Construction materials .1 STEEL

Steel is the major structural material in industrial buildings. Its strength, ductility, consistency, and availability render it uniquely desirable for structural framework and for concrete reinforcing.

However it is significantly weakened at fire temperatures such that very lightweight unprotected members can fail after only 5 – 10 minutes direct exposure to an intense fire (Fitzgerald, 1986).

The actual fire resistance of a steel member is determined by its structural load, size and shape, constraints, fire heat flux history, and material properties. Critical structural properties include yield strength and ultimate strength. Relevant thermal properties include the coefficient of thermal expansion, density, specific heat, and thermal conductivity.

The yield strength is generally defined (Fitzgerald, 1986, p. 126) as the stress that produces a permanent deformation of 0.2% of the original length of the test sample. It is also approximately equal to the stress at the plateau region of a stress-strain curve for steel at temperatures of 200^◦C (392^◦F) and lower. The ultimate strength is the maximum stress in a stress-strain curve. Beyond the ultimate stress, increasing deformation occurs with decreasing stress until the steel ruptures.

The room temperature yield stress for A-36 structural steel is approximately 36,000 psi (250 MPa),

Frequently occurring fire spread routes (Ref: ISO TC 92 SC4 Fire safety Engineering)

Through wall Through opening Over roof

Roof

Floor

Floor

Roof Roof / floor

Void Void

Floor Within roof

Through horizontal duct

= Fire

= Fire route

Through vertical duct

Through shaft (lifts, staircases, etc.)

Above ceiling Below floor

Figure 3.1. Fire spread paths (from ISO TC 92, SC4 Fire Safety Engineering)

and its room temperature ultimate stress is about 54,000 psi (370 MPa), although somewhat higher values are sometimes measured for both values (Lie, 1972).

The curves in Figure 3.2 show the ultimate strength and the yield strength variation with temperature, normalized by their respective values at room temperature, for St 37 mild structural steel. The yield strength decreases monotonically with temperature, while the ultimate stress peaks at about 250^◦C (482^◦F) and then decreases rapidly with increasing temperature. There is considerable spread in the data for both yield strength and ultimate strength. For example, the temperature at which the strength of St 37 is reduced by 40% is in the range 320 to 500^◦C (610 to 930^◦F) depending on the data scatter and whether we refer to yield strength or ultimate strength. The 40% reduction temperature is significant because the American Institute for Steel Construction specifies a maximum permissible design stress of approximately 60% of the yield strength for structural steel buildings (Milke, 1995).

The critical temperature for steel fire resistance is the steel temperature at which its strength is reduced to the point that it cannot support its applied load. This temperature depends on the precise structural failure criterion as well as the structure configuration, design load, and steel composition. Various examples listed in Table 3.1 span the range from 730 to 1220^◦F

s_x

s_y

Temperature, °C

Strength, percentage of original strength

100 200 300 400 500 600

20 40 60 80 100 120 140

Figure 3.2. Temperature effect on steel strength (modified with permission from Lie, 1972)

Table 3.1. Critical temperatures for steel structures

Steel Configuration Load

ratio^a

Critical temperature [F (C)]

Reference

ASTM A-36 Statically Determinate Beam 0.6 880 – 1110(470 – 600) Lie, p 162 ASTM A-36 Statically Indeterminate Beam 0.6 1080 – 1220(580 – 660) Lie, p 162

ASTM A-36 Statically Determinate Beam 0.6 890(475) Milke^b

ASTM A-36 Statically Determinate Beam 0.4 1050(565) Milke^b

ASTM A-36 Statically Indeterminate Beam 0.6 890 – 1040(475 – 560) Milke^b ASTM A-36 Statically Indeterminate Beam 0.4 1050 – 1140(565 – 615) Milke^b ST 37 Statically Determinate Beam 0.6 730 – 890(390 – 475) Lie, p 162 ST 37 Statically Indeterminate Beam 0.6 890 – 1020(475 – 550) Lie, p 162

ST 37 Long Column (L/r > 100) 0.3 970(520) Lie, p 168

ST 37 Short Column (L/r < 100) 0.3– 0.5 790(420) Lie, p 168

ASTM A-36 Long Column (L/r > 100) 0.52 940(505) Milke

Various Short Columns (L/r = 23 − 87) – 507 – 753 (945 – 1387) Talamona et al.

(1996)^c

aLoad ratio values are the ratio of the applied (design) load to the load that would generate a stress equal to the room temperature yield stress

bCritical temperatures calculated by Milke (1995) were based on the analysis recommended by the European Convention for Constructional Steelwork.

cCritical temperatures reported by Talamona et al. (1996) are based on measured temperatures at observed column failure times.

(388 to 660^◦C) for beams and from 790 to 1020^◦F (421 to 549^◦C) for columns. Values for statically indeterminate structures are higher than those for statically determinate structures. Lie (1972) recommends using a representative value of 790^◦F (421^◦C) for a statically determinate beam and 970^◦F (521^◦C) for a statically indeterminate beam.

