Willis Towers Watson Feature Stair
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Learn about the new feature stair in Willis Towers Watson's expanded tenant space in Willis Tower. An overview of the design challenges, such as building constraints, stair response to human excitation, and construction details, is discussed. Presented at the SEAOI 2017 May Dinner Meeting. The project is a finalist in the 2017 Excellence in Structural Engineering Awards.
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Willis Towers Watson Feature Stair
- 1. John Sweeney, Verderame | Cale Willis Towers Watson Feature Stair Eric Wheeler, S.E. Associate
- 2. John Sweeney, Verderame | Cale Original Design Intent
- 3. Design Constraints – Existing Building Conditions
- 4. Design Constraints – Existing Building Conditions New connection at panel point New slab opening
- 5. Design Constraints – Existing Building Conditions Existing truss below New stair load on slab
- 6. Analytical Model – Natural Frequency
- 7. Analytical Model – Computed Acceleration Response
- 8. Stair Construction Tube header to existing 19th Floor truss H- frame Tube header Existin g truss below
- 9. Tube Header to Existing 19th Floor Truss Detail Coordinated Connection Details Tube Header Exist truss diagonal Exist truss chord Torsional load Torsional load resolved into force couple
Editor's Notes
- Willis Towers Watson wanted their office expansion at Willis Tower to include a feature stair that made a statement when someone entered their space. The concept for the stair, developed by Verderam Cale Architects, is shown on this slide. It is a 1-story, steel stair with central stinger that wraps around an architectural element. Our challenge: Understand the constraints of the existing building on the stair design Design the stair so it’s insensitive to human induced vibration – people walking up and down the stairs Develop details that accommodate the stair loads and the existing Willis Tower framing
- We needed to understand the building constraints – existing framing or tenant occupied floors – before moving forward with the design. The existing floor framing in the tower consists primarily of composite steel trusses spaced at 15’ oc. They span 75’ between built-up columns. The trusses are typically 40” deep with tee sections for top and bottom chords and double angles for diagonals. We needed our new framing for the floor opening and stair support to frame into the truss panel points to facilitate a connection (dashed yellow line). We also recognized the existing slab would require reinforcing to support the new stair loads. Since the existing floor below is occupied, installing a new beam below the floor slab was not possible. A new frame would need to be installed to span over the slab and bear down on the slab at the truss locations below. So early in the project, we negotiated with the architect: To shift the stair layout so our connections align with the existing truss panel points Provide a podium to hide the steel framing that would need to go above the 18th floor slab. The framing spans above the 18th floor slab and bears down at the truss locations.
- We needed to understand the building constraints – existing framing or tenant occupied floors – before moving forward with the design. The existing floor framing in the tower consists primarily of composite steel trusses spaced at 15’ oc. They span 75’ between built-up columns. The trusses are typically 40” deep with tee sections for top and bottom chords and double angles for diagonals. We needed our new framing for the floor opening and stair support to frame into the truss panel points to facilitate a connection (dashed yellow line). We also recognized the existing slab would require reinforcing to support the new stair loads. Since the existing floor below is occupied, installing a new beam below the floor slab was not possible. A new frame would need to be installed to span over the slab and bear down on the slab at the truss locations below. So early in the project, we negotiated with the architect: To shift the stair layout so our connections align with the existing truss panel points Provide a podium to hide the steel framing that would need to go above the 18th floor slab. The framing spans above the 18th floor slab and bears down at the truss locations.
- We needed to understand the building constraints – existing framing or tenant occupied floors – before moving forward with the design. The existing floor framing in the tower consists primarily of composite steel trusses spaced at 15’ oc. They span 75’ between built-up columns. The trusses are typically 40” deep with tee sections for top and bottom chords and double angles for diagonals. We needed our new framing for the floor opening and stair support to frame into the truss panel points to facilitate a connection (dashed yellow line). We also recognized the existing slab would require reinforcing to support the new stair loads. Since the existing floor below is occupied, installing a new beam below the floor slab was not possible. A new frame would need to be installed to span over the slab and bear down on the slab at the truss locations below. So early in the project, we negotiated with the architect: To shift the stair layout so our connections align with the existing truss panel points Provide a podium to hide the steel framing that would need to go above the 18th floor slab. The framing spans above the 18th floor slab and bears down at the truss locations.
- We designed the stair to maintain a natural frequency above 5Hz, so human movements, typically between 2.5 Hz and 4 Hz, cannot excite the stair’s main modes of vibration. This also ensures that the stair response remains within an acceptable range. Since the natural frequency is sensitive to boundary conditions, we represented the stair supports as springs to account for the softness of the truss. If not, the software would give higher natural frequencies that would be unrealistic. Our final stair design had a natural frequency of 8.68 Hz.
- Once we established an acceptable natural frequency for the stair system, we checked that the stair’s response to various step frequencies were within an acceptable range. We applied a harmonic forcing function based on the step frequency to the model to predict the vertical acceleration response of the stair. We varied the step frequency to represent a person walking normally or rapidly down a stair. The design range for the stair was limited to a step frequency between 1 and 4 Hz. The upper bound was limited to 4 Hz, since it is impossible for a person to descend the stairs at rate higher than that. For a single walker descending normally, the step frequency was less than 2.5 Hz For a single walker descending rapidly, the step frequency used varied between 2.5 and 4 Hz This slide shows a chart of the acceleration response at the stair upper landing vs the step forcing function. The stair upper landing generated the highest acceleration responses. The blue line represents the computed acceleration response at the stair upper landing due to the step forcing function. The dashed green line represents that acceleration limit for non-perceptible stair movement. The lower dashed is the acceleration limit for a single person descending normally down the stair, or 1.2 %g. The upper dashed line is the acceleration limit for a single walker descending rapidly down the stair, or 2.1 %g. As shown, the stair response was within acceptable limits
- The following slide shows stair construction near completion. Starting at the bottom of slide, the picture shows the H-frame that spans above the existing floor slab and bears down only at the truss locations below. The existing angle connection in the center of the H-frame just transfers horizontal shear to the diaphragm. Moving to the top of the image is the tube header that receives the stair stringer. The header frames into the existing 19th floor truss that is highlighted on the next slide.
- We developed a connection detail for tube header to the existing truss to fit between the existing diagonals while also transferring the design forces to the existing truss. The torsional forces within the tube header were resolved into a force couple and transferred to the trusses top and bottom chords.
- The following slide is a comparison of the concept sketch vs the final stair construction. Thank you!