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Re: residential pier and grade beams

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Robert,
 
refer to section 1921.4.1 and verify if the axial force exceeds AgFc'/10.  If not, you can default back to the standard tie requirements in 1907 and 1911.
 
Paul Feather
----- Original Message -----
Sent: Thursday, June 08, 2000 12:37 PM
Subject: residential pier and grade beams

I am designing a foundation for a hillside residence and I am curious as to the requirements for transverse reinforcing in my c.i.p. piers in bored holes.  Section 1809.5.2.2 requires that transverse reinforcement per 1921.4 be provided in c.i.p. concrete piers in zones 3 and 4.  I am told that standard practice in the area is to only provide nominal hoops at large spacing ( # 3 @ 12 to 24"o.c. ) over the length of the pier ( 8' to 15' depth max).  Section 1921.4.4.6 would provide for a maximum hoop spacing of 3.75" ( 6*Db for #5 vert's ) where the more stringent(?) requirements do not apply.  I see that the 1999 Blue Book provides for some steel volume relief,  but not spacing.  Surface soil creep ( 2 to 4' of depth ) is the primary contributor for flexural loads and the retaining walls are supported on very wide footings which allow for staggered placement of piers.  My largest axial load may get up to 15 kips . So my questions are:
 
1.    Am I understanding the transverse reinforcing requirement correctly ?  ( Hoops at 3.75"o.c. )
 
2.    I remember seeing a design several years ago that used a welded wire fabric cage around the vertical rebar,  is this a good method considering that the axial loads are minimal ?
 
This requirement is also in 1994 UBC.
 
 
Robert Shaffer,  P.E.
Santa Cruz, CA