SFPE ENGINEERING GUIDE ON FIRE SAFETY FOR VERY TALL BUILDINGS: What’s New with the 2nd Edition?
By: JAMES QUITER, PE, FSFPE
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In the first quarter of 2022 SFPE published the 2nd Edition of its Engineering Guide: Fire Safety for Very Tall Buildings. The 2nd edition was published by SFPEs publishing partner Springer and can be found at https://link.springer.com/book/10.1007/978-3-030-79014-1?noAccess=true&page=2.
The Guide is intended to provoke thought in designing very tall buildings. It provides background, some code information, and engineering approaches that can be considered in the design of a very tall building.
The Guide very intentionally avoids prescriptive guidance because every tall building is different. Codes cannot adequately address the unique features of these buildings. They must be truly engineered, not approached with a cookbook or checklist solutions. Therefore, the Guide was written to avoid being adopted as Code, but instead provides options and things to think about.
Similarly, the Guide does not define a very tall building. It is up to the user to decide what comprises a very tall building and when to apply some of the concepts from the book. Codes are written around arbitrary thresholds. Good engineering should be performed using the principles of science. Whether some or all of the concepts discussed are appropriate for a very tall building will depend upon the features of the building, with height only one of the variables under consideration.
So, why prepare a second edition? There are two primary reasons: First, some of the content was outdated — either because new technology is available or because of events that have transpired. Second, as with any first edition, some areas were not covered or needed minor revisions. The remainder of this article discusses those changes.
Areas with major changes include the chapters on History, Situation Awareness, Emergency Egress, Fire Resistance, and Building Envelope (renamed from Facades). New chapters or sections were written on Special Features and Attractions, Energy Storage Systems, Existing Buildings, and Aerial Landing areas.
Several significant high-rise fires have occurred since the first edition of the Guide. Most of the fires included combustible facades. Brief descriptions of fires in Dubai, Australia, and the United Kingdom as well as an earlier fire in Shanghai have been added to the history section.
Situational awareness is perhaps the fastest-changing aspect of fires in very tall buildings. This is due to a variety of reasons. First, sensing equipment is changing and more information is available to responders (and therefore, perhaps, to occupants). This may include cameras — which are being used more widely and are becoming ever-more affordable — security devices, and, of course, smart fire alarm systems.
More important, social messaging like smartphone, texts, and other devices are used more and more by people during major events. The use of these devices needs to be considered as part of the overall plan for the building — either how they can be used, or how they can positively or negatively impact the approach. This area is rapidly changing, so designers and owners of the building will need to consider it as the building is designed, as well as periodically during the life of the building.
While most of the emergency egress chapter has remained the same, there have been several enhancements. New content discusses several key strategies:
- Explanation of evacuation times, what they mean, and how they can be compared. This also includes further discussion on how evacuation models are applied to very tall buildings and how they can be used to compare exit strategies. Some evacuation models have advanced significantly since the First Edition, so greater use and reliance can be placed on them.
- Discussion of what needs to go into an elevator egress system. When the First Edition was published, the US Code provisions on use of elevators for evacuation were being developed. They are now a part of the US Codes and several buildings around the world have been built and commissioned with elevators serving as part of the egress system. In some cases, they have been used in lieu of normally required exit stairs, and in other cases, as part of a series of trade-offs involving other fire protection features. Either way, the use of elevators as part of the egress system requires a great deal of thought, pre-planning, work with authorities, and education of occupants.
- Discussion on defending-in-place strategies. Theories on appropriate exit strategies in very tall buildings have abounded over the last few decades. As buildings got taller, it gradually became evident that partial evacuation was needed in lieu of full evacuation — at least in most events. On the other hand, the World Trade Center incident taught us that full evacuation may still sometimes be needed. As fire departments become more familiar with fires in these very tall buildings, they are also finding that for a well-protected and compartmented building, defending in place may be the best preliminary strategy.
Whichever strategies are used requires education and notification of the occupants, pre-fire drilling with the authorities, and cooperation by building ownership and management. These topics are all addressed in the Second Edition.
While there have been no substantial changes to what was included in the structural chapter, there have been some substantial additions. The primary addition is a discussion on the performance-based approached to structural design as opposed to prescriptive-based. In most cases, for a very tall building, a performance-based approach will yield a more robust design —often, for less money. A variety of approaches and models can be used in a performance-based approach (for instance, single element analysis versus more advanced methods), but all of them require input regarding fire size, fire scenarios, gravity, lateral and live loads, and cooling period. This will result in a structural system designed for the exposure rather than one based on a series of individual elements subjected to a fire test using a time-temperature curve developed 100 years ago.
