Stud Wall Check for out of plane wind loads 2

[medeek]

Up until now I have checked stud walls for axial and out of plane wind loading using MWFRS wind loads because I was of the understanding that when there is combined loading of a member (ie. axial and bending for a stud wall) one should use MWFRS instead of C&C wind loads.

Typical calcs for a stud wall look like the following link:

http://design.medeek.com/resources/framing/2014-2x6_STUDWALL.pdf

However, after reading through my recently purchased copy of "Guide to the Wind Load Provisions of ASCE 7-10" by Mehta and Coulbourne I've noticed that they are most often calculating wind loads for wall stud loads using C&C and not MWFRS.

Any enlightenment in this matter would be greatly appreciated.


Another related question that has been bugging me for a few days is why is there no C&C wind load diagram for hip roofs with θ > 27 degrees. I've seen many 12/12 pitch hip roofs so this omission seems very glaring to me.

 

[KootK]

C&C for the wall studs; not sure about the hip roof business. Wind pressure varies wildly over a surface. Statistically, for an element loaded by a smaller surface, the design pressure needs to be higher to account for a reduced opportunity to average out the variations.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[MileyDiley]

Agreed, C&C loads for studs. Exposure C or worse leads to some interesting stud wall calculations. I've seen many simple tract home plans with a wall or two that wouldn't calc out.

 

[JedClampett]

I don't have ASCE 7 here with me, but in UBC it used to say for higher sloped roofs, use wall coefficients.

 

[XR250]

C&C, but I use the trib area for wind as (span^2)/3, if needed, as is recommended in the commentary of ASCE7 for members with long and narrow tributary areas.

 

[medeek]

Sorry the previous link should have been:

2x6 Stud Wall Calc

Spent the whole weekend going through Mehta and Coulbourne's book. It all pretty much makes sense however going through some of the output from previous truss packages (Mitek Software used) I've noticed that the reactions for the trusses are always calculated using MWFRS and not C&C. The uplift and horizontal reactions are always a loadcase involving the MWFRS wind forces. Given the discussion above it would seem more correct to use the C&C wind loads for the truss reactions. I don't mind complicated rules, algorithms and codes provided there is some consistancy and logic to it.

 

[XR250]

see

http://www.eng-tips.com/viewthread.cfm?qid=328943

 

[medeek]

This comment from that thread help clarify how to use the effective area and tributary area:

[pre]You might have a roof truss that has a long span and a narrow spacing.

Example
Roof truss span = 50 feet.
Truss spacing = 8 feet.

Effective area = 50 x 50 /3 = 833 s.f.
Tributary area = 50 x 8 = 400 s.f.

The effective area is used ONLY to determine GCp values from the charts (which go to 1000 s.f.)
The tributary area is less than 700 s.f. limit so you would use C&C wind provisions vs. MWFRS provisions.[/pre]

However it still doesn't explain why the uplift and horizontal reactions are calculated using MWFRS for most residential trusses (ie. MPC wood truss 4/12 pitch 24' span, 24" o/c spacing).

 

[XR250]

Horizontal makes sense as MWFRS as it is dumping the load into the roof diaphragm at that point which is def. part of the MWFRS.

 

[manstrom]

There are a LOT of tricks to stud wall design with wind. At first glance, the wall capacity is significantly reduced with wind, especially with a 2x4. But, when you take all the factors into account, the reduction is relatively minor.

Cd = 1.6 for wind loading. Yes, I use C&C.

Cr = 1.15 for wall studs with gyp or 1.5 when sheathed with OSB (SDPWS)

Load factors will be D+L or D+0.75W+0.75L+0.75S. Never use 100% live with 100% wind. You are hurting yourself. You can work backwards to essentially get an increase to your allowable P.

C&C loading can be reduced by 0.7 (SCE 7-05) for deflection calcs

Also, make sure and use the proper studs height. Is it 9' or 8.72'?

Using these tricks, a 9' tall 2x4 @ 16" stud wall is good for 2241# without wind and 1890#/stud with 90 mph wind.

