Stud Wall Check for out of plane wind loads
Stud Wall Check for out of plane wind loads
(OP)
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:
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.
Typical calcs for a stud wall look like the following link:
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.






RE: Stud Wall Check for out of plane wind loads
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
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.
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
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).
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
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.
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
EIT
www.HowToEngineer.com
RE: Stud Wall Check for out of plane wind loads
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.
RE: Stud Wall Check for out of plane wind loads
DaveAtkins
RE: Stud Wall Check for out of plane wind loads
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.
RE: Stud Wall Check for out of plane wind loads
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.
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
Rich
RE: Stud Wall Check for out of plane wind loads
RE: Stud Wall Check for out of plane wind loads
1) check gravity axial loads. D + L or D + (Lr or S) or D + .75L + .75(Lr or S)
2) Then check bending loads from wind only using C&C at the worst case scenario (zone 5), no axial loads.
3) Then a combined check of axial and wind loads using (MWFRS) wind loads.
What I am missing in my spreadsheet is the C&C bending only load case check (2), which as stated in the whitepaper, is often the controlling load case.
RE: Stud Wall Check for out of plane wind loads
Updated Stud Wall Calculation
After giving it some thought about Load Case 3, I realized there is actually one more combined load case (D + W). Note that there is no 75% reduction in loads in this case. So to encompass this load case without having to insert yet another load case into the spreadsheet I decided to run the combined gravity and lateral load case all at 100%. This may be a bit conservative but it then ensures that I have covered myself for the D + W load case.
I also decided to use Zone 5 C&C so that I'm also conservative there as well. Note that in this particular case the C&C (bending only) load case governs as expected for a high wind location (155 mph ultimate wind speed).