THE BRACE CONNECTION THE BOLTED OPTION FIGURE 1 MODEL SHOWING THE BOILER AND THE STRUCTURAL SUPPORT SYSTEM (BOILER BUILDING)

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1 Comparative Study of Bolted versus Welded SCBF Connections Authors: Robert P. Krumpen III P.E., Bechtel Corporation, Dr. Peter J. Carrato P.E. S.E., Bechtel Corporation, INTRODUCTION Many large industrial projects are located in remote areas where the local labor pool does not include a significant number of skilled welders. As a result, field bolted connections are favored over field welded connections. Special Concentrically Braced Frames (SCFB) are a preferred structural system for many industrial structures. However to meet the detailing requirements for a SCBF structure in which member performance dictates the connection design loads, bolting can become complicated. The decision to use bolted versus welded field connections in industrial projects needs to be carefully evaluated before proceeding. A comparison of utilizing field bolted versus field welded connections for vertical braces is explored. A recent fossil power plant project employed a SCBF in a seismic design category D. It became evident in preliminary reviews of this project that the complexity of bolted bracing connections would create a challenge for the erector. The final conclusion for the project was that field bolted connections would be employed. However the actual design of the connections has cause the authors to revisit this decision and to closer evaluate alternatives for the next project that will require SCBF detailing.

2 FIGURE 1 MODEL SHOWING THE BOILER AND THE STRUCTURAL SUPPORT SYSTEM (BOILER BUILDING) THE BRACE CONNECTION The example used in doing the comparison study consists of a typical vertical brace associated with a boiler building. To give a sense of scale this structure is intended to support a coal fired boiler which is hung from the roof of the building at a height of approximately 300 feet. The brace is a 400mm x 400mm (16 x16 ) built up box section with a wall thickness of 18mm (approximately 11/16 thick). The material used was Chinese grade Q235 that has a yield stress of 32.6 ksi.. For this brace the connection needs to be designed to exceed the ductile capacity of the member, R y A g F y. Due to the variable nature of this material an R y of 1.5 was used, similar to ASTM A36. The connection therefore is required to be designed for a minimum capacity of 2167 kips. The connection is required to either provide the ability to resist the flexural capacity of the brace (Section 13.3 C AISC Seismic Provisions 1 ) or to permit inelastic rotation and withstand the compressive capacity of the brace (F cr A g ). The compression capacity of this brace is a significant load, around 750 kips. Providing a connection that has the necessary flexural rotational ability but is strong enough to withstand the compressive capacity is a bit tricky. With these requirements in place the two types of connection options can be described. THE BOLTED OPTION The bolted concept shown in Figure 2, consists of several elements; some typical for both seismic and non-seismic vertical brace connections and some that are unique to seismic connections. The brace is slotted with the knife plate welded to the brace. At the slot additional makeup plates are required to avoid tension rupture of the brace at the connection. Dual plates called sandwich plates are bolted to the knife plate and are also bolted to the gusset. The sandwich plates keep the load concentric along the axis of the brace and connection, and allow for the bolts to act in double shear to minimize the required number of fasteners. The gusset is stiffened by a series of plates. The load being carried from the brace to the column is such that it is advantageous for the beam intersecting the brace to be pulled back and replaced with a builtup stub beam. This is done to avoid the design problems in trying to transmit the large capacity loads of the brace into the floor system. The knife plate has a gap of two times the thickness of the plate to accommodate the inelastic rotation. Since the knife plate has to accommodate the rotation but be sufficiently strong to resist the compressive load and tensile capacity of the brace these plates tends to be thick. To ensure that the hinge is in the knife plate and not near the bolted areas, the sandwich plates are also stiffened.

3 FIGURE 2 BOLTED VERTICAL BRACE TO GUSSET OPTION Summary of Bolted Option Connection Brace to Knife Plate Weld - 5/8 Partial Joint Penetration Weld, 4 sides at 32 inches for a total effective length of 640 inches Knife Plate - 2 Thick Q345B (Similar to ASTM A572 Gr 50) 850 lbs Knife Plate to Sandwich Plate Bolts - (18) 1 1/4 A490SC Bolts Sandwich Plate - (2) 1 Thick Q345B (Similar to ASTM A572 Gr 50) 660 lbs Sandwich Plate Stiffener Welds 5/8 Fillet both sides at 26 inches for a total effective length of 1040 inches Sandwich Plate to Gusset Plate Bolts - (18) 1 1/4 A490SC Bolts Gusset Plate - 2 Thick Q345B (Similar to ASTM A572 Gr 50) 2000 lbs Total Effective Length of All Welds = 1680 inches Total Bolt Count = 36 bolts Total Weight of Connection = 3510 lbs THE WELDED OPTION The welded option shown in Figure 3 consists of a slotted brace field welded to the gusset. At the slot additional makeup plates are required to avoid tension rupture just like in the bolted option. Since the load being designed for is significant, the gusset is extended down and the intersecting beam is also pulled back. Stiffeners are required to strengthen the gusset plate. A gap three times the thickness of the gusset is provided to have a hinge.

