Flexural requirements (columns seismic: ACI 318)

Tekla Structural Designer
Modified: 2 Jun 2023
2024
Tekla Structural Designer

Flexural requirements (columns seismic: ACI 318)

Minimum flexural strength

Columns that are part of Special Moment Frames are required by ACI 318 to have a minimum amount of flexural strength depending on the connecting beams flexural capacities so as to promote the formation of beam yielding mechanisms in the case of an earthquake. This is done by establishing a ratio between the beam and column moment strengths in the moment resisting frame direction.

The design of the main reinforcement in a column is done for the top, middle and bottom region of the stack and moment capacity is calculated for the factored axial force in each region for the Major and Minor directions.

If SFRS Type = Special Moment Frame then flexural strength checks are performed at the joints:

  
Σ Mnc,l = Mnc,l,bot + Mnc,l,top
Σ Mnc,r = Mnc,r,bot + Mnc,r,top
where
Σ Mnc,l , Σ Mnc,r = Sum of the Nominal Flexural Strengths of the columns framing into the joint in the relevant direction.
Mnc,l,bot, Mnc,r,bot = Nominal Moment Strength of the stack below the joint obtained for the axial force value consistent with the minimum Nominal Moment Strength respectively for the left and right sway cases.
Mnc,l,top, Mnc,r,top = Nominal Moment Strength of the stack above converging on the same joint for the axial force value consistent with the minimum Nominal Moment Strength respectively for the left and right sway cases.
    

The sum of the beam strengths are obtained as follows:

  
Σ Mnb,l = Mnb,l- + Mnb,l+
Σ Mnb,r = Mnb,r- + Mnb,r+
where
Σ Mnb,l , Σ Mnb,r = Sum of the Nominal Flexural Strengths of the beams framing into the joint in the relevant direction.
Mnb,l- , Mnb,r- = Nominal Moment Strength at the joint from the beam on the left from current reinforcement arrangement respectively for the left and right sway cases.
Mnb,l+ , Mnb,r+ = Nominal Moment Strength at the joint from the beam on the right from current reinforcement arrangement respectively for the left and right sway cases.
    

The minimum strength ratio between columns and beams in both left and right sway cases is checked as follows:

  
Σ Mnc,l 6/5 * Σ Mnb,l
Σ Mnc,r 6/5 * Σ Mnb,r
    

Maximum allowed area of reinforcement

The maximum area of longitudinal reinforcement in columns part of Special Moment Frames is limited as follows.

If SFRS Type = Special Moment Frame

Then calculate maximum area of steel, As,max as follows:

  
As,max = 0.06 * Ag
  

where

  
As,max = Maximum allowed area of reinforcement
  
Ag = Gross area of the concrete section.
    

Maximum allowed longitudinal bar center spacing

Limitations on the longitudinal bar spacing emerge from the code requirement for maximum allowed cross-section center link spacing of the confinement reinforcement due to the method of link leg distribution across the column section.

If SFRS Type = Special Moment Frame

Then check maximum longitudinal reinforcement bar distance, scr,max as follows:

  
scr,max = 350 mm Metric-units
scr,max = 14 in US-units
    
Note: In ACI 318-14, scr,max is limited to 200mm for non-circular columns with, Pu > 0.3*Ag*fc OR fc > 70 MPa (10,000 psi).

Non-reversing plastic hinges

Non-reversing plastic hinges are regions along the stack of the column where flexural yielding is likely to occur.

Note: Non-reversing plastic hinges are beyond scope in the current release of Tekla Structural Designer.

Splices

Columns that are part of Special Moment Frames have restrictions on the allowed locations of lap splices.

Strength design of mechanical splices and restrictions to the use of welded splices as required by ACI 318-11 apply to Special Moment Frames

Note: These restrictions are not implemented in the current release of Tekla Structural Designer.
Was this helpful?
Previous
Next