Key Considerations for Assessing Structures at the Accidental Limit State in Fire Leon Sullivan Bsc Ceng Mistructe Mifiree S.E
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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 Structural Engineering 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 Eurocodes 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 Key considerations for assessing structures at the accidental limit state in fire 03 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.