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D1.F.5.6 Column Design in Pre-2014 Codes

Columns design in STAAD.Pro per the ACI code is performed for axial force and uniaxial as well as biaxial moments. All active loadings are checked to compute reinforcement. The loading which produces the largest amount of reinforcement is called the critical load. Column design is done for square, rectangular and circular sections. For rectangular and circular sections, reinforcement is always assumed to be equally distributed on all faces. This means that the total number of bars for these sections will always be a multiple of four (4). If the MMAG parameter is specified, the column moments are multiplied by the MMAG value to arrive at the ultimate moments on the column. Since the ACI code no longer requires any minimum eccentricity conditions to be satisfied, such checks are not made.

Method used

Bresler Load Contour Method

Known Values

Pu, Muy, Muz, B, D, Clear cover, Fc, Fy

Ultimate Strain for concrete : 0.003

Steps involved

  1. Assume some reinforcement. Minimum reinforcement (1%) is a good amount to start with.
  2. Find an approximate arrangement of bars for the assumed reinforcement.
  3. Calculate PNMAX = 0.85 Po, where Po is the maximum axial load capacity of the section. Ensure that the actual nominal load on the column does not exceed PNMAX. If PNMAX is less than Pu/PHI, (PHI is the strength reduction factor) increase the reinforcement and repeat steps 2 and 3. If the reinforcement exceeds 8%, the column cannot be designed with its current dimensions.
  4. For the assumed reinforcement, bar arrangement and axial load, find the uniaxial moment capacities of the column for the Y and the Z axes, independently. These values are referred to as MYCAP and MZCAP respectively.
  5. Solve the interaction equation:
    ( M n y M y c a p ) α + ( M n z M z c a p ) α ≤ 1.0
    where
    α
    		=
    The ALPHA parameter used in ACI 318 2011, 2008, 2005, and 2002. Refer to D1.F.3.1 ACI 318-2011 Design Parameters or D1.F.3.2 Pre ACI 318-2011 Design Parameters for details.

    If the column is subjected to a uniaxial moment,  a is chosen as 1.0

  6. If the Interaction equation is satisfied, find an arrangement with available bar sizes, find the uniaxial capacities and solve the interaction equation again. If the equation is satisfied now, the reinforcement details are written to the output file.
  7. If the interaction equation is not satisfied, the assumed reinforcement is increased (ensuring that it is under 8%) and steps 2 to 6 are repeated.
  8. The maximum spacing of reinforcement closest to the tension force, for purposes of crack control, is given by
    s = 15 ( 40 , 000 40 , 000 f s ) − 2.5 c c ≤ 12 ( 40 , 000 f s )

    with fs in psi and is permitted to be taken equal to (2/3) fy, rather than 60 percent of fy, as in ACI 318-02.

  9. Section 10.9.3 has been modified to permit the use of spiral reinforcement with specified yield strength of up to 100,000 psi. For spirals with fyt greater than 60,000 psi, only mechanical or welded splices may be used.

Column Interaction

The column interaction values may be obtained by using the design parameter TRACK 1.0 or TRACK 2.0 for the column member. If a value of 2.0 is used for the TRACK parameter, 12 different Pn-Mn pairs, each representing a different point on the Pn-Mn curve are printed. Each of these points represents one of the several Pn-Mn combinations that this column is capable of carrying about the given axis, for the actual reinforcement that the column has been designed for. In the case of circular columns, the values are for any of the radial axes. The values printed for the TRACK 1.0 output are:

P0
Maximum purely axial load carrying capacity of the column (zero moment).
Pnmax
Maximum allowable axial load on the column (Section 10.3.5 of ACI 318).
P-bal
Axial load capacity at balanced strain condition.
M-bal
Uniaxial moment capacity at balanced strain condition.
e-bal
M-bal / P-bal = Eccentricity at balanced strain condition.
M0
Moment capacity at zero axial load.
P-tens
Maximum permissible tensile load on the column.
Des. Pn
Pu/PHI where PHI is the Strength Reduction Factor and Pu is the axial load for the critical load case.
Des. Mn
Mu×MMAG/PHI where PHI is the Strength Reduction Factor and Mu is the bending moment for the appropriate axis for the critical load case.

