Prefab Wood Truss Uplift

[RWW0002]

Since we have several members active recently from the prefab wood truss industry I wanted to take a second to ask a question concerning the typical truss output.

For background - When specifying dead load to be used to resist wind uplift, I generally specify 0.6x an approximate of "actual" roof dead in my general notes. This generally equates to around 0.6 x 15 = 9 psf. So even for jobs where I am specifying 20 psf Dead Load (10 PSF @ top chord and 10 psf @ bottom chord), I generally specify for truss suppliers to use 8 psf dead load (DL) resistance for wind uplift calculations (5 PSF @ top chord and 3 psf @ bottom chord). For discussion purposes lets assume all of these loads are service-level ASD loading.

Even though I specify 8 psf DL resistance, we almost always see trusses come back with around 12 psf DL resistance.

Questions:
1. As far as truss output, when truss output shows 12 psf resistance (as shown below), is this already taking account the 0.6, or are the top and bottom chord DL given in the notes further reduced by 0.6 within the truss calculations? (See below for typical note.)

2. Assuming that the 0.6 is already taken into account in these loads provided and this is intended to be 0.6 x 20 = 12 psf, are others allowing this? I have to mark this up on practically every single truss shop I review, and even though I have asked for clarification, I have never been given a satisfactory answer to what exactly the numbers provided in the output represent. Is 12 psf DL for uplift calcs common?

I always run my own loads for truss ties @ bearing, but I would think this would have a significant effect on bottom chord design and required bracing.

 

[phamENG]

My understanding is that the load shown is the 'true' load. So if you add up the components and there's 12psf of material, the top chord dead load should say 12psf. The 0.6 is part of the standard ASD load combinations, and is accounted for in the background.

Every once in a while I'll get a sheet in the calc set that lays out all of the 'standard' load cases...but it's pretty rare.

 

[XR250]

Here is the thing, though. The actual top chord dead load is probably more like 6 psf and the bottom chord is probably like 4 psf.
So even with 0.6, you are probably unconservative. I used to specify the dead load to be used for uplift in my drawings which equated to the most likely dead load - not the worst case.

 

[RWW0002]

Backing into their reactions, I usually find that the load provided in the Wind Load note(DL resistance) is taken at full value and not reduced (at least in the reactions. This is why I generally request that they use 8 psf max for DL resistance. Makes me think that the 0.6 is not being applied "in the background"

XR250 - I agree - many times even 8 psf is not overly conservative.

 

[RontheRedneck]

RWW0002 - You said: "...I would think this would have a significant effect on bottom chord design and required bracing."

Most trusses I deal with have drywall on the BC, which means they're assumed as continually braced. So the dead load for wind design doesn't mean much.

If the BC has purlins, we can run the program with a "max purlin spacing". Then it comes back and says the BC purlins have to be XX" O.C.

The program I use allows up to 60% of the design dead load to be used in the wind cases. I suspect the designers you're dealing with are just using that.

From what you said, it sounds like you're specifying something (the 8 PSF) and the truss designer is not doing it. If I sent you trusses in a case like that and they came back annotated, I'd fix the problem and return revised drawings to you. If you're not getting that, then something is wrong with the process.

 

[phamENG]

So...I was writing up a post that got too long about how you guys are crazy and of course it's 0.6x the dead load listed. It probably still is, but I found this in TPI 1-14 and I really don't like it:

he dead load used in determining wind uplift shall not exceed the minimum expected actual weight of the materials, or 0.6 times the nominal design dead load if the minimum expected actual weight of the materials is not known.

This shouldn't be in there. It should simply reference the building code or, in the absence of a building code, ASCE 7. It does that, but keeps going with this section I quoted. What exactly is "nominal design dead load"? I'm guessing they mean the 10psf or 20psf "options" in the IRC for various load tables. But to suggest that you can take the actual anticipated dead load and use it without reducing it violates the IBC and reduces the factor of safety for the system.

From what Ron said it sounds like the software he uses applies the 0.6 to the dead load for combination with wind for uplift. That's a good thing, at least.

RWW0002 - when you back it out, are you using MWFRS load or C&C loads? Most truss manufacturers calculation their uplift using MWFRS loading, so that may be a reason you're getting less uplift than you expect.

 

[RWW0002]

@Ron. This is what I was guessing as well. I think your are right that many are using the max allowed DL until it is marked up on the submittal and then adjusting when asked..

