London Fire Brigade - Fire Resistance CPD Presentation
•Download as PPTX, PDF•
2 likes•743 views
This document discusses the history and future of structural fire safety design. It begins by outlining how fire resistance testing originated in response to large fires in the late 1800s and early 1900s. While initial fire tests aimed to standardize safety evaluations, the standard time-temperature curve used today does not realistically represent an actual building fire. New building materials and designs are challenging traditional fire resistance approaches. The document advocates a performance-based structural fire engineering strategy over prescriptive code compliance. It provides an example project that used computational modeling, thermal analysis and structural modeling to demonstrate the fire safety of a complex building with unusual architectural features.
Recommended
More Related Content
Viewers also liked
Viewers also liked (15)
Similar to London Fire Brigade - Fire Resistance CPD Presentation
Similar to London Fire Brigade - Fire Resistance CPD Presentation (20)
Recently uploaded
Recently uploaded (20)
London Fire Brigade - Fire Resistance CPD Presentation
- 1. Structural design for fire safety Where have we come from? & where are we going? Dr. Danny Hopkin – Associate Director London Fire Brigade – July 2016
- 2. Scope • Fire resistance – why? • Future trends – where next? • Design at the interface – an alternative • Summary • Questions
- 3. Fire resistance - History Modern application, dated origins
- 4. Why do we have FR? • Great conflagrations in the late 1800s and early 1900s; • Fire resilient buildings became a social expectation; • ‘Fire proof’ materials emerged; • A lack of trust in private testing; • A need to independently verify; • Emergence of federal & municipal fire test laboratories
- 5. The start of ‘standard’ • Earliest references of a ‘standard fire test’ – New York – late 1800s; • Five hours at 2000°F; • Post Baltimore - “no ordinary room would have enough inflammable material in it to maintain a 1700°F fire for more than 30 minutes”; • Anecdotal FF experience; • Still ‘non-standard’.
- 6. A benchmark test • Ira Woolson - NFPA - “unify all fire tests under one single standard and remove an immense amount of confusion within the fire testing community” (1917); • By the 1920s the time- temperature curve is standardised and fire resistance is born;
- 7. A concept discredited Ingberg 1928 Relates real fire severity to equivalent ‘standard’ durations
- 8. Woolson 1917 NFPA meeting – “We want to get it as nearly right as possible before it is finally adopted, because, after it is adopted by these various associations, it will be pretty hard to change it” Woolson’s premonition
- 9. Fast forward a century
- 10. Structural FR - Defined • Tests whether an isolated structural element does not violate particular performance criteria after a set period of time in a furnace, when subject to the standard time-temp curve; • Deflection limit span/20; • It cannot ever be a measure of survivability in a real fire; • The standard fire is not a standard fire, it’s not even a fire!
- 11. • Energy flow in is balanced against the losses to achieve ‘the standard fire curve’ Is it a ‘fair’ test? Pump some energy in Lose some energy to the furnace walls Energy absorbed into the specimen A concrete slab
- 12. A CLT slab • Less energy is required to balance the losses because the specimen is contributing Is it a ‘fair’ test? Pump some energy in Lose some energy to the furnace walls Energy absorbed into the specimen Specimen produces energy as it burns
- 13. Future trends, divergence and the renaissance of a familiar foe
- 14. Going up & urban
- 16. Sustainability
- 17. • 400+ towers (>20 storeys) proposed in London… • There will be features that are ‘unusual’ or sensitive to fire… • How will we approach their design?
- 18. • Wind – performance based assessment • Seismic – performance based assessment • Fire?............................
- 19. Lame substitutions* Fire safety engineering Structural engineering Structural design for fire safety *Credit – Guillermo Rein
- 20. The 1st kind Struct. engineer is replaced by pseudo-science Fire safety engineering Failure at x°C
- 21. Fire eng. replaced by pseudo-science Structural engineering Temperature Time Failure at x mins The 2nd kind
- 22. Both eng. replaced by pseudo-science Temperature Time Failure at x°C The 3rd kind
- 23. Solution – protect all steel members to a 120 minute standard for a limiting temperature of X°C Engineering…..done
- 24. • “intended to provide guidance for the more common building situations…” • “need to take into account the particular circumstances of the individual building…” A health warning
- 25. Apathy?
