SpeedCore is a revolutionary method of composite structural-steel framing, aiming to replace the traditional steel frame around a reinforced concrete core in office-tower construction.
- Cost savings
- Schedule reduction
- Wall thickness reduction
- More flexibility for adaptive re-use
- Increased blast resistance
How Does it Work?
What is the function of the steel plate? What is the function of the concrete? Who is responsible for designing the plate for the various loading conditions?
During construction, the steel faceplates with the steel cross-ties provide stability under construction loading during erection prior to the concrete infill being placed. The plates, by their nature, also provide permanent formwork for the concrete. After the concrete infill has cured, the plates, the cross-ties and the concrete act compositely with the plates. The steel acts as wall reinforcement and the primary resistance to tension and shear demands on the lateral system. The concrete infill, working compositely with the steel face plates, provides overall flexural and shear stiffness to the structure with the confined concrete having the ability to resist larger overturning compressive loads under lateral demands. The design responsibility of the phased loading of the module as a whole, the plate and the associated connections should be evaluated and discussed directly with the contractors involved. Most importantly, the conclusion of responsibility should be delineated in the final contract. Due to the special nature of the system, there should be open communication about design responsibility to ensure a successful project.
Rainier Square: For the case of the Rainier Square project, the design considers four stories of the empty module advancing ahead of the concrete fill operation. The contractor’s preferred erection logistics were considered in the design of the steel face plates along with cross-tie size, spacing and attachment. In addition, the four floors limitation was driven by OSHA requirements. 1926.754(b)2) states: “At no time shall there be more than four floors or 48 feet (14.6 m), whichever is less, of unfinished bolting or welding above the foundation or uppermost permanently secured floor, except where the structural integrity is maintained as a result of the design.”
At what building height would an owner begin to see measurable savings to a project's bottom line? Is there a function that could be applied to the Rainier Square Tower describing the savings as the theoretical building increases in height?
A savings of cost and schedule can be realized in buildings five stories and taller. The overall project savings primarily comes from the reduction in the construction schedule and the more accurate tolerances in steel erection as opposed to concrete placement. As the construction schedule is highly building dependent in terms of geometry and complexity as well as local construction practices and material availability, the measurable savings is likely not to be solely tied to building height.
How is it Different?
It is anticipated that once the SpeedCore system is more commonplace in the industry, the overall design schedule is expected to be on par with any other lateral force resisting system.
Rainier Square: Being the first project to implement the system in a high-seismic demand region, the requirements for a peer review of the system by the local building officials required additional review and approvals of the design, but the overall design, procurement, and construction schedules were coordinated with these additional efforts in mind.
How does the SpeedCore system compare to other lateral systems used in high rise buildings with respect to stiffness & strength?
SpeedCore is significantly stiffer than BRBF or ECBF framed cores. If proportioned similarly, SpeedCore would be comparable in stiffness and strength to a CIP concrete core.
When comparing SpeedCore walls and reinforced concrete core walls, what is the potential reduction in wall thickness?
The potential reduction in wall thickness between the systems is a function of the overall building configuration and lateral demands and would need to be evaluated on a project-by-project basis. Given the composite action between steel and concrete, there is potential for a reduction in overall wall thickness but will be highly tied to the overall reinforcing ratio of the face plates to the overall wall thickness.
Rainier Square: In the case of the Rainier Square project, the overall out-to-out of the wall assembly was similar between the concrete core and the SpeedCore alternative due to overall building stiffness and strength considerations.
Similar to adaptive re-use of a conventional reinforced concrete shear wall, drastic changes to major core wall openings at the lobby, stair, and doorways would be challenging to implement. However, there is more flexibility in the case of SpeedCore due to the predictability of the system. No scanning is required to locate reinforcing bars (there is none). New wall openings can be coordinated with the layout of the cross-ties in mind and field reinforced with additional cover plates as deemed necessary.
When comparing a composite plate shear wall to an equal thickness, conventionally reinforced concrete shear wall, the faceplates add increased blast resistance.
