Anchor bolts in Seismic Design Category D & requirements of ACI 318-05 1

[civilRoo]

I am working on anchor bolts for a pre-fab Pavilion at a sports-plex and also for a pre-fab pedestrian bridge at the same location and have been using Quick Anchor (which was recommended by a member of this forum--thanks, btw). The site is in a seismic zone (SDC D) and the anchors are not meeting the ductile failure requirement (as I presently understand it) of ACI 318-05 D.3.3.4 and D.3.3.5. The 2006 IBC has a modification to this requirement (whose jurisdiction would only apply to the Pavilion)in Section 1908.1.16 that allows for this ductile failure requirement to be dropped if the anchor's design strength is at least 2.5 times the factored load.

Does anyone have a different understanding of this or some advice on this that would not require what would seem to me to be impractically large anchor bots for a pavilion?

Or does someone have enough experience with this to point me in the right direction.


Also, does anyone know what guide spec is applicable to the foundation of a pedestrian bridge. I found an AASHTO spec that addressed the framing of pedestrian bridges but not the foundations.

Thanks.

 

[ishvaaag]

I read D.3.3.6 in ACI 318-08 and understand the same than you, that if you make the anchor not work but at .4 of its capacity as determined by D.3.3.3 you can forfeit the ductile failure requirement.

 

[Lion06]

You're correct, but I would also say that I believe you need to add an additional phi factor of 0.75. I don't have my code in front of me, but I believe that is required for SDC C or higher.

 

[StructJan]

That's right. The ACI318AppendixD for Seismic Design Categories C and higher requires to reduce anchor capacity by 0.75 and by 0.4 due to the ductility. The cracked concrete shall be also assumed.
It is possible to design ductile attachment (base plate) but watch out for the prying forces which can be very high.
Maybe somebody else has some experience with the ductile attachment design?

 

[msucog]

not sure if this helps (or is even cost effective to the project) but have you looked at having more sophisticated assessments performed to potentially improve the SDC? it could be as little as couple/few thousand dollars to see an improvement depending on your location and site conditions. will gladly pass along details if needed.

 

[blackmaddog]

I assume you can't increase embedment or decrease diameter to meet the ductile steel requirement. If your base plate will yield at design loads, I would argue that the ductile requirement of D3.3.5 is met.

Another option is to design reinforcement to resit the anchor loads with effective force transfer. For example in a pier you can extend the anchors below the development length of the rebar and ignore the concrete breakout portion of Appendix D.