Fire Resistance of Steel Structures

Fire Resistance of Steel Structures

Dr. N. SURESH, Profesor& Head, Syeda Nusrath Fathima, Post Graduate Student, Departameno of Civil Engineering, The National Institute of Engineering, Mysore

Structural steel frames are widely used in virtually all types of buildings and industrial installations. Fire-resistant design of structural steel framing is often required, depending on the fire risks associated with the structure, the magnitude of potential losses due to structural failures caused by fire, and the accumulated performance record of similar structures in past fire incidents.

Fire represents a transfer of energy from a stable condition to a transient condition as combustion occurs. During this process, the steel temporarily absorbs a significant amount of thermal energy. Subsequently, the steel structure returns either to a stable or unstable condition after cooling to ambient temperatures. During this cycle, individual members may become badly bent or damaged without affecting the stability of the whole structure. It is possible to predict the range of temperatures that a particular steel member of a building experienced during a fire using current heat transfer theories. Damaged members are indicative of energy redistribution within the member itself and possibly the whole structure. It is widely accepted that many steel structures retain their integrity after fire while their reuse and reinstatement is a critical economical issue.

This article presents an overview of how to conduct a forensic evaluation of a fire damaged steel structure and technical aspects of the assessment method, from which useful conclusions are drawn for the safe reuse of the structural elements and connection components, while the reinstatement survey is also comprehensively described.Various test procedures will be reviewed to examine their ability to determine metallurgical or structural degradation of the steel properties. These tests commonly include visual observations and measurements, surface hardness readings, residual stress measurements, metallographic sectioning and testing for chemical and physical properties. Additionally, several major groups of materials used for the fire protection of structural steel are discussed including insulating properties at elevated temperatures and practical guidance for their use.

Introduction

Perceptions of fire vary depending on the circumstance to which the structure is exposed. Controlled fires are rarely given much thought in our daily experiences. Uncontrolled fires, with the possibility of buildings collapsing, the implied damage, potential injury and loss of life have created a very serious dimension to Fire design, putting a moral responsibility on structural engineers. A negative connotation often exists that anything exposed to fire and heated to a high temperature must be damaged, regardless of the appearance of the structural members.

The strength of all engineering materials reduces as their temperature increases. Steel is no exception. Exposure to fire will subject structural steel to thermally induced environmental conditions that may alter its properties. Assessing these altered properties requires a combined knowledge of metallurgical and structural behavior as the fire raises the steel temperature and the steel later cools. Knowledge of steel properties and behavior developed from basic steel production, thermal cutting, thermal or mechanical straightening (or curving), heat-treating and welding provides the requisite information. However, a major advantage of steel is that it is incombustible and it can fully recover its strength following a fire, most of the times. During the fire steel absorbs a significant amount of thermal energy. After this exposure to fire, steel returns to a stable condition after cooling to ambient temperature. During this cycle of heating and cooling, individual steel members may become slightly bent or damaged, generally without affecting the stability of the whole structure. From the point of view of economy, a significant number of steel members may be salvaged following a post-fire review of a fire affected steel structure. Using the principle “If the member is straight after exposure to fire – the steel is O.K”, many steel members could be left undisturbed for the rest of their service life. Steel members which have slight distortions may be made dimensionally reusable by simple straightening methods and the member may be put to continued use with full expectancy of performance with its specified mechanical properties. The members which have become unusable due to excessive deformation may simply be scrapped. In effect, it is easy to retrofit steel structures after fire. However it is useful to know the behavior of steel at higher temperatures and methods available to protect it from damage due to fire.

Behavior of Steel at Elevated Temperatures

Fires are associated with sustained elevated temperatures and consequent high heat. The heat associated with a fire can cause many types of changes to structural steel elements such as member deformations, loss of normalized microstructure, stress relieving, sensitization of stainless steels, high residual stresses, or embrittlement due to rapid cooling associated with firefighting efforts.

