Double-deck lifts – a fleeting fad or the way of the future?

Author : Alan Cronin, director at Hilson Moran

19 August 2015

The double-deck lift, first installed in the early 1930s, is seeing resurgence as our cities look to the sky in search of living and workspace

Almost 60 years ago, Frank Lloyd Wright proposed the Mile High Illinois, a skyscraper that would tower 528 storeys above the earth. Such a structure would have required impressive vertical transportation, and Wright suggested five-deck elevators. The Illinois was never built and we’ve not seen triple-deck elevators, let alone five. However, the double-deck lift, first installed in the early 1930s, is seeing resurgence as our cities look to the sky in search of living and workspace.

The attraction of the double-deck lift is clear for the architects and engineers involved in the world’s tallest buildings, given the number of people that must be transported. But, historically a double-deck passenger lift wouldn’t have been considered in buildings less than 40 storeys in height; and in Europe, they’ve largely been used as shuttle lifts, limited to carrying goods and service staff.

So, what’s changed?
The 40-storey minimum standard before introducing double-deck lifts was largely down to the user experience. As a double-deck lift is simply two lift cars one atop the other, serving two vertically adjacent floors simultaneously, this often meant an odd floor/even floor service and a number of ghost stops where one car has passengers to unload or pick up, while the other is forced to sit and wait.

Destination control systems have now overcome this problem, making the double-deck a real option for buildings with fewer than 40 floors. Destination control is a simple concept, but the maths behind successful delivery isn’t – the system is programmed according to the physical environment and typical peak passenger demand. Then it groups passengers according to their destination, directing them to the most appropriate lift car. There are no controls in the lift itself – emergency-related buttons aside – and passengers enter their destination at a keypad or control screen in the lobby or on the floor they’re travelling from.

The destination control system is constantly monitoring the number of calls from each floor and the final destinations selected and, using proven formulae, works out which is the ideal car for each individual waiting passenger. The traditional odd/even deck number strategy still applies during peak traffic times, but off-peak the lifts can be programmed to call in a different sequence.

The result is greatly reduced average travel times for every passenger, and a much larger number of people being moved around at the same time. In some cases, you’ll see longer waiting times, because you’re not just jumping in the next lift to emit a ping and throw open its doors. And you won’t see the need for half the number of lifts, just because you have two lifts travelling together. Every building will be different, so the calculations for usage and space requirements need running carefully for every single project.

The impact on layout
So, if doubling theoretical capacity by moving from single deck to double deck doesn’t automatically halve the number of lifts – or indeed shafts – required, what are the potential impacts on layout and available space?

The good news is that, in most cases, double-deck elevators can result in a reduced number of lift wells, which means more lettable/saleable space on most floors – good news for developers and landlords. That’s a long-term gain that will offset and, in time, outweigh the greater initial financial cost associated with double-deck installations. And it’s worth remembering that double-deck cars driven by destination control can be more energy-efficient than traditional single-deck lifts, simply because of fewer journeys – which means reduced operating costs and environmental impact. Larger machines driving them mean more energy consumption, but there are fewer machines.

Here’s where you’ll see some critical differences in layout design, though:
A slightly larger plan area compared to conventional lifts, due to the increased sling around the lift car

Comparable pit depths and headroom – unless an upper car needs to serve the lowest floor or the lower car the highest floor – in which case those dimensions will need to increase dramatically

Double-deck lifts require a two-level lobby arrangement. Escalators and/or a single lift, to ensure all personal physical mobility issues are overcome, need to be factored in for travel between those two levels

Wayfinding needs to be thought through early and painstakingly; passengers need to be making their direction decisions intuitively, so signage and lobby layout must guide them flawlessly. Building security needs to be carefully coordinated too

Constant floor-to-floor heights for levels served by double-deck lifts

Minimum floor-to-floor distances of around 3750mm means lift car heights need to be at least 2400mm as there needs to be access between each car for maintenance

A double-deck lift needs a larger lift machine. So the machine room gets bigger and usually higher. And depending on positioning, the lift machine might need to occupy two levels.

And what about engineering and construction?
It goes without saying that in any engineering or construction project, the more information the project team is considering at concept design stage, the better informed the initial designs. With double-deck lifts, that early consideration is absolutely essential:

Floor-to-floor heights are genuinely critical. Disabled access codes require a level tolerance of +/-5mm for a lift. There’s no room for even the smallest of errors here
All the equipment involved is heavier. That means greater loads on the structure than conventional lifts

Larger motors are needed to drive larger, heavier lifts. That means an increase in electrical loads and heat output.

Are double-deck lifts worth the investment?
Judging by the number of planned double-deck lift installations in the pipeline in the UK and across Europe, the benefits are definitely being realised. As ever, it’s all about the numbers. And the options aren’t restricted to double-deck systems either.

There’s also the super double-deck, where the gap between the upper and lower cars can be extended to allow the cars to serve floors with differing heights. And Thyssen’s TWIN product puts two independently operated lift cars in the same lift well.

Destination control systems have made double-deck lifts a realistic, affordable option for a much greater variety of buildings. The greater stress on the building structure is offset by the reduced stress for peak-time passengers, the additional usable space within the building and the increased commercial value. But until someone takes on Frank Lloyd Wright’s mile-high monster, two decks is all we foresee for the time being….