DG 32: Design of Modular Steel-Plate Composite Walls for Safety-Related Nuclear Facilities §1.2.1(a)
Malushte, S.R. and Varma, A.H. (2015), “Rethinking Steel-Plate Composite (SC) Construction for Improved Sustainability and Resiliency of Nuclear Power Plants,” Nuclear Power International, Vol. 8, Issue 4."
How is it designed?
The design process published in the Pankow Foundation "Design Procedure" indicates a prescriptive approach using published material strengths. Pankow Foundation, who has been working with the developers of this system since 2006, has published a "Design Procedure for Dual-Plate Composite Shear Walls" and it is available for free download now.
AISC will be publishing a Design Guide (expected to be published in 2020) based on the research currently underway. The goals of the research include, but are not limited, to publishing a prescriptive method for SpeedCore design that takes advantage of composite action between the concrete and the steel.
For those interested in learning more about other design methods, AISC has recently published Design Guide 32: Design of Modular Steel-Plate Composite Walls for Safety- Related Nuclear Facilities. This design guide details a elastic finite element approach for the specific application of nuclear facilities, where walls tend to differ geometrically from common buildings.
The final in-place condition will always be the responsibility of the design professional. The design responsibility of the phased loading of the module as a whole, the plate and the associated connections should be evaluated and discussed directly with the contractors involved. Most importantly, the conclusion of responsibility should be delineated in the final contract. Due to the special nature of the system, there should be open communication about design responsibility to ensure a successful project.
What about the Rainier Square Tower geometry requires outriggers and belt trusses for lateral system reinforcement?
Rainier Square: The core is nominally 40’ wide by 93’ long. For a 850 foot tall building, this results in a core to building height aspect ratio of approximately 21:1 and 9:1 respectively. The ratio of 21:1 is far to slender for a building of this height and lateral demands. In order to supplement the core, the outriggers engage columns on the perimeter of the floor plate thus greatly increasing the ‘building stance’ resisting the lateral demands and lowering the lateral system to building height ratio to approximately 8:1.
General: A modeling software package, such as Autodesk Revit, is effective in documenting a SpeedCore project.
Rainier Square: Autodesk Revit was used for Rainier Square.
A FEMA P695 study (a recommended methodology for reliably quantifying building system performance and response parameters for use in seismic design) is underway which is expected to provide more guidance, but is aiming to justify an R = 8 for this system which is a higher R factor than any other shear wall system of any material.
Rainier Square Tower: Though the system is expected to test at R = 8, R = 6.5 was used in the seismic analysis in Rainier Square. Though a seismic analysis was necessary, the building is governed by wind loading due to its rectangular floor plate. R = 6.5 is in accordance with ASCE 7-10 Table 12.2-1 for a Building Frame System: Steel and Composite Plate Shear Wall SFRS.
How was this system approved, was it through use of the 2010 Seismic Provisions or the 2016 Seismic Provisions?
Rainier Square: The jurisdictional approval was tied to a rigorous peer review process along with the application of a performance-based seismic design methodology. Both the 2010 Provisions as well as the 2016 Provisions (draft Provisions at the time of the design) were references to the design basis, but supplemented to existing and ongoing research and testing.
How does it work in fire?
"Exposure to fire: The faceplate that is exposed to fire loses strength and stiffness; however, similar to Reinforced Concrete (RC) construction, the bulk of the concrete and the opposite faceplate maintain good strength and stiffness. As such, Steel-Plate Composite (SC) structures have better fire resistance than pure steel structures. Fire resistance of SC walls is potentially lower than RC structures; however, the fire resistance is enhanced if a suitable protection coating is applied to the faceplate(s) or fire-resistant steel is used. The member strength and stiffness during fire-exposure is also improved by embedding another steel plate within the concrete, that is, between the faceplates. ANSI/AISC N690 does not currently cover SC walls with embedded steel plates." - Design Guide 32 §1.2.1(d)
Fire protection requirements can vary by jurisdiction and will need to be addressed by jurisdiction. There may be opportunity in the near future to utilize the use of performance-based methods to design fire protection through the use of Appendix E of the next edition of ASCE/SEI 7.
