with your trusses, because, I am letting YOU design them. The blocking is for this little thing called a diaphragm. Now, unless you are doing a peer review, I am not going to explain this further to you.
G'bye...



Man I got almost annoyed for you as Lion reading this. That framer has a lot of nerve sticking out his hand for more money when he bid drawings. We as SEs have to bid on napkin sketches sometimes, and scope creep, fahgettaboutit!!!!

 

RE: Roof Diaphragm Dispute

I wonder if we could come back to the owner for more money saying, "What? You want a lateral system?  I'm going to need more money to do that."

I was extremely annoyed yesterday; now I'm just chuckling.   

RE: Roof Diaphragm Dispute

(OP)
Lion, thanks for the solidarity.  I appreciate it.

Yes, I was ticked yesterday.  First time I've ever had another engineer questioning my work.  I'm sure it happens to everyone from time to time, but I'm a younger guy and this was a first for me.  We all know that opinions vary greatly, even among engineers.  But this seemed as clear to me as whether or not you need a footer at the bottom of a load bearing wall.

I felt much better about the situation after actually talking to the engineer though.  It was just something he wasn't all that familiar with.

RE: Roof Diaphragm Dispute

All's well that ends well.  Good for you, UA.

BA

RE: Roof Diaphragm Dispute

Well Lion, we did just have an owner request windows at the ends of our shear walls to the tune of about 80% of our shear wall area, on a building where 75% of the foundations were already placed. Not those foundations thankfully, and they were tilt walls and actually worked out ok with some redesign. The kicker is this- the windows are there for just show!! They are actual windows, but the inside will be covered because it is warehouse space...

If we did not get a sweet ad serve our heads would have exploded.

I think even the architect finally had to tell the owner what a shear wall is, and stop tweaking a building that is being built for pete's sake. He literally had to tell the owner STOP, enough is enough!! :)

 

RE: Roof Diaphragm Dispute

a2mfk-

That's a riot......... coming from an architect!!

RE: Roof Diaphragm Dispute

Happy Lion...  Happy Lion...  Nice Lion.

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

RE: Roof Diaphragm Dispute

You might reread what the truss engineer said. Many times to cool off a client they will write something like "The truss designs assumes sheathing to be applied to the top and bottom chords of the truss, except as noted otherwise. This sheathing does not require blocking for the structural stability of the trusses."
This is what was called a make the client "feel good" letter.  

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Roof Diaphragm Dispute

I'm with Lion and others on this one.

As SE's, we need to stick to our guns more often. A change should be the exception, not the rule.

I'm not suggesting that contractors don't occasionally have good suggestions, but I'm getting tired of contractors wanting to make a change that will take us a few hours to verify simply so they can save 1% on something. Are they paying for our time to redesign something that is just fine to begin with?

RE: Roof Diaphragm Dispute

frv- I think what is different in this situation, and I may be misunderstanding it, is that the framer balked at the blocking because the truss engineer said it was unnecessary. Or the framer balked and the truss engineer said it is unnecessary.

Either way, a specialty engineer called out the EOR on his design to the contractor/owner, which in my mind is not how you handle things professionally. I have noticed "areas of concern" in EOR drawings as a specialty engineer before, and I have gotten with that engineer directly and privately to point it out to him, as a another set of eyes, not being confrontational. The response has usually been very cordial, professional, and thankful for my concern and for not making it public so that they can make the change as they see fit.

I think Woodman nailed it, pun intended, and the truss engineer either misspoke about the blocking or it was misunderstood by all but Unneutral.

RE: Roof Diaphragm Dispute

A2mfk....  you took the words right out of my mouth @ 9JUN11, 10:09, a day ago I wrote the following and then didn't posted it.

What is a truss engineer, anyway?  Are they really a full fledged one trick pony or not (per UA, 8JUN11, 22:19), or have they just been coached (teached) to run a pre-canned computer program that spits out a truss design, with some forces, stresses, deflections, and dimensions, and tells them when special bracing and the like are required to meet the programs design methodologies.  And, none of which he could even begin to imagine without the pre-canned program which he more than likely doesn't understand in the least.  If they took away his pre-canned programs he would be like a race horse without legs, not much of a winner.  I think the word engineer gets thrown around a bit loosely these days to stroke someone's ego.  You know..., like sanitary engineers drive garbage trucks, space engineers who look off into space, with glazed eyes, when being spoken to...  And, UA, you continue to perpetuate this by still calling him a truss engineer in your later posts, whatever he calls himself, he's just the truss drafter, and is way out of line discussing the plans and details with anyone other than the EOR.

The truss drafter/computer runner might have said, 'our truss design does not req'r. and special blocking or bracing for stability at the lengths, depths and stresses involved here,' otherwise, if you have questions talk with the guy who signed the plans.  By the way, I have worked with some truss drafters who could do some neat things with their trusses and programs to meet my special needs, once I explained what I needed and about how to do it; they could run the numbers and size the truss members and plates much quicker than I ever could, and to my satisfaction on shop drawing review.  So, they ain't all bad.
 

RE: Roof Diaphragm Dispute

(OP)
A2mfk,

I agree with most you just posted; however, this guy is an actual living, breathing, licensed P.E.  I looked him up on the state board's list to be sure.  So, he really is a truss engineer as he meets the same requirements I do to be licensed.  I can say though, that the man did not know about designing roof diaphragms.

Well, I think I've about beat this one to death.  Thanks everyone for your input.  Lot of knowledgable folks on here.

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