Designing a braced-frame column base

The Braced-frame base type (Wizard group Column bases, mark prefix BF) designs a column base plate at the foot of a braced frame, where a vertical brace lands on a gusset welded to the base plate and to the column. It combines two guides:

• AISC Design Guide 29, Vertical Bracing Connections, §4.3 and Example 5.12, for the brace, the gusset, the welds and the column;
• AISC Design Guide 1, 3rd Edition, Examples 4.7-7 and 4.7-8, for the base plate, the anchor rods, the shear transfer and the concrete (ACI 318-19 Chapter 17). These checks are the same as the Base plate type's.

How the brace force is split #

The work point sits on the column centre line, a height e above the top of the base plate. The brace force P (positive in tension) has a horizontal part H and a vertical part V. Following DG29 Figures 4-22 and 4-23:

• the gusset edge on the column carries all of V and a horizontal force Hc = (H e + V ec) / β, where β is the height of the middle of that edge above the plate and ec is half the column depth (0 for a gusset on the web);
• the gusset edge on the base plate carries Hb = H − Hc.

Putting the work point on the top of the plate (e = 0) and the gusset on the web gives the smallest forces in the column. The base plate is designed for the column's own forces plus the brace reaction: axial P − V (uplift for a tension brace), shear H and moment H e, in the plane of the brace (along N for a flange gusset, along B for a web gusset).

Braced-frame base inputs #

The editor is the base plate editor with three more sections after the loads.

• Loads: the column's own axial force, moment and shear, as for a base plate. Enter the brace force in the Brace force and geometry section: one value for required strengths, the service components D, L, Lr, S, W, E for load combinations, or one value per load case. With Brace force reverses ticked, every case is also checked with the brace in compression.
• Brace force and geometry: the column face the gusset is welded to (flange or web), the brace bevel (vertical and horizontal legs) and the height e of the work point above the plate.
• Seismic brace forces (AISC 341-22 D2.6a, D2.6b(a), D2.6c(a)): the base forces take the brace components from the required strengths of the brace connection. SCBF replaces the brace force by Ry Fy Ag, min(Ry Fy Ag, (1/0.877) Fne Ag) and 0.3 of that (F2.6c), from the brace Ry (0 = Table A3.2), length (end to end) and K; OCBF keeps your overstrength force but limits it to the expected brace strength (F1.6a Exception); BRBF uses ω Ry Pysc and β ω Ry Pysc (F4.6c.1) with Pysc, β and ω from the brace tests. All are divided by αs = 1.5 in ASD. The column forces you enter are added as before. With the story height H of the seismic design section, the column's own shear is limited to 2 Ry Fy Z / (αs H) (D2.6b(b)) and the total shear is at least 0.7 Fy Z / (αs H) (D2.6b(c)).
• Brace connection: a W brace whose web is spliced to the gusset with four angles (DG29 Ex. 5.12), or a double angle, single angle, WT or slotted HSS bolted or welded to the gusset. Bolts, lines, pitch, gage and edge distances; the distance from the work point to the first gusset bolt (0 places the connection just clear of the column and the plate).
• Gusset: thickness and material, the edge lengths on the column and on the plate, the corner clip, the two fillet sizes, and the buckling factor K (0.6 for this extended gusset, DG29 Table C-1) with the buckling length (0 = measured from the Whitmore section to the column face or the plate along the brace).

Braced-frame base checks #

• Brace and its connection: brace yielding and rupture, bolt bearing and tearout, block shear, the splice angles (yielding, rupture, block shear, bolt shear) and bearing and block shear on the gusset.
• Gusset: Whitmore yielding in tension, Whitmore buckling in compression, shear and tension yielding along the column, the Neal interaction, shear yielding along the plate.
• Welds: the gusset-to-column and gusset-to-plate fillets (Table J2.4 minimum sizes; ⅝ tg at a web), the column web and flange fillets to the base plate for the brace forces, with the base metal check of Manual Eq. 9-3.
• Column: web local yielding and crippling (flange gusset), the web yield line and web shear (web gusset).
• Base plate, anchor rods and concrete: every check of the base plate type for the combined reactions.

Braced-frame base drawings #

The sheet shows the base plate plan with the gusset, and an elevation in the plane of the brace: the column, base plate, grout, concrete, anchor rods, the gusset with its clip, the brace and its splice angles cut beyond the connection, the bolts, the gusset welds with their symbols and the work point.

Braced-frame base limits #

• The gusset is designed with the DG29 force field: no frame-distortion (seismic) forces. The seismic brace forces above give the D2.6 base forces; the other AISC 341 requirements of the brace connection (hinge zone, gusset welds) are checked in the bracing gusset type.
• Two-direction loading (a brace in each direction at the same column) is not combined; design each direction.
• The column-to-plate flange welds take the uplift less any column compression entered in the same case.

See Verification for the published examples reproduced by this type.