Escape Hatch

I was involved with the Escape Hatch project from the moment it came in as an enquiry.  It was immediately clear that it would be a fascinating job, as the client wanted a “Tracy Island style concealed hatch that would raise automatically to reveal a fire escape. Following an initial trip to London to meet the client’s team and discuss their requirements, I put together our quotation and we set to work.

The client already had a general concept for the hatch, but the design of the surrounding building had moved on and further refinement was needed to find space for all the machinery.  I therefore spent the early part of the project assisting the client to clarify the design envelope and coming up with various schemes for the layout of the main parts.  I put together calculations to size the cylinders and pivots, formulated the overall structural scheme and sized the main structural elements.  I also carried out an initial design risk assessment for discussion with the client at our fortnightly meetings.  By its nature the concept raises a number of hazards that would not exist in other projects and we had to exercise great care in the design to make sure the risks were minimised.

With the conceptual design well established, we moved onto the detail design phase.  One of the main challenges of the design was the width restriction: in order to comply with fire regulations a 6m wide staircase was needed, but the walls on either side were already under construction.  This left us with only a very narrow envelope to fit in any structure, mechanism and hand-rails, particularly once construction tolerance of the concrete walls and structural deflections of the hatch had been taken into account.  I was able to solve the mechanical problem by configuring the cylinders horizontally under the stairs, which allowed them to sit further from the wall than if they were mounted vertically.  This allowed larger, lower pressure cylinders to be used and reduced trapping hazards on the stairway.  My colleague Will had the task of solving the hand-rail problem, and was able to devise an ingenious linkage that fits within the required width and sweeps up as the hatch rises to provide a Building-Regs-compliant hand rail.  The linkage means the hand-rail is driven by the movement of the hatch itself, and no secondary prime mover is required, which increases the reliability.  We also used a finite element analysis model to optimise the design of the structure.  A box section deck similar to a bridge was selected for the main part of the hatch, as its torsional rigidity would minimise any twist in the event that one of the two hydraulic cylinder failed to open. A diaphragm at each end of the box deck provided a strong location for the connection of the tubular lifting frames on either side, and the main pivot.  The deck needed to be as light as reasonably possible to keep the hydraulic cylinders and the reactions back into the building structure to a manageable level.

I then produced the design documentation, including the detailed manufacturing drawings, and detailed performance specifications for the hydraulic, electrical and control systems.  Generating the drawings also meant finalising the design of dozens of secondary details, such as drainage and edging strips.  Managing the interface with several different sub-contractors involved with different aspects of the building was a big challenge!  Finally, I assisted the client with the selection of the contractor to manufacture and install the hatch, and provided ongoing engineering support during the construction phase.

In many ways this was a dream project – designing a unique moving structure almost from scratch, with some cunning mechanical design, sophisticated structural analysis and being able to see the whole job through to completion.  Seeing the hatch move for the first time, with everything doing exactly what it was supposed to, was a really rewarding experience!