4.6.2.3 Structural Model
Models for calculating the performance of structural elements exposed to fire are described in Chapter 5. Hand calculation methods can be used for simple elements but sophisticated com- puter models are necessary for the analysis of frames or larger structures. Computer‐based structural analysis models must be able to include the effects of thermal expansion, loading and unloading, large deformations and non‐linear material properties which are temperature‐
dependent, all for a framework of interconnected members of different materials. Hand calcu- lation methods for the main structural materials are given later in this book.
4.7.2 Floors
Floors are more difficult to test than walls because a larger furnace is necessary and the load- ing equipment is much more extensive. All floors are load bearing, so they all must be loaded during testing. Because of furnace size limitations, most floor systems are tested at spans less than commonly used in buildings. A tested floor system should have similar stresses and similar deformations to those expected in longer spans in a real building, to enable the test results to be extrapolated. Floors designed to span in only one direction must be free to behave in that way in the test furnace. Deformations are particularly important because the flexural curvature of a floor will affect the integrity of any fire resisting membrane or applied fire protection.
Many floor–ceiling assemblies rely on a ceiling membrane as an essential part of the fire resisting system, so these assemblies must be tested as a complete system. Flexural continuity and axial restraint can have a large influence on the results of fire resistance testing of floors, as described previously. Traditionally, floors have only been fire tested from below. This is because most fires tend to spread up, not down, and the most vulnerable part of the structure is usually on the underside. There has been some recent concern about the possibility of fires burning downwards through light timber floors clad on the top surface with particle board or plywood, but observations at real fires show that there is often a lot of debris on the floors, from collapsed ceilings, fittings and contents, which can provide some protection to the top surface of the floor system.
4.7.3 Beams
Beams are always tested for fire resistance as part of a floor or roof assembly, with fire exposure from below. The floor or roof may be structurally part of the beam, with composite action, or it may simply be a non‐structural component to seal off the top of the furnace. When assessing fire resistance of beams it is essential to know whether there is composite action. A major difficulty in fire testing of beams is the limited span available in almost all furnaces. As with floors, a tested beam should have similar stresses and similar deformations to those expected in longer spans in a real building, but this can be very difficult to achieve over a small span. Flexural continuity and axial restraint can be difficult to provide because of the large forces involved, but these can have a very large influence on the results. Effects such as these may be better assessed by calculation.
4.7.4 Columns
Columns are usually tested in special furnaces which expose the column to fire from all sides (Figure 4.11). There are only a few column furnaces in the world, so fire resistance of columns is often achieved by calculation rather than by test, or by using conservative generic ratings.
In real buildings, columns are often built into walls which protect one or more sides of the column from fire exposure. Partial fire protection of a column may reduce the load‐bearing capacity during a fire because of temperature gradients through the cross section leading to thermal distortion and eccentricity.
4.7.5 Penetrations
Penetrations through walls and floors can severely reduce the ability of these barriers to contain a fire. If fire enters a cavity in a wall or floor assembly through a penetration, it can also severely reduce the load capacity. Walls and floors in typical buildings have numerous penetrations for electrical, plumbing and air handling services. The fire resistance of all of these must be con- sidered as part of the fire safety design. There are standard methods of testing fire stops at penetrations through walls and floors (e.g. ASTM, 2012). Methods for protecting ‘poke‐
through’ penetrations are given by Gustaferro and Martin (1988). Particular problems can occur if unprotected penetrations are not visible, such as hidden penetrations through walls above suspended ceilings, and penetrations made at a later date during alterations to the building.
Many proprietary products are available for sealing gaps and openings in buildings, including fire resisting boards, paints, mastic sealants, intumescent strips and pillows. Products such as these can make the difference between success and failure of passive fire protection strategies.
Fire test performance of plastic pipe penetrations is described by England et al. (2000).
The fire resistance of walls and floors can also be reduced if the barrier is penetrated by a heat‐conducting member such as an unprotected steel beam. Fire on one side of the barrier can Figure 4.11 A special furnace for fire resistance testing of columns, with an unprotected steel column ready for testing. Reproduced by permission of Corus UK Ltd
cause the member to heat up, conducting heat to the unexposed side, where ignition can occur if combustible materials are in contact. Some codes prohibit this type of penetration. The danger of fire spread can be reduced by insulating the beam for a certain distance on either side of the wall. Deformations in steel beams passing through walls can also damage the wall unless the structure is specifically designed to prevent such damage.
4.7.6 Junctions and Gaps
Junctions between walls and floors are seldom fire tested. It is important for designers to assess the connecting details to ensure that the junctions do not give weaknesses in barriers that otherwise have excellent fire resistance. There are many proprietary products for providing fire resistance at junctions. Most common materials including concrete steel and wood can be used for preventing fire spread through junctions, provided that the materials have sufficient thickness and are well detailed. Aluminium and plastic materials are not suitable because they melt at low temperatures. Gaps between precast concrete panels can be fire rated using ceramic fibre blanket (Gustaferro and Martin, 1988; ICC, 2015).