The coefficient of thermal expansion for steel increases appreciably as the steel temperature is increased. The relationship quoted by Milke (1995) is

α= (6.1 + 0.0019T ) × 10⁻⁶ [3.1.1]

where α is the coefficient of thermal expansion (in/in-^◦F), and T is the steel temperature rise above 100^◦F.

The linear expansion corresponding to equation [3.1.1] can be substantial at temperatures approaching 538^◦C (1000^◦F). For example, a 15 m (50 ft) long steel beam would be elongated by about 10 cm (4 in), which could be enough to collapse constrained walls or ceilings. The forces on fire walls due to heated beam expansions are discussed in Section 3.4.

Thermal property data for steel and other construction and insulation materials are listed in Table 3.2. The thermal conductivity and the mass density of steel are significantly higher than for most other materials. On this basis, steel temperatures would be expected to be spatially uniform and to lag significantly behind the local gas temperatures. The actual time lag is analyzed in Section 3.2.

Table 3.2. Thermal properties of construction and insulation materials (data from Appendix A of SFPE Handbook (1995) for most materials)

Aluminum 20 204 2707 0.896 8.42

Asbestos Cement 20 0.175 750 – –

Ceramic Fiber Blanket 260 0.055 2.18

538 0.115 4.55

96.12 0.263

816 0.202 8.00

1093 0.208 11.40

Concrete

Light Weight 20 0.61 1200 0.84 60.

Normal Weight 20 1.64 2300 0.84 8.5

600 1.1 2300 1.25 3.8

Fiber Board 20 0.048 240 – –

Magnesia-85% 38 0.067 270 – –

Mineral Wool (Sprayed) 0.17 250 – –

Plaster

Cementitious 20 0.21 750 – –

Metal Lath 20 0.47 1440 0.84 4.0

Steel

1% Carbon 20 43 7800 0.473 1.17

300 40 7800 0.47 1.1

600 33 7800 0.47 0.90

Wood

Maple-Oak 30 0.17 540 2.4 1.3

Gypsum Board 20 0.24 678 0.90 3.9

100 0.24 649 3.0 1.2

300 0.12 675 0.80 2.2

3.1.2 STEEL INSULATION

Protective insulation is often applied to steel structures in order to achieve a desired level of fire resistance. Some of the protective materials employed include magnesia, vermiculite, concrete, sprayed mineral wool, and intumescent/ablative coatings. Thermal property data for several of these materials are listed in Table 3.2. Some of the materials have thermal conductivities four orders-of-magnitude lower than that of steel. Fire resistance calculations to account for the low thermal conductivities of insulating material effects are described in Section 3.2. The effect of steel insulation on fire resistance test results is discussed in Section 3.3.

There are several practical considerations associated with the selection and evaluation of fire resistant steel insulation. For example, many of the commercial insulations require careful applica-tion and curing procedures to keep the insulaapplica-tion intact and properly attached to the steel structure.

Furthermore, weathering, aging, and hose stream resistance tendencies of steel insulation are also important for certain applications. When warranted, special tests of these characteristics are some-times conducted to supplement the fire resistance tests. Commercial insulations that have been certified to achieve a specified level of fire resistance and to have passed certain of these supple-mental tests are listed in the Factory Mutual Approval Guide and/or the Underwriters Laboratories Fire Resistance Directory or in certification listings of other testing organizations.

3.1.3 CONCRETE

The inherent compressive strength of concrete (a typical room temperature ultimate compressive strength of 60,000 psi= 410 MPa) makes it an attractive material for columns and load bearing walls. Use of steel reinforcing bars to support tensile loads allows concrete to also be used extensively for beams and floor slabs. The ratio of reinforcing bar cross-sectional area to concrete area and the tensile strength and compressive strength of the steel and concrete, respectively, determine whether a beam will fail in tension or compression (Fitzgerald, 1986).

The variation of concrete compressive strength with temperature depends on the type of aggre-gate in the concrete. Siliceous aggreaggre-gate concrete with cement-to-aggreaggre-gate ratios of about 1:6 start weakening at temperatures of about 482^◦C (900^◦F) as indicated by the graphs in Figure 3.3.

Higher proportions of cement start weakening at lower temperatures. Carbonate and lightweight aggregates remain relatively unaffected by temperature until about 649^◦C (1200^◦F). According to Fleischmann (1995), the critical temperature at which concrete is rendered structurally inef-fective (as measured by strength reductions of about 50% of the room temperature values) is about 1200^◦F for siliceous concrete and about 760^◦C (1400^◦F) for carbonate and lightweight aggregates.

The thermal expansion of concrete is similar to that of steel for temperatures up to about 538^◦C (1000^◦F). Thermal expansion of a concrete floor slab heated from below can cause large thermal thrust forces to be exerted on surrounding structures restraining the concrete expansion.

These thermal thrust forces can be the limiting factor determining the fire resistance of concrete slabs. Calculation procedures to account for this effect are described by Fleischmann (1995).