Uplift on Wood Roof Trusses
Uplift on Wood Roof Trusses
(OP)
I've seen where some pre-engineered wood roof truss engineers will provide uplift reactions based on MWFRS pressures, and consequently, the hurricane ties at each truss support are sized based on MWFRS pressures.
What you do you guys think about that? I really wish ASCE 7 would further clarify the differences between MWFRS and C+C. I think this is one of those gray areas.
What you do you guys think about that? I really wish ASCE 7 would further clarify the differences between MWFRS and C+C. I think this is one of those gray areas.






RE: Uplift on Wood Roof Trusses
The hurricane ties, would have essentially 1/2 the area of the truss they support (I am simplifying, but this would not be too far from reality).
I would send the shops back and make them correct the trusses.
RE: Uplift on Wood Roof Trusses
Anything with larger tributary area is MWFRS.
RE: Uplift on Wood Roof Trusses
RE: Uplift on Wood Roof Trusses
Wind pressure doesn't "know" whether the truss has one or twelve layers between it and the truss - the wind would be the same regardless of layers.
RE: Uplift on Wood Roof Trusses
If you read the comentary to ASCE7-05, they give the example of a long span truss with the individual components designed based upon their tributary area. Then the chords, would be designed based upon the overall tributary area. If the area is large enough this may well be MWFRS>
RE: Uplift on Wood Roof Trusses
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: Uplift on Wood Roof Trusses
Read the wording in ASCE 7. There is something about MWFRS collecting load from more than one surface.
But it is affected by how long of a continuous surface it can attach to. With C+C loading, you are looking at peak pressures over a localized area. There is a difference if there are multiple surfaces within an area under consideration.
RE: Uplift on Wood Roof Trusses
So if I have a moment frame with <700 sf of tributary area, I need to design it for C+C loading? For example, for a 15' foot tall pre-engineered metal building with 20' frame spacing, each frame would have a maximum tributary area of 600 sf. You are going to make them design it for C+C loading?
RE: Uplift on Wood Roof Trusses
A lot of engineers in my area make the truss engineer design the truss to plate connections.
RE: Uplift on Wood Roof Trusses
The commentary is attempting to offer users a feel for elements which may receive load from multiple connections and "layers".
If you have a vertical column and footing that support only 100 s.f., I would use C&C wind load (roof uplift) on that column and footing, despite the fact that the column receives the wind load through roofing, insulation boards, screws attaching the boards, metal deck, joists, beams, bolts connecting beams, etc.
These layers do not PHYSICALLY change anything about what sort of wind pressure arrives at the top of the column.
For an element that is part of a MWFRS, you would design the brace system for MWFRS loads, AND, if the area on the brace is low, I would also design check for C&C as well. These are not mutually exclusive necessarily.
RE: Uplift on Wood Roof Trusses
RE: Uplift on Wood Roof Trusses
Quote: "A lot of engineers in my area make the truss engineer design the truss to plate connections."
Are you sure about this?
I have heard of truss manufacturers typing on hangers and anchor clips onto sealed drawings without the truss engineer knowledge. And that others will mark up unsealed truss layout drawings with connections and send them out with the truss package.
Hopefully, the truss/layout drawings you are getting have not been changed like this.
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: Uplift on Wood Roof Trusses
I hope that hasn't been occurring. I usually spend time critiquing the selection of ties to make sure they meet the reported uplift loads and to make sure they are providing something that is compatible with the structure at special cases like heavy girder truss connections.
RE: Uplift on Wood Roof Trusses
2. Truss to structure connections should ALWAYS be designated by the EOR, we are the only ones with the overall structural knowledge of that particular building. Many people, including some SEs, neglect to design truss connections for the combined forces due to uplift:
>>> shear from the roof diaphragm to the wall (unless you transfer it via blocking)
>>> out-of-plane shear due to lateral forces on the wall (transfer mechanism for lateral forces from the wall into the roof diaphragm).
Simpson for example requires this check in their tech notes and provides an interaction formula. These are often listed as F1, F2, and F3 and diagrams are provided in the manual. I know, probably preaching to the choir....
