Fire engineering designing of structures has been proven to be an effective method of designing constructions. However, that does not mean the elimination of protective methods, but the efficient use of protective products. This paper reviews the most common method which can be used for protecting beams with web openings as a common structural element, along with an introduction to a software developed at Warringtonfire that could optimise design of such structural elements under fire conditions.
Performance based design of structures is a concept which has been used around the world for some time and considers the performance of structures under various kinds of loading. One of the areas where such a design method has been employed is in designing structures under fire conditions. The goal for such design is to improve structural safety, increase the flexibility in design and reduce the need fire protection products by utilising engineering approaches (Wang, Burgess, Wald, & Gillie, 2012). While these engineering approaches helps to mitigate some cost they should not compromise the structural safety, which means, such design will need complex structural modelling (Angus, 2016) to ensure the structural response under fire conditions. Using such approach allows the key structural elements to be identified and be provided with a sufficient level of protection. The key structural elements, however, depend on the proposed construction and how the structure has been detailed.
One of the most common types of constructional method in today’s multi story buildings is the use of composite construction (Lawson & Hicks, 2011). Using this construction method not only has allowed the designers to have flexibility (over a wider range of span that they can design for) but also the ability to enhance the structural performance under fire conditions (Bailey and More (2000a) (2000b)). However, in normal construction the finished storey height, especially in multi-storey buildings, can become a restriction for building services engineers. In these situations, specifically in steel structures, the most common solution is to use beams with web openings, also referred to as cellular beams. Under such situations to make sure that the composite system can provide an adequate level of support under fire conditions, the system beam needs to be protected.
Fire protection for cellular beams
To be able to identify the adequate level of protection required for a beam with web openings, as any structural element, it needs to be analysed in both ambient and fire conditions. However, due to the nature of these systems where some parts of the web have been removed, cellular beams would need to be checked for additional failure modes compare to a solid beam. These failure modes include bending failure, web post buckling of the web, shear buckling of the web and Vierendeel bending around the opening (ASFP-YellowBook, 2015) (CEN-BS-EN13381-9, 2015) (Lawson & Hicks, 2011) (SCI-RT1356, 2012).
This indicates that beams with openings are more vulnerable to failure than solid beams and this point can be pronounced even more when the section is exposed to fire. This is because unlike a solid beam where the temperature (up to certain depth, EN 1994-1-2 (CEN, 2005)) can be assumed to be uniform along the height, the temperature distribution along the depth of the section is non uniform. This nonuniformity leads to non-uniform stress distribution around the openings leading to lower critical temperatures of such beams compared to beams with a solid web. Possible failure modes of beams with web openings under fire and ambient conditions can be seen in Figure 1.
In the UK, to make sure that beams with web openings have been adequately designed and checked for all possible failure modes, two design guides, namely SCI P355 (Lawson & Hicks, 2011) for ambient design and RT1356 (SCI-RT1356, 2012) for elevated temperatures, have been published. However, those checks are given for beams without fire protection. This means if the beam was protected the temperature distribution would be different and because the strength of steel sections are sensitive to temperature, it is crucial to determine the temperature distribution along the depth and length of the section.
This issue has been recognised and addressed by BS EN 13381-9 (2015) and ASFP Yellow Book 5th Edition (2015). These documents acknowledge the fact that the temperature distribution is different as a result of the opening, even when protected by a fire protection product. Thus, a series of beams with openings of different web post length should be tested and using those results, the temperature distribution for each scenario can be mapped. The results allow a designer to follow one of the three methods detailed within those standards/guidelines to determine the required protection thickness, see Figure 2.
Calculating product-based fire protection for beams with web opening
The procedures given in both BS EN 13381-9 (2015) and ASFP Yellow Book 5th Edition (2015) are the same and reiterate the fact that depending on the fire protection used the temperature distribution and hence the failure time of the section could be different. However, the next question which could be critical is what is the optimum level of protection for a cellular beam to have certain fire resistance performance?
The answer to this question can be quite critical as could have a direct impact not only on the safety of the design but also on the overall cost of the system. In such cases, the designer should make sure that the proposed configuration of the opening has been formed in a way that the cost of the steel work could be reduced. However, this reduction should not be to the level that a high level of protection has to be used to make sure that the fire resistance of the beam could be achieved (Jowsey, Scott, & Torero, 2013).
Due to the complexity of the problem and higher number checks in order to have an optimum design, it is required that the designer runs a series of calculations to identify the best combination of the opening position, spacing between them and required level of protection, see Figure 3.
However, finding the best design configuration can be a tedious procedure. The designer would need to repeat numerous time-consuming sets of checks which could heighten the chance of error in calculation.
To this end, Warringtonfire collaborated with ASFP by testing cellular beams in order to establish the temperature distribution for cellular beams (Figure 4), and developed a software called Thermcalc (Guo & Yuan, 2015). This software follows the structural analysis protocols given in RT1356 and also has been designed to be product-based , in line with EN13381-9 and ASFP Yellow Book 5th edition, see Figure 5.
The software provides the designer the chance to analyse numerous configurations of composite beams under fire conditions within a short timeframe. In addition, ThermCalc has the advantage of being a product-based software which means that the programme can calculate the results based on the features of each individual product, which is extremely beneficial. Such software provides designers with the opportunity to make more and more use of performance-based design in order to have an optimised design without compromising on safety.
This paper draws attention towards accurate designing of beams with web openings in line with recommendations given within standards. Also, it highlights the fact that if designers have appropriate tools, reaching an optimum design, which could reduce the cost of the construction and enhance the structural performance of the system, is achievable and on this line the authors introduced the software developed at Warringtonfire as a viable solution.
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Dr. Mostafa Jafarian
Dr. Jifeng Yuan