- 27. Design at the interface A structural fire engineering strategy for an expressed Cor-Ten frame
- 28. Fire safety engineering Structural engineering Structural fire engineering The interfaces What? Who? How? An interface between disciplines The interfaces between facets of a successful delivery
- 29. Regulations Responsibility Skill & Care Structural engineers understood they were responsible for ensuring “stability for a reasonable period” in fire Those responsible for construction were engaged at an early stage and became familiar with the requirements Design team understood that the fire performance demands were beyond their competency & delegated Competence – A prerequisite for success
- 30. 4 Pancras Square For an industrial buildingAn industrial site
- 31. The building • A 10 storey office – 46 m in height; • Predominantly a concrete frame – cast insitu & PT; • Architectural feature – external Cor-Ten frame; • A huge Cor-Ten transfer structure; • Tricky interfaces.
- 32. A successful solution A melange of competing goals, obligations & constraints, of varying intelligibility
- 33. The life safety goal • "Stability for a reasonable period"; • Consistency of risk – Kirby, et. al; • Overall reliability requirement of 97%; • Active reliability contribution of 93%; • Passive reliability requirement of 49%; • All 50% have the potential to fully develop. 0 20 40 60 80 100 0 50 100 150 200 Fractile(%) Height (m)
- 35. Thermal conditions • A lack of guidance – Law & O’Brien – SOA; • Steady state analysis – overly conservative; • A need to quantify transient behaviour; • Consider the impact of wind; • Quantifying thermal gradients, etc., key.
- 36. Thermal conditions Side 1 Side 2 Rear Front A 0.75 0.75 0.96 0.41 B 0.67 0.67 0.95 0.40 C 0.86 0.86 0.98 0.57 0.00 0.20 0.40 0.60 0.80 1.00 Relativeproportionofcompartment temperature(-) Elevation of element 0 200 400 600 800 1000 1200 0 30 60 90 120 150 180 210AST(°C) Time (min) Fire Compartment Sides Rear Front - BS EN 1991-1-2 Annex B as a ‘scalar’ - Benchmarked against CFD models - Adequately conservative. Element orientation influences exposure:
- 37. • Location ‘manages’ exposure; • Sections still very hot; • Concrete filling, where practicable; • Shielding, where permissible; & • Otherwise, plate sizing. Fire Floor Floor Above 120 mins 180 mins 240 mins Managing temperature
- 38. Materials – Cor-Ten • Cor-Ten is not a typical material; • The scale of the section is not typical;
- 39. Structural response • Two key areas: • Vierendeel transfer; & • Columns • Other complications: • Connections; • PT; • Bi-metallic corrosion & PFP.
- 40. Vierendeel behaviour -4000 -3000 -2000 -1000 0 1000 2000 3000 0 5000 10000 15000 20000 Axialforce(kN) Time (s) • Expansion governed; • Very sensitive to TFs; • Doesn’t deflect excessively; • Plastic strain tension; • A building that needs to ‘breathe’; • Matching ‘actual’ vs. ‘idealised’.
- 41. Column behaviour • Concrete filling; • Explored rebar vs. T; • T more ‘buildable’; • UC 254x127x84 (S355); • Actions influenced by curvature & slab ‘push-out’; • Sensitivity to vertical fire spread explored; -150% -100% -50% 0% 50% 100% 150% 0 30 60 90 120 150 180 InnerTeeUtilisation(%) Time (min) BF WEB_C TF MAX Min
- 42. Lessons & key points • Struct. Eng. understood their responsibility & limits; • “Stability for a reasonable period” not FR120 + sprinklers; • They understood the expertise req’d & delegated; • Those responsible for delivery were involved in design. • Quantification of the goal -> rational basis -> rational process; • Thermal tools are inadequate for external exposure; • Cor-Ten does not behave like regular carbon steel; • Bigger is not always better.
- 44. Skill & care • Successful fire engineering doesn’t end when a report is issued….
- 45. Summary Considered structural design for fire safety
- 46. Final remarks • Fire resistance is ‘old hat’; • Prescriptive guidance caters for the simple; • The legal requirement is stability for a reasonable period not a predefined level of FR; • An approach commensurate with complexity; • Competence is a prerequisite for successful design; • A great report ≠ a great solution.
- 47. "If you always do what you've always done, you'll always get what you've always got.“ H. Ford
- 48. Thanks for your time • Danny.Hopkin@trentonfire.co.uk • http://uk.linkedin.com/in/dannyjhopkin • https://twitter.com/DannyHopkin • http://www.slideshare.net/DannyHopkin
Editor's Notes
- Fire engineering technical lead for Trenton Fire; An advocate for considered FR design; Educated as a structural engineer first and a fire engineer second; Chair the IFE Special Interest Group on Fire Resistance & I’m involved in a few FR standards committees.
- Gareth Steele asked me to come and speak at this event on the topic of FR; Audience is a melange of characters & backgrounds; Conscious of this when developing a presentation; I’ll forewarn you as I’m deliberately going to say some provocative things.
