Concrete Gravity Columns

So I was looking through the ACI 318-08 at the tie requirements for concrete gravity columns and saw that the entire Chapter 21 was re-written.  So don't go looking for revision bars along the side of the pages because there aren't any.  Some changes have been made to the tie requirements that I thought would be nice to summarize.  I'm going to pull information straight from the code so please forgive if it is too redundant.  Thanks

Concrete gravity columns by definition are not part of the lateral force resisting system and therefore are detailed per section 21.13.

Depending on the design method and level of forces, columns are to be details per 21.13.3 or 21.13.4.  If one determines the design displacement, the resulting member and shear forces on the column, and if the capacity is greater than the demand then 21.13.3 can be followed.  Otherwise 21.13.4 must be followed which provides a level of ductility and strength that match a column which is part of the LFRS.

21.13.3 - where induced moments/shears from the design displacements are less than the member moment/shear strength capacity then to following sections shall be satisfied.

• 21.13.3.1 - for lightly loaded members where Pu < Ag f'c/ 10 shall satisfy
• 21.5.2.1 - provides minimum reinforcement for flexural members. 
• Stirrups shall be placed no more then d/2 for length of member.
• 21.13.3.2 - for moderately loaded members where Pu > Ag f'c/10 shall satisfy
• 21.6.3.1 - longitudinal reinforcement to be between 1 and 6 percent.
• 21.6.4.2 - arrangement of hoops and crossties in column, 14" max.
• 21.6.5    - shear strength requirements at faces of joint for column.
• Tie spacing So shall not exceed smaller of 6 times diameter of smallest longitudinal bar and 6 inches.
• 21.13.3.3 - for heavily loaded members where Pu > .35Po shall satisfy
• 21.13.3.2 - see above for moderately loaded column.
• 21.6.4.7 -  cover limitation of 4" along with additional transverse reinforcement to confine additional cover.
• Amount of transverse reinforcement shall be 1/2 that required by 21.6.4.4 but shall be spaced not greater than So for the full member length.

 21.13.4 - where induced moments/shears are not calculated or where the demand exceeds the capacity of the column then the following sections shall apply.  Note that this would be our typical design approach.

• 21.13.4.1 - materials shall satisfy
• 21.1.4.2 - compressive strength f'c shall not be less than 3000 psi.
• 21.1.4.3 - compressive strength for light weight shall not be greater than 5000 psi.
• 21.1.5.2 - deformed reinforcement shall be ASTM A706.
• 21.1.5.5 - value of fy or fyt shall not exceed 60 ksi. 
• Mechanical splices shall satisfy 21.1.6 and welded splices shall satisfy 21.1.7.1
• 21.13.4.2 - for column load Pu < Ag f'c/10 shall satisfy
• 21.5.2.1 - provides minimum reinforcement for flexural members.
• 21.5.4 - shear strength requirements for flexural member
• Stirrups shall be placed no more than d/2 for length of member.
• 21.13.4.3 -  Where gravity axial load exceeds Ag f'c/10 the following shall be satisfied.
• 21.6.3.1 - longitudinal reinforcement to be between 1 and 6 percent.
• 21.6.3.2 - lap splices are permitted only within the center half of the member length, shall be designed as tension lap splices, shall be enclosed transverse reinforcement conforming to 21.6.4.2 and 21.6.4.3.  This is a major change from previous codes. 
• 21.6.4 - all sections must be followed therefore tie reinforcement similar to column that is part of the LFRS.
• 21.6.5 - all sections must be followed therefore shear strength requirements similar to column that is part of the LFRS.
• 21.7.3.1 - joint reinforcement shall satisfy
• 21.6.4.4(a) or 21.6.4.4(b)
• 21.6.4.2
• 21.6.4.3
• 21.6.4.7
• except as permitted by 21.7.3.2 

As can tell from section 21.13.4.3 when the design displacement forces are not checked the detailing requirements become much more significant.