15.3.1 Beam Limitations
The DuraFuse Frames connection has been investigated with a variety of beam sizes. The specimens with the smallest beam size [W14×38 (W360×64)] were early prototype tests and had rectangular fuse plates. The smallest beam size that has been tested with fuse plates with extensions, similar to Figure 15-1, is a W21×50 (W530×74). On the bigger side, more than 10 tests have been performed with W33 (W840), W36 (W920), and W40 (W1000) beams. The deepest beam that has been tested is a W40×167 (W1000×249). The heaviest beam that has been tested is a W36×232 (W920×345).
Although the AISC Seismic Provisions permit limited increase in beam depth compared to the maximum sections tested, the prequalification limit for maximum beam depth was not extrapolated beyond the test data for a W40x167 (W1000x249) to be consistent with other connections in this standard. The strain demands on the fuse plate are directly related to the beam depth, so it is conservative to base the beam depth prequalification limit on the maximum tested specimen.
The prequalification limit for beam weight, 309 lb/ft (460 kg/m), was established based on the weight increase permitted in the AISC Seismic Provisions (1.33 times the heaviest beam tested). Connections with beams that have thicker flanges also have thicker fuse plates and external continuity plates. Experiments have not shown negative consequences of thicker plates.
Because there is no beam yielding in the DuraFuse Frames connection, it is not necessary for the beam to satisfy the width-to-thickness ratios given in the AISc Seismic Provisions. Several tests have been performed with beams with higher width-to-thickness ratios than . The greatest flange width-to-thickness ratio that has been tested is 7.80 [W30x99 (W760x147)]. The greatest web width-to-thickness ratio that has been tested is 51.9 [W30x99 and W36x150 (W760x147 and W920x223)]. The requirement that the beam flange width-to-thickness ratio should not exceed is deemed conservative because no plastic hinge formation is expected in the beam.
All experiments for the DuraFuse Frames connections have demonstrated good performance without the presence of lateral bracing near the beam-column connection. The only beam restraint in the tests was at the beam tip. The DuraFuse Frames connection is much less susceptible to beam lateral-torsional buckling or column twisting because the beam does not yield and there is no local buckling in the fuse plate to disrupt symmetry of the load path at the connection. Thus, the beam stability bracing can be determined using the AIS C Specification, neglecting the prescriptive requirements for SMF that assume beam yielding.
15.3.2 Column Limitations
The DuraFuse Frames connection has been investigated with a variety of column shapes and sizes. The W14 (W360) columns that have been tested are W14x38 (W360x57.8) and W14x68 (W360x101). The W21 (W530) column that has been tested is W21x132 (W530x196). The W24 (W610) columns that have been tested are W24x103 (W610x153) and W24x250 (W610x372). The W36 (W920) columns that have been tested are W36x150 (W920x223) and W36x231 (W920x344). The box columns that were tested were 24 in. (600 mm) deep, 17 in. (430 mm) wide, with a plate thickness of 1 3/4 in. (45 mm). The connection details for box columns and HSS columns are the same, so testing of box columns also represents HSS columns. The prequalification limit for column depth for DuraFuse Frames connections is based on the maximum tested specimen, even though the AISC Seismic Provisions permit an 11% increase in depth beyond what has been tested.
As with most other prequalified connections, the DuraFuse Frames moment connection has not been tested with built-up I-shaped or cruciform column sections. However, the CPRP has deemed such columns to be similar enough to rolled shapes to justify prequalification.
15.3.3 Plate Limitations
All tests that have been performed for the DuraFuse Frames connection have used plate material that satisfies ASTM A572/A572M Grade 50 (Grade 345) for the cover plates, fuse plates, top plates, and external continuity plates. In the absence of experimental data for other plates, prequalification is limited to plates that satisfy ASTM A572/A572M Grade 50 (Grade 345).
The fuse plates have the additional requirement that the material not have a tensile strength greater than 85 ksi (590 MPa). Because there is no upper limit on the tensile strength of ASTM A572/A572M Grade 50 (Grade 345) steel in the ASTM specifications, it is prudent to have some protection against steel that is too strong in the fuse. The strongest steel that was used for fuses in testing had a mill certified tensile strength of 86.5 ksi (600 MPa).
A variety of geometries for the yielding regions (YR) of the fuse plates have been investigated and found to provide sufficient deformation capacity. The limits on the thickness and width of the YR are based on the dimensions of the YR that have been tested. Figure C-15.1 shows the range of yielding region (YR) width and depths from the qualification tests at UCSD and BYU. The limits for the width [not greater than 4 in. (100 mm) and not less than 1.5 in. (38 mm)] are based on the limits of what has been tested, with some extension because excess rotation capacity was observed for all specimens where fuse yielding or fracture was the governing limit state.
Figure C-15.2 shows the YR width-to-thickness ratios from the qualification tests at UCSD and BYU. The limits for the width-to-thickness ratio (not greater than 4.25 and not less than 1.5) are based on the limits of what has been tested, with some extension because excess rotation capacity was observed for all specimens where fuse yielding or fracture was the governing limit state.
Figure C-15.3 shows the YR width-depth ratios from the qualification tests at UCSD and BYU. The limits for the width-depth ratio (not greater than 1.25 and not less than 0.5) are based on the limits of what has been tested, with some extension because excess rotation capacity was observed for all specimens where fuse yielding or fracture was the governing limit state.