Slotted Web (SW) vs. Dogbone (RBS) Connection Savings/Costs
1. RBS moment frame beams are heavier than SW beams. This is because seismic lateral story drift controls the design of Special Moment Frames so that RBS beams, due to their reduction in stiffness, are typically one to two sizes heavier than SW moment frame beams. For example, in Reference (1) parametric studies were conducted on six-story two- and three-bay moment frames subjected to seismic base shears. Lateral story drift coefficients of the frames with and without RBS beams were compared. The frames with RBS beams were designed with a 40% reduction in their flange areas. When compared to frames without the RBS cuts, these flange reductions showed a 10.6% increase in story drift for the two-bay frame and a 10.3% increase in the story drift for the three-bay frame.
2. Fabrication of SW beams is more economical than for RBS beams. Only two web cuts are required for the SW beams whereas four flange cuts are required for the RBS beams. Moreover, web cuts involve the thinner web material than the thicker flange material. Additionally, no special surface preparation is required for the SW weld access holes.
3. Field inspection of the connection weldments is significantly reduced. For the SW moment frame connections, the nondestructive testing rate for an individual welder or welding operator may be reduced from 100% to 25%, provided the reject rate is demonstrated to be 5% or less for the welds tested for the welder or weld operator. A sampling of at least 40 completed welds for a job shall be made for such reduced evaluation. (See Reference 2, Section 2.6)
The SSDA SW connection design fee is typically 10% to 30% of the savings resulting from lower steel tonnage, fabrication, and field inspection over the RBS beams and connections.
1. Chambers, J.J., et al., "Effect of Reduced Beam Section Frame Elements on the Stiffness of Moment Frames", Journal of Structural Engineering, ASCE, March 2003, pp. 383-393.
2. Engineering Report 5861, ICBO Evaluation Service, Inc., Whittier, CA, May 1, 2002.