40 Broadway: a focus on our feature staircase

Austin Young, our Structural Design Engineer, has put a spotlight on the self-supporting steel staircase we’ve designed for 40 Broadway, the 115,000 sq. ft commercial scheme in St James’s Park, Westminster:

Walking out of St. James Park Underground Station into the spacious reception of 40 Broadway will be a visual delight thanks to the stunning double height lobby and self-supporting steel staircase flowing from ground to first floor. Designed as an eye-catching feature, steel stringers will act as both the structural framework and the architectural balustrade for this sculpture-like stairway.

Creating the conceptual design

Due to the structure being exposed, it has been conceptually designed in a way that suits architectural constraints. The completed look is a supple, continuous ‘ribbon’. However, being too large to be brought to site in one piece, it has been designed as two seamless segments. Final assembly will be on site with welded joints, while the connections will be full penetration prepped butt welds. These are to be cut down and then coated in the desired finish to give the effect of this continuous strip of metal. Midway, a refuge area will be concealed within the depth of the stringer, appearing as though there is a constant incline from the outside.

Detailed REVIT design of the feature staircase for 40 Broadway

Finite element analysis has been used to ensure suitable stresses within the steelwork design and to understand the behaviour of the structure. One of the biggest complexities is ensuring comfortability whilst the stair is in use. Significant deflections or vibrations will make the user feel uneasy, especially as there is not a traditional support. A bespoke connection has been conceptualised to overcome this.

Lifting bolts at the point the stringers meet the first floor will be engaged to pre-stress the staircase before being locked in place. By raising the top by approximately 10mm during installation, movements are reduced as the structure is effectively ‘stiffened’. By stretching the staircase initially, it becomes harder to move. Consequently, this increases user comfort without increasing the thickness of the steel stringers required.

Imagine stretching a spring – the further it is stretched the harder it is to extend.

Detailed REVIT design of the feature staircase for 40 Broadway

This feature is part of a combined system consisting of the staircase and supporting steelwork and slab. Therefore, the supports need to be sufficiently rigid to limit their own deflections as ‘lifting’ the staircase up will also pull down the support. Deflections will be controlled in two ways. Firstly, the staircase will land on cantilevered box sections with backspans beyond the main supporting beam and openings to allow them to be filled with concrete. Secondly, the main supporting beam will be a composite steel and concrete beam where the edge of the slab wraps around the side of the beam.

We’re looking forward to seeing this design in fruition – it’ll certainly give us a spring in our step.

For further information on our involvement in 40 Broadway, click here.