structure design is also evident in the construction of the current tallest skyscraper, the Burj Khalifa. The elevator conundrum The invention of the elevator was a precondition for the invention of skyscrapers, given that most people would not (or could not) climb more than a few flights of stairs at a time. The elevators in a skyscraper are not simply a necessary utility like running water and electricity, but are in fact closely related to the design of the whole structure. A taller building requires more elevators to service the additional floors, but the elevator shafts consume valuable floor space. If the service core (which contains the elevator shafts) becomes too big, it can reduce the profitability of the building. Architects must therefore balance the value gained by adding height against the value lost to the expanding service core. Many tall buildings use elevators in a non-standard configuration to reduce their footprint. Buildings such as the former World Trade Center's Twin Towers and Chicago's John Hancock Center use sky lobbies, where express elevators take passengers to upper floors which serve as the base for local elevators. This allows architects and engineers to place elevator shafts on top of each other, saving space. Sky lobbies and express elevators take up a significant amount of space and add to the amount of time spent commuting between floors. Other buildings such as the Petronas Towers use double-deck elevators allowing more people to fit in a single elevator and reaching two floors at every stop. It is possible to use even more than two levels on an elevator although this has yet to be tried. The main problem with double-deck elevators is that they cause all elevator occupants to stop when only people on one level need to get off at a given floor. Another solution, employed by the Shanghai Tower and the under-construction (2019) Jeddah Tower is for buildings to be created for mixed-use, putting office space in the lower floors as it uses more floorspace. Multistory penthouses and atria—which require little cross-sectional floorspace—are placed towards the top. Other difficulties when building skyscrapers Building skyscrapers can be difficult due to factors other than complexity and cost. For example, in European cities like Paris, the difference between the appearance of old architecture and modern skyscrapers can make it hard to get approval from local authorities to construct new skyscrapers. Building skyscrapers in an old