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uplift on steel beams
14

uplift on steel beams

uplift on steel beams

(OP)
When calculating wind uplift from wood trusses that goes to beams, is it proper to just use MWFRS or C&C? Thank you in advance.

RE: uplift on steel beams

You ought to also post this in the Truss Engineering forum.

My understanding is the first few ones should be checked for C&C

Check the other forum where there are guys who do this for a living

RE: uplift on steel beams

3
It's totally based upon the tributary area that the beam supports.  If the beam takes more that 700 sf of area, (per ASCE 7) then you can use the MWFRS values.  See ASCE 7-02, section 6.5.12.1.3.

If the area is < 700 sf, then you'd use the C&C values, but if the beam is also part of the MWFRS (say a collector beam) then you ALSO have to check it under its MWFRS loads as well.

RE: uplift on steel beams

2
JAE is correct.  I believe the point of the 700 ft^2 trib area is that the MWFRS pressure is the LOWEST it can see.  Basically, if you check it for C&C (assuming a small enough trib area that it doesn't qualify for MWFRS loads), there is no need to check it for MWFRS because that will be lower.  

RE: uplift on steel beams

Well...there is a need to check the MWFRS if it is part of the lateral system.

What I mean is, you may have an eave beam on the edge of a roof that sees uplift under C&C wind.  But also, it may serve as a collector beam or a part of an X-brace that is under that eave beam where the overall building lateral load is proceeding down through your beam and into the X.  In that case you really have two wind load situations to verify as they are taking wind in different manners.

RE: uplift on steel beams

It is a little confusing based on the definition ASCE provides for components. Intuitively it seems that based on the tributary area it could be classified as a component, but by the definition of C and C:

"Components and Cladding: Components receive
wind loads directly or from cladding and transfer
the load to the main wind force-resisting system....Examples of components include fasteners, purlins, girts, studs, roof decking, and roof trusses..."

So if the roof truss is a component, then the supporting roof beam would always be designed as MWFRS since it is receiving wind from a component(the trusses). Do you agree?

RE: uplift on steel beams

I got into a big discussion of this over at the ICC bulletin board and a lot was said about the definition of a member being designed by C&C wind if it takes wind directly and a member being designed by MWFRS wind if it takes wind via other members that are directly taking wind.

I know the ASCE 7 commentary seems to imply this but based upon the theory of the wind pressure concepts, what is important to remember here is that the higher C&C pressures are due simply...simply...to the fact that you get wind pressure peaks over small areas and as the tributary area increases, the variations of wind pressure average out and the overall pressure goes down.

I think the concept of saying "a member will be designed by either C&C or MWFRS wind based upon whether it receives wind directly" is not accurate.

The use of C&C or MWFRS depends on function of the member (is it truly a part of the MWFRS) and based upon tributary area.  

Member with At < 700 sf and not part of MWFRS:  Use C&C

Member with At < 700 sf and part of MWFRS:  Check both C&C and MWFRS wind loadings

Member with At > 700 sf and not part of MWFRS:  Use MWFRS

Member with At > 700 sf and part of MWFRS:  Use MWFRS

RE: uplift on steel beams

I agree, it doesn't make sense to exclude a member from higher localized effects based on how the wind is received.

So in this line of thinking, a lot of the foundations that we design should also be checked as components (At<700).

RE: uplift on steel beams

I'd say yes if the At < 700.  Think about a small one story bay where an interior tube column supports a light roof.  The uplift on that part of the roof might be quite higher than the dead load under the .6D + W combination so the footing does then try to hold the column down under C&C.

RE: uplift on steel beams

Can anyone elaborate where the 700 sf value comes from?  I find this an interesting topic because if you believe that C&C or MWFRS is only based on tributary areas, than footings, shearwalls, and diaphragms would need to be designed for C&C loading if they had small enough tribs.  My point is I don't agree with this since the C&C graph has trib areas ranging from 1 sq foot to 1000 sq feet.  If the intended cutoff was 700 feet why doesnt the graph stop at that point?  What wind loads would you use to design a wall stud connection to the foundation that is supporting roof trusses? Do you use C&C wall pressures based on the trib of the stud (shear connection) and C&C uplift based on the trib of the truss (tension connection)?  Do you use C&C based on the trib of the entire assembly so one pressure for shear and tension calculation?

With that said, we tend to use the surface approach.  If something receives its wind load from one surface, than C&C is appropriate, if it's from more than one surface a MWFRS approach is used.

So in my opinion, the answer to the question is C&C needs to be checked for the beam since it receives wind only from one surface, the roof truss (I consider the truss top one surface, even if it's dual pitched).  The trib area should not determine what type of loading to use, only how much loading if it's determined to be C&C.  And MWFRS would need to be checked if it was also part of the lateral resisting system.

As a final comment I should state all my wind opinions are based on ASCE 7, so they may not apply to other codes I have not read or studied

RE: uplift on steel beams

It says it is permitted to be treated as MWFRS when > 700, it doesn't have to be. It may be that some of those that wrote the ASCE provisions agree with using a surface based definition, what we are talking about is more engineering intuition.