4.7.7 Seismic Gaps
In seismic regions, buildings are provided with seismic gaps to allow differential movements to occur in the event of an earthquake. These seismic gaps can be within a building (to separate non‐structural items and prevent damage when the structure moves) or between buildings (to allow separate parts of the building to move independently). Expected movement on one floor within a building can be 50 mm or more, and expected movement between parts of multi‐
storey buildings can be up to half a metre. It is very difficult to provide details and flexible filling materials to accommodate these movements, and also provide fire resistance before, during and after an earthquake. A review of this problem is given by James and Buchanan (2000). Many proprietary products are available for filling seismic gaps, but their fire performance after large movements is often not proven.
4.7.8 Fire Doors
Doors are a very important part of the passive fire protection in many buildings. There are many proprietary fire resisting doors on the market, but they are usually expensive and have to meet different requirements in different countries. If a fire door is to match the fire resistance of the wall in which it is installed, the whole door assembly must be able to meet the integrity and insulation requirements for the specified fire resistance period. Solid core doors can easily be made with sufficient fire resistance, but weaknesses occur at the handle, hinges and all around the door edges. Many countries require that fire‐rated doors be tested with exactly the same hardware as will be used in practice (Figure 4.12). The edge of the door or the frame is often fitted with a strip of intumescent material that swells into a foam when heated, to prevent flames penetrating the gap around the door.
Glazed doors are only required to meet integrity requirements because glazed panels cannot meet insulation requirements. Various codes have different limitations on the maximum size of the glass panel in fire doors. To meet the integrity requirements, the glass must be special fire resistant glass or be wired glass. There are an increasing number of proprietary fire resistant
glazing products on the market. Fire safety requirements for doors are very different in differ- ent countries. Some fire doors are required to prevent spread of smoke, in which case they must pass an air leakage test as well as a fire test. Many aspects of the performance of fire doors under test are described by England et al. (2000), who also propose an improved test method.
Real fire experience has shown that steel roller‐shutter doors maintain excellent integrity in severe fires. No insulation rating is possible because the thin steel of the door heats up very rapidly, but a roller‐shutter door can restrict the spread of fire provided that there are no com- bustible materials near the unexposed face of the door.
4.7.9 Ducts
Air handling ducts are potential paths for fire spread in buildings. Some authorities require ducts to be provided with fire resistance. Typical steel ducts can only provide an integrity rating, which can be improved to an insulation rating with insulating material such as ceramic fibre blanket placed internally or externally. More fire resistant ducts can be made from mul- tiple layers of material such as gypsum board. There is no standard test method for ducts, but some systems have been tested successfully in non‐standard tests.
An air‐handling duct passing through a barrier can cause a serious reduction in the fire resistance of the barrier. This can be prevented by placing a ‘fire damper’ inside the duct where it passes through the barrier. Some fire dampers are also designed to control smoke movement. The dampers are designed to close automatically. Small dampers operate when a Figure 4.12 Fire resistance test of two doors. The door on the left has had an integrity failure, as shown by penetration of flames and hot gases. Reproduced by permission of Building Research Association of New Zealand
spring‐loaded blade or curtain inside the duct is released by melting of a heat‐activated fusible link. Dampers in large ducts may have motorized closers which are activated by the fire detec- tion system in the building. Another type of system has blades covered with intumescent material which swells up to close the duct at high temperatures. Testing requirements are described by England et al. (2000).
When there is a severe fire on one side of a wall penetrated by a duct, the collapsing duct on the hot side of the wall may cause damage to the wall itself, reducing the fire resistance. To prevent such damage, the fire damper should be firmly attached to the wall, and the duct should be con- structed with joints which allow the duct to pull away from the damper, leaving the damper intact as part of the wall. This approach cannot be easily applied to fixed services such as cable trays.
4.7.10 Glass
Glass is a vitreous solid material with crystal structure similar to a liquid. On heating, it goes through a series of phases of decreasing viscosity. Most typical glass softens or melts at temper- atures from 600 to 800 °C, but it will crack or break if exposed to thermal shock at much lower temperatures, due to differential temperatures within the glass or because of expansion of the surrounding frame. Normal window glass is assumed to break and fall out of the windows at the time of flashover (typically around temperatures of 500–600 °C), although tests have shown that this does not always occur. Toughened glass or heat strengthened glass may not shatter at high temperatures. Double glazing tends to remain in place much longer than single layers of glass.