Forces that develop in a steel member, as the temperature increases, are primarily dependent on the end restraint, and to a lesser degree on the intermediate bracing and supports. Steel expansion is temperature dependent and as the temperature of the steel increases, an unrestrained member will elongate according to,

ΔL = αLΔT
Where, L= original length in meters, ΔT= change in temperature in oC

A member fully restrained in the axial direction will develop axial stresses as the temperature increases according to,
fa = EαΔT

Although structural steel offers the advantage of being noncombustible, the effective yield strength and modulus of elasticity reduce at elevated temperatures. The yield strength of structural steel maintains at least 85 percent of its normal value up to temperatures of approximately 427°C. The strength continues to diminish as temperatures increase and at temperatures in the range of 704°C, the yield strength may be only 20percent of the maximum value. The modulus of elasticity also diminishes at elevated temperatures. Thus, both strength and stiffness decrease with increases in temperature.

Dill (1960) has stated, “Steel which has been through a fire but which can be made dimensionally re-usable by straightening with the methods that are available may be continued in use with full expectance of performance in accordance with its specified mechanical properties.” Recent researchers, Avent (1992), Wright (1990), confirms that this statement is as true today as when it was first stated over 30 years ago and suggests that the criteria for evaluating fire exposed and damaged steel is a function of repairability, rather than inconsequential metallurgical changes.

For severely damaged structural steel members, where the deflections are excessive, metallurgical degradation is a moot point. It is usually more economical to replace the member than attempt salvage operations by heat straightening. But as noted above, the member can be heat straightened with an acceptable amount of metallurgical and physical property degradation.

Reinstatement of Structural Steel After Fire

The uncertainties encountered in conducting a fire damage assessment, mainly during the procedure of the in-site investigation, usually lead to conservative decisions, concerning no severely affected structural elements mostly. Moreover, significant redistribution of forces through undamaged parts of the structure from the fire affects the appraisal, the reliability of which depends largely on the experience of the inspector. For such reasons, a well-established guidance accompanying the site visit is considered mandatory.

Assessment Process

The complete assessment determines the nature and extent of the fire damage and whether repairs are required. Prior to making a site visit, the building’s history and construction should be determined, which includes obtaining copies of the drawings and other pertinent documents. The first crucial point of the assessment is to collect information about the fire severity. Building occupancy conditions and the fire history should be documented shortly after the fire. This information will provide an indication as to fire load and temperature exposure for each member. Steel protection means (fireproofing) and compartment ventilation conditions can affect the time-temperature history. Many procedures are available to assess structural steel integrity after fire exposure, including visual observations, non-destructive testing and destructive testing (removing samples). The last method can incorporate chemical composition analysis, obtaining physical properties (Fy, Fu, ductility, toughness, etc.), residual stress determination and distribution, and metallographic observations. Each procedure will be assessed to examine its relative value in predicting structural steel conditions after fire exposure. The procedure is completed with the suggestion of the strengthening or the replacement of critical members.

The Evaluation process involves,
  • Determining when to observe the fire-damaged structure including both before and after cleaning.
  • How to evaluate the post-fire conditions either visually, non-destructively, or destructively.
  • Assessing the structure to determine the kind of repairs if any is required.
The Repair process involves,
  • Evaluating the options-remove and replace, salvage/repair, or no action.
  • Detailing the repairs-preparation, installation, and quality control.
Unless working with the fire department to fight the fire, no one should enter the structure until it is safe to do so. Fire damage to the structure is not always obvious. By entering as soon as practical, the engineer can uncover hidden distress and observe the collateral damage to contents. Collateral damages frequently provide good indications of maximum fire temperatures.

Evaluation

The three stages of evaluation are visual assessment, non-destructive testing, and partially-destructive testing.

