Components & Cladding on all members?

[Johns20188]

Hey everyone,

I have two quick questions:

1. When designing individual roof members (joists, rafters, purlins, etc), I know we need to use components & cladding (C&C) to find the uplift and downward wind on the member. However, do we also use C&C loads on the girders that carry those roof members, on the posts that carry the girders, or on the footings that carry the posts? Or do we use MWFRS loads for those (even if its trib area is less than 700sf)? I feel that C&C wind loads add a significant amount of downward weight, and can really force the use of larger members.

2. When calculating C&C uplift forces to design uplift roof connectors for a member, do you take the effective wind area of the entire member to find the total uplift force and then calculate the reactions to get the uplift demand on the connector? Or do you take the effective wind area of the connector only? I'm asking this because taking the effective wind area of the entire member will most likely be greater than just the effective wind area going to the connector and this can change the uplift magnitude.

Thank you.

 

[thaidavid40]

There was a good summary explanation article related to these questions a few years ago in the SBC Magazine. Here is the link:

http://www.sbcmag.info/article/2012/wind-load-analysis-mwfrs-vs-cc

Hopefully, this will explain in fuller detail the ins and outs of deciding on C&C versus MWFRS loadings.
Dave

Thaidavid

 

[BUGGAR]

Do wood truss manufacturers design their top chords for C&C forces?

 

[Hokie93]

In response to question #1, I would say you need to provide a continuous, rational load path for the forces resulting from the C&C wind pressures. You can, however, use the wind pressure associated with the effective wind area of the component in question as you follow the load path. The result is that you may use a different (and lesser) wind pressure for joists, girders supporting the joists, posts supporting the girders, etc.

For fasteners associated with a member/component, the design force on the connection would be based on the effective wind area for the member and the resulting end reaction of the component. For the case of cladding fasteners, the design force is based on the area tributary to the fastener.

The above responses assume you are using ASCE 7 for the determination of wind pressures.

 

[JAE]

I think Hokie93 answered #1 as I would have. And yes, the C&C downward component should be considered in adding to the dead/snow/roof live using the applicable load combinations that include those effects.

On #2 -

do you take the effective wind area of the entire member to find the total uplift force and then calculate the reactions to get the uplift demand on the connector? Or do you take the effective wind area of the connector only?

The C&C wind design concept/philosophy is that smaller areas of applied wind can get statistically higher pressures than larger. For an example - a roof purlin spanning 20 feet and spaced at 4 feet on center would have an effective area of 20 x 4 = 800 s.f. You would use that area to design the purlin itself. But the connection of that purlin to a supporting wall or girder would have half that area so you would use 400 s.f. for the wind pressure on that connection.



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[DaveAtkins]

I agree with all that has been said. I would be very surprised, however, to see downward wind on a roof member control the design, since when combined with snow, both wind and snow get multiplied by 0.75 (for ASD design).

DaveAtkins

 

[KootK]

Logically, I don't feel that a member or connection further along a load path should ever need to be designed for higher wind pressures than an element higher up the load path. I see that as an outgrowth of the statistical reasoning that JAE mentioned above. As such, in JAE's example, I would use the wind pressure based on 800 sf rather than 400 sf for the design of the wall connection..

I think that much of the confusion around this issue would be eliminated if we used an "influence area" definition rather than a tributary area definition, similar to what we do for live load reduction (and for similar statistical reasons). And, in fact, that is what I usually do. By that reasoning, the pulin and the wall connection in JAE's example would have identical influence areas. Part of the elegance of this is that you have one standard to apply to all members, relatively unambiguously.

I should add that I'm in the Bahamas right now so I don't have the ability to fact check myself against ASCE7 etc. I'm sure someone will school me if I've gone two far off reservation.

To digress somewhat, I loathe the combination approach proposed for trusses. I firmly believe that code provisions should be established assuming hand calculation procedures regardless of what tools designers will likely have available to them. The combined approach would be nightmare in the context of hand calculations. Technical merit aside, I find the combined approach unacceptable on this basis alone. Any reasonably designed "truss" should be predominantly an axial thing anyhow. If a truss is really going to be governed, locally, by C&C wind, I'd argue that it wasn't much of a truss to begin with.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

KootK - I agree - simplicity is much preferred. I was responding to the question from a purely code-compliant stance as that was how the question was framed.

One of my local engineer friends simply uses 20 psf for MWFRS wind and 30 psf for C&C on everything, thinking that ASCE 7 has gotten ridiculous. I sort of agree but with spreadsheets - well - I get a bit geeky and go with the actual code forces.

As for the connection - a 20 ft. long purlin would have all sorts of wind pressure variations along its length. But with those variations, one half of the span could see a peak gust pressure much higher than the average load over its length - thus the smaller area on the connection makes sense to me.

I would almost be more inclined, to keep things simple, to use 400 sf for the whole purlin design and connections....definitely not the 800 sf.



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[KootK]

As for the connection - a 20 ft. long purlin would have all sorts of wind pressure variations along its length. But with those variations, one half of the span could see a peak gust pressure much higher than the average load over its length - thus the smaller area on the connection makes sense to me.

Without considering local pressures in excess of q_400sf, the only way for the connection load to physically be q_400sf X TA is for q_400sf to be applied over 100% of the purlin's influence are. And, by definition (code definition), that wouldn't make statistical sense, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

Yes that is technically true - but a span loaded with a uniform 30 plf across its length (purlin area design) vs. a span with 45 plf on one half span and 15 plf on the other half span (smaller area connection design) will have a higher end reaction than the uniform loading.



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[JAE]

The above is illustrative of the condition drives the higher wind vs. lower area concept. In design practice you would be comparing the lower uniform load - 30 plf over the whole span, for the member design, and a higher 45 plf over the whole length for the connection design to account for a condition where high peak variations in pressure could occur on one-half the span near the connection. (made up numbers to illustrate the concept).



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