For circular columns,

M u = M u y 2 + M u z 2

e/h = (Mn/Pn)/h

where
h
=
the length of the column

Example

Column design per the ACI 318-2005 code 

UNIT KIP INCH
START CONCRETE DESIGN
CODE ACI 2005
FYMAIN 58 ALL
MAXMAIN 10 ALL
CLB 2.5 ALL
DESIGN COLUMN 23 25
END CONCRETE DESIGN

Example

Column design per the ACI 318-2002 code 

UNIT KIP INCH
START CONCRETE DESIGN
CODE ACI 2002
FYMAIN 58 ALL
MAXMAIN 10 ALL
CLB 2.5 ALL
DESIGN COLUMN 23 25
END CONCRETE DESIGN

Example

Column design per the ACI 318-1999 code 

UNIT KIP INCH
START CONCRETE DESIGN
CODE ACI 1999
FYMAIN 58 ALL
MAXMAIN 10 ALL
CLB 2.5 ALL
DESIGN COLUMN 23 25
END CONCRETE DESIGN

Column Design Output

The samples illustrate different levels of the column design output. The following output is generated without any TRACK definition (i.e., using the default of TRACK 0.0): 

   ====================================================================

        COLUMN  NO.     5  DESIGN PER ACI 318-05 - AXIAL + BENDING

   FY - 60000  FC - 4000 PSI,  SQRE SIZE - 12.00 X 12.00 INCHES, TIED
           AREA OF STEEL REQUIRED =   7.589  SQ. IN.

   BAR CONFIGURATION       REINF PCT.   LOAD   LOCATION   PHI
   ----------------------------------------------------------

     8 - NUMBER  9           5.556        2      STA     0.650
   (PROVIDE EQUAL NUMBER OF BARS ON EACH FACE)
   TIE BAR NUMBER    4 SPACING   8.00 IN

TRACK 1.0 generates the following additional output: 

       COLUMN INTERACTION: MOMENT ABOUT Z -AXIS (KIP-FT)

    --------------------------------------------------------
      P0        Pn max    P-bal.    M-bal.   e-bal.(inch)
     942.40    753.92    179.59    170.75   11.41
       M0       P-tens.   Des.Pn    Des.Mn     e/h
     148.52   -480.00    350.15     10.47    0.00249
    --------------------------------------------------------

       COLUMN INTERACTION: MOMENT ABOUT Y -AXIS (KIP-FT)

    --------------------------------------------------------
      P0        Pn max    P-bal.    M-bal.   e-bal.(inch)
     942.40    753.92    179.59    170.75   11.41
       M0       P-tens.   Des.Pn    Des.Mn     e/h
     148.52   -480.00    350.15    136.51    0.03249
    --------------------------------------------------------

TRACK 2.0 generates the following output in addition to the above examples: 

                            Pn       Mn       Pn       Mn    (@ Z )
            |              695.93    77.23   347.96   148.53
         P0 |*             637.93    93.16   289.97   157.71
            | *            579.94   107.06   231.98   164.41
      Pn,max|__*           521.94   118.23   173.98   170.18
            |   *          463.95   129.01   115.99   163.66
      Pn    |    *         405.96   139.03    57.99   156.37
     NOMINAL|     *         Pn       Mn       Pn       Mn    (@ Y )
       AXIAL|      *       695.93    77.23   347.96   148.53
 COMPRESSION|       *      637.93    93.16   289.97   157.71
          Pb|-------*Mb    579.94   107.06   231.98   164.41
            |      *       521.94   118.23   173.98   170.18
 ___________|____*_______  463.95   129.01   115.99   163.66
            |  * M0   Mn,  405.96   139.03    57.99   156.37
            | *   BENDING
      P-tens|*     MOMENT
            |
D1.F.5.6.1 Slenderness Effects and Analysis Consideration
Parent topic: D1.F.5 Column Design
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