Visual Assessment

Proper evaluation of steels subjected to fire typically requires estimating the temperature and duration of the fire. In case there is no data available, the engineer is advised to identify physical effects that will be visible from the structural elements.In some cases, well documented, visual observations in the fire exposed building area can provide the most useful information concerning the predicted fire temperature and the resultant structural steel peak temperature. The steel surface itself can provide fire temperature information. For clean unpainted steel, a yellowish brown color indicates a temperature of 460-480°F while a blue color indicates a temperature of 600-640ºF. Noticeable, tightly adhering mill scale indicates a steel temperature considerably below 1200ºF. Structural steel exposed to temperatures above 1200ºF will develop a coarse, eroded surface markedly different from the appearance produced by mill rolling, Dill (1960). Although there are numerous types of coatings, markers and paints, which may be used on structural steel, they usually are not designed to withstand elevated temperatures. At temperatures above 600ºF they usually will have blistered, discolored or even flaked-off depending on the exposure to open flames. Any identifiable residue is an indication that the steel has not reached its critical temperature.

A temperature increase will cause an unrestrained member to increase in length or large forces will develop when restraint exists. The Deformation of the entire structure should be monitored in order to categorize the holistic damage, beyond the aesthetic aspects linked to the fire visual impact. For Steel members, the visual observations are extended with distinguishing the members into three categories of deformation;

Category 1: Straight members that appear unaffected by the fire. This includes members that have slight deformations not easily detected by visual observations (within 4 or 5 times ASTM A6 rolling tolerances).

Category 2: Members noticeably deformed but could be heat straightened if economically justified.

Category 3: Members severely deformed that only under extreme circumstances would repair be given any consideration.

Camber and sweep of each fire exposed, structural steel member should be determined using appropriate measurement techniques (plumb bob, string line, laser). A Category 1, 2 or 3 designation should then be assigned to each member. Most often, the Category 2 or 3 designation can be assigned without measuring because of severe local buckles or excessive deflections.

Slightly deformed Category 1 members, with deformations greater than rolling tolerances, must be analyzed to determine the repair level. Depending on individual circumstances, the analysis will determine if these members can be accepted unconditionally, heat straightened, stabilized with supplemental braces, or reinforced with plates and shapes.

Category 2 members require additional attention because the decision to repair or replace is often a function of the nearby members’ condition. If a Category 2 member is heat straightened, the change in metallurgical and structural properties will be inconsequential. Rehabilitation or replacement of Category 2 members is usually dependent on expediency, economics or overcoming the human psychological rejection of what appears to be damaged steel.In most cases, Category 3 members are obvious and usually rejected without much consideration. Salvaging a Category 3 member would most likely occur in a very critical location where removal is inappropriate or impossible. Repair and replacement is then implemented as required.

Visual inspection of connections that are to remain is critical since connections may have fractured due to the fire event. Inspecting connections is imperative for beams that will be retained. Connection behaviour is different than main member behaviour when the temperature increases because of their relative compactness. The axial force developed by a restrained member will impose large forces on the end connections. Generally, the beam will buckle or deform to accommodate the axial force. Under these conditions, connection distress is easy to identify; when a Category 1 steel beam cools, if the connection has fractured, the steel beam will pull away from the adjacent member revealing the damage. Connections at the ends of Category 2 and 3 members to be salvaged are usually refurbished along with the beam; therefore evaluation of the connections is not warranted. If the beams are restored in place, then the bolts, brackets and welds should be given special attention.

Non-Destructive Testing

Following the preliminary checks, special techniques including both destructive and non-destructive operations can be utilized. Non-destructive evaluation techniques for steel structures include distortion measurements, plumbness or straightness checks, liquid-penetration examination-detecting discontinuities on welds, metallographic observations and hardness testing. If the member distortion is minor, it is unlikely the member was exposed to a temperature of more than 1200ºF for any length of time and therefore no consequential metallurgical changes will occur. Measurements of distortion, such as buckling of restrained plates and out-of-plumbness, can provide an indication of maximum temperature reached.

Weld inspection techniques such as ultrasonic testing, magnetic particle testing, and dye penetration testing may also be useful in determining the integrity of welded connections. Special attention should be given to bolted connections nearby distorted members, since it is evident that the members suffer axial forces beyond the yield strength, most likely during both the heating and the cooling phase.

