Paul Kassabian, PE, Paul Rosenstrauch, PE, Harry Dodakian, And Michael Chase 2023-12-18 21:59:46
Careful analysis and thoughtful steel details made a new monumental staircase an architectural and serviceability success.
MONUMENTAL STAIRCASES add a signature element to a building—but also an element of complexity.
Such a scenario can be found in Cambridge, Mass.: the three-story staircase in The Ragon Institute of Mass General, MIT, and Harvard’s soon-to-be-opened 323,000-sq.-ft headquarters. The stair posed several structural challenges, the most prominent being its triangular helix geometry and the base landing on structural steel framing rather than a slab on grade.
The new building will be a collaborative research center that brings together scientists, clinicians, and engineers from diverse fields to harness the immune system to combat and cure human disease. The stair will connect these various disciplines and foster collaboration.
The stair is comprised of three runs from the second to the fifth floor. Its geometry is a single helix that, when viewed from above or below in plan, forms a triangular shape. It is supported at one point of the triangle at each floor level. The interior and exterior stringers are ¾ in. thick by 5 ft deep ASTM A572 steel plates. The width of the stair is 5 ft, 2 in. Treads and risers are comprised of 3⁄8 in. steel plates supported by HSS4×4×¼ beams spanning between stringers and will be covered by a terrazzo finish. The stair will support glass guardrails at both stringers.
Stair and Building Structure
The stair’s structural interaction with the building structure was one of the major design challenges. The bottom of the stair starts at an elevated level, removing considerable stiffness associated with stairs bearing on grade. The stair is anchored into each floor level at the middle of the floor plate within 35 ft column bays. The floor construction is 3¼-in. lightweight concrete slab on 2-in. composite metal deck—40% less stiff than a typical 6¼-in. total depth composite deck.
Even though the stringers are 5 ft deep, the structure depth for connections was limited to 16 in. due to architectural constraints. Each stair anchorage point at Levels 2 through 4 is cantilevered 13 ft, placing base building support beams in negative flexure and removing the stiffness benefit of their composite action. The Level 5 cantilever is more pronounced and extends 18 ft from the girder that spans between building columns.
Vibration
Although the stair stringer’s 5 ft depth is significant, analysis early in the design process indicated that vibrations of the stair-and-building system would drive the design. The building’s overall flexibility and the location of the stair mass within the building bay compounded the vibration challenges.
During the design phase, the team consulted AISC Facts for Steel Buildings Number 5 (aisc.org/facts) and AISC Design Guide 11: Vibrations of Steel-Framed Structural Systems Due to Human Activity (aisc.org/dg), both of which recommend a vertical frequency minimum of 5 Hz for monumental stairs. Modal analysis of framing local to the monumental stair indicated the building could achieve a natural frequency of 6.25 Hz.
However, the stair as originally designed was attached with bolted shear connections (standard for a steel stringer stair) and could only achieve a fundamental frequency of just under 3 Hz—unsuitable for user comfort when combined with the large associated period. In response, the design team explored options to stiffen the system and achieve a fundamental frequency that exceeded 5 Hz.
The first step in stiffening was to update the spandrel beams within the atrium from W16×26 to HSS16×12×3⁄8 sections, which could resist torsional moments. The Simpson Gumpertz & Heger (SGH) design team modeled rigid attachments to these spandrels, which led to some beneficial stiffening but did not sufficiently raise the frequency. The team also proposed stiffening kickers, but head clearance and visibility constraints rendered it an unviable option.
The design team focused on two areas to solve the vibrations: the Level 5 support and the connections between the stairs and the structural frame.
The Level 5 support exhibited the most vibrational excitement in modal analysis. The structure supports the stair in this area with cantilevered W24×162 beams, which are large but still flexible given the cantilever distance. To stiffen the region, the design team extended the stair stringer along and parallel to these cantilevered beams, increasing the stiffness by a factor of 3.6 to 18,000 in.4
The plate continued to the edge of the floor deck, but the deck prevented it from continuing into the steel structure. A lower plate was added across the two W24×162 beams, stiffening the cantilevers 1.3 times to 6,600 in4. Incorporating these stiffness adjustments increased the frequency to 4.75 Hz.
At Levels 2 through 4, the design team incorporated fixity into the connection between the stair and the structure and added supplemental framing. The supplemental framing—made of additional cantilevers and backspans—provided additional rigidity at the connection points and helped engage the base building stiffness beyond the spandrel area with the stair connections.
To model the size and rigidity of the connections, the team used constraints in the analytical model to tie stringer shell element nodes to the supporting beam element nodes. The number of nodes selected for the constraints in the stair corresponded to the size of the base building beam. These modeling choices returned a fundamental frequency of 5.2 Hz.
The design team recognized that linear analysis would not capture the local behavior of thin plate elements, which can buckle under nonlinear instability. In response, the team performed a nonlinear buckling analysis to determine an alpha value: a number representing a multiple of the applied load at which the model becomes unstable. The loads applied included the self-weight of the stairs and terrazzo finish, the dead load of the handrail, and the 100 psf occupancy live load for assembly areas required by ASCE 7-10, Minimum Design Loads for Buildings and Other Structures.
Fabrication and Installation
Each of the three stair runs were fabricated in three individual segments. Each segment weighed approximately 4 tons, with nine total segments that resulted in 36 tons. Of the three segments, two end segments anchor directly into the base building at the lower floor and upper floor, respectively, and the middle segment anchors between the end segments.
Temporary bolted connections capable of transferring the stair segment’s self-weight were incorporated into the steel’s stringers to provide mechanical connections.
Crane time came at a premium, so fabricator DeAngelis Iron Work asked SGH to design a temporary rigging frame. SGH recognized the challenge of temporarily supporting the three tiers of the stair during erection. SGH and detailer Drafting One, LLC designed a temporary frame that would support the stair segments during erection and created a concept that erected and bolted the stair in place before fully welding the connections.
The stair was erected from Level 2 up to Level 5, lifting the segments sequentially from the crane. The first segment on the lower run was lifted and bolted to the base building at the low end and supported on the temporary frame at the other. The stair was not designed to support levels above, so the frame remained in place throughout the segment erections.
The temporary frame incorporated braces between levels and fixed moment connections around the stair, as the frame needed to be stable and accommodate the stair itself passing through it.
The final product is a self-supporting stair consisting of 36 tons of steel and 15 tons of terrazzo and other finishes. The stair adds a dramatic structural element to the new Ragon Institute building entrance and, just as importantly, showcases the collaborative achievement that can be realized when fabricators and engineers work closely with each other on unique projects.
Owner
The Ragon Institute
General Contractor
Consigli
Stair Design Engineer
Simpson Gumpertz & Heger, Inc. (SGH)
Architect
Payette
Structural Engineer
Arup
Steel Team
Fabricator and Erector
DeAngelis Iron Work, Inc.
Detailer
Drafting One, LLC
Paul Kassabian (PEKassabian@sgh.com) is a principal and Paul Rosenstrauch (PLRosenstrauch@sgh.com) is a project engineer, both with SGH. Michael Chase (mac@deangelisiron.com) is a project engineer and Harry Dodakian (hmd@deangelisiron.com) is a project executive, both with DeAngelis Iron Work.
©AISC. View All Articles.