Designing a double-tee moment connection

The Double-tee moment type (Wizard group Moment, mark prefix DT) designs the prequalified double-tee connection of AISC 358-22 Chapter 13 with the AISC 341-22 Chapter E requirements. Two identical T-stubs, cut from a rolled W shape, are bolted to the column flange with tension bolts and to the beam flanges with shear bolts; the beam web has a bolted single plate welded to the column flange. Yielding is intended in the beam just past the last shear bolt.

There is no non-seismic option. Neither AISC 358 nor the Steel Construction Manual covers a double-tee moment connection outside a seismic system, so ILX does not offer one.

The design is a capacity design: the forces follow from the beam's probable maximum moment Mpr at the plastic hinge, not from entered loads, so the editor has no Loads section. The connection is designed by LRFD whatever the design method (358-22 1.3, φd = 1.00, φn = 0.90); choosing ASD changes only the AISC 341 terms that carry αs = 1.5 (Ca of Table D1.1b, Eq. E3-2 and the brace forces).

Double-tee inputs #

Every value AISC 358 leaves to the engineer is an input; its default is listed here.

• Frame and capacity design: the seismic system (SMF); one beam or two identical beams on both column flanges; the clear span Lcf (360 in.; Lcf / d ≥ 9, 13.3.1(5)); the beam length Lo between column work points for the stiffness check (0 = Lcf + dc); a moment connection at this end only (off; 2.4.4 User Note); the gravity line load wu = 1.2D + f1L + 0.15S (1.25 kip/ft) and a further concentrated gravity shear Vg (0) for Vgravity (2.4.4); Ry, Rt (0 = AISC 341-22 Table A3.2) and Cpr (0 = Eq. 2.4-2); a concrete structural slab (off) and, in an SMF with a slab, its isolation from the column (13.2 Exception); the E3.4a moment-ratio exception (off).
• Beam and column: rolled W shapes (beam W21×50, column W14×159 by default), their materials, the column axial load Pr and story shear Vc (both 0), whether the column ends at the joint, and the J10.6 panel-zone option.
• T-stubs: the W shape both T-stubs are cut from (W21×93), its material (A992; 13.4.1 also permits A913 Gr. 50), the T-stub width WT along the column flange (14 1/2 in.; the stem is not tapered), a shim between the stem and the beam flange (0, at most 1/4 in., 13.4.5(1)) and a finger shim between the T-flange and the column (0).
• Shear bolts: grade (A490), diameter (7/8 in.), threads (excluded), the number per flange nvb (0 = the least even number meeting Eq. 13.5-4 and the Step 15 back-check), spacing svb (3 in.), gage gvb (4 in.), the beam flange end distance Lehb (1 3/4 in.), the stem end distance (1 3/4 in.), the beam setback SB (0 = clear of the T-flange fillet and the tension bolt heads plus 1/2 in.), the stem holes (standard or oversized) and the faying surface class (A; μ enters Eq. 13.5-31).
• Tension bolts: grade (A490), diameter (1 in.), four or eight per T-stub (eight: two rows of four), the gage gtb across the stem (5 in.; gtb / tft ≤ 7.0), the inner gage gic across the column flange (5 1/2 in.), the spacing of the outer bolts with eight bolts (3 in.), the T-flange holes (standard, oversized or short slots parallel to the column), and which Step 22 equation checks the column flange (Eq. 13.5-53; Eq. 13.5-59 is the alternative the standard allows).
• Continuity plates and doubler: continuity plates are always required (13.4.2). Designed sizes them at least tbf thick and to E3.6f.2; provided uses your thickness. Width (0 = to the column flange edge less 1/4 in.), fillet size, corner clip along the web (0 = AISC 358-22 3.6). The panel-zone doubler: design when required, none or provided, with its material, thickness, one or two plates, welds and extension.
• Web single plate: the design procedure (Manual Part 10 with the eccentricity at the bolts, or direct shear with the T-stubs restraining the rotation), the configuration (auto = conventional when a ≤ 3 1/2 in., else extended; the Step 21 User Note expects an extended plate because of the setback), bolts, spacing, thickness, material, edge distances, the plate-to-column weld (two-sided fillets, two-sided PJP or CJP, 13.4.3), the clearance to the T-flanges, the web bolts and the hole types (standard or short slots parallel to the beam).

