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Concrete Q&A: Design Considerations for Fire Resistance of Structural Strengthening System
Structural Engineer , July 2004
Author(s): Tarek Alkhrdaji, Ph.D.

The repair and strengthening of concrete structures is a challenging and growing segment of the concrete repair industry for both engineers and contractors. To achieve a successful upgrade, the design should involve four basic elements: concepts used in design, detailing of the upgrading system, compatibility and composite behavior of the existing structure and the strengthening materials, and proper field application methods. Determining design parameters is the first and most important step toward achieving adequate and durable strengthening solutions. In addition to strength and serviceability, design parameters should address environmental exposure and fire rating issues. This may be achieved only by understanding the physical and mechanical properties of the selected construction materials at elevated temperature and their influence on the overall performance of the structure.

Structural strengthening may be achieved by section enlargement, externally bonded fiber reinforced polymer (FRP) composites or steel elements, internal or external post tensioning, span shortening, or a combination of these techniques. These upgrading systems should be tailored not only to produce a structural system that meets strength and aesthetic requirements of the structure, but also to meet the service performance and safety requirements. The latter typically includes fire rating of the upgraded structure or structural elements. It is common that, while focusing on strength requirements, the design engineer may overlook the fact that the upgraded structure is carrying higher loads and may require a more stringent fire rating than the original structure or structural elements.

Compared with normal temperature design, the behavior and rating of the structural member (existing or upgraded) at the time of fire is different because the applied loads are smaller, material strengths may be reduced, smaller load factors can be used, and serviceability requirements are not important. As a result, the strength design of a structure for fire safety is different than that for ultimate capacity and may incorporatelower material strengths and load factors and no strength reduction.

The first step in designing structural upgrade to achieve adequate fire rating is to verify that the residual strength of fire-exposed structural elements, RFire, is greater than the factored external load, UFire.

The residual strength, RFire, is calculated using reduced material strengths that are determined based upon the maximum expected temperature in the event of fire and a given fire duration. The yield strength of reinforcing steel is reduced, and the compressive strength of concrete is reduced. As a result, the overall resistance of the reinforced concrete member to external loads is reduced. This concept is used in the American Concrete Institute's Guide for Determining the Fire Endurance of Concrete Elements (ACI 216R) to provide a method of computing the fire endurance of concrete members. Also, most national and international codes specify a strength reduction factor f = 1.0.

In the event of a fire, the applied loads are most likely - lower than the conservative design loads specified for normal temperature conditions. When addressing fire rating, design codes typically specify load magnification factors (during the event of a fire Ufire) that are lower than those used in design for normal temperature conditions. For example, dead load (DL) and live load (LL) factors for fire design by ASCE and Eurocode are as follows:

1.2DL + 0.5LL (ASCE, 1995)

1.0DL + 0.9LL (ECI, 1994)

Similar values have been established by other design codes. Although it produces smaller ultimate loads, this approach for fire design is considered adequate. Evaluations of service loads under normal day-to-day conditions indicated that most buildings have a (DL+LL)/(UFire) ratio of 0.5 or less (Buchanan, 2001).

Ratings of commercial fire protection systems are typically established based on full-scale, load-bearing testing. The test must demonstrate that the insulating material will not debond or deteriorate for the duration of the fire. In addition, depending on materials used in their construction, strengthening systems may require insulation to different levels as the materials used in their construction may begin to deteriorate at different temperatures. The following is a general description of design and detailing considerations for conventional structural strengthening systems focusing on fire performance and methods of achieving code-specified fire rating.

Concrete section enlargement

Enlargement is the placement of additional concrete on an existing structural concrete member (in the form of overlay or jacket). The additional concrete may be "structural" concrete reinforced with steel bars or wire mesh and designed to be a load-carrying element, or "protective" concrete used to increase durability, fireproofing, or to encase post-tensioning or bonded steel elements to protect them from mechanical and environmental damage. Using this method, columns, beams, slabs, and walls can be enlarged to add load-carrying capacity, or to increase stiffness.

Concrete structural elements are known to have good fire performance because the cement paste undergoes an endothermic reaction when heated, and reduces the temperature rise. As a result, catastrophic failure of concrete structures due to fire seldom occurs. Failure is almost always related to the inability of other structural members to absorb the large thermal deformations rather than the loss of strength of concrete and steel.

Many national codes and professional organizations list generic ratings for concrete structural elements, giving the minimum thickness of concrete cover needed to protect the main steel reinforcement from the effects of fire. These ratings are equally applicable to additional concrete used to enlarge the section as a method of structural strengthening. However, the designer should consider the elevated temperature properties and variation in the coefficient of thermal expansion of new and existing concrete materials. For example, high-strength concrete tends to have a higher rate of strength loss than normal concrete at temperatures up to 750°F, while lightweight concrete exhibits excellent fire resistance due to its low thermal conductivity.