RE: Uplift on Wood Roof Trusses
So why do the truss programs show the MWFRS uplifts? They do because the truss program C&C uplift is often very conservative and greater than the building requires. If the C&C uplift is sent to the building department they may require the building designer to not only increase the building holddowns (which is not required per the builder designer) but in some cases have wanted the building designer to redesign the building for this higher uplift. Rather than trying to make the truss program more accurate for C&C loading, the truss plate companies has allowed an option to show only the MWFRS reaction (the truss is still designed for the C&C loading) and leave the required C&C reaction and connection up to the building designer.
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: Uplift on Wood Roof Trusses
The MWFRS vs. C+C decision has implications on more than just the truss to structure connections. What about top plates to stud, stud to bottom plate, bottom plate to anchors, etc.??? It is all part of the same load path with similar tributary areas. Do you design all that C+C loading?
If I understand JAE's interpretation, all this would be designed for C+C loading, simply by virtue of its tributary area, regardless of how far it is down the load path.
What else does the truss engineer need to know?
With regard to other forces at that joint:
- We provide blocking whenever there are significant diaphragm shear forces to transfer. How else are you going to get the load down from the roof diaphragm to top of the wall without inducing bending about the truss to wall connection?
- We call for a few toenails at the truss to wall connection to account for the out-of-plane loading separately.
RE: Uplift on Wood Roof Trusses
Also most structures I have designed are in Florida where CMU is predominant, so wind uplift transfer is usually an afterthought. In a really high uplift situation that effects walls and foundations I would recalc for MWFRS.
Not everyone specifies blocking for shear transfer, but I usually do. Unless you have a small enough heel height, then there are connections that can transfer the roof diaph. shear load into the top of the wall.
And for out of plane I suppose toe nails would work, I just don't like them so I never specify them. And many times that out of plane force can be quite critical. Also, can't toe nail into a bond beam if its a CMU wall with no plate.
Different regions, different methodologies, as long as it is thought out and somebody uses engineering judgment... Feel like I am beating a dead horse here. Sorry.
RE: Uplift on Wood Roof Trusses
a2mfk I am in South Florida and I find that truss to wall connections typically require the heaviest connectors that suppliers produce. Before some of the heavier connectors came on the market (Simpson DETAL for instance) it would often require more than one connector per truss.
RE: Uplift on Wood Roof Trusses
That's not beating a dead horse. That's just applying a little common sense, and engineering judgement, and realizing that the client probably wants the design done during your lifetime, and his. All this despite the current state of the building codes. Soon the wind loading design on a small building will take longer and involve more pages of calcs. than an entire warehouse building design did 20 years ago. And, surprise, surprise that warehouse is doing just fine after several significant wind events, it still has its roof, and doesn't leak or lean too much.
RE: Uplift on Wood Roof Trusses
I have this gut feeling very few people do that combined check but I hope I am wrong. I can see a CMU wall blowing in because it sheared off an inadequate uplift strap that was only designed for uplift, or maybe lateral, but not both. But I think there must be some good engineers at Simpson, it seems they have every detail covered. Well, its good business for them too.
I've done forensics after hurricanes and tornadoes, and therefore I am overly conservative in my roof pressures and connection design.
Horse dead now...?
RE: Uplift on Wood Roof Trusses
Architects and contractors in my area take notice when you put a strap at the top and bottom of every stud.
I just wish ASCE 7 would provide better guidance on whether or not situations like this should fall under MWFRS or C+C. The codes hints at the "layers" factor and the "multiple surfaces" factor. I wish they would clarify these considerations. I think the cost of all the additional strapping could be somewhat significant on a large project. I'm sure if you asked an owner whether or not he cared about a extra couple grand, he would say yes.
Some engineers may argue for MWFRS because they think the modern wind code is conservative. In my short time in this profession, I've seen so many poor designs with regard to wind that have surprisingly lasted many years. I'm talking about things so underdesigned per modern code that you could not even prove it would hold under the unfactored loading - nevermind a factor of safety. I know these cases are anecdotal, but I've seen enough of them in a short time to make me scratch my head.