RE: uplift on steel beams

CFSEng - I really disagree with you.  Wind varies extremely up close but averages out over large areas.  M

My example above - designing a footing to resist C&C wind uplift on a roof.  The footing is separated from the application of the direct wind by what...5 or 6 elements?  (anchor bolts, base plate, column, beam, joist, deck, etc.)

But if the bay is only 10' x 10' then the potential for a large variation in wind pressure at that column location via the roof uplift is much greater than if the bay was 50' x 50'.

Using MWFRS for that footing would be foolish.  

My take is that the basic wind that you use for EVERYTHING is C&C wind.  If you have large trib areas - or if it is truly part of the main wind force resisting system then use MWFRS wind.

This isn't all that complicated to understand.  If you've ever had a wind tunnel test done on one of your structures you would see how there are peak variations of wind that occur all over the place.

I agree that ASCE 7 isn't all that consistent.  In fact - I think the wind provisions are a complete mess.

RE: uplift on steel beams

I think what's important about distinguishing between MW and CC is that MW is providing stability for the globl structure, CC is for a local element of the structure.  Getting hung up on the "receives wind from more than one surface" pseudo-definition is that ASCE does not use this as a definition, but rather only as a hint toward intent.  LFRS do receive wind from more than one surface generally, but that does not suggest that every element that receives wind from more than one surface should be construed as MW.  

I agree with JAE and have always followed using MW for things like braced frames, shear walls, diaphragms and so one are MW because these elements of the structure provide stability for the global structure.  All others, including trusses and footings, are CC unless the tributary area is greater than 700 ft2.

RE: uplift on steel beams

Well as professionals we have the right to disagree.  My take is you do not use CC wind for everything.  I agree this is not too complicated, CC is for localized pressures, MWFRS is for envelope.  In JAE's example, I agree the footing would be checked for CC uplift since it receives wind from only one surface (the roof).  That's not to say it couldn't also be checked for MWFRS if it was part of the lateral system used to resist overturning, base shear, etc

My point is I would check this for CC not because of its trib area, but because of how it receives the load.

RE: uplift on steel beams

CFSEng - well put.  I agree we disagree on this and this is nothing new - I know a lot of engineers who go in circles trying to meet the intent of ASCE 7 wind - I'm one of them.

Thanks for your input.

RE: uplift on steel beams

Reading the definition of components in the ASCE7 Commentary, I think the key word is "directly."  Unfortunately, "directly" is not defined. Examples are given.  The beam is not one of the examples.

In my mind, "directly" means recieving the wind through a rigid path, that is, through members that work in tension, not bending.  A bending member, such as a wood truss, has the ability to relieve local peak pressures before dumping the force to the beam.  The shock element is gone, the way I see it.  I don't believe ASCE intended this member to be designed for C&C pressures.

RE: uplift on steel beams

Just a comment about CC vs MWFRS (I apologize for hijacking this thread but I feel this is important to understand)

Take a look Figure C6-6 in the commentary of ASCE 7-05.  Note where the peak pressures occur in the picture.  You can see why they require high edge zone pressures when looking at a deck attachment at the windward edge or on the leeward side of the ridge.  Now you can see on the opposite side (leeward) near the wall the uplift suction is very low.  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.  This is why you have to consider how something gets its wind load, not just its trib area.  This is also why the truss people argue the truss reactions should be designed for MWFRS and not CC pressures.

Trib areas on the CC table only account for load averaging, they do not account for wind directionality

RE: uplift on steel beams

Girts are mentioned in the Commentary as a "component" taking wind load from the siding which is considered "cladding". Is a wind column with eff. area < 700 sf also considered a "component"?

RE: uplift on steel beams

If the wind column gets its wind load from the siding, I would say yes.  If it gets its wind load from the girts, I would say no.  By "Components recieve loads directly from cladding", I think the code means "in a straight line."  Once the loads have to turn a corner and put a member (the girt) in bending, that is no longer direct.

RE: uplift on steel beams

The wind column receiving load from a girt is not really part of the MWFRS. It seems more like a component but does not receive load directly from the cladding.

Are you saying it is part of the MWFRS or to just design it for the loads from MWFRS?

RE: uplift on steel beams

jike-

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.

RE: uplift on steel beams

CFSEng,
Looking at Figure C6-6 - it does show low pressures on the leeward roof.  

You said:  "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"

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.



RE: uplift on steel beams

JAE


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.



RE: uplift on steel beams

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 include...".  It was not meant to be an comprehensive list.




RE: uplift on steel beams

All right ladies...

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.  

And how can you be more arbitrary than arbitrary? yawn

RE: uplift on steel beams

....dramatic pause to catch our breath....


yawn

RE: uplift on steel beams

Ahh, this has turned into quite the discussion...

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.

RE: uplift on steel beams

A comment on the definitions.

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.

RE: uplift on steel beams

JAE-

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.

RE: uplift on steel beams

CFSEng

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.

RE: uplift on steel beams

Let me get my 2 cents in here.

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.

RE: uplift on steel beams

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...

RE: uplift on steel beams

2
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.

RE: uplift on steel beams

In my opinion charliealphabravo has summed it up.

RE: uplift on steel beams

I think charliealphabravo has a lot of good points...agree with most of it. (a star for you too!)

But this assertion:  

"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"

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.).

 

RE: uplift on steel beams

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.

RE: uplift on steel beams

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.

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