MInimum ACI 318 Shear Reinforcement for drilled concrete piers

[KGJ]

I am designing some laterally loaded drilled reinforced concrete piers (on the order of 6' to 9' in diameter). I am using ACI 318-02 (I do have 08 on order - so I apologize). Section 11.5.5.1 requires a minimum amount of shear reinforcement if the factored shear force, Vu, exceeds 1/2 of the shear strength provided by concrete. Exception a to this section includes Slabs and Footings.

The equation for the minimum amount of shear reinf. is in section 11.5.5.3, which is based upon diameter of pier, strengths of concrete and steel and the spacing. Using 4ksi conc. and 60 ksi #5 ties, I am getting exceptionally tight tie spacings with this size of piers. #5's @ 10" o.c. for a 6'6" diameter pier 35' deep. My question is this: Does a drilled reinforced concrete pier qualify as a footing and is thus exempt from this particular minimum.

I will be putting ties in to meet shear stresses and confinement minimums. I apologize for my question of semantics, but any help is much appreciated. Thanks in advance!

 

[frv]

I'm not familiar with 02, but I suspect it is almost the same as 05. In 05, ACI specifically states that Drilled Piers are outside the scope of ACI 318. I believe ACI 336 deals with Drilled Piers.

For the portions above the soil, I would just treat it as a circular column, meaning your shear reinforcing spacing would NOT be exempt.

 

[Teguci]

IBC has the requirements you are looking for. Look under chapter 18 regarding piles and piers (your 6'-0" diameter pier may be classified as a pile per IBC definitions) for required reinforcing. CRSI will also have guiding information for non-laterally loaded piers.

ACI 336 will give you some construction information (ex. use 4" clearance to rebar when removing a steel casing). I would also point you to FHWA publications dealing with drilled piers and piles (009752 and 009966 respectively free to download from FHWA).

For laterally loaded piers, closely spaced piers will shadow each other and reduce their effectiveness for resisting lateral loads.

 

[JLNJ]

Often shear reinforcing is not required but it sounds like you are fighting some gigantic loads.

What is the magnitude of your shear? More than 250 kips? Vertical load? 3000 kips?

 

[KGJ]

Thank you all for your input and suggestions (very helpful as usual). Again I apologize in that I am using older codes (need to shell out some cash for new codes). I was not aware that drilled piers were not in the scope of ACI 318. What is strange is that Section 1810 of the 2006 IBC refers to ACI 318 Sections 21.4.4.1 through ~.3 for Seismic Category D, which are for Special Moment Frames.

My project is a transmission line in the rural area around Phoenix. The conductors (wires) on these lines are typically viewed as huge dampeners, so I am not too concerned with seismic failure. I've heard of many t-lines being insignificantly affected in major seismic events.

I have shear loads on the order of 361 kips to 1,061 kips (in conjunction with Axials in the range of 40 kips and moments in the range of 7,200 ft-kips).

Thanks again.

 

[KGJ]

One more thing, for a good majority of my piers the shear stresses aren't driving a significant amount of shear reinforcing. I am just worried about meeting the minimums which may exceed what is needed from a stress resistance point of view.

From a stress resistance aspect, I need #5 ties @ 20" o.c. for some of my 6' and 7' piers. Using the IBC Section 1810 minimums I would be at 12" o.c. for most of the pier and 6" o.c. in top 6' of pier.

Side note: the base plates for the steel poles on this line are around 5'6" to 7'6 in diameter.

 

[JAE]

KGJ - have you looked at ACI 318, section 1.1.5?

 

[KGJ]

Good point, JAE! The way I read the 02 version of section 1.1.5 is that only piers in high seismic regions are covered by ACI 318-08. Which is my situation. This section then refers me to 21.10.4, which I am then directed to 21.4.4. Thank you all!

 

[jhenry]

There is a formula in the ACI which allows you to use the Axial load to mitigate a portion of the shear, because it allows a higher shear value for the concrete. Read thru the sheat section of the curerent code.

 

[hawkaz]

I haven't looked too closely yet- but what is the definition of a high seismic design category per ACI318 1.1.5?

Being a transmission line, I wouldn’t be surprised if you are seismic design category C in Phoenix (with the increased I factor)

 

[KGJ]

Thank you jhenry and hawkaz. I did see the formula that takes into account the axial load, thank goodness.

I am not sure what they define as "High" seimic in 1.1.5, but if you end up going to 21.10.4 they have two sections, 1 refers to Seismic C and the other Seismic D, E and F. I will check into the impact of the I factor, but the geotech has already issued a study report stating Cat. D.

 

[hawkaz]

Was the geotech refering to site class D or Seismic Design group D?