The Twin Towers: 10 Years, 10 lessons on sustainable architecture

The history of technological evolution is filled with failures and the lessons learnt from them. Many will even claim that “design by disaster” is one of the most effective methods for progress. The World Trade Center is no exception. The collapse of the World Trade Center buildings had the potential to question the mere nature of tall buildings and mark tall building design in ways that we could have never anticipated. Nevertheless, tall building design in the last decade was not driven by September 11th 2001, but by a strong impetus towards sustainability and a thriving real estate market. The result has been an unprecedented growth in the number of tall buildings and unprecedented innovation driven by sustainability. The World Trade Center failures have not driven the evolution of tall buildings; nevertheless, a series of more subtle, but no less important lessons have emerged.

Lesson 1:

Sustainability is a managed life cycle where proactive decisions are made to reduce consumption and negative impact from the inception of a project to the disposal of the infrastructure. In those terms the World Trade Center collapses do not reflect the definition of sustainability. When addressing fire, the design of tall buildings needs to embrace these principles.

Lesson 2:

Fire safety is a social responsibility that guarantees the citizen a safe environment, thus it is an integral part of sustainable building design. This social responsibility has been historically translated into codes and standards that establish prescriptive requirements for buildings. These prescriptive requirements, if followed carefully, provide the minimum level of safety required by society.

The nature of codes and standards is responsive, thus their development is not a driver for construction technology but a reaction to the introduction of novel ideas. Because of this, there have been periods in which codes and standards had enough embedded knowledge that they could respond to all variants of construction innovation. In these periods, infrastructure can be comprehensively classified into some group that is fully addressed by a specific set of rules. Few exceptions appear outside the codes and standards and require individualised solutions. In periods of great urban or technological development, codes and standards cannot cope with the evolution imposed by the drivers of the construction industry. In these periods, codes and standards fall behind, standard solutions only concern few buildings and, in most cases, individualised solutions are necessary. Explicit definitions of safety produced using engineering tools need to complement codes and standards to provide individualised solutions.

The WTC epitomised innovation, and most technical solutions were evaluated using the most sophisticated engineering tools of the time. Fire safety was established in a purely prescriptive manner. The last decade has been a period of great innovation for tall buildings, thus fire safety cannot be based on a purely prescriptive analysis.

Lesson 3:

Sustainability requires building in the infrastructure’s life cycle through an optimised design process. The WTC did not optimise fire safety within the structural design process, thus it was not optimised correctly. The failure to understand the structural behaviour in fire resulted in disproportionate and unpredictable consequences. Tall buildings are by nature optimised structures, thus their design needs to incorporate fire safety as an integral design component.

Lesson 4:

Sustainability requires optimised utilisation of tools and resources. Problems need to be understood and resolved by means of the most adequate methodologies. The WTC demonstrated that, for tall buildings, egress and structural performance are the pillars on which fire safety stands. But for tall buildings, egress is of a similar time scale to the deterioration of the structure by the fire. Thus, both components of the fire safety strategy are coupled. Egress times can be reduced but, for tall buildings, they can never be made much shorter than structural failure times. Thus improvements in egress can be legislated through code requirements. In contrast, innovative structures need a proper engineering analysis, because they do not conform to standard practices but mainly because their integrity is the guarantee for safe egress. In the absence of an adequate structural design, enhanced egress capabilities cannot be used as compensation. The safety of tall buildings requires an explicit structural analysis conducted using state-of-the-art engineering tools.

Lesson 5:

Sustainability is about shared responsibility. WTC showed that the responsibility for integrating structural integrity to the fire safety strategy was undefined. While in a prescriptive environment the architect is responsible for the definition of thermal protection to the structure, nobody is responsible for the assurance that the structure will perform adequately. The implicit assumption that thermal protection is sufficient to guarantee safety was proven inadequate. For tall buildings, the structural engineer needs to assume responsibility for the adequate performance of the structure in a fire.

Lesson 6:

Innovation introduces complexity, thus the drive for sustainable tall buildings is introducing fundamental changes in structural design, material selection and potential fire conditions. The WTC demonstrated that to establish an adequate fire safety strategy, there was a need for professionals of great knowledge in all fields involved. Questions of competence emerged when the analysis of recognised professionals was put into question. What does a structural engineer need to know to be able to design a tall building that will be safe in a fire? What does a fire safety engineer need to know to be able to design a proper fire safety strategy for tall buildings? Sustainable tall buildings require the involvement of professionals competent in the design of a comprehensive fire safety strategy.

Lesson 7:

Throughout the WTC investigation, it became very clear that the framework educating professionals involved in the design of innovative buildings leaves a gap of knowledge when it comes to the assessment of safety. It is designed to operate within a prescriptive environment and does not incorporate the knowledge base necessary for engineering-based safety. If we are to continue to strive for sustainable infrastructure and the associated innovation, then we need to support the development of an educational framework that will enable our professionals to deliver safe infrastructure.

The current framework is not sustainable.

Lesson 8:

The WTC showed that we lack an adequate definition of competence. Our current definition of competence not only leaves enormous knowledge gaps, but also is structured around incorrect objectives. The development of sustainable tall buildings needs a definition of who is competent to deliver the engineered fire safety strategy.

Lesson 9:

Extracting from a failure all the knowledge that will enable professionals not to make the same mistakes requires a minimum level of prior understanding. The most successful investigations are those conducted in an atmosphere where all those involved have sufficient knowledge to make the most of the investigation and to transfer that new knowledge into the design process. In the past, fire investigations have been conducted in such an atmosphere. The unprecedented magnitude and novelty of the WTC failure caught the fire safety and structural communities unprepared for the investigation. Thus, over the last decade, these professional communities have produced the science to unveil many of the phenomena, but not to transform that knowledge into design methodologies and tools. The gaps of knowledge are now evident; thus the future of tall building design depends on our capability to continue filling these knowledge gaps at a pace faster than our capability to innovate.

Lesson 10:

The WTC showed that if we want sustainable tall buildings, we need to develop the knowledge base and the technological tools that can adequately assess the performance of a fire safety strategy. We need to incorporate this knowledge, not by legislating new rules, but through adequate professionals structured within a relevant definition of competence. We need to legislate competence, not standardised solutions.