The Structural Performance of Cable Supported-Structures During a Fire MEng Thesis Project
Papers of Relevance Version 2.0 Prepared by : Hector Haines and Jonathan Noblett 31st October 2011
Hector Haines (s0791309) Jonathan Noblett (s0791627)
School of Engineering The University of Edinburgh Faraday Building The King's Buildings Mayfield Road Edinburgh EH9 3JL
INTRODUCTION In the outset of our project it was decided that the scope of ‘The Structural Performance of Cable-Supported Structures During a Fire’ was probably too large a subject area to cover in a single year-long thesis. Therefore, research was required into previous work, to inform our decision on the specificity of our thesis work as well as giving us the general background knowledge required to undertake research at the forefront of our field.
BRITISH STANDARDS AND EUROCODE REVIEW The British Standard and Eurocode that relate to the design and testing of steel structures in fire are BS 59508:2003 (previously BS 5950-8:1990), BS EN 1993 1-2:2005 (previously ENV 1993 1-2:1995). Both documents cover the same scope in a similar fashion. Shown below is the scope from BS 5950-8:2003: This part of BS 5950 gives recommendations for: — steel beams, columns and tension members designed to BS 5950-1; — steel/concrete composite beams designed to BS 5950-3; — concrete-filled steel hollow sections designed to BS 5400-5; — composite floors designed to BS 5950-4. For each type of member, recommendations are given for the load carrying capacity and, where appropriate insulation performance, for a given fire exposure assuming they act in isolation and restraint to thermal expansion is ignored. These recommendations are based on: a) fire resistance derived from standard fire tests; b) fire resistance derived from calculations.
NOTE These recommendations may also be applied to members for which the fire exposure has been determined from natural fires.
Although tension members are mentioned in the scope of both documents, little detail or instruction is made in either, with most of the sections referring to bracing. General detail seems relevant for cable structures, and the Fire Design Flow Chart can help simplify understanding of the process of design for any structural member type. The documents do make good use of experimental data to show figures such as Thermal Elongation, Variation in Specific Heat Capacity and Thermal Conductivity with Temperature. The Eurocode goes into even more detail and shows figures associated to the stress-strain relationship, and stain hardening of steel at elevated temperatures. Useful tables such as strength retention factors for varying strains and design temperatures for different sections and fire resistance period for tension members are also included. Nevertheless, the bulk of these documents are devoted to ‘standard structural elements’ such as beams, columns and composite floors. Where mention of tension elements does occur, it is usually in just a general context such as: 18.104.22.168 Tension members Where thermal expansion can cause gaps in the fire protection materials, consideration should be given to the penetration of heat.
In relation to our project ‘cable supported structures in fire’, there is little to go on. However, some mention is made of external steelwork (relevant in cable bridges, and cable membrane structures) in fire, but again this is mainly in the context of external beams and columns. Indeed an admission is made in BS 5950-8:2003, section 8.8 External bare steel where;
“Methods of assessment are beyond the scope of this document and specialist literature should be consulted. The engineer should, however, be satisfied that the procedure and assumptions made are applicable to the structure in question.”
Other document sections such as ‘Connections’ have some relevant information cited generally, but detail is only given on standard type bolted connections; B.3 Connections B.3.1 General Connections in the fire affected zone and adjoining areas should be examined to ensure that there has been no distortion or damage due to heat or thermal expansion. During heating and cooling high stress levels may be generated at the connections with the result that deformation and failure of bolts and welds may occur in either shear or tension. For bolts in particular, this might sometimes not be obvious until they are removed for inspection, in which case it is recommended new bolts are inserted. Note: BS 5950 is not the current standard, it has been superseded by BS EN 1993 however, some structures which we look at will have been designed to BS 5950 so it is important to make note of both standards.
PAPERS OF RELEVANCE An Inverse Heat Transfer Model of Thermal Degradation within Multifunctional Tensioned Cable Structures, P.G. Moore, H.N. Jones, III, and S.G. Lambrakos, (Submitted June 2, 2004) This paper, sponsored by the Office of Naval Research, sets out an inverse parametric model for the analysis of electrically air heated tensioned cable structures. The prototype model was based on multifunctional wires, and is described as a “silicone rubber-impregnated coated assembly with carbon fiber strands of rectangular cross section”. The relevance of this paper to tensile steel cable structures is small, although the general parametric equations could be modified or used to inform or predict certain characteristics of our experiment. However, the general thrust of this paper seems to be about the thermal degradation of the composite wires rather than the structural performance, and it is for this reason that its relevance is minor.
Evaluation of the Impact of Potential Fire Scenarios on Structural Elements of a CableStayed Bridge, Ian Bennetts, Khalid Moinuddin, Journal of Fire Protection Engineering 2009 19: 85 This paper provides an in depth analysis of potential fire risks associated with a cable stayed bridge. Fire scenarios are logically identified and modelled, and analysis completed into the effect of these. In addition to this, various cable configurations and a simplified main support are included in the analysis. From the computational analysis Temperature-Time and Load Ratio-Time curves are drawn. These curves seem to be in accordance with the British Standard and Eurocode curves. This paper could provide useful information and insight if the specific thrust of our project is the analysis of bridge cables. Other cable supported structures, such as the O2 arena, or Heathrow Terminal 5, may have little specific relevance but nevertheless the general analysis of the structures could be derived from similar equations and instances to those used throughout this paper.