 

[structSU10]

ASCE 7 has a provision that you can use a larger effective area for the studs, as 1/3*h*h instead of spacing*h. I don't have ASCE in front of me right now, but I know it is discussed early in the wind chapter (at least for 7-05).

 

[RFreund]

This is a good question when you consider the uplift/horizontal reaction from the roof truss. Now you have wind that is being applied to the roof and transferred to the stud as well as the wind directly on the stud (out-of-plane). I'm trying to imagine an instance where you would have a horizontal reaction not taken by the roof diaphragm though. However in any case, I think if you are checking the stud with for axial due to wind and bending due to wind, you could make a case to use MWFRS axial load and C&C for out-of-plane bending load. Otherwise what tributary area do you use? Two separate tribs for truss and stud?



EIT

http://www.HowToEngineer.com

 

[medeek]

For my Stud wall calc I'm looking at axial load (dead load + snow load) and bending load (wind load). I can see right now that there is a perfectly good argument for not taking 100% of the live loads for the combined stress case (which it appears I am doing). However for the gravity only case I would take the full D + S. My combined stress case at 100% is too conservative.

However my initial question has been answered. C&C for the bending on the stud wall. Evaluate the effective area as L*L/3 then apply the distributed force using the tributary area. One additional question though... Is it more appropriate to evaluate the wind load at Zone 5 or Zone 4 assuming the maximum axial load is present in each zone on the wall? It would seem that Zone 4 wind loads would be more representative of what the stud wall on average might see however Zone 5 would be more conservative (worst case scenario).

I am still confused as to the reason the truss manufacturers use the MWFRS for the truss uplift at the bearing points. Logically the C&C wind load should apply. However, they do use the C&C for engineering the truss plates and members as expected.

 

[DaveAtkins]

If the stud can occur in Zone 5, it should be designed for Zone 5 loads.

DaveAtkins

 

[manstrom]

Truss manufacturers typically design for MWFRS incorrectly in my opinion. On high wind roofs, I usually mark this up to C&C.

Regarding C&C zones. I use a weighted average between 4&5. I know it's not conservative, but there is plenty of redundancy in the amount of studs. Designing everything for zone 5 is a killer. Designing for both conditions isn't practical.

 

[woodman88]

When I was working for a truss plate manufacturer, the truss designs were always designed for MWFRS and C&C wind loads. When we showed the C&C uplifts the building departments wanted the building redesign for the truss uplifts. Rather than trying to input the correct data for each truss, the truss manufacturers just showed the MWFRS information on the truss designs.
My opinion is that as the truss designs are defered submittials the EOR/Building Designer has already calculated the C&C uplifts for the building at the trusses so showing the MWFRS uplifts on the designs is acceptable.
Please understand that the wind condition rarely changes anything in the truss design except for the uplifts.

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.

 

[medeek]

This is why I like posting questions to this site. The answers often vary but it is interesting and definitely educational in the way different individuals handle specific engineering problems. As I've come to realize there is usually a lot of engineering judgement when it comes to interpretation of the building codes and the ASCE7-10. After a discussion like this I am usually able to weigh all of the responses and come up with a solution that I feel pretty solid about. Thank-you for all of your responses and for sharing your knowledge and experience.

 

[Hokie93]

A decent answer to your lingering question about truss manufacturers using MWFRS pressures for truss reactions is found in the Frequently Asked Questions chapter of the reference you have been reviewing (Guide to the Wind Load Provisions of ASCE 7-10 by Mehta and Coulbourne). Please see Chapter 14.4, question #11. The basic premise is that a GABLE truss technically qualifies for MWFRS pressures because it receives wind load on more than one surface (i.e., it satisfies the Main Wind-Force Resisting System definition in ASCE 7). Note the same logic would not apply to a monoslope truss.

 

[rlewistx]

American Wood Council (AWC) has produced a document with recommendations for wind design on wood studs. I have attached a copy to this post. Look at the commentary on page 7. It says to use combined wind and MWFRS wind as one case and CC wind alone, no axial load not by wind, for bending. Interesting discussion in this paper.

Rich

 

[medeek]

Thank-you for posting that doc Rlewistx, I've downloaded it and comparing to what I currently doing in my calculator.