Even if the tributary area of the wall or foundation is less than 700 sf??
Right, file that under another thing a lot of engineers neglect. Even if the heel height is small, in order to transfer the shear you still have to make some argument that the truss connection is taking some moment. How do you prove that?
Don't worry about it. I enjoy the discussion.
RE: Uplift on Wood Roof Trusses
What about the rest of the uplift load path? Top plates to stud, stud to bottom plate, bottom plate to anchor bolts? Should that be C+C as well?
RE: Uplift on Wood Roof Trusses
So for example lets take steel roof deck connections. You must actually design the deck connections for two different load cases.
For the components and cladding load case you design the deck and the deck connections for the uplift components and cladding load based on an area of less than 10 ft2. (span * span / 3)
Then you must also check the deck connections for MFWRS. Here you would check the deck for MFWRS uplift in combination with your diaprhagm shear (this checks the deck for overall effects of wind on the entire building. It is based on wind loads spread over the entire building envelope walls and roof) The deck at the boundaries must transfer the diaphragm shear to the shear walls similar to how a steel beam connects to a column with bolts. I find that the final line of welds often must have shear collectors which many engineers are failing to do. One puddle weld to a joist every five feet sometimes doesn't cut it. You can do two welds and often and that will work but then the joist seat must be designed for rollover.
Lets take timber truss connections now. Again there are two load cases. In one load case you have only uplift components and cladding force acting. When the wind hits the building your are checking that one truss that gets higher wind because a gust hit it and only it in isolation. The pressure depends again either on the tributary area of the truss or span * span /3.
A second components and cladding load case check would be the out of plane shear transfer from the wall. This is for high isolated gust hitting a wall. Again you check this in isolation. So if the connection of that one truss can tranfer that small gust area it is OK. It would be very unlikely that the same truss would handle both a high concentrated gust on the wall and the roof at the same time.
For the MFWRS load case you must combine both MFWRS uplift and MFWRS out of plane and MRWRS in plane pressures. This would be a correct methodology according to how I was taught and from everything I've read since. For trusses dumping diaphragm shear out of the deck into the shear walls or for trusses receiving shear from the walls being sucked or pushed on by wind this is a macro event. Kind of like the repetive member factor in wood design. All the trusses act in unison to transfer these lateral loads. Therefore you use the MFWRS shear loads in combination with the MFWRS roof laods.
In practice it takes time to come up with all these loads. I usually will combine the components and cladding loads and connect for that to be conservative. I will only usually seek out the MFWRS loads when checking combined loads if I'm really having trouble getting something to work at a reasonable cost. For example if I have to use multiple connectors or something really impracticle I'll sharpen the pencil and use the MFWRS loads in combination and the components and cladding loads in isolation. If you place shear collectors between the truss seats than you can completely eliminate the in plane diaprhagm shear transfer from the combination. Often I will pour concrete tie beams to support steel joist since you can get much stronger connections into concrete versus masonry.
So in summary you really must always check everything for both the components and cladding load case and the MFWRS load case. But by definition components and cladding is a singular load because its intent is to capture the effects a concentrated but high pressure gust that effects only a small area. When checking the MRWRS systems load path you must check the truss connections for shears in combination with uplift.
You will notice however that as areas increase the components and cladding pressures (for roof and walls) will approach the worst case MFWRS pressures. So again for saving time its usually practicle to work with the components and cladding loads for both your isolated checks and your combinded shear and uplift checks. If you get into trouble you can use the MFWRS loads as that is the intent of the codes. It can reduce your loads sometimes by 10 - 20% which can help when checking the combined shears and uplift.
Roof deck connections have become kind of a specialty of mine so I've become very intimate with ASCE 7-05, etc... I'm pretty darn certain what I've written above will never guide you wrong.
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: Uplift on Wood Roof Trusses
Top plates to stud will be controlled by your components and cladding uplift load from your trusses. I would based this on the trusses area since the top plate has some redistribution qualities as well. And lets face it that is the component the load is coming from. The load is from the truss not the wind hitting the top plate connector directly.
