Wood Studs - Reducing Calculated Deflection by 30%

[conradlovejoy]

We are dealing with the ever-increasing issue of brick veneer climbing higher than the typical 30ft max of yesteryear and it is causing our company to look into our designs to find out what we can be doing differently for these special cases. I am looking into deflection of wood studs backing brick veneer and ran into an old set of calcs within which the engineer reduced the calculated deflection by 30% before determining if it met the deflection criteria. Obviously this created a pretty large difference between The required M.O.I. that I calculated and that found using this old spreadsheet. I am not aware of where this "move" came from and am assuming it is some NDS or IBC/ASCE stipulation that I haven't come across and was hoping someone a bit more seasoned might recognize and be able to explain it to me.

 

[Wallache]

Probably from IBC table 1604.3, note f.

 

[conradlovejoy]

Thanks Wallache. The 2012 and 2015 IBC have increased the reduction. The load may now be multiple by 0.42 which is even better news for my calcs. I appreciate the quick response and code help.

 

[Wallache]

Not a problem. I've taken the 0.42 factor to include the 0.6 wind load combo factor (i.e. 0.7x0.6=0.42)

 

[mike20793]

The 0.7 (0.84 factor applied to the velocity and then squared with the pressure equation) reduces the the mean recurrence interval to 10 years. The 0.6 factor reduces the pressures from a strength limit state to a service limit state as Wallache indicated.