Rainier Square: Rainier Square Tower has currently detailed the walls to receive spray-applied cementitious fire protection on both sides of the wall.
Fire protection/rating is a next step in the design and research considerations for the system. Traditional fire testing associated with a prescriptive fire protection strategy is more likely to be replaced and/or supplemented with a performance-based approach.
How is it built?
What is the overall wall thickness in the Rainier Square Tower project? How thick are the plates? What are the material strengths?
Total Wall Thickness: 21 inches thick at the top - 45 inches thick at the base
- ASTM A1043 Grade 50, Fy = 50 ksi - 62 ksi*
- ½ inch thick throughout
- ¾ inch thick at localized high-demand regions
- Rod Fy = 55 ksi
- 1 inch diameter
- 12 inches on-center
What is the relative site fabrication (erector) intensity of this system when compared to other steel systems? How is the increased site work affect project schedule and cost?
It is expected that the erection crew would be larger than a conventionally framed building. As with any project, the crew size would be specific to the job and highly dependent on the number of picks required between the non-lateral system gravity steel framing as compared to the lateral system core elements. That being said, the reduction of all site labor is expected to be a less. The lack of site-placed temporary formwork and reinforcing bars has a significant impact on site labor intensity.
Rainier Square: For the case of Rainier Square, depending on location in the building, the steel erection crew size varies. It is estimated to be anywhere between 50% to 100% larger than the size required to erect just the gravity system for this project. The overall project schedule and cost was reduced significantly through the use of the SpeedCore system.
Are the plates able to be joined by field bolting or welding? What specifically is done to ensure proper fit-up in the field? Are there any special tolerance considerations?
Field welded or bolted connections can be developed for a particular project’s needs. Given that all the components of the core are pre-fabricated, a tight shop tolerance is needed to ensure proper fit up in the field for either connection type.
Rainier Square: The Rainier Square project utilizes a field splice weld capable of developing the tensile strength of the faceplates.
Specially trained staff or specific machinery are not required for fabricating a SpeedCore module. As with any larger fabricated element, a fabricator would need to coordinate adequate shop working areas and crane capacities.
In the Rainier Square Tower project, where are the modules spliced? Where are the concrete cold joints in the system?
As depicted in the rendering of the SpeedCore system, the splices are made at approximately +4'-0" above finished floor, just as a column would be spliced. The concrete contractor plans to fill the walls on a floor-by-floor basis, roughly matching the elevation of the steel plate horizontal splices.
As with other construction aspects of the SpeedCore system, order of module erection and concrete placement should be evaluated on a project-by-project basis. Steel erection can advance up to four stories beyond the last concreted level per OSHA.
Rainier Square: The panels that comprise the core are erected in sequence with the columns. The panels are connected to the core boundary columns and erected one level at a time. Concrete is placed after the panels and columns have been erected, plumbed, and welded.
What qualities of concrete are necessary to achieve the desired behavior in concrete placement and structural behavior?
Rainier Square: 10 ksi concrete strength was required by design; however the cylinder breaks are coming in at 14-15 ksi. Concrete strength and stiffness were integral to the design of the system based on the lateral demands of the building. In terms of placement, depending on the planned methods for placement, a highly flowable mix that can adequately consolidate into all voids within the panels is required.
Architects are often looking to use high recycled content in their building materials. Given the highly variable material properties for recycled aggregate, it’s not recommended to use recycled aggregate in the concrete mix for critical lateral systems that are stiffness controlled.
Similar to the coordination efforts required for a conventional reinforced concrete walls, utility penetrations are coordinated in the design phase. In this system, the openings are incorporated into the pre-fabrication of the panels. Utilities are run perpendicular through the plates and concrete infill. The lack of internal reinforcing in SpeedCore should make the accommodation of utilities less of a challenge than placing them in a conventional reinforced concrete shear wall.
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