Reducing the risk and mitigating the effects of fire in tall buildings
13 March 2014
Authors: Barbara Lane, director of fire engineering, Arup and Susan Lamont, associate director of fire engineering, Arup. Additional modelling team members: Graeme Flint, senior engineer, Arup and Allan Jowsey, fire engineering manager, Akzonobel. Thanks also to University of Edinburgh fire research centre
Barbara Lane is one of the speakers at the upcoming CPD seminar on ‘New Developments and Challenges in Fire Safety’ (26 March). Presentations will address the new I.S. 3218 Fire Detection and Alarm Systems standard and the new Building Control (Amendment) Regulations. There will be a presentation on the latest developments in fire suppression systems and Lane will examine structural failure on 9/11. Click for more details.
Arup Fire has investigated structural response in fire for many years, so after the events of September 11, 2001 we considered it essential to develop a clear understanding of possible structural collapse modes in severe fires.
We therefore commenced a major program of numerical analysis of the response of tall building structural forms to multiple floor fires. Arup, as well as many other consultancies around the world, has a policy of supporting and being part of cutting-edge research as we believe it forms the basis of our advancing profession.
Therefore our structural fire research and analysis discussed in this paper was carried out with our main research partner, the Centre of Fire Research Excellence, at the University of Edinburgh and the structural fire safety team there led by Prof Jose Torero.
Since 9/11, there has been a greater interest in the safety of tall buildings and how increased safety can be achieved without compromising on aesthetics or unnecessary costs. We will discuss how both these can be achieved with an increase in life safety compared with prescriptive solutions when fire engineered solutions founded within a risk assessment are used.
RESPONSE TO EXTREME EVENTS
The events of 9/11 changed the perceptions of building designers, contractors, owners and occupiers with respect to safety and security issues in buildings. Everybody had a reaction.
Tall building design moved out of the technical domain and now also forms part of the realm of public interest, due to the heightened awareness of building performance since 9/11. Codes and standards have historically evolved as a result of reactions to major events. It is to be expected that major disasters will provoke knee-jerk reactions. One example of this is the call for much longer periods of fire resistance on tall buildings immediately after 9/11.
This is an understandable, emotion-driven response, but we would propose instead that designing a structure with fire as a design load provides a more robust design solution. Simply increasing fire-proofing thickness without understanding the actual structural response to heat provides no guarantees of increased safety.
So what has the reaction of Arup been since the events of 9/11? Where can we do more to give reassurance to people living and working in tall buildings and how can we help our tall building clients?
Following 9/11, a number of questions were asked by the many stakeholders in the design process. These questions related to people concerns as well as commercial questions relating to insurance and lettable values. Some examples of these concerns were:
• What are the life safety and insurance issues associated with these extreme events?
• What is the best approach to understanding the buildings’ real performance in fire or other security events?
• What tools are available to satisfactorily resolve the issues and therefore people’s concern?
• Might this type of disaster occur again, even without the extreme cause?
The Extreme Events Mitigation Task Force (EEMTF) was set up in Arup to address our clients’ concerns worldwide. It was a specially created network of specialists tasked with identifying and solving the design concerns and new issues 9/11 posed.
On 9/12, life went on but people were nervous. Tenants became uneasy about occupying the upper floors of buildings. Some suggested the way forward was building low and only in concrete. Practical implications for businesses in high-rise city centres were therefore high, and it is our view that this remains the case. Therefore, it is important to react to lessons learned, but not to over-react.
AN INTEGRATED DESIGN APPROACH
1. It must be stressed that it is not possible to design for every conceivable or inconceivable event. Therefore a threat and risk assessment can be used to quantify real risks, in order to develop suitable mitigation measures, on a project-by-project basis.
2. The evacuation of buildings and the ability of new and existing buildings designed for a phased evacuation to now accommodate a total evacuation in a non-fire event is a practical reality in tall building design now.
3. Understanding the role of structure and its real response to fire, along with the performance of fireproofing materials in real events, is also key – even more so as events such as the Madrid fire enhance our understanding of real structural performance.
The design process can be summarised here in an integrated design approach: establishing the real risks, analysing their impact on the building performance using the tools available to us, and developing designs to accommodate this. Beneficial mitigating options to clearly defined scenarios are the goal. There is no point in spending large sums of money on a protection system that may offer little extra benefit.
EXAMPLE OF A RISK-PROFILE APPROACH
The graph to the right is an example of threat and risk assessment or risk profiling work of a tall building in London for a major commercial bank. It was used to help the client select a building to occupy or to assess various new building designs.
In this case, the client occupied building 2 but was due to move to building 1. He wanted to understand the risk of a fire or other event in building 2 and how it compared to their new proposed building (building 1) and another selected tall building in London (building 3).
Semi-quantified risk techniques were used to look at various life safety and business hazards. Results were used as a choice indicator for establishing design solutions and system selection. T hey could also be used to help determine insurance loss predictions. This is an example of a risk-informed decision.
BUILDING EVACUATION ISSUES
The ability to evacuate tall buildings in an imminent catastrophic event was highlighted by the Arup Task Force as a key lesson. Key considerations are:
• Evacuating whole buildings via stairs that are designed to evacuate only two or three floors in a fire event.
