Seismic - Are footings considered cracked elements? 2

[JAE]

Here's the situation:

Wood framed building
Wood shear walls using proprietary hold downs.
Hold downs using cast-in or post-installed hold down anchor bolts
Design of the bolts per Appendix D of ACI 318
Bolts extending into a continuous trenched concrete footing.

Question:

Would the embedded bolts be considered to be in a "Cracked" concrete section or an "uncracked" concrete section? This makes a huge difference in load capacity.

Previous assumptions:
I always thought that the cracked condition was typically for the bottom of flexural beams with the concrete in tension. For a footing, the top of the footing isn't in tension - except in a seismic event, the footing may be uplifted by the shear wall and the deformed shape of the footing implies a tensile condition near the hold down anchor bolt.

Also - if the answer is to simply check to see if there is net uplift on the footing at the hold down - do you use an R = 1? Or do you use the R for the shear wall system?

Do you use 0.6D + .7E? Or the ACTUAL condition of D + .7E. Or do you use D + 1.0E?

 

[MiketheEngineer]

I am rather conservative - so I ALWAYS use the worst case scenario.

Kept me out of trouble for 40 years!!

 

[jdgengineer]

It's my understanding that your thinking of cracked concrete was common in 1997 UBC (ie bottom of slab considered to be cracked as it was in tensile zone). However with IBC and adoption of appendix d its my understanding the concrete must be assumed cracked unless proven otherwise. Typically, we assume concrete is cracked in your situation (obviously conservative) and its my understanding this is typical.

 

[jdgengineer]

We've also been using the SSTB bolts and SB bolts from Simpson for these conditions. They are ICC approved and have much higher values than Appendix D. Haven't had any issues with plan reviewers accepting these bolts instead of conventional nut sandwich.

 

[JAE]

jdgengineer.

I follow you. However, do you have an IBC section you can point me to that defines "cracked" conditions? I couldn't find one.

I understand they say "where concrete is cracked under service load conditions" in the Appendix D, but WHICH service load condition (see above about which R, which combination).

 

[JoshPlumSE]

JAE -

ACI D3.3.4.4: The anchor design tensile strength for resisting eqrthquake forces shall be determined from consideration of a through e for failrue modes given in table D4.1.1 assuming the concrete is cracked unless it can be demonstrated that the concrete remains uncracked

But, for non-seismic loading, I think we can go back to JAE's original assumption (about it only applying to the bottom of beams or such).