Finding Effective Wind Area for C&C

[JLMartinez20]

Good afternoon all,
This is my first post on the forum, so I apologize for any incorrect post etiquette.

I am struggling to determine the Effective Wind Area for a set of calcs to determine roof pressures. Here is a breakdown of the information I have:
Building Dimensions
- h = 24.2'
- L = 104'
- B = 138'
- θ < 7°
- Joist Spacing: 5'
- Roofing Material: 1.5B20 Roof Deck (15' Length) Fastened at supports with a 36/5 pattern with 5/8" puddle welds. Sidelaps fastened at 8" O.C with 5/8 welds. End laps occur at supports only. Minimum lap = 3 inches.

I am using ASCE 7-22 for my reference material. Since h<60' and θ<7°, I am using Figure 30.3-2A (pg. 319) to determine my external pressure coefficients. Initially, I was just using the entirety of the Zone areas for my effective wind area, but that didn't seem correct. I found the definition for Effective Wind Area on pg. 263, and am now very confused on how to determine the value. On one hand, I keep reading it is the larger value of: Span Length (15') * Joist Spacing (5') OR Span Length (15') * Span Length/Three (15'/3). These values are equal at 75 sqft. HOWEVER, in the definition of Effective Wind Area, the last part states: "For cladding fasteners, the effective wind area shall not be greater than the area that is tributary to an individual fastener." I interpret that as Joist Spacing (5') * Fastening Spacing (8" or .67') meaning and Effective Wing Area of 3.35 sqft. This is much different than the 75 sqft. I am also confused as to how, once I have my effective wind area, how that is applied to my 4 Zones on the roof.

Any help is much appreciated on understanding how to approach this issue. Thank you all for your time and have a great day!

 

[structSU10]

Each component is designed for a different pressure based on that tributary area. The fasteners may be for 50 psf while the joist is designed for 30 psf. It accounts for the localization of higher pressures that can occur that gets averaged out over larger regions.

 

[strucbells]

It sounds like you are confused about how C&C loads are generally applied and used in the code. I will assume you are familiar with MWFRS design, applying global wind loads to the building and designing the diaphragm and overall lateral force resisting system for shears etc. and carrying the loads to the foundation. These are typically "in plane" loads.

C&C loading is a completely separate check from the MWFRS analysis used to ensure that individual building envelope elements directly subject to "out of plane" wind loading (such as purlins, girts, roof deck, roof fasteners, etc.) are able to resist localized peak wind gusts and transfer these local forces to the main building frame. Engineering judgement typically dictates at what point you can stop following the C&C load path. C&C wind pressures are higher than MWFRS wind pressures due to the higher likelihood that a wind gust may peak locally compared to averaging over a larger area. Statistically, the smaller the tributary area of the element, the higher wind force it should be designed for, and vice versa.

Therefore, you will run your MWFRS design first. Then come back and check each building envelope element individually for its proportional C&C load based on its tributary area and its location in the building. Your fasteners will have one tributary area, your roof deck another, and your purlins or trusses etc another. The variation in tributary areas means you have different pressures subject to each element.

This can get complicated and time consuming even for relatively simple buildings. Depending on your building construction, it may be more efficient to take a worst case pressure and apply it globally or define certain zones where fastening, etc. varies to resist this loading.

Disclaimer: I have not started using ASCE 7-22 yet, but I believe my understanding from using 7-05 through 7-16 about how these loads are generally applied is still valid.

 

[andrews2]

Previous replies are correct, you need a pressure that is specific to each component, not a single value for the whole building. If your question is what value to provide on your drawings for others to use in the design of non-structural components and attachments, then it is best to provide a table with a range of values depending on the roof zone and effective area. Your table may have pressures for effective areas of 10, 20, 50, 100, and 500 square feet, and whoever uses that table would be responsible for interpolating correctly. This is more economical than providing a single conservative value for the entire building.

For my own curiosity, what state is this project in? Florida is the only state I'm aware of that has adopted 7-22 and I am curious if others are starting to adopt it.

 

[lexpatrie]

The fasteners are probably 10 ft[sup]2[/sup] effective area, as that's the minimum one is required to consider. (i.e. puddle welds in uplift)

Joist would be based on the span[sup]2[/sup]/3 or span*spacing/3, whichever is larger. This is what the joist supplier needs to know, though generally one provides a diagram, maybe give the values for 10 ft[sup]2[/sup] and say they can adjust them downward for effective area per ASCE 7-22, they should know the length, but you need to give them net uplift, not gross uplift, and make that clear, and if it's ASD/LRFD, so 0.6D+/-0.6W. You can often get joists that cross zones, so it's not straightforward to just give a loading diagram unless you want to be fairly conservative and exceed code in a fair portion of the roof.

Girders would generally be the bay area, or twice the bay area, or span2/3 whichever is larger.

Columns probably over 700 ft[sup]2[/sup] so MWFRS applies not C&C.