AkzoNobel Fire Engineering and Estimation Services

Key considerations for assessing structures at the accidental limit state in fire Leon Sullivan BSc CEng MIStructE MIFireE S.E. M.ASCE MSFPE Manager, AkzoNobel

Synopsis state (ULS) requirements for strength and sprinkler systems, passive fire protection Most structures in Europe are constructed stability, in addition to serviceability limit such as intumescent coatings, fire-rated using limit state design methods. Most of state (SLS) requirements for deflection and boards and mineral wool etc. All of these these structures are protected against some vibrations. Designing to these limit state protection systems will contribute towards form of specified fire scenario. However, criteria is a process that engineers are both controlling the temperature of the structural only a small minority of projects link these familiar with and feel comfortable using. elements to remain within the limits of the two major considerations together to form specified design temperature by the end of part of a unified structural fire design When it comes to designing to meet the the resistance period. process. requirements for accidental limit states (ALS) relating to scenarios such as fire, Although this approach will produce a The provide designers with the blast and seismic events, the process can satisfactory fire protection system, it will be necessary procedures to undertake an often be overlooked, unless it is a specific overly conservative for the overall objectives accurate and economical structural fire hazard identified in the project specification. of the fire strategy in terms of maintaining design, yet few engineers ever consider structural stability for the duration of the fire undertaking such an assessment. Compliance with fire regulations is a resistance period. Taking the passive fire structural design requirement prevalent protection (PFP) systems as an example, to This article will focus on the load actions throughout the world, so how is this typically limit the temperature of a structural member and combinations to be considered that addressed on projects and how can we as to 400°C in a design fire exceeding 1000°C enable the engineer to perform an adequate engineers improve our knowledge and after 90 minutes will require more PFP structural assessment for the accidental limit working practices to produce more accurate material applied than if that same member state in fire. Importantly, it will also cover structural fire engineered designs. were allowed to reach 600°C during the important considerations to ensure that any same period. This means that an excessive structural fire engineering strategy is Current limited state amount of PFP may be used throughout the appropriately aligned, and the key The approach currently adopted throughout building, causing unnecessary cost and information that is required within the the UK and Europe to protect a steel framed wasteful use of materials, if the limiting steel contract chain to facilitate this performance- structure against fire involves specifying a temperature is specified without due based approach. fire resistance time period, then stipulating a consideration. limiting temperature for each member based Introduction on its structural use and fire exposure The alternative to this method is to adopt a A colleague of mine recently asked me “At classification. These temperatures vary performance-based approach incorporating what point during your degree did you learn between countries around the world, but are a structural fire design assessment to about structures in fire?” to which I gave the generally based on the conservative produce a robust fire protection strategy that rather sheepish, yet predictable response… assumption of the structure being fully will also be accurate and efficient in terms of “I didn’t”. loaded during a fire. the material used, as well as the associated Sadly though, a similar response could be time and labour costs to the project. received from many highly experienced The fire strategy developed for the building engineers when questioned about the level will determine how the structure will be When less may be more of structural fire engineering knowledge they protected to ensure that the temperatures of Throughout any project contract, there is have attained over the course of their the members are prevented from exceeding always a desire to reduce unnecessary professional lives. the values attributable to each member costs where possible. This can manifest category type. This strategy may specify the itself in driving investigations into tasks such Engineers often spend most of their careers use of either or a combination of the as reducing materials used, streamlining designing structures to satisfy ultimate limit following: active fire protection such as construction methods, assessing the Key considerations for assessing structures at the accidental limit state in fire 02

payback periods of mechanical equipment installed etc. As structural engineers, we have all received the request from the contractor at some point, to review our designs to see if there are any unnecessary member redundancies in our structure, in a perceived bid to reduce steel tonnage, concrete volumes and other material costs to increase the contractor’s chance of an acceptable financial return.

