Tabular diaphragm values for wind design??
Tabular diaphragm values for wind design??
(OP)
In the "pole barn" industry, we depend very heavily on metal-clad roof and wall diaphragms. For those not familiar with this method of construction, we use mechanically laminated dimensional lumber columns in the ground typically 8' o.c. Wood trusses are typically 8' o.c. as well. The framing on the roof then is 2x4 purlins (on edge) and on the walls it is 2x4's flat. Corrugated steel panels are screwed to this wood framing to provide a diaphragm assembly.
An associate and I were having a discussion about whether or not we can take the tabular shear values of tested diaphragm assemblies and increase their capacity by a 4/3 or 1.6 factor. I'm kind of torn. I read a thread about the 4/3 factor, but I believe that is more of a reduction in loads due to the likelihood of a combination of two or more events happening simultaneously. Chap 16 of the IBC states clearly that "increases in allowable stresses specified in the appropriate materials section of this code or referenced standard shall not be used with the load combinations of Section 1605.3.1 (IBC 2000) except that duration of load increase shall be permitted in accordance with Chap 23."
The IBC does not publish steel-clad diaphragms like it does wood sheathing clad diaphragms, but does reference a standard called Diaphragm Design of Metal-Clad, Post-Frame Rectangular buildings. Nowhere in that document does it give any values or state that tabular diaphragm values from tested assemblies may or may not be increased.
For wood-on-wood diaphragms, the IBC permits a 40% increase in tabular values for wind design. Does it seem reasonable that since the metal-cladding is attached to wood framing that I can use the load duration factors for wood design (NDS), or 1.4 like the IBC allows for wood panel diaphragms, or even a 1.33 increase? Any thoughts would be appreciated.
Thank you.
An associate and I were having a discussion about whether or not we can take the tabular shear values of tested diaphragm assemblies and increase their capacity by a 4/3 or 1.6 factor. I'm kind of torn. I read a thread about the 4/3 factor, but I believe that is more of a reduction in loads due to the likelihood of a combination of two or more events happening simultaneously. Chap 16 of the IBC states clearly that "increases in allowable stresses specified in the appropriate materials section of this code or referenced standard shall not be used with the load combinations of Section 1605.3.1 (IBC 2000) except that duration of load increase shall be permitted in accordance with Chap 23."
The IBC does not publish steel-clad diaphragms like it does wood sheathing clad diaphragms, but does reference a standard called Diaphragm Design of Metal-Clad, Post-Frame Rectangular buildings. Nowhere in that document does it give any values or state that tabular diaphragm values from tested assemblies may or may not be increased.
For wood-on-wood diaphragms, the IBC permits a 40% increase in tabular values for wind design. Does it seem reasonable that since the metal-cladding is attached to wood framing that I can use the load duration factors for wood design (NDS), or 1.4 like the IBC allows for wood panel diaphragms, or even a 1.33 increase? Any thoughts would be appreciated.
Thank you.






RE: Tabular diaphragm values for wind design??
DaveAtkins
RE: Tabular diaphragm values for wind design??
I know you said that you've never done that, and now I'm wondering why not? Does today's code prohibit us from doing it? Did you just want to be extra safe? Do you do lots of PF buildings?
Thanks.
RE: Tabular diaphragm values for wind design??
DaveAtkins
RE: Tabular diaphragm values for wind design??
RE: Tabular diaphragm values for wind design??
Mike McCann
McCann Engineering
RE: Tabular diaphragm values for wind design??
Seeing then that we have to live with that 10psf loading, which doesn't account for the Kd (wind directionality factor) or any .75 factor or such like, is there anything that I can do to either the load side or the material side of the equation (legitimitely) to counteract those (in my opinion) unnecessarily high loads?
Just for example, on the buildings that we typically do, the 10psf load is equivalent to a calculated 110mph wind speed as opposed to the 90mph windspeed. Furthermore, if we have a building that is Category 1, such as a storage facility, the 10psf minimum totally ignores that and makes the comparable "effective" wind speed 115mph or so.
How do other practicing engineers apply the 10psf minimum load? I'd appreciate hearing some discussion on that.
Thank you.
RE: Tabular diaphragm values for wind design??
DaveAtkins
RE: Tabular diaphragm values for wind design??
Also, freak storms can occur, anywhere. It's good to have some minimum of design, even if it is arbitrary.
Mike McCann
McCann Engineering
RE: Tabular diaphragm values for wind design??
I understand that the roof loads go away in the vertical direction, but the 10psf still needs to be applied horizontally to the vertical roof projection. That really is what "hurts."
I understand as well that it never hurts to have some extra safety factor built in, but don't we have enough already with diaphragm values with a 2.5X safety factor, along with all of the other published values that an engineer uses in design with their associated safety factors?
I appreciate each of the inputs to this thread. Anybody else care to share your perspective on this? Has everybody else just accepted this 10psf thing and I'm still fighting it years later?
Thanks.
RE: Tabular diaphragm values for wind design??
DaveAtkins