An existing or enlarged concrete structural element can be designed to have a given fire rating using design charts relating the residual strength of concrete and steel to standard fire exposure time. The residual strength of the member calculated using material strengths determined from these charts should be equal to or larger than the ultimate force, calculated using reduced load factors. Typical design charts are given in ACI 216R. In addition to increasing the cover size, fire performance of an enlarged concrete member may be enhanced by applying gypsum board or trowelled on plaster.

Strengthening with FRP composites

As with any other composite system, the bond between FRP composites and existing concrete is critical to ensure load sharing by the externally bonded reinforcement.

In FRP composites, the fibers can continue to support some load in the longitudinal direction until the temperature threshold of the fibers is reached. This can occur at temperatures near 1800°F for glass fibers and 350°F for aramid fibers. Carbon fibers are capable of resisting temperatures in excess of 500°F. Epoxies used in their construction, on the other hand, have a threshold temperature known as the glass transition temperature or Tg. The value of Tg depends on the type of epoxy. For most types used in FRP construction, Tg varies within the range of 140°F to 180°F. When the temperature of the epoxy rises beyond its Tg, the elastic modulus of a polymer is reduced due to changes in its molecular structure. This also results in a reduction in the force transfer between fibers through bond to the resin, the tensile properties of the overall composite are reduced. Testsby Kumahara et al. (1993) have indicated that temperatures of 480°F, which are much higher than the resin's Tg, will reduce the tensile strength of glass FRP and carbon FRP materials by more than 20%. Wang and Evans (1995) indicated that other properties affected by the shear transfer through the resin, such as bending strength, are reduced significantly at lower temperatures.

According to ACI 440, externally bonded FRP reinforcement should be designed to supplement existing interior reinforcement. In other words, if under any circumstance, the FRP reinforcement is compromised, such as in the event of a fire, vandalism, or other, the structural elements must be able to carry a certain ratio of the existing service loads without collapse. Using this philosophy, ACI 440 recommends that the existing strength of the structure be sufficient to resist a level of load described by RFire>= (1.2DL + 0.85LL) new loads.

This design approach is similar to the conventional fire design approach that was discussed earlier. It is noted that the level of strengthening that can be achieved using FRP reinforcement is often limited by the fire-resistance of the existing structure (without FRP). Nonetheless, such requirement would still allow for an upgrade of up to 60%.

Due to its low temperature resistance, the FRP system will not be capable of enduring a fire for any appreciable amount of time. Furthermore, it is most often not feasible to insulate the FRP system to substantially increase its fire endurance, because the amount of insulation that would be required to limit the FRP temperature below 180°F is currently cost prohibitive. However, new fire proofing systems for FRP strengthened members have been developed. These systems have been shown to limit the temperature of the FRP to 200°F or less. Although promising, these systems require further testing to account for various design parameters. In addition, testing of these systems is limited to temperature variation investigations and do no include loaded test member, per several guidelines. In addition, very limited test data are available for the designer to evaluate their performance.

The nominal resistance of the member at an elevated temperature, RFire, can be determined using the guidelines outlined in ACI 216R. This resistance should be computed for the time period required by the structure's fire-resistance rating, for example, a two-hour fire rating, and should disallow the contribution of the FRP system.

Internal/external post-tensioning

Structural strengthening using post-tensioning involves the application of an active force or forces to decrease distress caused by external loads. These forces are typically delivered by means of prestressing tendons or high strength steel bars located inside (internal post-tensioning) or outside (external post-tensioning) the concrete section.

Internal post-tensioning systems have the inherent benefit of full encapsulation in concrete. Accordingly, the same fire-resistance design and detailing concepts that applies to section enlargement applies to this technique. External prestressing, on the other hand, has the disadvantage of being located outside the structure, rendering it susceptible to fire. Unprotected strand is particularly susceptible to fire damage. Approximately one-half of a tendon's strength is retained at 800°F. This can be easily resolved by encasing the system in concrete or by using shotcrete.

The post-tension strengthening system should be designed to provide a fire rating similar to that of the existing structure. Strength, modulus of elasticity, expansion, thermal conductivity, creep and stress relaxation are all affected to some degree by elevated temperatures.

Calculations of fire rating and fire rating of prestressed steel elements follow the same concepts discussed earlier. It should be noted, however, that prestressing steel is more sensitive to elevated temperature than mild steel reinforcement. Charts for determining strength reduction due to fire exposure are available in ACI 216R.

Bonded steel elements

Unprotected steel elements tend to perform poorly in fires compared to concrete structures because structural steel elements are typically thin and have high thermal conductivity. When exposed to fire, the steel temperature increases and depending on the fire exposure time and intensity, its strength and stiffness may be significantly reduced. However, structural steel elements with adequate design and a fire protection system can be designed to acquire excellent fire resistance.