What many contractors and structural engineers are unaware of is that carrying out this process invariably increases the cost of the fire protection to the project. As the contractor incurs raw material and erection costs for the structural frame at the front-end of the construction programme, attention is often drawn to reducing these cost elements as much as possible without sufficient consideration to the resulting Figure 1 - Structural fire design team members effect on subsequent project processes. repetition in serial sizes when possible and By introducing a structural fire engineer to Up until the point of contract award, the using simple connection detailing during the design team, a vital link is created engineer may have spent many months, fabrication are often lost to the more between the structural design engineer potentially years, developing their design immediate concern of material savings. responsible for the stability of the building in from first concepts through to what they the event of a fire, the architect who is believe is their ‘final’ design. Post-contract A similar concern is shared by fire protection invariably responsible for the successful award they can find themselves in the manufacturers. The temperature of an execution of the fire protection strategy and situation where they are given a small unprotected small section size rises more the contractor responsible for installing the fraction of that time by the contractor, to quickly than that of an unprotected large specified fire protection. revise the submitted design in an effort to section. This results in more fire protection reduce costs. The engineer is then faced material required to maintain the limiting This expanded form of the design team with the task of reviewing their design and steel temperature when that value has been structure creates the opportunity to ensure substituting members where sufficient specified simply on the member’s structural the fire protection strategy is executed to redundancy exists in member capacity or function and exposure classification. It bring the greatest potential cost benefit to deflection limitations to warrant the use of a follows that, if the contractor’s objective is to the project. For the structural fire engineer smaller, lighter section size. This reduces reduce the holistic cost of the structural to bring maximum value to the team, they the steel tonnage attributable to the project frame, the greatest opportunity for this is must work closely with the structural design and the contractor believes that savings created by considering the fire protection engineer to gain an understanding of the have been made. strategy as an integral part of the structural loads acting on the building for the fire design process. scenario. Steel fabricators often raise their concerns on this issue; that using a wide variety of The new recruit In order to carry out this process, the smaller sections sizes can cause To maximise the benefits of such a process, structural design engineer must carry out unnecessary complexity and cost to the a new member needs to be introduced to the necessary analysis for this accidental resulting fabrication and connection the traditional design team: the structural limit state event, but how? designs. The benefits of rationalising the fire engineer. range of section sizes used, adopting

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Table 1 - Example of ALS load combinations for typical office developments

1.0 Gk,j + 0.5 Qk,1 1.0 x Permanent self-weight 0.5 x Occupancy load considered as leading variable load Wind load not required as accompanying variable load = 0

1.0 Gk,j + 0.2 Qk,1 + 0.3 Qk,i 1.0 x Permanent self-weight 0.2 x Wind actions considered as leading variable load 0.3 x Occupancy load considered as accompanying variable load

(2) Accidental limit state in fire expansion’, how should this be addressed In accordance with Clause A1.3.2 1, the The Eurocodes have provided engineers by the engineer? partial factors  applied to load actions at the with the best means yet to carry out accidental limit state should be 1.0, with any structural fire assessments on their Simplifications before implications relevant  factors taken from Table A1.1. In structures, using methods and principles Before we begin to contemplate the the case of the lead variable action, the that they are already familiar with for typical implications of thermal expansion frequent value  or the quasi-permanent

ULS and SLS design checks. Despite this, phenomena and the resulting time- value  may be selected, although the UK very few engineers even consider carrying dependent analyses that would ensue using National Annex recommends the use of the out such an assessment. Generally this non-linear material properties, we should frequent value4. appears to stem from the fact that they do first consider the simplifications that the not view this as part of their project remit, design standards offer when dealing with If this is applied to an example of a typical but also in many cases that they are simply this issue. office development for a UK project unaware of what is actually involved. considering permanent self-weight loads, The Eurocodes classify load actions from One such simplification is given by Clause Category B variable occupancy loads and fire as accidental events, as they are 2.4.1(3) of EN 1993-1-2 3 which states that wind actions with no pre-stressing loads statistically unlikely to occur during the ‘for verifying standard fire resistance present, the relevant load combinations design life of the building. As a result, when requirements, a member analysis is required for consideration would be as considered in conjunction with other sufficient’. This means that for buildings shown in the table below. coexistent actions on the structure during where fire strategy requirements are based the fire scenario, they are assigned lower upon meeting the heat regimes of the From this simplified approach, the task of combination safety factors than those standard temperature-time curve defined in producing an accurate assessment for the considered for normal operating conditions. Clause 3.2.1(1) 2, a satisfactory solution is accidental limit state appears more As with other ultimate and serviceability limit obtained by considering each structural achievable, but not without further state designs using the Eurocodes, the load element in isolation, and the more complex consideration to other aspects that may combinations required to make structural global analysis method accounting for affect governing structural failure modes. assessments for the accidental limit state in thermal expansion effects is not necessary. fire are given in EN 1990 1. Clause 6.4.3.3 Structural fire design analysis gives the following expression for the load When using the member analysis method3, models combination to be considered. Clause 2.4.2(4) states that ‘effects of axial Carrying out a structural fire assessment or in-plain thermal expansions may be may not necessarily be as simple as adding

