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RE: IBC Section 1908.1.16 is a bugger

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I don't understand your steps 1 and 2.  The 2.5 factor is applied to the factored design force, not to the "capacity of the anchor".
 
I believe that the intent is to allow design of the anchor for a force less than the ductile failure load where applied loads are significantly lower than the anchor capacity.  For example, you may use 3/4" column anchor bolts as a minimum size but have relatively low uplift loads on the anchors.  The anchor embedment for uplift could be designed for 2.5*factored loads in lieu of the depth required for ductile failure.
 
Seems like a practical alternative for design?
 
Bill Sherman
CH2M HILL / DEN
720-286-2792
 


From: SGE Structural [mailto:sgordin(--nospam--at)sgeconsulting.com]
Sent: Tuesday, February 03, 2009 3:09 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: IBC Section 1908.1.16 is a bugger

Still trying to find some logic in all this...
  1. Find the capacity of the anchor.
  2. Apply 2.5 factor to the capacity and find the "backward-design" force.
  3. Apply the "backward-design" force to the anchored structure.
  4. Find that the structure will fail at a fraction of that force.
In most cases, this means that the structure fails long before anything (brittle or ductile) will ever happen to the anchor.
The failure of the anchored structure appears to be the "trigger" embedded into the problem itself.
 
Why are we designing the anchors for the force that the anchored structure is incapable of resisting?  Why are we concerned about the brittle failure if the structure will fail in whatever mode long before the anchor is be loaded to any noteworthy portion of its capacity?
 
Is there logic here, after all?
 
V. Steve Gordin, SE
Irvine CA
 
  
----- Original Message -----
From: john yang
Sent: Tuesday, February 03, 2009 13:49
Subject: Re: IBC Section 1908.1.16 is a bugger

However, by code, you have to prove the failure mode is not in the brittle failure and this is not limited to steel anchor only.  Therefore, if the failure happens in the concrete like breakout, pullout and so on, 2.5 factor should be applied.  2.5 factor is kind of safety factor to ensure that, when the brittle failure mode is governed, the material is in the elastic range.

For example, if I use very strong anchor (high capacity anchor), the concrete pullout strength will be governed.  Therefore, regardless of anchor type or material, 2.5 factor should be applied.

In contrary, if I use relatively weak anchor and the anchor capacity is governed "not concrete", 2.5 factor may be eliminated. 

Thanks.

John Yang

On Tue, Feb 3, 2009 at 7:41 AM, Larry Hauer <lhauer(--nospam--at)live.com> wrote:
Bill,
 
I haven't gotten any "post installed" anchors in concrete to work using the 2.5 increase per CBC 1908.1.16, so I am falling back on the definition of "Ductile Steel Element" from page 379 of the ACI which states that ASTM A307 "shall be considered ductile." The way I read ACI Sec. D.3.3.4, the A307 threaded rod, (for epoxy applications), or anchor bolt would meet the criteria of this section, (I hope).
 
Larry Hauer S.E.




From: t.w.allen(--nospam--at)cox.net
To: seaint(--nospam--at)seaint.org
Subject: IBC Section 1908.1.16 is a bugger
Date: Fri, 30 Jan 2009 10:26:36 -0800



Now that new construction is slowing down, I wonder how big of a role this will play in the remodel world.

 

Does anyone know the source of the 2.5 factor mentioned in the modification to ACI 318 Appendix D.3.3.5? It seems rather arbitrary to me. But, what do I know?

 

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
V (949) 248-8588 F(949) 209-2509

 



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