×
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

Allowable Stress Increase?
2

Allowable Stress Increase?

Allowable Stress Increase?

(OP)
I am currently using IBC 2000 that references ASCE 7-98 for wind loads. ASCE 7 has eliminated using the 1/3 increase for the D+W load case. Does this mean when designing Simpson holdowns for wood framing subject to uplift or shear walls that we cannot use the values listed in the Simpson catalog as 1.33 or 1.6 times the allowables?  

RE: Allowable Stress Increase?

I have been using the ASCE 7-98 since shortly after it came out and have since switched to the ASCE 7-02.  When I design any part of a structure I use the load combinations as the code specifies.  Thus if some component only sees the lateral load due to wind, then I apply the full force with standard allowable stresses and no increase.  As far as catalog values go I factor the allowables back down to normal allowables and use that for design.

RE: Allowable Stress Increase?

ASCE 7 specifically allows the 1.6 increase for wood.  The 1/3 increase has only been eliminated for masonry and steel design.

DaveAtkins

RE: Allowable Stress Increase?

(OP)
Dave:

Thanks for your input! Could you reference a code section in ASCE 7 that states that?

RE: Allowable Stress Increase?

In the ASCE 7-98 Commentary, Section C2.4.3, it states that "...allowable stress increases that are based upon duration of load...may be combined with the reduction factor for combining multiple transient loads."  This means that you can use the 1.6 increase for the load case of D + 0.75L + 0.75W.  I don't see why you cannot also use it for 0.6D + W.  Also, IBC Section 1605.3.1.1 specifically allows the duration of load increase for wood, for the case of 0.6D + W.

DaveAtkins

RE: Allowable Stress Increase?

I believe DaveAtkins is mistaken.  The overstress factor is allowed for wood due only to duration of load coefficients, as permitted by the NDS.
This overstress factor is not allowed for connectors in wood construction, because the connectors are not made from wood.  (I suppose if the connections were mortise and tenon, you may be able to make the argument.)

Unfortunately, it appears that IBC 2000 has done away with the probably 100 year old practice of 4/3 allowable overstress when wind is a major factor.

RE: Allowable Stress Increase?

Just a side note. An article published by Duane Ellifritt
in 1977 called "The Mysterious 1/3rd Stress Increase" on the AISC e-pubs website. Free to read if your a member. It tries to explain the history of its origin and use from 1896.Good Luck.

RE: Allowable Stress Increase?

epr - the Duration of Load Factor, Cd, in the NDS wood spec does apply to connectors such as bolts, lag screws, shear rings, nails, etc.  Whether you should apply these to Simpson (or other similar) products may be a legitimate question.

RE: Allowable Stress Increase?

JAE - Thanks, I stand corrected.  I was thinking about pre-engineered metal connectors.  Other connectors can be designed using these duration of load (and other adjustment) factors.  Sorry, I did not intend to mislead anyone.

RE: Allowable Stress Increase?

Since Simpson has always included the 1.6 increase in their tables, I will continue to assume that they allow this increase for duration of load.

DaveAtkins

RE: Allowable Stress Increase?

(OP)
I would imagine that the controlling factor for most Simpson Connectors is the connection to the wood with nails or bolts. It would seem to me that an allowable increase in the shear value for those nails and bolts would be allowed under the NDS provisions.

I tried to call Simpson today to discuss this, but they are off for President's Day. I will try to talk to them tomorrow.

RE: Allowable Stress Increase?

Real engineers work on President's Day.

DaveAtkins

RE: Allowable Stress Increase?

I had to research the very question a couple weeks ago. IBC 2000 has exceptions to the stress increase, specifically wood, and more specifically load duration stress increases. At the end of the research, all of NDS for wood in it's entirety is adapted as code by IBC.

The old stress increase of 1.33 for wind or seismic increased to 1.6. Simpson reflects this. The capacity of the Simpson connectors is limited by the engineered preperties of wood, not the bolts or the steel that make the connectors. That's why the design values are peculiar in their catalogs for species and load duration.

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