I'm saying it does not meet my definition of a component so I don't believe the code requires it to be designed for C&C loads.
At the same time, I recognize that there are some very knowledgeable engineers here that disagree, and I'm not absolutely sure that I'm right. After reading this thread, and seeing that it wouldn't in any way benefit me to design for MWFRS pressures, I would probably go ahead and design the wind column for C&C pressures.
I wish we could clear this issue up, as I hate being overly conservative.
CFSEng,
Looking at Figure C6-6 - it does show low pressures on the leeward roof.
You said: [blue]"If you were to analyze this frame using just the CC tables, your pressure distribution would be much higher than the picture shows since you would use edge zone pressures in areas that do not recieve edge zone pressures simultaneously"[/blue]
For all but a very very small building, the directionality doesn't matter. The wind can come from any direction and this figure is for only one direction.
In this picture, the wind is very high on the roof on the windward side, so the deck, purlins, beams, columns, footings - anything that takes this HIGH wind over a small trib area should be designed for that HIGH wind.
The wind can come from another direction so that the formerly "leeward" roof would then have HIGH pressures and its deck, purlins, etc. should also be designed for that high wind.
None of this depends, or matters, whether the wind is "directly" applied to the element in question. Why the hell would it matter? The footing doesn't "know" whether the wind uplift force is due to direct pressure on it or via the column...it's simply a net upward force that the footing must resist.
Why would you use a "MAIN WIND FORCE RESISTING SYSTEM" wind on something that isn't a part of the main wind force resisting system? And if not, what other wind would you use? The only "other" wind is C&C wind.
jmiec - your statement: "I'm saying it does not meet my definition of a component so I don't believe the code requires it to be designed for C&C loads.".
I could turn that around and say the same thing - ...it does not meet my definition of MWFRS so I don't believe the code requires it to be designed for MWFRS loads.
therefore - must by C&C.
C&C isn't all that excessively conservative once you get "back" into the structure for wind columns, footings, etc. The area goes up so the C&C begins to approach the MWFRS loads anyway.
Wind "turning corners" or being beyond some personal definition of C&C's isn't consistent with ASCE 7.
If being beyond some personal definition of C&C is not consistent with ASCE7, then neither is being beyond some personal definition of MWFRS. Remember, wind column was not one of the examples. Why wasn't it? Whose definition is more arbitrary?
In my first post in this thread, I said that I believed the key word in ASCE's definition of Components is "directly".
As ASCE doesn't define directly, other than to give examples of elements that are "components", I think it's enlightening to find the features common to the examples. The six examples given; fasteners, purlins, girts, studs, roof decking, roof trusses, all have one thing in common: they all receive wind load from cladding through a rigid path, that is, through members, usually fasteners, that work in tension, not bending. Perhaps that is the only feature they have in common, in which case, I think it may be the defining feature of the term directly.
jmiec,
Good points. I would just direct you, though, to the definition in ASCE 7-05 in section 6.2:
Components and Cladding:
Elements of the building envelope that do not qualify as part of the MWFRS.
Pretty clear cut - if its not part of the MWFRS then it is C&C by default. Per 6.5.12.1.3, if the At > 700 then you are permitted to use MWFRS wind....but only then. And that is based entirely on tributary area...not direct or non-direct application of the wind.
I think that wind columns not being included doesn't mean anything as the definition reads, "Examples of components [red]include[/red]...". It was not meant to be an comprehensive list.
Going with the "directly" term in the commentary, not the code, definitions, let's think about roof framing.
1. Plywood on roof trusses.
2. Plywood on purlins on roof trusses.
Taking the ASCE's commentary statement, case 1 would make the sheathing and the trusses both CC since the sheathing receives wind directly and the trusses receive wind from the cladding.
Case 2, however, things are different. The sheathing receives wind directly so it's CC, the purlins receieve wind from the cladding so that's CC but the trusses receive wind from another component now. Does that mean the trusses are now MW? Does it make sense that by adding framing members and changing nothing else that all of a sudden the trusses are magically in the MW category? Should adding more levels of framing change the force somewhere in the load path? NO!
The commentary is just that, commentary, a discussion about the code to help understand intent. Taking the above example, it doesn't make sense that the wind loads on a component will change just because more framing is added, further removing the member in question from the wind surface. It does make sense, however, that a member may "see" different loads depending on it's function in the building structural system or it's shear size.
OK, lets continue on the truss analogy. I prefer to take a more rational approach than one based on terminology or attempts at definitions in the code.
We have a standard truss pitched 5:12 from a ridge. According to the CC design principle you would apply a higher pressure on each end and a higher pressure each side of the ridge. Please explain to me how this loading can occur from wind in ONE direction. It can't. For that load case to occur, the wind would have to come from two directions, so you are applying a worst case loading from two directions at the same time. Sounds conservative to me.
I agree the truss top chord, sheathing, any purlins, etc. need to be checked for CC since they can see a high local pressure, but the truss reaction would be conservative if designed for CC. Last time I checked, the reaction of a truss on a beam is based on the loading over the entire truss, not just the loading directly over the beam.
CC: Elements of the building ENVELOPE that do not...
Last time I checked, an interior column is not part of the envelope, so would that make it MWFRS by default?
MWFRS: An assemblage of structural elements assigned to provide support and stability for the overall structure...
So a footing supporting a column supporting beams supporting joists supporting deck would not be an assemblage of elements supporting the overall structure?
Since CC is definied as any elements that do not qualify as MWFRS, would that mean the roof plywood (clearly MWFRS when used in the diaphragm), is not designed as CC?
These questions are not meant to be answered. My point with this post is please don't try to warp the definitions to fit personal interpretations (heck, I just did it). As engineers, we need to understand the fundamentals of how the CC and MWFRS tables are derived and use engineering judgement to properly apply those loads.
You're right, the Code, without the Commentary, is clear and precise, C&C. It's too bad they wrote that Commentary, because it seems (to me) to contradict the Code.
I agree that the example list is not exhaustive, but the list does provide information. An example list provides guidance as to how to treat items not listed. Items similar to those listed are components. When writing such a list, one would think the writer would make a concerted effort to make the list representative. So, yes, whether or not a structural element appears on the list is meaningless, but whether a structural element has the characteristics of the items on the list is important. I don't think the wind column, or the beam match the characteristics of the examples given.
UcfSE
Good Example. In the second case, the trusses may not be C&C. Magic has nothing to do with it. Physics does. In the first case, the force is applied directly to the wood trusses. In the second case, the purlins bend, and the wind performs work on the purlin. The energy that reaches the truss is diminished by the work required to bend the purlin. Same argument applies, by the way, to the connection of the wood truss.
Your roof truss example is fine. I see your point.
I would just add that for a truss spanning all the way across the building, the tributary area of the truss is probably getting rather large and approaching MWFRS loads anyway. And wind blowing down the length of the ridge may actually produce high uplift on both eaves.
And an exterior column is certainly part of the envelope - especially if it supports horizontal wind girts. It would then be a key integral part of the wall's lateral wind resisting load path, and definitely not part of a MWFRS taking wind from the overall building.
The reason that the C&C charts have area values beyond 700sq. ft. is because the values are effective areas not trib areas.
I agree that ASCE7 isn't clear enough on this and other areas. But I think that the intent of C&C loading is to cover the localized high pressures. MWFRS is intended for more global stability elements like shearwalls. I design footings for MWFRS uplift and I design main girders in a large building for MWFRS. I can't justify my opinion based on specific language in the code but it is more of a judgement call. I do think that is the intent of the code but they sure leave us out on a limb by being so vague. It seems almost everything in wind design is "engineering judgement".
In the example of the very small building I would probably use the larger values but for sure the masonry shearwalls and the diaphragm would be MWFRS.
I live in Florida and do most of my work using 140mph. If we make conservative assumptions everything that we design gets crazy. When you are designing a normal bar joist type of building and it takes an 8'x8'x3' footing 3' below grade to hold the column down you can't afford to make overly conservative assumptions.