Hardness testing aids in determining the loss of tensile strength and to a lesser extent changes in ductility and toughness. Scaling of carbon steels typically begins above 1000ºF and can affect hardness readings. Above 1200°F the surface will develop a coarse, eroded surface markedly different from mill scale.

Partially-Destructive Testing

Sample removal from the fire-damaged steel members can provide specific physical property and residual stress information. The primary destructive tests for steel are tensile tests to determine yield and ultimate tensile strength. Temperature effects on the physical properties (yield and ultimate tensile strength, ductility, toughness), residual stress distribution and grain structure can be reviewed.

Although a compendium of indirect information about the structural capability of individual elements can be obtained, it may still become necessary, and is indeed frequently desirable to determine the actual loading capacity of an integrated system of elements. For this an actual load test must be performed. That test can be made before reinstatement has been completed, afterward, or both. Tests completed prior to reinstatement will establish what repairs, if any may be required. Tests completed after reinstatement will indicate whether corrections have been adequate.

Residual stress evaluation is performed by removing instrumented areas or coupon samples from the fire exposed steel member. Steel coupon samples are removed and the surface is polished to a mirror-like finish. The surface is then etched with a weak acid solution, which exposes the steel grain structure. A grain structure that differs from the usual mill rolled grain configuration would suggest a metallurgical or structural change in the steel.

Engineering Analysis

The assessment phase compares the findings of the evaluation phase (damage plots, temperature plots, and allowable stresses based on non-destructive and/or destructive testing) with the analysis findings to determine if the applied stresses in various elements exceed allowable levels. This then determines the extent if any, of required removals, replacements, and/or repairs. Based on the results of the assessment, repair materials can be selected, details developed, and repairs installed.

Methods of Fire Protection

The capacity for a building to remain functional for a specified length of time during a fire is of utmost importance for life safety and fire department access. However, all conventional construction materials begin to degrade when exposed to elevated temperatures for prolonged periods of time. Therefore, it is often necessary to provide means of fire protection to the building’s structural elements in order for them to properly carry load during this important time period.

Building fire protection may be categorized into two main systems, active and passive. An obvious approach is to eliminate the heat source by extinguishing the fire or by generating an alert so that an extinguishing action can be initiated. Extinguishing systems such as sprinklers and smoke and heat detection devices are responses to this approach, and are classified as active fire protection systems. Active protection relies on devices requiring external activation to alert occupants of a fire and to control building fire conditions. Automatic sprinkler systems, smoke detectors, and fire department suppression are all examples of active systems. Alternatively, another approach for improving the fire safety of a steel structure is to delay the rate of temperature increase to the steel to provide time for evacuation of the environment, to allow combustibles to be exhausted without structural consequence, and/or to increase the time for extinguishing the fire. This approach, which involves insulating the steel or providing a heat sink, is classified as a passive fire protection system. Passive protection provides fire protection by relying on in-place elements, and requires no external activation. Examples of passive protection include fire-rated ceilings, gypsum board or lath and plaster systems, spray-applied fire resistive materials (SFRM), and mastic coatings.

There are three generic types of fire protection for structural steel:
  • cementitious products
  • board and casing systems
  • intumescent coatings
Cementitious Products

Cementitious products based on gypsum or Portland cement binders are normally applied by low pressure spray techniques to the profile of the steel section to be protected. These materials contain low density aggregates and rheological aids to help the application characteristics. Fire protection is provided to the steel by these materials in two ways, the first being the ‘cooling effect’ as the trapped moisture (physically and chemically bound) evaporates as the temperature of the surrounding fire increases. Once all the moisture has turned to steam the product then behaves as a thermal insulation material. Low density mineral and synthetic aggregates are used in these products since they are efficient in allowing the steam to escape, while denser materials might impede its progress and cause the product to spall.

Board and casing systems

Board and casing systems use materials such as ceramic wool, mineral wool, fire resistant plasterboard, calcium silicate and vermiculite to provide fire protection to steel. These products provide fire protection in much the same way as the cementitious products and are dry fixed around the steel using clip, pin, noggin and screw systems.