• The interaction of fire fighters and escapees.
• The possibility of using lifts for evacuation.
Many people are now unwilling to stay in a building on fire even if it is remote from their location and want to be reassured that they can evacuate in a timely fashion. Therefore a new approach to designing for evacuation must be considered. It is becoming a key tenant requirement, in the UK for example, that tall buildings are designed to have the capacity for a total evacuation.
This has resulted in whole building evacuation studies using tools such as STEPS (evacuation software) and ELEVATE (lift software) to understand real evacuation times and therefore the real capacity of the core design in a building.
We are increasingly proposing the use of lifts as part of evacuation in fire and non-fire events and we continue to work with other interested parties in developing an acceptable design standard to allow this to become a reality in fire design. Currently the Arup approach is to protect the lifts for evacuation to the same standard as fire fighting lifts. There is lots of interest in this use of lifts. Approximately 15% of people may have a problem walking down protected staircases of any height. In a tall building, where physical conditioning may become an issue that percentage rises considerably.
Enhanced lifts are a potential option and can result in a 40% decrease in overall evacuation times. It is estimated that it takes 20–30 seconds per floor by foot on stairs. A standard high-rise has lifts capable of moving about 15% of the population in five minutes.
Firefighters use lifts during ‘high-rise’ fires at a much later stage in the fire than those evacuating so using the lifts for occupant evacuation has minimal impact. We have found that they are most efficient in shuttle mode, e.g. between a refuge floor and ground.
The Mega Tower in Hong Kong and the Shanghai Tower are both in excess of 100 storeys high and lift evacuation has been proposed by Arup Fire on these projects. In a high-rise building currently under design in London, there is a large number of occupants expected in viewing galleries, restaurants and retail on the higher floors. The prescriptive solution would be four protected stairs for a phased evacuation. The proposed solution is two protected stairs plus double deck lifts (protected to fire fighting standard) and dedicated refuge floors.
The aim is for people to be guided by marshals to either use the stairs to the ground or to travel via the stairs to protected refuge zones and then queue for a lift. This will result in evacuating the original floors as quickly as current designs, but a much quicker total evacuation of the building.
There is currently no guidance on acceptable waiting times in refuge floors. On these projects, Arup has proposed eight minutes based on evacuation times from stadia as per UK guidance. It is assumed this will meet the patience levels of the occupants.
This solution provides for faster means of escape times in a total evacuation and also reduces the area required for stair cores, which increases the potential lettable area.
WHAT IS STRUCTURAL FIRE ENGINEERING?
The third key issue identified since 9/11 is structural fire engineering. As part of this topic, we will also relay some views on the National Institute of Standards and Technology’s (NIST’s) 2005 probable collapse theory for World Trade Center (WTC) 1 and 2.
The picture (right) showing spray applied protection is the extent of structural fire engineering on most buildings, tall or otherwise. Structural engineers do not traditionally consider fire as an actual load on the structural frame. What are we doing as an industry to allow this to happen?
Seismic design relies on modelling, risk analysis and changes to the structural stiffness. Wind design relies on additional structural members and wind tunnel tests. Current fire design relies on very simple, single element tests and adding insulating material to the frame. Thermal induced forces are not calculated or designed for.
So what is structural fire engineering? It is the ability to determine credible fire scenarios and then calculate the thermal and mechanical response of the structure to fire. In other words, the fire becomes a design load on the structure.
Structural fire engineering – definition:
- Determine realistic fires
- Calculate heat transfer to structural elements – steel/concrete
- Calculate mechanical response of full frame
- Determine load carrying mechanisms in the fire case
- Determine any weak points in the structural design
- Determine where fire proofing appropriate
- Determine structural alterations if required
After a major programme of research and development in the UK by Edinburgh University, Sheffield University, Building Research Establishment, CORUS, Imperial College, Arup Fire, the Fire Engineering Design and Risk Assessment Group et al, designers have the ability to analyse real structural response to fire. So what does this really mean on a project?
At Plantation Place South in London, based on the specific structural performance to fire, we could demonstrate no fire proofing was required on the secondary steelwork. We used computer modelling to predict the whole frame load carrying mechanisms in fire. These were catenary action in the beams and tensile membrane action in the slab supported by cooler edge beams and columns.
This lower reliance on passive fire protection is in contrast to the NIST work where the amount of fire protection on the truss elements is believed to be a significant factor in defining the time to collapse.
However there is no evidence in NIST’s preliminary report that this is backed up by structural modelling in response to fire. It appears that only heat transfer modelling considering different levels of fire protection have been carried out and the failure of the individual elements has been related to loss in strength and stiffness only. Thermal expansion and the response of the whole frame to this effect has not been described as yet
In the second part of this article, the authors analyse high-rise structures in multiple-floor fires and examine the collapse mechanism proposed by NIST in its presentation report.http://www.engineersjournal.ie/2014/03/13/reducing-the-risk-and-mitigating-the-effects-of-fire-in-tall-buildings/http://www.engineersjournal.ie/wp-content/uploads/2014/03/Twin-Towers-1024x680.jpghttp://www.engineersjournal.ie/wp-content/uploads/2014/03/Twin-Towers-300x300.jpgCivilArup,construction,fire,safety