The Guide now discusses the limitation of the prescriptive approach and compares it to a performance approach. It also describes systems for reducing drift and accelerations due to wind.
In addition to performance-based approaches, the Guide now discusses several of the features that may drive the use of such approaches. These include complex atria, either on lower floors or high in the building; large open floor plates; long structural spans; complex structural systems or geometries; or large structural members.
Also addressed are changes in materials that may be used in very tall buildings. This includes very-high-strength concrete and the use of mass timber. Though mass timber is not yet approved in most codes for very tall buildings, efforts are underway across the globe to extend its use and to develop mass timber products with fire resistance suitable for taller buildings. While the use of mass timber could (and probably will) generate its own engineering guide, it was important to address it in this Guide, along with some considerations connected with its use.
Lastly, the fire resistance chapter has added information about consideration for resilience and robustness of the structural systems. Typical codes are primarily written around life safety, so avoidance of collapse is a primary goal of the structural system. However, owners and designers should recognize what will happen after a fire. While the building may still be standing, is it re-useable? If something worse than the expected fire occurs, what happens? These are topics for discussion at the design stage, so that the stakeholders can decide and understand the resilience and robustness of the building.
Perhaps the most significant changes to the Guide have been made to what was formerly the chapter on facades. It has now been retitled Building Envelope and significantly expanded.
Many of the high-rise fires across the globe since the publication of the First Edition have been greatly impacted by the building envelope. Several of these fires are discussed in the revisions to the History chapter. They have all been characterized by spectacular images broadcast around the world showing major damage and, in some cases, loss of life.
One means of determining relative risk of a facade system has been developed by NFPA and is available on its website. The tool is called EFFECT (Exterior Façade Fire Evaluation and Comparison Tool). The Guide describes this tool and how it may be used.
More broadly, there is an expanded description of facade types and some risks they may pose. These include curtain walls, double-skin facades, and built-up walls with or without a cavity. Once the wall type is known, fires must be considered from an internal source, an external source, or a source inside the cavity. Once there is ignition, the spread is considered and will be very different depending on the material chosen and how it is constructed. Will it spread up the side of the building? Will it spread into the building on one or more floors? Will the building systems be able to control interior spread? Are there other buildings exposed that may be susceptible? These cases are all discussed in the Guide.
Of course, all of these depend on the material being used. If combustibles are contained within the exterior façade, a variety of fire tests may apply. The Guide discusses these tests for both facades and roofs and describes some considerations to discuss prior to their use. Those may include adequacy of the test, behavior of the material, quality of construction, and rigor of the approval and inspection process.
In addition, the Building Envelope chapter also discusses the use of exterior walls for LED display screens, other signage, photo-voltaic systems, and green walls. All of these uses are increasing substantially in major cities across the globe
Other New Chapters
Several other new chapters feature unique and very tall buildings often covered by other codes and standards. A new chapter addresses Special Features and Attractions. As tall buildings proliferate, so do the number of interesting attractions that may be on the top of them. This may include restaurants and bars, observation decks, pools, amusement rides, thrill rides, or other such features. Each of these features must consider egress, life safety systems, fall hazards, first responder safety and effectiveness, and the effect on other parts of the building. The Guide also addresses use of the top of the building as a base for fireworks shows.
Another topic covered for the first time is energy storage systems. This concept and application is not unique to very tall buildings, but such systems are certainly more likely to be included in them and need to be considered for fire risk and exposure. The section mostly references other developing codes and standards.
A new chapter has been added to address existing buildings. Many very tall buildings were built prior to modern technology and approaches. Retroactive improvements have primarily been required on a city by city (if not building by building) basis. This new chapter addresses key things to consider in an existing building and also discusses scoring systems used to evaluate protection features.
Finally, the Guide includes an aerial landing access discussion. Many very tall buildings have such facilities either due to local code requirements or due to operational needs. It is likely such uses will increase with drones, aerial taxis, and other such devices. This section of the Guide addresses egress, fire equipment, and first responder issues for the various uses that may occur on the top of the building.
James Quiter, PE, FSFPE, Retired-Arup