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Special Moment Frame – Member Design

Strong Frame Special Moment Frame

Moment Frame Design Requirements and Assumptions

Member Design

Similar to the connection design, members (beam and column) are designed for frame mechanism forces, assuming links at both ends of the beam are at their probable maximum rupture strength. The beam is designed and tested as unbraced from column to column. There are no requirements for stability bracing of the beams at the link locations. Columns are designed so bracing is only required near the top ange of the beam. Since the frame members are not dissipating energy (i.e., plastic hinges do not form), members are designed in accordance with AISC Steel Construction Manual (AISC 360). This means b/t and h/tw ratios in AISC 341 are not applicable to our beam and column members in the frame when designed using a pinned-base design. However, if the base is designed as xed or partially xed, i.e., so the columns may yield at the base, then AISC 341 slenderness ratios will be met for the columns at the base level.

Base Plate Design

The capacity design approach also extends to the design of the column base plates. Pinned column base connection demand loads (axial and shear) are calculated from the lesser of the frame mechanism forces and the forces from code omega load combinations.

Design capacity for the base plate is calculated from AISC Design Guide #1 (DG #1) and Design Guide #16 (DG #16). Base plate compressive capacity is calculated per DG #1, whereas base plate tension capacity is calculated assuming two-way action using the method in DG #16. Welds in the base plate are checked for shear and tension interaction using capacity-level loads as noted above.

Oversized holes in base plates are required for erection tolerance. Per DG #1 Section 3.5, AISC recommends use of oversized holes for anchor rods. For the Strong Frame, the column base plate holes typically exceed the anchor rod diameter by 1⁄4". When oversized holes are used for erection, considerable slip in the base plate may occur before the plate bears against the anchor rods. In addition, due to anchor placement tolerance and potential for anchor movement during concrete placement, it is not likely that all the anchor rods will resist the same load. AISC DG #1 Section 3.5.3 has two separate recommendations for shear load transfer from the base plate with oversized holes to the anchor rods:

  1. Use half of all anchor rods to transfer the shear force at each column.
  2. Weld a plate washer with standard oversized holes (+1/16") to the top of the base plate.

In order to minimize welding at the jobsite, Simpson Strong-Tie currently uses the first approach in our design for the anchor rods in shear. However, the Designer can coordinate with Simpson Strong-Tie if they prefer to use the second method. Please note that, for this option, welding and welding inspection are required in the eld. The effect of oversized holes in the frame and structural movement shall be evaluated by the Designer.