UBC minimum reinforcement1907.12.2.1, 1910.5.1 1

[EngineerofSteel]

I am designing the second phase of a pipe-column (or "pole barn"), shade structure. All footings are cylindrical pier footings with #3 hoops holding (8) #5 bars vertically. the #5 bars are placed 3" from the outside of the 48" diameter pier footing.

The reviewer is requesting I "check concrete footings for minimum reinforcing per U.B.C. sections 1907.12.2.1 and 1910.5.1."

the x-section area is 1810 in^2. *.0018 = 3.26 in^2 required. my (8)#5 bars provide 2.45 in^2.

The 8.625 O.D. XS pipe has an area of 12.80 in^2 and this column extends to within 3" of the bottom of the footing.

How can I justify the existing reinforcement? This is a U-3, "occupied agricultural" building.

Can I include the area of the steel in the column as part of my "reinforcement"?



Any suggestions?

-DD

 

[dik]

Why not? less the area enclosed by the pipe column also...
I'm not sure, but the 0.0018Ac is based on minimum shrinkage steel. I doubt that your 48" shaft is long enough to warrant shrinkage steel. also, the shaft is likely in compression (might not be; with reinforcing, the concrete could be in tension). Is it possible to consider the shaft as a plain concrete item?

Dik

 

[JAE]

Dairy Designer.

chapter 19 of the UBC is based upon ACI 318.

ACI 318 specifically excludes Drilled Piers so the sections that are indicated probably don't apply to your piers.

Drilled piers are covered by a different ACI committee - 336. Here's a link to the ACI website:

ACI 336 page

For most of the 1000's of drilled piers that I've designed, we used different design parameters - usually driven by expansive clays that would expand and pull up the pier so we used the rebar to resist the resulting tension. Generally we used about 0.75% of the gross area, though, which is much higher than your 0.0018 (0.18%).

8 - #5 in a 48" dia. pier seems a bit meager.

But I'd check out 336.

 

[BigInch]

IMO, the real problem is the 48" diameter piers. They are far to big for an 8" column. Would it not be better to use a 16" diameter pedestal sitting on a 48" diameter footing? I presume the 48" diam head-to-toe allows for easy quick construction, but the penalty that it brings is a higher min steel requirement.

I would not consider that a pipe column in the middle of a 48" diameter pier could possible provide minimum shrinkage steel. My reason is that shrinkage does not only occur in the middle of the column and you have provided relatively little steel elsewhere. Shrinkage will be possible basically everywhere else and needs to be controlled.

As long as I've started.... Something else that I personally never like to see are steel columns embedded in concrete near ground level rather than using proper base plates and anchor bolts ... like streetlight foundations. Sooner or later the steel column will corrode at the interface and with embedded columns, repairs are almost impossible. I can just imagine what that's going to look like in 4 or 5 years after the cows huddling up around the poles saturate the soil with uric acids and salts...

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[EngineerofSteel]

I believe the information JAE gave is the info I need. On the bottom of worksheets submitted as part of engineering calculations (submitted by the veteran engineer preceeding me) is the following comment:

"for agricultural structures; pipe column extends full length of pier footing except for three inch clearance at the bottom of footing; therefore reinforcing is not required."

Does this wording appear somewhere in ACI 318?

An 8" schedule 80 pipe has an OD of 8.625". In a 48" dia. footing, this is 18% of the width. Does "middle", which experiences no compression or tension really span this much?

I am sure the steel column, extended to within 3" of the bottom, strengthens the pier footing. Just makes sense.

The bottom 18" of steel columns are always protected from corrosive elements using PVC sleeves. This barns last decades, as built by us. Others barns with similar use have been in place 50+ years. We design the shade higher than adjacent corrals.... as you know, manure flows downhill!

Thanks for the concern. This was noted before my time, and is a standard practice issue.

Thanks all! I have what I need.

-DD