Allowable Stress Increase? 2

[jike]

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?

 

[aggman]

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.

 

[DaveAtkins]

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

DaveAtkins

 

[jike]

Dave:

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

 

[DaveAtkins]

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

 

[epr]

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.

 

[cap4000]

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.

 

[JAE]

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.

 

[epr]

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.

 

[DaveAtkins]

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

 

[jike]

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.

 

[DaveAtkins]

Real engineers work on President's Day.

DaveAtkins

 

[AlohaBob]

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.