Thermal Behaviour of Steel Cables in Prestressed Steel Structures, Zhihua Chen; Zhansheng Liu; and Guojun Sun, Journal of Materials in Civil Engineering, ASCE, September 2011 The main focus of this paper was to look at the effects of heating prestressed steel structures. The investigation looked at determining the thermal expansion co-efficients for different steel cables as the parameter has not been adequately defined even though a value is given in the two codes (ASCE and Eurocode) it compared. The determined values of thermal expansion co-efficient were found by testing four different types of steel
cable; steel wire rope cable, steel strand cable, semi-parallel steel tendon cable and Steel rod. These values were then used to perform a numerical analysis of a cable-supported beam as used as a component of the roof of the Qian’an Conference and Exhibition Centre in China. The results from this analysis showed that; “the thermal expansion of steel cables has a direct influence on the value of cable prestressing force when temperature changes” (Chen et al 2011) The paper demonstrates that structures utilising either steel tendon or steel-wire rope cables for support will have significant displacements under heating whereas steel strand and steel rod cable structures have a more moderate change to the cable forces under temperature change. This paper is relevant to our project as it provides a good method for the determination of thermal expansion co-efficients. It also gives insight into the behaviour of the pre-stressed steel under temperature changes. However, it looks at the global structure, as opposed to initial thoughts that we should focus on local parts of structures, such as the connections of the cable to the beam, and how they perform under heating.
Analytical and Experimental Investigation of Thermal Expansion Mechanism of Steel Cables, Zhihua Chen; Zhansheng Liu; and Guojun Sun, Journal of Materials in Civil Engineering, ASCE, July 2011 This paper, by the same authors as the last, and published just two months previous, discusses many of the same key points, and concludes on the similar thermal expansion coefficients. However, the additional work shown in this paper is that the cables were not highly pre-stressed, and as such a relationship between the temperature, lay pitch and diameter of the wire was established, where an increase in diameter saw an increase in thermal expansion coefficient and an increase in lay pitch saw the expansion coefficient decrease. The relevance of this paper is not as high as the previous paper, but findings such as those mentioned above could be useful.
The Security Assessment of Cable Assemblies In Structures, Klein Timothy, International Bridge Conference 2008 This paper has not had a peer review however; it was presented at the International Bridge Conference in 2008. The main points of the paper is to demonstrate the effectiveness of a new composite socketing medium for the use in bridge applications, it would not be limited to just this use. The paper discusses the limitations of High Grade Zinc and Resin as socketing medium at elevated temperatures if a structural cable on a bridge is subjected to a fire and how the new composite medium called CM can perform up to much higher temperatures of up to 2,500 degrees Fahrenheit. The paper discusses the method of performing the tests and touches lightly on protection of the sockets using a material to deflect the heat. This paper will be useful in our experiments to decide on our method for testing our samples as it details issues which occurred during the testing and how they adapted the heating to get the results they required. The purpose of the research differs to ours as we want to look at the relationship between resin filled sockets at different temperatures and different loads.
Theoretical analysis and numerical simulation on behaviour of large span cablesupported structures under fire conditions BAI Yin, SHI Yong Jui and WANG YuanQing, Sci China Ser E-Tech Sci, August 2009, Volume 52, No. 8 This paper investigates the performance of three types of cable-supported structures by developing formulae in terms of geometrical and material nonlinearity with high temperature effects. The paper uses the modulus of elasticity reduction factor of pre-stressed cold-draw rebars from EC4 then finds
the change in the pre-tensioning force and displacement under increasing thermal loads for; cable truss structures, beam string structures and pre-stressed cable net structures. The majority of the paper is devoted to the derivation of the equations used for the analysis and the conclusion of the paper mainly discusses the need for FEA as numerical analysis relied on the supports of the structure being fixed where in practical engineering this is difficult to achieve - the rigidity of the supports heavily influence the performance of cable-supported structures. When the structure is subjected to the effects of a fire, it always leads to weakening of supports (BAI et al 2009). In reality the support condition would be unknown, so an iterative approach is required, which would become too complex without the use of FEA approach. Although this paper does demonstrate a good use of numerical analysis it has some, but not a lot of, relevance to our project. This is because it uses values already defined by EC4 and does not consider exposed components of the structure which may be more susceptible to fire.
ADDITIONAL PAPERS If in our project we decide to analyse cable-membrane structures then good information about their design and performance is available in; Tensile fabric structures: concepts, practice & developments, B. N. Bridgens, P .D. Gosling & M. J. S. Birchall, The Structural Engineer, 20 July 2004 or the seminal work by Frei Otto, Tensile Structures. Additionally, papers such as Examples of Fire Engineering Design for Steel Members, using a Standard Curve versus a new Parametric Curve, C R Barnett, G C Clifton, Second International Workshop, Structures in Fire, Christchurch, March 2002 gives some suggestions for generating design fires using a BFD curve to more accurately represent a natural fire situation.
CONCLUSIONS The majority of past research efforts have focussed mainly on the determination of various thermal properties or the numerical analysis of Global cable structures under thermal load. Therefore it seems natural, and most exciting to take a different look at the structures and take an experimental approach too. The use of resin filled sockets to fix the end of the steel cables is a recent innovation and is being used on such structures as the Forth Road Bridge. However, the behaviour of these assemblies in fire at different loads has not been examined and this is where we are going to focus our efforts this year.