ctbuh.org/papers Title: Bridging the gap: An analysis of proposed evacuation links at height in the World Trade Centre design competition entries Authors: Antony Wood, Professor, University of Nottingham Philip Oldfield, Professor, University of Nottingham Subjects: Building Case Study Fire & Safety Keywords: Skybridges Urban Design Publication Date: 2005 Original Publication: CTBUH New York 7th World Congress 2005 Paper Type: 1. Book chapter/Part chapter 2. Journal paper 3. Conference proceeding 4. Unpublished conference paper 5. Magazine article 6. Unpublished © Council on Tall Buildings and Urban Habitat / Antony Wood; Philip Oldfield © 2007 University of Sydney. All rights reserved. Architectural Science Review www.arch.usyd.edu.au/asr Volume 50.2, pp xx-xx Bridging the Gap: An Analysis of Proposed Evacuation Links at Height in the World Trade Centre Design Competition Entries Antony Wood† and Philip Oldfield School of the Built Environment, University of Nottingham, Nottingham NG7 2RD, England † Corresponding Author: Tel: (00 44) 115 9513111; Fax: (00 44) 115 9513159; E-mail: [email protected] Received 5 October 2006; accepted 16 February 2007 Abstract: The 9/11 collapse of the World Trade Centre Towers has created the largest single retrospective analysis of tall building de- sign in the past 40 years. In no field is this more relevant than in the field of evacuation. The case for an ‘alternative’ design solution for tall building evacuation – allowing horizontal evacuation at height through skybridge linkages – has already been made. However, to many people – and despite the real advantages as exemplified in built examples such as the Petronas Towers – the skybridge seems a purely fantastical proposition, with no relevance beyond isolated, one-off scenarios. This paper analyses the skybridge links of the numerous re-design proposals for the World Trade Centre Towers, with the aim of establishing their advantages and how they could be incorporated into tall building design in the future. Keywords: Tall buildings, Design, Evacuation, Skybridges Introduction The terrorist attacks on September 11th 2001, resulting in the New York and New Jersey (Stephens, 2004), five of the designs collapse of the World Trade Centre towers have arguably created proposed include some form of linkage at height between towers. the largest single retrospective analysis of the design of tall build- In addition, many of the ‘unofficial’ proposals also incorporated ings in the past 40 years. All aspects of tall building design – safety some form of skybridge. systems, structural systems, façade materials, siting and layout – are What all these proposals – and especially the five of the seven being questioned, and significant research has and continues to ‘official’ submissions – show is that, far from the idea of the be undertaken in the quest to validate and improve the viability skybridge being purely a fictional notion, it is actually a very real of the high rise. solution for improved evacuation efficiency. Evacuation efficiency In no field is this more relevant than in the field of tall building now is at the cutting edge of new tall building design. This paper evacuation. Significant advances have been made in investigating analyses the skybridge connections in each of the proposals, before the opportunities for the improvement of existing systems such as drawing conclusions on the role of physical connections at height stairs, and the viability of ‘new’ systems, e.g. the use of elevators in tall building design. in the event of fire (CTBUH, 2004). Another of these ‘alterna- tive’ systems is the use of horizontal evacuation at height through Background to the World Trade Centre ‘skybridge’ linkages between towers. The case for the skybridge, Design Replacement Proposals including the historical precedent and study of tall buildings cur- rently employing skybridges, has already been made (Wood, 2003). The many varied proposals for the replacement of the World Trade However, to many people – and despite the very real advantages Centre (WTC) Towers in New York mark an important watershed as exemplified in built examples such as the Petronas Towers – the in the evolution of tall building design. In the same way that the skybridge seems a proposition from the realms of fantasy, with no Chicago Tribune Tower competition of 1922 (Tigerman, 1981) is relevance beyond isolated, one-off scenarios. established as a seminal snapshot of tall building design thinking It is interesting then that, of the seven official proposals for the internationally at the time, so will the year of frenzied design activity World Trade Centre re-design competition sponsored by the Lower following September 11th, 2001 be viewed as a ‘stock take’ of early Manhattan Development Corporation and the Port Authority of 21st Century design thought in the field of the high-rise. 2 Architectural Science Review Volume 50, Number 2, June 2007 For the purpose of this study, it is necessary to categorise the conferences, community facilities and skylobbies. This approach