∑ 퐺푘,푗+ 푃 + 퐴푑 + (1,1 표푟 2,1) 푄푘,1 + ∑ 2,푖푄푘,푖 neglected’, which means that the term for a couple of extra load combinations into the 푗≥1 푖>1 Ad, shown above in Equation (6.11b), model and then clicking ‘Analyse’. Through (1) effectively becomes redundant in the collaboration with the architect and This expression remains consistent with accidental limit state load combination for a structural fire engineer, the structural design those given for ultimate and serviceability member analysis. engineer must ensure that the requirements limit states, although now with the inclusion of the fire protection strategy are reflected in of the term Ad to represent the design value The net result of this approach is to reduce the analysis model used to produce an of the indirect effects of thermal actions due the load combination for the accidental limit accurate structural fire assessment. to fire. Now although Clause 1.5.1.7 of EN state in fire to the following expression: 1991-1-2 2 defines this phrase as ‘internal A project fire protection strategy may forces and moments caused by thermal ∑ 퐺푘,푗 + 푃 + (1,1 표푟 2,1) 푄푘,1+ ∑ 2,푖푄푘,푖 stipulate that some secondary members do

푗≥1 푖>1 not require protecting or only require partial

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facilities, this introduces the additional consideration to be given to effective buckling length values used in lateral torsional buckling calculations.

If the ultimate limit state design of a primary member relies on the lateral restraint of unprotected or partially protected secondary members, as shown in Figure 3, the accidental limit state design must reflect any change in the length considered for lateral torsional buckling checks.

When considering the partially protected arrangement shown in Figure 4 for the ALS in fire, deactivating the members in the analysis model must also increase the Figure 2 - Reduction factors for carbon steel at elevated temperatures buckling length considered for the primary members where adequate lateral restraint is protection, an approach commonly adopted structures where concrete slabs are not no longer present. Care must be taken that in offshore oil and gas platforms, but also present to provide the necessary lateral deactivating unprotected members does not found in some commercial construction restraint to floor beams, commonly the case result in loads being removed from the buildings. In scenarios involving unprotected in offshore structures or onshore industrial analysis. The unprotected steel may have or partially protected steelwork, what further facilities. lost the necessary yield strength and considerations should the structural design stiffness required to provide lateral restraint engineer give to their existing design? Where concrete slabs provide lateral to the primary , but it will still be restraint to unprotected secondary transferring gravity loads via the In the case of steel frames, the structural members, studies have explored the effects connections. design engineer requires an understanding of the resulting membrane actions of of how the mechanical properties of carbon composite structures in fire. This work is Although the buckling length for the primary steel change during a fire and how quickly beyond the scope of this article, along with member has increased, this does not mean these changes may arise. Elevated the general design principles of steel that the beam is now going to be deemed temperatures have detrimental effects on structures in fire, but further knowledge can inadequate. It must be remembered that the both the yield strength and stiffness of be obtained from available sources design load actions considered for the carbon steel, as illustrated in Figure 2, referenced at the end of this article. accidental limit state are considerably lower which affect the governing and To perform a more appropriate global than those used for the ultimate limit state, buckling failure modes. Class 4 sections are analysis for the accidental limit state, due to the reduced safety factors applied. affected to an even greater extent and have unprotected and partially protected This reduction in the loads considered to be their own reduction factors given in Annex E members should be deactivated from the acting during the fire may mean that the of EN 1993-1-2. model where such member could still be considered as suitable members are providing the only form of following the structural fire engineering When unprotected steel is exposed to the lateral restraint. This process helps produce analysis. time-temperature curve associated with fires a more accurate stiffness matrix in the from hydrocarbon fuel sources, the yield model which only considers fully protected Adopting these principles gives the strength can be reduced to just 5% of its members that are capable of distributing structural design engineer greater original value in less than ten minutes. For loads and resisting these actions. confidence that accidental event scenarios this reason, it is not unreasonable to have been addressed more suggest that unprotected or partially In the case of unprotected secondary comprehensively and that this assessment protected members are unable to perform members that provide the only form of can be used in further processes to reduce the same function in the fire that they were lateral restraint to primary members for the project costs. designed for at ambient room temperatures. normal operating condition of the structure, This is a particularly important point in e.g. in offshore structures or industrial

