Pier and grade beam system for Residential building in Texas

[Mina_GS]

Hi All,

I am working on a residential building in TX, and I have seen some samples showing that the piers are 12" diameter and just reinforced with 2-#6 long rebars and no ties were included there, the piers will be disconnected from the grade beams and using void boxes under the grade beams only, so the piers will be axially loaded only.

FYI: the soil is expansive and the building is low seismic and low wind.

The question is, where can I find the code reference that is supporting the design for axially loaded piers to be reinforced with just two rebars and without ties, and how will the piers confined?

 

[1503-44]

That could be simply a very low stress design, such that the only reinforcement required is that for temperature and shrinkage. Such elements are very common. If stresses are less than what concrete alone will carry without any steel at all, you only need to provide "minimum temperature and shrinkage steel".

https://www.aboutcivil.org/concrete-minimum-steel-reinforcement.html

Try ACI 318 Chapter 22

https://up.codes/viewer/nevada/aci-318-2014/chapter/22/sectional-strength#22

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[JAE]

In south TX where I used to work for years, we would typically use 0.75% rebar.
For 12" diameter that equates to 0.848 sq. in. or (2) #6.

The 1% minimum required for columns in ACI 318 technically doesn't apply as 318 excludes drilled piers.

The geotechnical reports also many times recommended 0.75% as a minimum but also provided a force formula for expansive clay uplift on the piers (i.e. the clays "grabbed" the piers and lifted them up. The resistance below was provided by a belled end - usually 3 times the shaft diameter.

Confinement was usually wire hoops at 12" diameter - again - not related to 318.

ACI does have 336 which deals with piers. Here's a link to an older 1993 version: ACI 336.3R-93

 

[Mina_GS]

@JAE

I understand that the two rebars will work with the pier reinforcement ratio for long rebars, but what I saw that the piers does not have any hoops or circular ties, which is really a big question here, how should the confinement happen?

Also, it is required by AASHTO and ACI to provide at least 4 rebars for long reinforcement in case of circular hoops, and 6 in case of spiral, so how should it work with just two rebars?

 

[JAE]

We never used two bars - always at least 4.
We typically specified 3/8" dia. hooping at a 12" pitch but over the years got push back from contractors preferring #3 ties @ 12" o.c.

 

[Mina_GS]

Thank you for your response, actually I was working on a project there and found multiple samples provided by the client showing 12" belled piers, reinforced with 2-#5 long reinforcement and no ties, and the soil engineer and the contractor as well confirmed that they are always using this type for piers in the projects.

I tried to find any code reference for that but nothing confirmed it, it seems like it is a standard practice in this area.

 

[1503-44]

It is, or was. As long as load is light, and expansive clay is not a factor.
Sometimes you just find drilled piers filled with packed caliche, crushed limestone, or oyster shells, or just rocks, if there any rocks in the area. No bars. Especially with agricultural buildings. Sometimes just a caliche pad, for light oilfield worksheds, parts stock, ie shipping containers, etc.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."