many thousands of design proposals for the WTC replacement is far from unique in tall building design and is utilised in many submitted, to place the case studies examined in this paper in context. built examples which are often arch-like in shape so as to minimise The design proposals can be categorised into three areas: the tower’s footprint whilst still incorporating larger floor areas (i) The ‘Official’ entries, as managed by the Lower Manhattan above (examples include the Shanghai Stock Exchange Building by Development Corporation and the Port Authority of New WZMH Architects and La Grande Arche de La Défense by Johann York and New Jersey (seven team entries, first submitted in Otto von Spreckelsen). The obvious and more commonly used December 2002 – see Stephens, 2004). alternative to this solution is to construct the larger floor plates as (ii) The ‘Unofficial’ entries, as organised by organisations such a podium below the towers – a structurally simpler and cheaper as New York Magazine (September 2002), New York Times option, but at the expense of the open public realm. Magazine (September 2002) and the Max Protech Exhibition One could imagine that the vast size of the super floors in Meier (January 2002): “A New World Trade Centre: Design Propos- et al.,’s proposal, the full width of the building, may actually have als” (Protech, 2002). a detrimental effect on this extra ground floor area, in a manner (iii) ‘Independent’ proposals created by individuals / or- similar to the numerous elevated concrete highways around the ganisations but outside any ‘set’ competition or organising world, cutting out the light and limiting the view from beneath structure. them. However, the lowest of the super floors occurs at over For reasons of quality/depth of subject, the emphasis of this 79 meters above the ground, much higher than typical elevated paper is on category (i) above. roads, so in fact below them monumental gateways are created, directing pedestrian flow into the site, whilst above them – on Official Proposals top of the skybridges – terraced skygardens offer recreation and views out to the city. Team Peter Eisenman, Charles Gwathmey, Each horizontal connecting element is three storeys in height, Steven Holl and Richard Meier. with many of the floor plates planned simply as an extension of the open-plan office space that is present in the vertical elements Perhaps the most obvious skybridges in any of the official of the design. This offers a more varied range of tenant occupancy competition entries were those proposed by the ‘New York’ team programmes than available in typical towers, where one is often led by Richard Meier et al. The design created two grid-like restricted to occupying a series of floors in vertical sections. structures perpendicular to each other, the first grid composed “A tenant could occupy a typical horizontal floor plate or lease of three vertical towers and four levels of horizontal connecting spaces that are arranged vertically, or staggered, or even looped elements running north-south along the site, the second grid of around one of the voids.” (Nordenson and Riley, 2003) two vertical towers orientated east-west, with the same four levels The top and second-from-bottom horizontal elements incor- of horizontal connecting elements (see Figure 1). porate skylobbies where horizontal non-emergency circulation In this scheme large areas of the World Trade Centre site between towers is possible. Double-decker express lifts would is designed as external public space as the five towers’ foot- transport occupants from the ground floor to these large open prints only occupy about a quarter of the site. However, this small skylobbies where occupants can move into adjacent towers to lo- urban footprint has not restricted the inclusion of large open floor cal elevators. However, this incorporation of skylobbies and new areas at higher levels, as the horizontal skybridge elements of the tenant arrangements raises the important question of security as scheme are super floors. These large linear floor plates, spanning the horizontal circulation from adjacent towers means the entry between the vertical towers, offer space suitable for trading floors, of occupants to any particular tower is no longer restricted to the ground plane. Since the skylobby level also contains public facilities there would be the need for reception desks and security barriers between the express lift lobbies (that provide vertical transportation to the public areas) and the local lift lobbies (that provide vertical transportation to the private office floors). Within the two sky- links that do not contain a skylobby, at least one of the three floors would also need to incorporate some form of protected corridor connecting the cores, allowing for a horizontal escape route, whilst still securing the surrounding office environment. In terms of impact on evacuation, this arrangement of vertical towers and horizontal skybridges offers a multitude of evacuation routes, thus improving evacuation efficiency significantly.