Glass is sometimes used in fire resisting barriers, where it can only provide an integrity rating, because it has no structural capability at elevated temperatures and cannot provide an insulation rating unless it is coated with some sort of intumescent coating. If glazing is to be used in a fire resisting barrier, it must be assembled with special glass, either wired glass (reinforced with fine wires in both directions) or specially formulated fire resistant glass. Fire resisting glazing is usually installed in steel frames which clamp the glass and prevent it from deforming excessively when it gets hot. Aluminium frames cannot be used because of low melting temperatures. Glazed assemblies can be tested in full‐scale fire resistance tests, but the assessment is only for the integrity criterion.
A number of proprietary insulated glazing systems have recently been developed, consist- ing of alternating layers of glass or sodium silicate with transparent intumescent materials.
These products are transparent at room temperatures, but become opaque at high tempera- tures, achieving fire resistance of up to 2 h. Glass walls and windows can provide resistance to fire spread if they are sprayed continuously with water from a properly design sprinkler system (Kim et al., 1998; England et al., 2000).
4.7.11 Historical Buildings
Fire engineers are sometimes asked to report on the fire safety of historical buildings. This often requires information on fire resistance of old materials and obsolete building systems. This information can often be obtained from many current listings, and calculations can be made using the information in this book. A useful reference is Appendix L to NFPA 909 (NFPA, 2010b) which gives extensive lists of fire ratings of elements such as masonry walls, hollow clay tile floors, old‐style doors and cast iron columns, which are no longer used in new construction.
For concrete
Thermal conductivity k 1 6. W/mK
Density 2300kg/m3
Specific heat c 980 J/kgK
Thermal inertia k cp 1900Ws /m K0 5. 2 (medium) Conversion factor kb 0 055.
Window height Hv 2 0. m Window width B 3 0. m
Window area Av H Bv 2 0 3 0 6 0. . . m2 Horizontal vent area Ah 0 (no ceiling opening)
v A Av/ f 6 0 24 0 0 25. / . .
h A Ah/ f 0
bv 12 5 1 10. v v2 43 0.
Ventilation factor w 6 0
3 0 0 62 90 0 4 0 25
1 43 0 0 0 820
0 3 4
.
. . . .
. .
.
m0 3. Equivalent fire severity te e k wf b 800 0 055 0 820 36 1. . . min
4.8.2 Worked Example 4.2
Repeat Worked Example 4.1 with an additional ceiling opening of 3.0 m2. Ceiling opening area Ah 3 0. m2
h A Ah/ f 3 0 24 0 125. / .
Ventilation factor w 6 0
3 0 0 62 90 0 4 0 25 1 43 0 0 125 0
0 3 4
.
. . . .
. . .
.
7772m 0 3. Equivalent fire severity te e k wf b 800 0 055 0 772 34. . min
4.8.3 Worked Example 4.3
Repeat Worked Example 4.1 using the CIB formula and the Law formula.
CIB formula
Length of room l1 6 0. m
Width of room l2 4 0. m
Floor area Af l l1 2 6 0 4 0 24 0. . . m2
Height of room Hr 3 0. m
Fuel load energy density ef 800MJ/m2
Total area of the internal surface At 2 l l1 2 l H1 r l H2 r 2 6 4 6 3 4 3 108m2 For concrete
Thermal inertia k cp 1900Ws /m K0 5. 2 (medium) Conversion factor kc 0 07. minm2 25. /MJ
Window height Hv 2 0. m Window width B 3 0. m
Window area Av H Bv 2 0 3 0 6 0. . . m2 Ventilation factor w A
A A H
f
v t v
24 0
6 0 108 0. 2 0 0 793 0 25
. . . . m .
Equivalent fire severity te e k wf c 800 0 07 0 793 44 4. . . min Law formula
Net calorific value of wood Hc 16 MJ/kg Equivalent fire severity t A e
H A A A
e
f f
c v t v
24 0 800
16 0 6 0 108 0 6 0. 48 6
. . . min
Structural Design for Fire Safety, Second Edition. Andrew H. Buchanan and Anthony K. Abu.
© 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd.
This chapter describes the process of designing structures to resist fire exposure. It also describes some simple tools for making structural calculations, and explains the importance of loads and support conditions in estimating load capacity under fire conditions. These proce- dures can be used for verifying structural fire performance in the strength domain.
Building structures are made up of a number of elements such as walls, floors and roofs, often supported by structural members such as beams and columns. To avoid collapse of a building structure, the combination of elements and their supporting members must perform their load‐bearing function for the duration of the fire.
In many simple structures, collapse of one member can result in total collapse of the struc- ture. Hence in a fire, structural failure can occur if the applied load exceeds the load capacity of a critical member at any time during the fire. In more complex structures it may be possible for the structure to survive a fire even if one or more members loses its load‐carrying capacity. This is more likely to occur in a redundant structure with a number of alternative load paths.