As far as the bottom chord, many commercial truss jobs around here do not have gyp attached to bottom chord (drop ceiling). So worng net uplift may make a difference in bracing (usually just an extra runs maybe some additional web bracing sometimes.. Honestly I dont pay that much attention to the changes, just make sure they have the right load)

@Pham - I was using the load noted in the noted referenced.. MWF Int zone envelope procedure. And in that case seemed to confirm that the noted load was not reduced by additional 0.6. The noted load was total DL resistance.

It sounds like I have been right to mark this up each submittal but began to doubt myself since it is incorrect on almist all sets I get (design drawings call for 8 and initial submittal shows 12)

 

[RontheRedneck]

phamENG said: "From what Ron said it sounds like the software he uses applies the 0.6 to the dead load for combination with wind for uplift."

To clarify a little - 60% is the max allowed. If I go into the defaults, put in 80% then click "OK", the program gives me a warning that it can't be more than 60% and then changes the setting to 60%.

 

[RWW0002]

Thanks Ron. I have not problem with the program defaulting to 0.6x the input dead load, I just wish more would pay attention to my note in design drawings and adjust it down to 8 for wind uplift cases before submitting so I don't have to mark it up and all the time.. I was beginning to think I was misunderstanding the output.

As a side note, this may become a much bigger deal than it is now with the transition to 105 mph wind in many areas with newer code editions. For MWF interior wind for an Exposure B building and 105 mph wind the service-level uplift load may very well be less than 12 psf. So if 12 psf DL is used to resist uplift then this may result in no net uplift on the designed truss.. Just something to watch out for.

 

[RontheRedneck]

RWW0002 - I understand your frustration with notes being misses in your design drawing. I certainly miss them at times.

In my defense, the contractors seem to perpetually be in a rush to get the sealed drawings. They seem to think we can kick them out on the spur of the moment with little effort.

And often I feel like they're a waste of time. They often just get rubber stamped, and no one looks at them.

Maybe that's fodder for a different thread.

 

[RWW0002]

@ Ron - We all miss things from time to time. Maybe I need to make these things a little more clear on the drawings..

I am used to marking this up on truss shops and its no big deal (although when not picked up the 3rd or 4th go-around while I am hearing from the contractor that I am holding up the whole job for not letting things go.. now that is another thing entirely..) The intent of the thread was to make sure I was not double dipping with the 0.6 factor - me reducing dead load resistance and then the program reducing them even further - and I think you have answered that. Thanks for the help.

 

[JLSE]

RontheRedneck - What software do you use?
And all, is the concern that the truss will fail? Or that you dont have an accurate holdown force to work with?
I'll certainly be adding this requirement and taking a look at this in the future... thanks.

 

[RWW0002]

I generally compute my own holdown forces @ bearings using design wind and spot check with the truss drawings - designing for worst case (also this spot check can help make sure you and the truss supplier are on the same page)

Just like any other design criteria/loading issue, it could lead to some kind of failure depending on the truss and severity of load discrepancy, but I am not sure that it is super likely. Others correct me if I am wrong, but the biggest think this is likely to affect beyond the end reactions is bottom chord bracing and (potentially) some web bracing in the case of reversal of forces.

Here is an example where I could see it potentially leading to some kind of failure in a high wind event-
-Say a light roof (basically selfweight + sheathing + metal roofing - maybe 7 or 8 psf ) with long span trusses and no hard-attached ceiling
-105 mph wind load Exposure B resulting in maybe 9 psf (service-level)uplift
-20 psf dead load specified so 12 psf used for DL resistance
-Resulting truss designed tfor 0 net uplift (9psf-12 psf)
-Wind event causes compression in bottom chord and reversals in the web leading to buckling...

Even in this case standard erection bracing would help mitigate against this, but it is worth considering..

 

[JLSE]

RWW0002- Okay. I get that. If Im not mistaken, the truss designers usually assume either direct applied ceiling or they specify a nominal bracing. In the situation you describe, Id recommend bracing the BC either way. A continuous runner is easy and cheap, add some bay blocking and youre there.

 

[RontheRedneck]

JLSE asked: "What software do you use?"

Alpine.

JLSE also asked:
And all, is the concern that the truss will fail? Or that you don't have an accurate holdown force to work with?

In my rather jaded opinion, trusses virtually never fail due to wind. The only exception I've personally seen was a poorly braced gable end truss failing when the rather tall studs broke.

The wind design generates a number for uplift. Contractors either do not install hold-downs at all or install them wrong. Which makes the whole process of designing for wind a waste of time.


However I realize there are places with inspectors who actually check those things and they probably do get installed right. It just does not happen here.