Double-tee checks #

The 358-22 13.5 procedure, step by step:

1. Mpr = Cpr Ry Fy Zb at the hinge, Sh = S1 + Lvb (Eqs. 13.5-5, 13.5-6), Vh (2.4.4), Mf = Mpr + Vh Sh (2.4.5) and the beam shear Vu (2.5).
2. Beam flange net section Zb,net Rt Fub ≥ Zb Ry Fyb with the holes in both flanges (Eq. 13.5-1), and the largest hole of Eq. 13.5-2.
3. Shear bolts: φrnv (Eq. 13.5-3), the Step 4 estimate (Eq. 13.5-4) and the back-check nvb φrnv ≥ Ff (Eq. 13.5-37) with Ff = Mf / (db + tst) (Eq. 13.5-36).
4. T-stem: the Step 9 sizing at Fpr = Mf / 1.05db (Eqs. 13.5-7 to 13.5-11), then gross yielding, net fracture and flexural buckling at Ff on the lesser of WT and the Whitmore width (Eqs. 13.5-38 to 13.5-41; AISC 360 E3 above KL/r = 25).
5. Tension bolts and T-flange: the Step 10 and 11 sizing (Eqs. 13.5-12 to 13.5-23) and the Step 17 back-check ntb φT ≥ Ff, φT the least of the plastic mechanism, mixed mode and bolt fracture (Eqs. 13.5-42 to 13.5-52).
6. Stiffness: the connection must be fully restrained, Ki ≥ 18 E Ibeam / Lo (Eqs. 13.5-24 to 13.5-35).
7. Bearing and tearout of the shear bolts on the beam flange and the stem, and block shear of both (Steps 18 and 19, φd: ductile modes). The alternate mechanism of Fig. 13.7 is not checked, as the standard says.
8. Beam shear at Vu (Step 20) and the web single plate at Vu through the single-plate design (Step 21), with the beam web block shear of Commentary Fig. C-13.2.
9. Column flange yield lines (Step 22, Eqs. 13.5-53 to 13.5-59), continuity plates (13.4.2 and 341-22 E3.6f.2), the SMF panel zone (E3.6e) and column-beam moment ratio (E3.4a).

Prequalification limits (13.3: beam ≤ W24, ≤ 55 lb/ft, tbf ≤ 5/8 in., column ≤ W36 with a slab or W14 without, width-to-thickness by AISC 341), detailing (13.4: materials, two shear bolts per row, the hole types, gtb / tft ≤ 7, pretensioned bolts, Class A surfaces, shims), spacing, edge distances and tightening clearances, the lateral bracing at the hinge (13.3.1(7)) and the protected zone (the T-stubs and the beam to one beam depth past the last shear bolt, 13.3.1(8)) complete the report. Steps 9 to 11 and the Step 4 estimate are shown as recommendations: the back-checks of Steps 15 to 17 at the actual flange force decide adequacy.

Double-tee drawings #

The elevation shows the column with its continuity plates and doubler, both T-stubs with the tension bolts through the column flange and the shear bolts through the beam flanges, the beam set back past the T-flanges, the web single plate with its weld symbol, the dimension chain from the column face (setback SB, the beam flange end distance, the shear-bolt pitch, the stem end distance), the gage gtb and the hatched protected zone. The T-STUB (PLAN) view gives the stem from above with the shear-bolt pattern (gvb, WT, the bolt rows) over the beam flange; the T-FLANGE (FROM BEAM) view, on the drawing sheets, gives the tension-bolt pattern (gtb, gic, the outer spacing, the edge distances). On screen the elevation and the plan are shown. The drawing set (PDF and DXF) has the elevation on the connection's sheet and the plan and T-flange on a second sheet, T-STUB DETAILS.

Double-tee limits #

• Rolled W shapes only: built-up beams and columns and cruciform columns (358-22 2.3) are not designed; the T-stubs are cut from rolled sections as 13.4.1 requires. Tapered stems are not modelled (the stem width is WT).
• Interpretations, stated in the report: Eq. 13.5-19 is printed b′ = b + dtb/2 while Eq. 13.5-48 gives b′ = b − dtb/2; ILX uses b − dtb/2 throughout. Eq. 13.5-21 is printed with tst for tft. Eq. 13.5-53 is printed with tfc to the first power and is read as tfc² (Yc is dimensionless; Eq. 13.5-59 is its square-root form). The "b" of Eq. 13.5-54 is read as bc. Lstem in Eq. 13.5-29 is the stem length from the T-flange to the stem end.
• Fatigue (AISC 360 Appendix 3) is not evaluated for this type.
• No worked example exists in AISC 358-22 or its Commentary; the tests are hand calculations of each equation.