×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Load Duration
3

Load Duration

Load Duration

(OP)
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!

RE: Load Duration

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.

RE: Load Duration

ACI 530 2.1.2.3 does allow a one-third increase.

MBones

RE: Load Duration

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. (???)

RE: Load Duration

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.

RE: Load Duration

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

RE: Load Duration

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?

 

RE: Load Duration

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

RE: Load Duration

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?  

RE: Load Duration

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

RE: Load Duration

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.  


  

RE: Load Duration

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?

RE: Load Duration

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.

RE: Load Duration

(OP)
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!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources