Seismic Design to ACI 318

Tekla Structural Designer
2021
Tekla Structural Designer

Seismic Design to ACI 318

Reinforced concrete structures in buildings subjected to earthquake effects are designed elastically to the strains and displacements both from static and dynamic forces which they are subjected to. It is recognized that during an earthquake the building and its structural elements are very likely to be exposed to displacements well into their inelastic range and special precautions need to be taken as to increase the strength of critical sections in members which contribute to the building's lateral resistance while also contributing to the ductile behavior of the building in order to allow for the dissipation of induced stresses.

In the case of reinforced concrete structures, particular design and detailing requirements (provisions) need to be fulfilled beyond the conventional design of the elements as to provide them with ductile response capabilities. Such requirements are mainly addressed to structural elements part of structural systems built for the purpose of resisting seismic lateral forces - Seismic Force Resisting Systems [SFRS]. Seismic provisions also apply to reinforced concrete elements not part of the SFRS when the building is assigned to a higher Seismic Design Category - SDC D, E or F.

The seismic design checks and detailing requirements of reinforced concrete members in Tekla Structural Designer are based mainly on ACI318, Chapter 21 - Earthquake-Resisting Structures.

Seismic Force Resisting Systems

The level of design and detailing required of members that are part of a seismic resisting structural system can differ depending on the amount of toughness they are intended to provide to the building. ACI318-11 groups the main structural systems into "Ordinary", "Intermediate" and "Special" groups. Different types of structural systems have limitations to their application in each of the Seismic Design Categories.

Structural System Allowed in SDC
Ordinary Moment Frames A, B
Ordinary Cast in Place Structural Walls A, B, C
Intermediate Moment Frames A, B, C
Intermediate Precast Walls A, B, C
Special Moment frames A, B, C, D, E, F
Special Structural Walls (Precast / Cast in Place) A, B, C, D, E, F

As the current release of Tekla Structural Designer does not fully include the design requirements for all the Seismic Force Resisting Systems, SFRS types have been classified as included or excluded from the member design.

Members in included SFRS types are fully covered for seismic design provisions while those in excluded types are covered to a limited extent only.

  • Seismic Force Resisting Systems included in the design:
    • Intermediate Moment Frames
    • Ordinary Moment Frames
    • Ordinary Reinforced Concrete Structural Walls
  • Seismic Resisting Systems excluded from the design:
    • Special Moment Frames
    • Special Reinforced Concrete Structural Walls
    • Intermediate Precast Structural Walls

Consequently the current release of Tekla Structural Designer can be said to consider the design requirements for each of the Seismic Design Categories as follows:

Seismic Design Category Seismic Requirements
SDC A N/A
SDC B Considered
SDC C Considered
SDC D, E, F Not Considered

Materials

Additional seismic material requirements apply to:

  • concrete beams and columns assigned to a Special Moment Frame SFRS
  • concrete walls assigned as Special Reinforced Concrete Structural Walls

Concrete Compressive Strength

The requirements for compressive strength of concrete are limited:

Minimum compressive strength of normal weight concrete f'c
  =  
21 MPa (3,000 psi)
Maximum compressive strength of lightweight concrete f'c
  =  
35 MPa (5,000 psi)

Reinforcement Steel

Reinforcement steel shall comply with ASTM 706(M), Grade 420 (60,000 psi).

(fy)actual - (fy)specified
125 MPa (18,000 psi)
(fu)actual / (fy)actual
1.25

where,

(fy)actual
  =  
Actual yielding strength of the reinforcement based on mill tests, MPa (psi)
(fy)specified
  =  
Specified yield strength of reinforcement, MPa (psi)
(fu)actual
  =  
Actual ultimate tensile strength of the reinforcement, MPa (psi)

Reinforcement Characteristic Yield Strength

Requirements for the characteristic yield strength of the reinforcement steel are:

  • Maximum allowed characteristic yield strength of longitudinal reinforcement: fy = 420 MPa (Grade 60 - 60,000 psi);
  • Maximum allowed characteristic yield strength of shear reinforcement: fyt = 420 MPa (Grade 60 - 60,000 psi);

ACI 318-19

If working to ACI 318-19 click the below links to view additional important information:

Was this helpful?
Previous
Next