into the nightclub without paying. Buildings Local building codes will often dictate the number of fire exits required for a building of a given size. This may include specifying the number of stairs. For any building bigger than a private house, modern codes invariably specify at least two sets of stairs. Furthermore, such stairs must be completely separate from each other. Some architects meet this requirement by housing two stairs in a "double helix" configuration where two stairs occupy the same floor space, intertwined. For old buildings that predate modern fire code requirements and lack space for a second staircase, having intertwining stairs so close to each other may allow firefighters going up and evacuees going down to use separate staircases. Westfield Stratford City uses this configuration in the Upper car park. This part of the building has eight storeys: LG, G and 1 are part of the shopping centre; 2 has some offices and a storage area; CP1, CP2, CP3 and CP4 are a multi-storey car park. The floors are served by the main public lifts and escalators, and by 1 set of a double-helix stairway and lift per 1000 square metres, going into the service areas. The main public escalators don't count as fire exits, as the doors may be locked during less busy periods. The building, therefore, has one fire exit per 4000m2 of floor space. Knowing where the emergency exits are in buildings can save lives. Some buildings, such as schools, have fire drills to practice using emergency exits. Many disasters could have been prevented if people had known where fire escapes were and if emergency exits had not been blocked. For example, in the September 11, 2001, attacks on the World Trade Center, some of the emergency exits inside the building were inaccessible, while others were locked. In the Stardust Disaster and the 2006 Moscow hospital fire, the emergency exits were locked and most windows barred shut. In the case of the Station Nightclub, the premises was over capacity the night fire broke out, the front exit was not designed well (right outside the door, the concrete approach split 90 degrees and a railing ran along the edge), and an emergency exit swung inward, not outward as code requires. In many countries, it is required that all new commercial buildings include well-marked emergency exits. Older buildings must be retrofitted with fire escapes. In countries where