Special Moment Frame – Member Design
Moment Frame Design Requirements and Assumptions
D5. Member Design
Similar to the connection design, members (beam and column) are designed for frame mechanism forces, assuming Yield-Links at both ends of the beam are at their maximum probable tensile 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 Yield-Link® locations. Columns are designed so bracing is only required near the top flange of the beam. Since the frame members are not dissipating energy (i.e., beam 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 fixed or partially fixed, 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. Please note, for the Strong Frame column design, the demand forces are from overstrength load combinations. This is similar to other SMF column design, however, for Strong Frame columns, axial + moment interaction check is required, whereas typical SMF column design is permitted to ignore the bending moment (unless the moment results from loads applied between points of support).
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 overstrength 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:
- Use half of all anchor rods to transfer the shear force at each column.
- 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 field. The effect