MInimum ACI 318 Shear Reinforcement for drilled concrete piers
MInimum ACI 318 Shear Reinforcement for drilled concrete piers
(OP)
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!
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!






RE: MInimum ACI 318 Shear Reinforcement for drilled concrete 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.
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete 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.
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
What is the magnitude of your shear? More than 250 kips? Vertical load? 3000 kips?
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
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.
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
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.
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
Being a transmission line, I wouldn't be surprised if you are seismic design category C in Phoenix (with the increased I factor)
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers
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.
RE: MInimum ACI 318 Shear Reinforcement for drilled concrete piers