13.4.1 T-Stub Material Specifications
T-stubs tested at Georgia Tech were cut from W-shapes rolled from either ASTM A572/A572M Grade 50 (Grade 345) or dual grade ASTM A36/A572 Grade 50 steel. In all cases, the yield and tensile strengths of the steel used for the T-stubs were consistent with ASTM A572/A572M Grade 50 (Grade 345) or ASTM A992/A992M material. Details are available in Swanson (1999).
13.4.2 Continuity Plates
All of the experiments upon which these provisions are based were conducted with continuity plates present. As a result, continuity plates are required in the prequalified double-tee connections. The continuity plates are required to control column-flange bending that may result in column-flange prying forces in the tension bolts between the T-flange and column flange. While T-flange prying is considered explicitly in the design procedure, column-flange prying is not. Further, when two rows of four tension bolts are used, it is speculated that the absence of continuity plates may result in column-flange bending that could produce larger forces in the interior tension bolts than the exterior tension bolts. More research is needed before the requirement of including continuity plates can be relaxed.
13.4.4 Bolts
All of the tests used as a basis for prequalification employed two identical T-stubs and bolt patterns that were arranged symmetrically with respect to the axes of the beam and column (with the exception of the web bolts, which are slightly off center with respect to the plane of the beam web and column web). Further, all prequalification tests used two rows of bolts between the T-stem and beam flange and between the T-flange and column flange. Significant questions remain regarding whether additional rows of bolts between the T-flange and column flange would participate in carrying load without the presence of stiffeners on the T-stub.
All of the six full-scale assemblies tested at Georgia Tech made use of ASTM F3125/ F3125M Grade A490 bolts, but a number of the T-stub component tests included ASTM F3125/F3125M Grade A325 fasteners. As a result, Grade A325, A325M, A490, or A490M bolts, or their tension control equivalents, are permitted in the double-tee connections. While all of the tested configurations used the same diameter and grade of fasteners throughout the connections, there is no requirement that all bolts in the prequalified connection be the same diameter or grade. It is expected that all bolts will likely be Grade A490 or A490M, but there may be advantages to using larger-diameter tension bolts between the T-flange and column flange and smaller- diameter shear bolts between the T-stem and beam flange.
Oversize holes are not permitted in the beam flange, because if they are used, it is difficult to achieve adequate moment capacity in the beam at the row of shear bolts farthest from the column face.
Of the 22 experiments conducted at the University of Wyoming, 18 components failed with a fracture of the flange of the T-stub. Of the 60 T-stub components tested at Georgia Tech (including the 12 T-stubs in the full-scale assembly tests), no fractures in the flanges were observed. An analysis of the 82 experiments revealed that the T-stubs that failed via flange fracture all had a ratio of tension bolt gage to T-flange thickness, , that was greater than 10. No flange fractures were observed in T-stub components with a ratio of of less than 9. After considering the data, the CPRP elected to limit the ratio of to be not greater than 7.0, a value that includes all of the components tested at Georgia Tech.
All bolts are required to be installed as pretensioned fasteners and faying surfaces are required to be prepared as they would be in slip-critical connections. The connections are not intended to be designed as slip critical, however. The required pretension and faying surface preparations are intended to provide a reasonable level of friction to prevent slip at service loads and provide slip resistance conducive to energy dissipation at design loads. Prevention of slip at design loads is not considered desirable as this would limit a robust energy dissipation mechanism of the connection.
13.4.5 T-Stub Shims
The thickness of shims between the T-flange and column flange is limited to a maximum of ¼ in. (6 mm) to preclude the use of shims that would be similar in configuration to the shimmed configuration in the University of Wyoming experiments where the T-flange was permitted to flex both when the T-stub was in tension and when it was in compression. Further, the use of shims that extend the full width and breadth of the T-flange is encouraged.