Seismic force resisting systems

Tekla Structural Designer
2021
Tekla Structural Designer

Seismic force resisting systems

Available SFRS types

The seismic design requirements for a particular member are based upon which Seismic Force Resisting System (SFRS) the member forms part. Hence, in Tekla Structural Designer you can set appropriate members as part of one of the following systems:

SFRS types included for steel members

Moment Frame Systems

  • Special Moment Frames (SMF)
  • Intermediate Moment Frames (IMF)
  • Ordinary Moment Frames (OMF)

Braced Frame and Shear Wall Systems

  • Ordinary Concentrically Braced Frames (OCBF)
  • Special Concentrically Braced Frames (SCBF)
  • Special Concentrically Braced Frames (SCBF)

Other Seismic Frame Type

  • Not specified - this is a 'catch all' for all other types- but no seismic design performed.

SFRS types available for concrete members

Moment Frame Systems

  • Special Moment Frames (SMF) - limited design.
  • Intermediate Moment Frames (IMF)
  • Ordinary Moment Frames (OMF)

Walls

  • Special reinforced concrete structural wall - limited design
  • Intermediate precast structural wall - no seismic design performed
  • Ordinary reinforced concrete structural wall

Other Seismic Frame Type

  • Not specified - this is a 'catch all' for all other types- but no seismic design performed.

SFRS types excluded

Everything else

  • e.g. Eccentrically Braced Frames

Members allowed in the SFRS

The following member types are allowed to be part of a SFRS in Tekla Structural Designer

  • Steel columns
  • Steel beams
  • Steel braces
  • Concrete columns
  • Concrete beams
  • Concrete walls

The following member types are not allowed to be part of a SFRS in Tekla Structural Designer

  • Any timber, cold-formed, general
  • All other “Characteristics”, e.g. steel joists, truss members, purlins
  • Composite members
  • Plated, Westok, Fabsec, concrete filled, concrete encased – selectable but no design (i.e. only rolled)

Assigning members to the SFRS

The choice of members to be part of a SFRS is entirely the engineer’s responsibility.
  • It is expected that all members in a frame are assigned to the SFRS.
  • The assigned members should be specified to act in building Direction 1 or Direction 2

Special Moment Frames - assigning connection types at steel beam ends

For SFRS comprising of steel SMF it is necessary to ensure that the beams fail before the columns. To this end, an assessment of plastic moment capacity is made at each floor. The moment capacity is dependent upon the position of the plastic hinge, typically (dcol + dbeam)/2. These locations can be selected appropriate to each beam end either in the beam properties.

Options are provided as follows:

  • Plastic hinge position at start

    • Plastic hinge position at end

Either, (dcol + dbeam)/2 (default)

or, dcol /2 + L

L = 0 (default)

Validation of the SFRS

There is only a small amount of validity checking for an SFRS that can be performed automatically; it remains in large part the user’s responsibility to ensure that each SFRS is defined appropriately.

The following validation conditions are however detected:

  1. Any A or V brace in a Seismic Force Resisting System must have the A or V as vertically released. A warning is provided in validation if this is not the case.
  2. X type bracing is defined as more than one V or inverted V (A) type brace pair on the beam. When more than one A or V brace pair is detected, the additional checks required by AISC 341-05 and AISC 341-10 given in Section 8.3 are out of scope. This situation is not detected during validation, but it is identified in the seismic design, so that the beam is given a “beyond scope” status.
  3. The use of K braces is not allowed in AISC 341. An error is provided in validation.
  4. Tension only braces were permitted to the 05 version but had no additional requirements. In the 10 version they are only are allowed for OCBF. Thus, an error is provided in validation when a tension only brace is set as part of a SCBF and the code is the 10 version. (The same validation is also applied to compression only braces.)
  5. If seismic loadcases are included in combinations and there is not at least one member assigned to each of Direction 1 and Direction 2 then a warning is issued.

Auto design of SFRS members

All SFRS members can be auto-designed to the conventional design requirements both for seismic and non-seismic combinations;

During the automatic design procedure, besides the conventional auto-design, and for seismic combination results only:

  • Steel members in the SFRS are checked for seismic provisions;
  • Normal weight reinforced concrete members in the SFRS are auto-designed for seismic provisions
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