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Figure 3 - Buckling length considered for ULS design Figure 4 - Buckling length considered for ALS design

Reap what you sow frame costs, fire protection costs, labour have a significant impact on project costs As part of this collaborative design team costs, productivity, etc. to ensure a holistic and productivity, with single coat solutions effort, the structural fire engineer has solution is obtained rather than one which bringing several benefits to a project. already worked with the architect and the considers each element as an isolated Without considering these aspects as part of structural design engineer to ensure that the process. a combined process that incorporates the fire strategy requirements have been whole frame cost and construction considered correctly. The structural fire To provide the most cost effective passive programme, unnecessary expenditure can engineer now begins the process of working fire protection solution, many aspects need escalate quickly and schedule delays with the structural design engineer and the consideration such as protection type, incurred. contractor to ensure that a solution is found project environment, offsite or onsite that considers all aspects of the fire application, number of coats or layers Summary protection process to bring maximum benefit applied etc. In the case of intumescent Too often in construction, structural design to the project. This will involve balancing coatings, the number of coats required can engineers are asked to focus on making savings in the material costs of structural elements without being afforded the time to fully consider the subsequent cost impact of making such changes. By considering fire protection costs as part of the holistic cost of the structural frame, a performance-based approach can help deliver the greater benefit to the project.

Introducing a structural fire engineer as an inherent member of the design team offers the greatest opportunity to realise such additional project value. By close collaboration with the structural design engineer and the necessary assessment carried out for the accidental limit state in fire, savings can be made that will benefit Figure 5 - Fire protection solution the design team as a whole.

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References and further reading

British Standards Institute. (2002). BS EN 1991-1-2:2002 Eurocode 1: Actions on structures - Part 1-2: General actions - Actions on structures exposed to fire. London: BSI.

British Standards Institute. (2005). BS EN 1990:2001+A1:2005 Eurocode - Basis of structural design. London: BSI.

British Standards Institute. (2005). BS EN 1993-1-2:2005 Eurocode 3: Design of steel structures - Part 1-2: General rules - Structural fire design. London: BSI.

British Standards Institute. (2005). NA to BS EN 1990:2002+A1:2005 UK National Annex for Eurocode - Basis of structural design. London: BSI.

Franssen, J.-M. (2005). Design of Steel Structures subjected to Fire: Background and Design Guide to Eurocode 3. Les Éditions de l'Université de Liège.

Franssen, J.-M., & Real, P. V. (2010). Fire Design of Steel Structures (1st Edition ed.). ECCS - European Convention of Constructional Steelwork.

Lennon, T. (2011). Structural Fire Engineering. London: ICE Publishing.

Lennon, T., Moore, D., Wang, Y., & Bailey, C. (2007). Designers' guide to EN1991-1-2, EN1992-1-2, EN1993-1-2 and EN1994-1-2. London: Thomas Telford Publishing.

Simms, W. (2012). Fire Resistance Design Of Steel Framed Buildings. Ascot: The Steel Construction Institute.

Vassart, O., & Zhao, B. (2013). Membrane Action of Composite Structures in Case of Fire (1st Edition ed.). ECCS - European Convention for Constructional Steelwork.