The same thing applied for the stud to bottom plate.
However for the bottom plate to your foundation I find it is usually controlled by the combination of MFWRS out of plane shear in combination with MRWRS uplift. In practice I combine the components and cladding loads instead since they are higher and easier to grab. I think most engineers are simply sizing the bolts here for diaphragm shear transfer. But again in reality I find that the bottom plate often shears due to the out of plane wind pressures. I had to program a spreadsheet for houses cause otherwise designing all the parts and pieces becomes daunting. Most people aren't actually engineering houses. They just do the same thing every time. I try to avoid designing houses anymore. Of course right now I take what comes.
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: Uplift on Wood Roof Trusses
Roof trusses receive their load from the roof. So does the roof deck. Most of the time the columns are also receiving their wind load from the roof.
Truss connections receive their load from two surfaces. The walls and the roof.
If you design almost everything with components and cladding based loads you will be fine. But when doing combined checks of shear and uplift you can use the MFWRS loads. The worst case MRWRS load however is usually not that much smaller than the components and cladding load. In fact the diaprhragm shear is always a MFWRS load.
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: Uplift on Wood Roof Trusses
Based on the current ASCE7 I don't see how you could argue that the wall to truss connection would be anything but C&C loading. So it follows to me that the components of a stud wall would be based on C&C loading. I also can visualize a localized peak gust that would envelope both a portion of a wall and roof.
We live and practice in a world where lawyers rule. It sucks but it is the real world. If a person was defending their design in a hostile environment I don't find anything in ASCE7-05 that could be used to defend MWFRS loads for this condition.
RE: Uplift on Wood Roof Trusses
RE: Uplift on Wood Roof Trusses
The attached technical note from the CFSEI (cold-formed steel engineering institute) basically says that you design rafters, trusses, and the individual truss components for C&C but you design the uplift connections to the stud walls for MWFRS. Of course, it is acceptable and up to the designer if he chooses to use C&C for uplift tie design as it is more conservative and saves time in not having to re-calculate for MWFRS forces at the uplift connections. I think the article gives a pretty good rationale for this and references some other design standards that substantiate this position.
RE: Uplift on Wood Roof Trusses
I still think C&C loads should be used for truss hold-down connections at their ends. To me that is because the "typical" failure of roofs in high wind events is either due to deck/diaphragm uplift failure (causing instability in other members and then collapse) or due to connection failures at roof edges from roof framing separating from the wall.
However, the document you posted brings up an interesting aspect of the condition of a truss hold-down on a roof...the last paragraphs of your posted document suggest that due to the long, slender area of a roof truss member, the statistical variations over the area do actually average out more like MWFRS wind pressures.
I can see that aspect of it.
RE: Uplift on Wood Roof Trusses
It is important that the trusses be checked for C&C not only for the uplift connectors but also for possible stress reversal in the webs and chords. Typically they don't brace webs and chords in tension but if uplift causes compression in the bottom chord or web members it could result in a failure in longer members. The steel joist industry provides uplift bracing for joists when uplift is a problem but I rarely see this in the wood truss industry. I also rarely see truss programs that properly design gable end trusses since they don't consider loads on the side of the trusses. The EOR needs to make sure that all possible load cases are properly addressed by the truss manufactures. I frequently require the shop drawings to be revised and resubmitted because the technicians that typically plug and chug don't know how to handle special cases and many engineers that stamp the drawings seem to be on autopilot.
RE: Uplift on Wood Roof Trusses
Thanks for posting that paper. That is the first time I have ever seen this issue discussed in a technical paper. The references cited in the paper certainly indicate that MWFRS pressures are the basis for roof to wall anchorage in prescriptive methods of wood design. Very interesting.
I would still like to see something definitive from ASCE 7 on this matter.
RE: Uplift on Wood Roof Trusses
Is it that hard to make wood framing work in really high wind speeds? Or is the CMU more of a preference for durability?