Wood Shear Wall Capacities - 40% for Wind? 1

[JAE]

All of the past IBC codes have allowed a 40% increase in shear wall capacities given in the tables (for wind) for plywood and OSB (but not gypsum board).

The 1997 UBC doesn't have the same provision for the 40% increase for wind capacity, yet the shear wall capacity values are identical.

Does anyone know the source or reason for this additional capacity under wind loading? The UBC load combinations and the IBC load combinations both simply have "W" so there is nothing different there. Both UBC and IBC have the same values in the tables (older APA developed capacities).

Under both codes, there is no applicable duration of load factor for shearwalls (the NDS doesn't include shear walls).

Under the UBC there was a footnote implying that the 1/3 stress increase was already in the values of the tables. So since the IBC has the same values in its tables, the 1/3 stress increase is still within the values.

So why the added 40% increase in the IBC when compared with the UBC?

Here's a past thread that mentions this as well:

thread337-180012

 

[UcfSE]

Try this.

Second question and answer pair. It looks like it was due to a change in the factor of safety for wind load for the tables. From what I can tell, this is a relatively recent change compared to when the UBC was last published. Of course, it could be that the writers for the 1997 UBC deliberately ignored the increase.

 

[JAE]

Thanks, UcfSE, that is what I needed. It appears that the code officials just decided that the wood shear wall tables (which had been developed with a safety factor of 2.8) is now adequate for wind with a 2.0 safety factor.

2.8 / 2.0 = 1.4 ...thus a 40% increase.

Gratitude....

 

[UcfSE]

 

[dm3415]

Also, I think this "increase" also acts as a restriction. When I've used the increase in the past I have to check that any seismic shear to a shear wall is less than what is permitted under the increased wind shear loads, which about half the time in my neck of the woods (Nevada) seems to be the case.

 

[OldPaperMaker]

dm3415,
I think that you are making more work than is required in the IBC. If one follows your approach then anytime the wind loads are less than 40% greater than the seismic loads then you would be forced to design the nailing, etc for the seismic loads.
It seems to me that after you have calculated the wind and seismic forces to the base & various levels and converted the seismic forces to ASD and concluded that the wind governs the design then you use the values in table 4.1A (of the 2001 ASD Supplements) or 1.4 times the values in Table 2306.4.1 of the 2006 IBC to determine the nailing, sheathing thickness, etc. for your shearwalls.

 

[JAE]

and concluded that the wind governs the design then you use the values in table 4.1A

OPM - I think you are mistaken here. It is not correct that you somehow determine a global "wind or seismic controls" check based on the base shear or similar value, and then ignore one or the other from then out.

dm3415 is correct that under all situations the various load combinations must be satisfied. In some cases in the building wind will govern the design of various elements and in other areas of the building seismic may govern. It is not an all or nothing proposition.

An example - A diagonal steel braced bay may have its main member design controlled by wind, but with the required overstrength factor on brace connections, the connection may be controlled by seismic.

In high wind areas, or in high seismic areas, you may have cases where one or the other will tend to generally govern, but you still need to check them. Using just the base shear to check everything isn't correct.

 

[OldPaperMaker]

JAE,
Thanks for the correction. I was responding to the thread by making the assumption that this was for a Light Framed bearing wall system without Structural Irregularities and in which the wind loads clearly governs the design of the entire structure and its individual elements.

Do you agree with dm3415 that if the wind load to a shearwall is less than 40% more than the seismic load that one should use the Seismic Allowable Shear for Wood Structural Panel values(Table 4.1B or IBC Table 2306.4.1 without the 40% multiplier)? for determining the required sheathing, nail size and spacing?

 

[JAE]

I think I agree with that....convoluted semantics but I think I follow. You have a wind load W on a shear wall and an E on the same wall.

The W is less than 1.4E.

In a logical fashion you would check the table with W using 40% higher values than the table values (or use W/1.4 and enter the table directly)

Then, using E - you enter the table directly.

Whichever is worse controls.

This is (I think ) consistent with what you said...I hope.