When strengthening is achieved using externally bonded steel plates, the designer should detail the system to include the effects of fire. For example, thermal expansion of the steel elements may cause damage to adjacent concrete members. Although it could be well protected, when exposed to fire, elongation of the external steel elements may produce pressure which consequently could cause failure of the adjacent concrete members. In addition to bonding the external steel system to the concrete surface by two-component epoxy adhesive, mechanical anchors must be installed to maintain part of the composite behavior in case of epoxy failure due to fire.

There are many methods of protecting structural steel elements from the effects of fire. Most of these methods are equally applicable to steel-based strengthening systems such as externally attached steel elements and supplemental steel frames. Generic ratings are typically used for structural steel elements, which assign a time of fire resistance to materials with no reference to manufacturers or to detailed specifications. Ratings are given for encasement in concrete or other material in terms of thickness that need to be applied to produce certain rating. For concrete encasement, nominal reinforcement may be needed to hold the concrete together and keep it in place in the event of a fire. Gypsum boards are also good insulating materials. The behavior of gypsum is enhanced by water that results from crystallization and is driven off as temperature rises, which gives an additional time delay at about 212°F. Empirical formulas for calculating the required gypsum board thickness to achieve a given fire rating can be found in the Uniform Building Code (UBC, 1997).

Proprietary cement-based spray-on material with some form of glass or cellulosic fibrous reinforcement is usually the most economical form of passive protection for steel elements. The required thickness of these materials is typically provided by the manufacturer and may be found in some trade publications. Intumescent paint is another fire protection system that could be used with steel strengthening systems. When heated, these special paint materials swell up into a thick charry mass that provides insulation to the steel. Several coats of paint may be needed to achieve a specific rating. Intumescent paint systems are proprietary systems that could be more expensive compared to gypsum board and spray-on materials. Also, they may not be suited for exterior or harsh environment applications due to their unknown durability.

Span shortening

Span shortening may be accomplished by erecting additional supports some distance away from the existing ones. The new support may be constructed using reinforced concrete or structural steel elements. Either system can be fire-protected using one of the methods discussed in section enlargement of supplemental steel strengthening methods.

Conclusion

Structural strengthening of concrete structures is an art form that has evolved into a complex science. It involves the use of conventional cement and steel based materials, as well as new techniques that utilize the advanced composite materials commonly used in aerospace and military applications. Regardless of the system used, performance of the structure under fire must be evaluated and the system must be detailed to provide the specified fire rating. We must recognize that strengthening design and detailing is much more complex than new construction. In addition to dealing with the unknown actual structural state, the degree to which added new materials and an existing structure share the effects of the system must be evaluated and properly addressed. Fortunately, the same concepts used in structural design for fire safety of new construction applies to strengthening design.

Although conventional methods of fireproofing are impractical for FRP, the continuous research by academic institutions and manufacturers may make existing systems more effective in the future. FRP fireproofing systems must demonstrate through full-scale testing their ability to increase the fire endurance of the upgraded element under new (increased) service loads. It should be emphasized however, that the FRP strengthening limit is based on the concept that the existing structural elements will provide an adequate level of fire endurance for the new loads. This is solely attributed to the inherent fire endurance of the existing concrete structure. Fire proofing of FRP will allow the use of FRP as primary reinforcement, which would result in higher levels of upgrades using these systems.

Selected references

  • ACI 216R-89, Guide for Determining the Fire Endurance of Concrete Elements, American Concrete Institute, Farmington Hills, Michigan.

  • ECI 1994, Eurocode 1: Basis of Design and Design Actions on Structures, Part 2-2: Actions of Structures Exposed to Fire, European Committee for Standardization, Brussels, Belgium.

  • Buchanan, A. H., 2001 (editor), Fire Engineering Design Guide, Center for Advanced Engineering, University of Canterbury, New Zealand.

  • Buchanan, A. H., 2001, Structural Design for Fire Safety, John Wiley and Sons, LTD.

  • Kumahara, S.; Masuda, Y.; and Tanano, Y., 1993, Tensile Strength of Continuous Fiber Bar Under High Temperature, International Symposium on Fiber-Reinforced-Plastic Reinforcement for Concrete Structures, SP-138, American Concrete Institute, Farmington Hills, Michigan.

  • Wang, N., and Evans, J.T., 1995, Collapse of Continuous Fiber Composite Beam at Elevated Temperatures, Composites magazine, Volume 26, No. 1.

Tarek Alkhrdaji, Ph.D., is a design engineer with Baltimore-based Structural Group. He has been involved in numerous projects involving structural repair and upgrade as well as full-scale, in-situ load testing. He can be reached via e-mail at talkhrdaji@structural.net.


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