- If we’re going to talk fire resistance, we have to go right back to the beginning; It sets the scene for where we’re going and what must be considered moving forward;
- We have fire resistance because of various great fires, all, to a large extent, involving timber; These great fires increased societal awareness of fire and, as a result, fire resilient buildings became an expectation (both in terms of personal safety + insurance); Proclaimed fire proof materials flood the marketplace but all of the testing was ‘in-house’ and adhoc; Acknowledged need for independent verification of claims and we see the emergence of test laboraties;
- The earliest references on FR testing come from New York and the NY floor test (or the so called fire + water tests); 2000F is 1093°C; 1700F is 927°C; Test regime is informed by anecdotal FF experience; Tests were conducted in various locations at this time, but were not standardised.
- An NFPA committee was formed on fire resistance, with a clear goal…..; During this time, the time-temperature curve for furnaces is standardised and reflects what most will be familiar with today; However, at this point, there isn’t any great consideration of time.
- Soon thereafter, the standard heating regime is discredited as not being a representative fire; Simon Ingberg sets about trying to relate real fires to durations of exposure in the so-called ‘standard fire’; Our fire resistance grades and the relationship with time is established.
- That’s the end of the history lesson, other than to point at Woolson’s 1917 premonition; In reference to the standard fire, he states…. And How right he was!
- Because, in 100 years, the only significant change to the fire resistive principle is the technology; The time-temperature curve, albeit massaged with time, still reflects the NY anecdotal evidence;
- Let’s be clear about what structural fire resistance is…. The performance criteria are generalised & do not represent failure; The fire resistance achieved by elements forming a structure has not correlation with when the structure would fail in a real fire; Fire resistance is a benchmarking measure for contrasting performance.
- With age and experience, I would now question whether it even succeeds at being a fair comparative concept; Let’s take two cases….
- What happens if we take a different specimen? If it were a good insulator, such as plasterboard, heat flow into the specimen would be limited and less energy would need to be pumped in… But what if the specimen is combustible? Despite me standing here and pointing out the flaws, we have to say the idea of fire resistance hasn’t served us too badly. Buildings aren’t falling from the sky; Key question, will it continue to serve us well in face of unyielding innovation?
- We build more with timber, the ambition is to build higher with timber…. Murray Grove in London at 10 storeys used to be the world’s tallest timber frame building. Soon the world’s first 40 storey timber building will be complete.
- Achieving a sustainable building is a key goal for most clients. This means influencing material choices and their thermal performance, which all impact upon fire development and fire performance…
- Structural fire resistance is delivered at the interface of two disciplines. Perhaps the best articulation of how we will approach the design of these future challenging buildings is that of the lame substitutions proposed by Dr. Guillermo Rein at imperial college, London. Designing at the interface is tough, as it requires two disciplines to work together.
- For a moment I shall wear my fire engineer’s hat, and you will be the proxy of the structural engineer. I’ve no interest in understanding your structure, so instead I dilute the problem, first from a complex structure to a single element and then, in lieu of trying to understanding this element, I’ll assign a temperature at which I consider it might fail.
- Next, I will wear my structural engineers hat and you shall be the proxy of the fire engineers. Again, I’ve not interest in understanding your discipline, fires are complicate, so I will dilute all of the possible manifestations of a fire and massage them into a single time-temperature curve….the standard fire curve.
- Then there is the 3rd kind, where both parties are replaced by pseudo-science. Where neither the structure’s performance or the fire’s characteristics are represented in any realistic form. This, unfortunately, is what some might suggest is representative of how fire resistance is dealt with…
- Our guidance is informed by the PWBS, published at a time where building’s weren’t tall or complex; But they haven’t evolved in line with our ambitions, as evidenced by the changes in London’s skyline; So what will we do in the future?
- We will work more at the interface referenced; I wanted to come to this CPD event and present an alternative path for complex or unusual buildings; It’s a path that will become increasingly important if we construct less common / straightforward buildings; I think it’s a successful example of considered FR design, but I’ll let you be the judges at the end.
- This aspect of the presentation has a common theme….interfaces; It’s primarily about structural fire engineering, i.e. the interface between two disciplines; The project was setup to be successful before we got involved; Niggles at the interface between facets of successful delivery.
- We have regs that tell us what we’re trying to achieve; We need someone to realise they are responsible and to take responsibility; Those that take responsibility need the skill.
- We move on to the project. 4PS is part of the largest regeneration project in the UK, sandwiched between St. Pancras International & King’s cross railway station.