Intumescent coatings

Intumescent coatings derive their name from the Latin verb ‘tumescere’, which means to begin to swell. In a fire situation, these thin film products swell up to form a char which protects the steel, thanks to its insulating properties. Using various types of industrial coating equipment, these materials are applied as a thin film and are often available with a range of topcoats in different colours so that the designer can achieve his or her aesthetic needs as well as those of fire protection on visible steel. Intumescent coatings are particularly effective for steel that requires up to 90 minutes’ fire protection.

Summary and Conclusions

A first impression upon arriving at a fire scene is usually very negative because of the immense destruction and adverse environmental conditions. Field assessment of fire damaged steel members requires a systematic approach to determine their condition. Based on practical experience, a member inventory should be performed and each member should be classified. A classification system consisting of the following three categories has been suggested, Category 1— essentially straight members; Category 2—noticeably deformed, but repairable members; and Category 3—severely deformed members that generally are uneconomical to repair. Visual observation and measuring the member geometry provide the most practical means to classify and assess the potential for damage in a fire-exposed member. The repair or replacement of an individual member is dependent on classification, location in structure, and economics.

This paper investigates the assessment of the post-fire properties of Fire damaged steel structure as well as their subsequent reuse after a fire event. A review of the reinstatement of structural use is unfolded. All structures subjected to fire should be evaluated in a systematic manner to determine the extent, if any, of required repairs. The intensity and duration of the fire can be estimated by observing the collateral damage. A variety of testing methods and tools are available to evaluate the effects of the fire on both the materials and structural elements. These evaluations, combined with an engineering analysis, allow effective and economical repair details to be developed and installed as needed.In conclusion, it can be simply stated: “If it is still straight after exposure to fire—the steel is OK”. A similar statement was made over 50 years ago, and is still applicable to this day. With this statement in mind and the points raised in this paper, assessing the structural capacity or integrity of a fire exposed steel structure or steel member can be adequately determined.

References
  1. Arthur Nestor R., Iwankiw, J. Parker, and Jesse J. Beitel, STRUCTURE magazine, February 2005.
  2. Avent, R.R., “Designing Heat-Straightening Repairs,”Proceedings of National Steel Construction Conference,June 3–5, Las Vegas, Nevada, American Institute of Steel Construction, Inc., Chicago, IL, 1992.
  3. C. Maraveas, Z. Fasoulakis, K.D. Tsavdaridis, “Post-fire assessment and reinstatement of steel structures”, Journal of Structural Fire Engineering.
  4. Chen, J., B. Young, and B. Uy (2006), Behavior of high strength structural steel at elevated temperatures. J. Struct. Eng. ASCE, 132(12):1948–1954.
  5. Cooke, G.M.E. (1988), “An Introduction to the Mechanical Properties of Structural Steel at Elevated Temperatures,” Fire Safety Journal 13 (1988) 45-54.
  6. Dill, F. H., “Structural Steel After a Fire,” Proceedings of National Steel Construction Conference, May 5–6, Denver, CO, American Institute of Steel Construction, Chicago, IL, 1960.
  7. Long T. Phan, Therese P. McAllister, John L. Gross, Morgan J. Hurley Best Practice Guidelines for Structural Fire Resistance Design of Concrete and Steel Buildings, NIST Technical Note 1681, November 2010.
  8. Narendra K Gosain, Ray F Drexler, DilipChoudhuri, “Evaluation and Repair of Fire Damaged Buildings”, Structure Magazine-September 2008.
  9. Ruddy, J., J.P. Marlo, S.A. Ioannides, and F. Alfawakhiri (2003), Fire Resistance of Structural Steel Framing, AISC Steel Design Guide 19, Chicago: American Institute of Steel Construction.
  10. Tide, R. H. R., “Integrity of Structural Steel after Exposure to Fire,” AISC Engineering Journal, First Quarter 1998, pp.26-38.

NBM&CW November 2017

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