Two-Dimensional Modular Steel to Optimize Critical Path Steel Erection

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Since the early 2000s, modular design and construction have expanded from two-dimensional grating floor panels to modular composite floor and roof panels, modular girt trusses, and partially fabricated stair towers. Over the past 25 years, approximately 10,000 modular steel panels have been used in fossil and nuclear power plants, as well as oil, gas, and chemical plants. Potential applications include AI data centers, chip manufacturing plants, and large-scale global airport expansions (i.e., to mitigate operational impacts).

Truck-transported modular steel (Fig. 1) includes the most labor-intensive miscellaneous steel components. The shop assemblies concurrently optimize weight and volume criteria, thus enabling truck transport without police escort or cost premium. Cost benefit includes a substantial reduction in schedule, which reduces interest costs, cranes, and other equipment costs; field supervision and engineering; home office support; and (at remote sites) per diems.

Steel composite panels (Fig. 2) are used in lieu of concrete floor and roof slabs. Panel components include two primary beams, infill beams at 12 feet maximum spacing (with dual use for commodity support), nominally 30 composite deck panels, penetrations, hundreds of steel-headed stud anchors and select HSS stubs above the floor for equipment or commodity support.
All structural and miscellaneous steel for a floor or roof bay is erected with only three modular composite panels, an incremental girder, and a greatly reduced bolt quantity relative to conventional “stick-built” construction. The web at the ends of the primary beams is typically coped to a depth of only 8 inches to mitigate impacts to the overall floor depth. The coped web is reinforced by two 4-inch-deep angles to provide for increased shear capacity. Only two bolts are used at panel seated connections, regardless of load. Oversize (OVS) holes all plies address cumulative tolerances.

Paradoxically, the modular panels greatly improve both safety and erection speed relative to “stick-built” conventional construction. Ironworkers may walk on top of the composite deck within the floor panels and install one-sided ASTM F3148 TNA bolt assemblies from above the floor panels. In conventional construction, ironworkers straddle girders and walk on their bottom flanges. All bolt installation is made by reaching below the girder top of steel elevation. The enormous quantity of composite deck sections is not installed until much later.

The composite deck orientation within the floor panels enables 7/8-inch diameter steel headed stud anchors on the two primary panel beams. This reduces the required number of steel headed stud anchors by a factor of two relative to conventional construction. For conventional construction, the flutes of the composite deck above beams limit the maximum diameter to ¾. In addition, design codes impose significant deck reduction factors.

Additional benefits of the modular steel composite panels include:

The composite deck is typically 16 gage to mitigate wet concrete deflection and increase durability in shipment and handling. The thickness also mitigates cumulative concrete ponding from the composite deck, primary panel beams, infill beams, and girders. Longer span primary panel beams are selectively cambered to preclude contributing to cumulative concrete ponding. Reinforcing steel installation and concrete placement is consistent with conventional construction.

The horizontal legs of seated connection angles are coped away from the girder flange to improve hand access underneath (Fig. 3). Bolted connections are made only to the outer angle of each seated connection. Where up to four primary panel beams come together, the outer portion of each beam top flange is coped to allow for hand access. After bolt installation, a thin formwork plate (with shop welded bar “stops” underneath) is “dropped-in” prior to concrete placement. This plate is not shown for clarity.

Closure for concrete placement between adjacent panel flanges is provided via light-gage steel strips. The plate is attached at grade to the latter panel to be installed with powder actuated fasteners (PAF).

Column line composite stub girders have lengths of WT shapes welded to the top flange to facilitate composite action (Fig. 4). Between WT segments, column line primary panel beams bear on the girder top flange. However, at braced bay column lines, panel beams are not coped but bear on erection angles, thereby facilitating conventional bracing connections.

Modular girt trusses are used (Fig. 5) in conjunction with thin gage, 3-inch deep, steel siding panels.

Truss configurations are typically established for two upper bound bay sizes. The shorter trusses have three truss panels and longer trusses use four panels. Truss chord members are W12s with horizontal webs. Truss verticals are channels which also serve as lateral-torsional restraint to the chord members. Truss diagonals are angles. Seated connections are used at truss ends.
The optional variation in vertical member spacing provides a common Design to Capacity ratio along the entire length. Historically, this yields a nominal ten percent tonnage reduction versus equal spacing.

Partially shop fabricated stair tower assemblies (Fig. 6) are used to maximize shop labor but mitigate transport volume. Assemblies include two columns, vertical and horizontal bracing, girts on two of three sides, platform framing, grating, and pipe penetrations (e.g., fire water). The panels are mirrored and mated for shipment. Cable tray, firewater pipe, and other commodities may be attached at grade prior to uplifting.

Cap and base plates facilitate seated connections for module stacking (Fig. 7). Tower segments provide rapid permanent stair access for craft. Moment frames are used in lieu of braced frames in the longitudinal direction to eliminate interferences with walkway access to building interiors. However, vertical bracing may be used for the exterior column line, with moment frames for the interior column line.

Based on successful implementation, the cited modular steel approaches and improvements shared above are recommended for expanded industry use in all non-commercial buildings and structures. Primarily for commercial purposes, a third category of steel (Modular Steel) has been used to distinguish the design/build approach from the Structural Steel and Miscellaneous Steel categories of AISC 303, the Code of Standard Practice. This new category captures the significant shift of field labor costs to shop fabrication costs. ■

About the Author

James L. Ryan, PE, is a retired Principal Engineer specializing in Steel Design and Modular Steel. His career included 40 years at Bechtel Corporation, with an intermediate 5 years at a commercial design firm. He welcomes any questions or further discussion and may be reached at JimR21157@gmail.com.