Design of piers without Transvers Rebars and with 2-#5 Long Reinf.

[Mina_GS]

Hi Folks,

I am working on a project in Texas, low wind and low seismic, the soil is expansive soil and the soil engineer recommended the system to be grade beams and belled piers, the Architect and GC showed me a sample of another project and wanted to follow that one, the system was done as follows:-

* Grade beams 12x30 to be placed as waffle system and spaced 12 ft to 13 ft on center in each direction.
* 12 ft length and 12" diameter piers to be placed under the grade beams (mainly under the grade beams intersection points).
* The piers should not have reinforcement into the grade beams to avoid any stresses due to swelling of the soil, so the reinforcement will just stop under the grade beam surface.

As a result of the above, the piers will just take compression forces and the lateral shear is less than 250 lbs, so we are under the 50% of concrete section shear capacity.

THE QUESTION IS: Can we design the pier to be reinforced with just 2-#5 Longitudinal rebars and no transverse rebars at all?

ACI318-19 mentioned in 13.4.3.2 to design the piers as columns (per chapter 10.5), so we need long and transvers rebars, also if no transvers rebars added, how to be sure that the section is confined, and no buckling will happen as well?

 

[BulbTheBuilder]

Can we design the pier to be reinforced with just 2-#5 Longitudinal rebars and no transverse rebars at all?

A minimum of longitudinal reinforcement ratio of 0.005 is needed since it is primarily subjected to axial compression. Also, for circular shapes you will need a minimum of 6 rebars so I wouldn't put only 2#5 rebars. Else, you literally have your pier being "plain concrete" which can't be done for piers.

also if no transvers rebars added, how to be sure that the section is confined, and no buckling will happen as well?

Good question. Since you can't be sure do provide transverse rebars.