Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: uplift in footings

[Subject Prev][Subject Next][Thread Prev][Thread Next]
In Australia we probably have that:

0.9D where it resists
1.2D where it loads
(nothing specific about uplift or over turning.)

Thus beam with over hang:
 1.2D for overhang
 0.9D for back span.

My understanding is that limit state codes are based on 5% probability of
design load being exceeded, and a 5% probability of not achieving design
strength.

So implication is that have 5% probability of not achieving 90% of nominal
dead load. Due to variations in constructed dimensions and nominal
densities. On the other hand 5% probability that weight will 120% greater
than nominal. (Or at least that's the Aussie code writers current guess,
their first was 0.8 and 1.25, who knows what they will dream up next.)

Visit Joint Committee on Structural Safety website at:
http://www.jcss.ethz.ch/

Under publications is the Probabilistic Model Code (pdf format). It explains
the derivation of partial load factors and the likes, and calibration of
codes. I think it mainly concerns the EuroCodes.

Their next meeting appears to be local to most of you guys:
Next JCSS Meeting March 28, 2007, Stanford University, California, USA


Regards
Steven CONRAD Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
Roy Harrison & Associates
Consulting Engineers (Structural)
PO Box 104
Para Hills
SA 5096
South Australia
tel: 8395 2177
fax: 8395 8477


-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com] 
Sent: Tuesday, 23 January 2007 00:02
To: seaint(--nospam--at)seaint.org
Subject: Re: uplift in footings

In this case I go by the letter of the load combinations. Personally, 
given the "accuracy" applied to the wind and snow provisions of late, 
I'm a bit surprised they haven't introduced a Dh and Dl for the load 
combinations. Dh would correspond to a "high" dead load estimate of the 
system, and used for gravity and lateral seismic loading, and Dl 
corresponding to a "low" dead load estimate to be used with wind and 
seismic uplift. 

I agree that it would seem that 0.6 on a known footing size (and likely 
a good bit heavier unless the footings were carefully formed) is a bit 
conservative. The exceptions are exceedingly rare. Besides - if you 
believe that 7 cy of concrete you order actually gets delivered to the 
site, there's at least an extra 10%-20% concrete on practically every 
job I've seen.  I'm not aware of a contractor who ordered the exact 
volume of concrete and ended up with leftovers.

Jordan



Jim Wilson wrote:
> While we're on the subject of footing uplift, why is the 0.6D factor 
> applied to the weight of concrete footings?  Isn't this factor 
> primarily intended to cover the unpredictability of future 
> conditions.  As long as footings are inspected for size, it can be 
> considered a given fact that the footings will weigh so much and 
> provide so much resistance. 
>  
> Jim Wilson, PE
> Stroudsburg, PA
>  
>

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ******** 



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********