The effective wind area is a good point. If the trib to the joist is 500ft^2 and the effective area (1/3 length provision) gives 800ft^2, you still have to design it as a component but are allowed to use the 800ft^2 instead of 500ft^2. So that would kind of make sense as to why the difference between the 700ft^2 provision and why the graph goes up to 1000ft^2. I guess...
A structural element should be designed for MWFRS pressures if it is part of the MWFR system and for C&C pressures if it is not. This determination is made using the definitions of MWFRS and C&C given in the Standard (ASCE 7). Note that these definitions do not mention anything about 700sf or Effective Wind Area. The language about 700sf is an exception that allows C&C to be designed using MWFRS pressures if the Effective Wind Area of the C&C is 700sf or more. This provision should not be interpreted to mean that no elements in the MWFRS will have wind areas less than 700sf. In fact, Effective Wind Area is not really relevant to MWFRS analysis. The tabulated pressures for MWFRS are not a function of Effective Wind Area; nor are Effective Wind Areas the same as tributary areas.
Also note that, to be precise, the definitions of MWFRS and C&C are mutually exclusive. That is to say, the way the definitions are written, an element cannot be both MWFRS and C&C. I am not aware of any provision in the Standard that speaks to the design of structural elements to both MWFRS and C&C pressures; or any language in the Standard that requires small MWFRS to be designed for C&C pressures. You can find language that recommends this in Kishor’s Guide to ASCE7 and you might possibly infer it from language in the Standard Commentary. It is left to your professional engineering judgment to determine if MWFRS pressures or C&C pressures or both must be used. It is left to your professional engineering judgment to determine whether a structural element is C&C or part of the MWFRS system; notwithstanding the lists in the Commentary that would seem to imply that a certain structural element (a truss for example) is both C&C and MWFRS. This cannot be determined without knowing how the structural element is loaded which is why this language is in the Commentary and not in the Standard.
With all due respect, it does not take any twisting of the definitions to arrive at this conclusion. I believe an attorney or law judge would read them the same way. In fact, I must confess that my opinion is based almost entirely on what the Standard says. I don’t have extensive knowledge of the inner workings of the Committee or of how the numbers were derived, except what I have learned from Kishor, Minor, McDonald and others, and this is not always encouraging. This really is an issue that the ASCE7 Wind Load Committee should take up. If the Committee believes that Effective Wind Area should be part of the MWFRS and C&C definitions they should explicitly make it so.
I think charliealphabravo has a lot of good points...agree with most of it. (a star for you too!)
But this assertion:
[green]"an element cannot be both MWFRS and C&C. I am not aware of any provision in the Standard that speaks to the design of structural elements to both MWFRS and C&C pressures"[/green]
is not correct.
Check out the commentary in C6 of ASCE 7 where it specifically states situations where an element could be part of a MWFRS AND be considered a component.
My example of this would be a column in an exterior wall that is part of an X-brace for the entire building. It is definitely part of the MWFRS and should be designed as such. But it also may receive lateral wind load from wall girts and should also be checked for C&C wind (assuming the trib area < 700 sf.).
For me this type of question isn't covered in the code. Obviously you would use MWFRS loads directly to the column when considering the building stability but how could you have a condition where it would it would see C&C loading while it isn't also receiving MWFRS loading. Sometimes you have to make a conservative assumptions. I quess you can't write a wind code that covers everything.
I think we all agree that if you read the standard and ignore the commentary, that the definitions are clear.
At the same time, the commentary and the appendices, while not a part of the standard, are written to provide explanatory information. The commentary to ASCE7 states "a considerable amount of detailed information is needed to put the provisions into effect. This commentary provides a place for supplying material that can be used in these situations and is intended to create a better understanding of the recommended requirements through brief explanations of the reasoning employed in arriving at them."
This is generally true for most structural design codes and standards. The commentary generally provides a more in depth description of code requirements. I think it is imperative to understand and apply the commentary to the standard. In this case, I think the commentary is unclear.
Finally, in this type of discussion, it's irrelevant how lawyers or judges interpret structural codes or standards. They have little understanding of our profession.
![[yawn]](/data/assets/smilies/yawn.gif "[yawn] [yawn]")