- It’s a 10 storey office, that’s not exceptionally tall. Despite the veneer, it’s primarily a concrete frame building, with cast insitu columns and PT slabs. However, it’s uncommon because the perimeter structure is weathering steel. Once again, the topic of interfaces raises its head, because some quite innovative things happen where materials meet.
- I pay homage to Arup and Angus Law at this point, by expressing our process as a Veen diagram. In ethereal speak, we had a very demanding architect, who wasn’t satisfied with cheating. The frame had to express the engineering. If you stick steel outside, it rusts. The solution, therefore, was to adopt an unconventional material. In terms of our input, the building had to stand up when subject to large enclosure fires. All of this combined led us down particular solution paths, which we will arrive at by the end.
- The min. goal is the legal one, i.e. “stability for a reasonable period”. If we were to suggest that the building is simple / typical, then a prescriptive solution would comprise FR120 + sprinklers; (a) this conflicts with our ambitions for the building and (b) the building is not simple. So we expressed or goal as a reliability. Put simply, as buildings get bigger and thus the consequence of failure more significant, a greater array of fires must be resisted. In this case, 4PS is a 97th percentile building. Accounting for sprinkler protection, this means the structure is required to resist 49% of “fully developed” fires.
- What do these fires look like? This is where Monte Carlo tools are useful. We have a series of inputs for our fire dynamics models that all invariably feature uncertainty. So we can generate an array of design fires. We can then rank these in order of severity using the common time equivalence concept. At the 49th percentile, there will be 100s of fires, so we make a selection that elicit a spectrum of behaviours, i.e. different durations, temperatures and modes.
- We know our fire ‘constraints’ at this point. The next step is how they manifest as thermal exposure. For external steel, this is the point at which we find that the existing tools fail us. The guidance is crude, it’s steady state, we need something more efficient. But the existing methods are a good starting point.
- We have confidence that the ‘steady state’ heat flux can be quantified. We, therefore, asked ourselves, if we express this as an AST and normalise it relative to the fire compartment temperature, what does it tell us? If we apply these scalars to the enclosure time temperature curve, can this be our thermal condition? Through an extensive CFD study, we were able to conclude yes, with an acceptable degree of confidence that we were being conservative.
- The next step was relatively straightforward because the thermal conditions become an input to the heat transfer model for different elements. This is the point at which we can qualitatively say temperatures are too high and we need to provide some mitigation, i.e. through concrete filling and / or shielding.
- We’re now at a stage where we can seek to quantify structural performance. Here we encounter more challenges because (a) Cor-Ten doesn’t behave like regular mild steel, (b) some of our sections are class 3 or 4, so susceptible to local buckling and (c) we’re interested in cooling, so any variation to the residual strength must be quantified.
- So what of the structure. Before we got to the point of any form of quantitative analysis, we made some changes. At conception, there were no internal columns at ground floor level. We felt that had to change to reduce the loading to the Vierendeel transfer structure. Primarily, we were interested in two areas, the Vierendeel and the perimeter columns. However, other complications existed that I don’t have time to elaborate on today.
- In the case of the stability critical Vierendeel, the challenges centre on its size. It’s a storey deep. The top chord is 800 x 1000 mm, the bottom 800 x 1500 mm. Whilst it doesn’t get excessively hot, the forces it generates when expansion and contraction is restrained are significant. For this reason, it’s really sensitive to non-uniform heating. A key part of the solution is, therefore, allowing some connections to slip. We also encountered some challenges matching what exists in practice vs. what is simulated, i.e. construction tolerances, slip, hole elongation.
- In terms of the columns, they aren’t huge like the Vierendeel. So they get hotter and, for this reason, they are concrete filled. However, the concrete isn’t ‘structural’, it is there to protect a mild tee section that is welded within and it is this tee, combined with the weathering steel, that gives the columns their resilience in fire. The columns solution was numerically studied under a range of heating conditions, including a fire spreading vertically.
- We learnt a lot from this project both technically and in terms of managing the design process. Ultimately, it’s a good example not because we were involved but because it was acknowledged that we were needed! The process we went through is increasingly typically and rational. This isn’t a 30, 60 or 90 min building. It’s a 97th percentile building. I don’t think we would be the first to have suggested that the tools for external steel and crude and, generally, inadequate. So we had to be creative.
- I’m just going to briefly touch on what might have happened if we had followed fire resistance convention, which is characterised by disciplines ignoring each others existence.
- The quality of our input won’t be measured by the quality of our report; We shouldn’t lose sight of the fact something needs to get built when the calcs are done.
- You can deliver something challenging, such as that envisaged by Eric Parry, but we can’t often do it by accepting the status-quo. That is, within the constraints of prescriptive rules and magic numbers.