4 FIGURE 3 WELDED VERTICAL BRACE TO GUSSET OPTION Summary of Welded Option Connection Brace to Gusset Plate Weld - 5/8 Partial Joint Penetration Weld, 4 sides at 32 inches for a total effective length of 640 inches Gusset Plate - 2 Thick Q345B (Similar to ASTM A572 Gr 50) 3700 lbs Total Effective Length of All Welds = 640 inches Total Bolt Count = 0 bolts Total Weight of Connection = 3700 lbs (Based on Rectangular Dimensioning) THE IMPACT TO THE ENGINEER While the two brace connection options have very different limit states, there are some worth comparing. The bolted option will trend to larger connection materials due to the reduction in effective cross sectional areas. For our example if we look at tension rupture, the bolted option has (6) 1 ¼ dia bolts. These holes effectively reduce the capacity of the plate to around 73% to that of a solid plate. Another example is based on block shear. Both the bolted option and welded option have block shear failure modes. With the bolt pattern shown in Figure 2, the bolted option has an effective area of around 66% to that of a solid area. The magnitude of the brace loads typically requires the beam to column connection to be pulled away from the column. This presents some unique issues. If the continuity of the member is to be maintained to the centerline of the column, then the beam to column connection needs to be a moment connection. An alternative is to maintain the connection as a simple shear connection

5 and require the column to handle the moment being generated by the eccentricity of the connection. The later option can present several coordination problems and often requires revisiting the structural analysis. Therefore utilizing moment connections can be the clearest path forward. The constructability of utilizing moment connections can be improved if the opposite end of the beam is an extended shear tab connection. This configuration works provided the opposite end is not also part of another gusset plate (See Figure 6). FIGURE 4 BEAM TO COLUMN CONNECTION SHOWING THE MOMENT DEMAND THE CONNECTION REQUIRES FIGURE 6 BRACE FRAME CONFIGURATION THAT REQUIRES MOMENT CONNECTIONS ON BOTH ENDS OF THE BEAM REDUCING CONSTRUCTABILITY THE IMPACT TO THE FABRICATOR

6 SCBF vertical brace connections have an impact on the fabricators performance. The bolted option has some significant issues. There are stiffeners with the sandwich plates as well as the drilling of holes in both the plates. The latest seismic specification now permits an oversized hole in one ply which allows for more adjustability. Also all of the welds are required to be done in the shop. The welded option has significantly less complexity. The major issue associated with this option is that the gusset plate is required to be shaped to include the plastic hinge. Since the fabricated weight traditionally includes waste which in the case of gussets, the rectangular dimensions are used (The largest width times the larges height). While the physical gusset plate actually weighs 2400lb the fabricated weight is significantly higher at 3700 lbs. While it is surprising that the higher weight for the welded option is actually very similar to the bolted option, it is clear that the complexity of the bolted connection is the more costly fabricated connection. Since fabricated steel is purchased usually by the ton, the cost can be significant. Typically the average weight of a boiler building s connections is 10-13% of the total weight of fabricated steel, high seismic connections will increase this ratio to 20-25%. The reduced complexity of the welded option should result in lower fabrication cost for this option. THE IMPACT TO THE FIELD The field s impact on the choice of connection design is a function of a number of variables. In many industrial projects, the availability of adequate number of qualified welders to work in remote regions encourages the use of bolted connections. The bolted option is traditionally preferred based on that key factor. However the bracing connection associated with SCBF structures are significantly different from typical bracing connections and other aspects of constructability should be considered. Some issues of constructability can include erection tolerances, number of parts being handled, and clearance for installation. Depending on whether the bolted or welded option is selected other consideration include wrench clearance, welding position down hand or over hear, as well as associated inspection requirements. In focusing on comparing just the act of bolting versus field welding some basic evaluation can be done. If we assume that a 5/8 weld is a five pass weld and that the weld rate is 3 inch/min, then the field weld option would take 3.5 hours to perform. Also cleaning of slag between passes would further increase the weld time. If we assume that the bolt up rate is 10 bolts/hour then for the bolted option would take 3.6 hours. This does not include the initial fit up and inspection for either concept, but this does show that the installation time can be potentially similar.

7 FIGURE 7 PHOTO OF COMPLETED VERTICAL BRACE CONNECTION The bolted option has the issue of that it has more loose parts. More loose parts equates into more time in fitting up the connection. The welded option requires that the brace be knifed onto the gusset plate. Careful attention needs to be paid to being able to install the brace. CONCLUSION While traditional approach for industrial projects has been on bolted connections, it appears that field welded connections for SCBF may offer some competitive advantages. Concerns associated with the time required to perform the field welding needs to be weighed against the increased number of bolts and higher fabrication costs. REFERENCES [1] AISC S 341, SEISMIC PROVISIONS FOR STRUCTURAL STEEL BUILDINGS, AMERICAN INSTITUTE FOR STEEL CONSTRUCTION, CHICAGO.