Load Duration 3

[EIT2]

I often see 1.33 load duration for masonry. What type of duration would this apply to?

Can a 1.6 wind load duration also apply to steel and masonry as it does to wood?

Thank you!

 

[JAE]

The 1.6 factor, and all the Cd factors in the NDS, apply to wood due to its ability to withstand short term loads better than long term loads. You would not use 1.6 for any other material.

The 1.33 (1/3 stress increase) factor has traditionally been used for various materials other than wood. However, the new IBC building code does not permit this factor unless you have two or more "transient" loads applied....and it doesn't define what a transient load is so there is debate there about when to use it.

 

[MBones]

ACI 530 2.1.2.3 does allow a one-third increase.

MBones

 

[jheidt2543]

Actually, I always thought that LOAD DURATION was most commonly a consideration for design of wood members. I thought the 1.33 and 1.6 factors for masonry design were factors used for wind loading or wind gusts. (???)

 

[Stress02]

Current BOCA also does Not allow 1.33 increase for seismic loading conditions unless using Load Factor design.
I have stopped using the increased allowable stresses as that will be the standard in the near future here.

 

[MBones]

I agree with Stress02. The days of the 1.33 stress increase are numbered.

 

[shemp]

ACI 530 does allow the 1/3 stress increase, but the increase is only applicable to the load combinations used in ACI 530. The only way you would be using these load combinations is if the "legally adopted building code does not provide load combinations" (ACI 530, 2.1.2.1).

If you are using the IBC, then you will not be using the ACI 530 load combinations. The IBC's ban of the 1/3 increase will govern.

anyone disagree?

 

[boo1]

Additionally, some states (Florida) prohibit the material strength increase.

 

[mjohan]

Can someone comment on the 1.4 increase allowed for wood diaphrams and plywood sheathed shearwalls (IBC 2000 code)?

Does the increase only apply to wind, or can that also apply to seismic?

 

[boo1]

see:

http://www.strongtie.com/products/highwind/hw-load-determination.html

 

[mjohan]

Thats a good site for info. Thanks.

When I follow the IBC, I have been using the basic load combinations "ASD". From those equations, I mult. the seismic force by .7 and compare to wind.

Specifically, using plywood shearwalls with 2x studs, a bit of confusion, to me, lies in section 2306.4.1 "The allowable shear cpacities.... in accordance with Table 2306.4.1 These capacities are permitted to be increased 40 percent for wind design."

I am wondering if others have taken an increase in table 2306.4.1 when using seismic forces? Or just for when wind is governing the design? Does APA comment on these increases when using ASD?

TIA

 

[haynewp]

The IBC 2000 allows a 1/3 increase for wind or seimic WITHOUT another transient load included if this is so referenced in the material code section according to the alternate combinations. It becomes no help with wind since you have to multiply the wind load by 1.3, however I do not see that stipulation with seismic.

The AISC ASD supplement that removed the 1/3 increase from the ASD 9th didn't come out until the end of 2001.

So shouldn't we still be able to use the 1/3 increase for steel in a dead plus seismic combination according to the alternate combinations in IBC 2000? Since the 1/3 increase wasn't removed from the ASD 9th until after the IBC 2000 came out?

The ASD supplement doesn't mention IBC 2000 alternate combinations being affected.

 

[mjohan]

haynewp,

you should look at boo1's link. Very helpful with those types of explanations.

However, I am asking about wood shearwalls which does have a 1.4 wind increase for shearwall capacities... I would like to know if others use this interpretation to justify this increase in capacity with regards to seismic?

 

[boo1]

Check this link

http://www.aisc.org/MSCTemplate.cfm...Management/ContentDisplay.cfm&ContentID=24932

 

[haynewp]

Well that clears it up. For IBC 2000, you can still use the 1/3 increase for 0.9D+E/1.4 case in the alternate combinations for steel.

 

[EIT2]

mjohan,

You may want to start a new thread. I believe there is a significant percent of forum participants that do not revisit threads for various reasons, such as, the original question has been answered, the thread is old ... There may be many who are not viewing your question!