This section gives basic advice on the assessment and repair of fire damaged buildings, because structural engineers are often engaged to report on options for re‐use or repair. If there is a danger of local collapse, immediate concerns about the stability of free‐standing residual parts of a fire damaged building will have to be addressed very quickly. More often the owner will want to know if the damaged building can be rehabilitated, in which case there will be time for a more complete investigation.
Figure 2.17 One of many severe fires which destroyed buildings after the Hanshin‐Awaji earthquake, Kobe, Japan, 1995
2.7.1 Inspection
It is important to visit the fire scene as soon as possible after the fire, while all the fire debris and non‐structural damage is visible. This visit can provide essential information on the extent of the fire, the location of the most severe burning and the maximum temperatures reached during the fire. It is also important to revisit the fire scene after debris and non‐structural items have been removed, when it becomes possible to inspect structural members in more detail.
It is very important to inspect the details of connections between structural members for cracking of concrete, damage to welded connections or distortion of bolts.
Maximum local temperatures reached in a fire can be estimated from an inspection of mate- rials which have melted. The approximate melting temperatures of several materials are given in Table 2.2. The duration of the fully developed period of the fire can be roughly estimated from the residual size of heavy timber members which will have charred at approximately 0.7 mm per minute as described in Chapter 9.
Most of the significant fire damage in a structure will be readily visible. With the exception of temperature‐related loss of material strength, significant damage will usually be visible as large deflections, local deformations, spalling of concrete or charring of timber. Most mem- bers which have deformed during the fire will have to be replaced, unless the deformations do not affect the future use of the building. Allowance must be made for deflections which may have existed before the fire. If a large number of members have significant distortion the entire structure may have to be demolished.
2.7.2 Steel
Unprotected steel members often suffer large deformations in fully developed fires, whereas well protected members usually exhibit little or no damage. In most cases no further assessment is necessary for fire‐exposed steel members which remain straight after cooling (Tide, 1998).
The most common grades of structural steel do not suffer significant loss of strength when Table 2.2 Approximate melting temperature of materials
Material Approximate melting
temperature (°C)
Polyethylene 110–120
Lead 330
Zinc 420
Aluminium alloys 500–650
Aluminium 650
Glass 600–750
Silver 950
Brass and bronze 850–1000
Copper 1100
Cast iron 1150–1300
Steel >1400
Source: Reproduced from Gustaferro and Martin (1988) by permission of Precast/Prestressed Concrete Institute.
badly cracked, can be replaced with poured or sprayed concrete, incorporating additional reinforcing if necessary. Concrete members exhibiting no visible damage may have reduced strength due to elevated temperatures of the concrete or the reinforcing. Typical mild steel reinforcing regains any lost strength when it cools. High strength steels, especially cold‐drawn prestressing tendons, are susceptible to strength loss if they are heated to temperatures above 400 °C. Prestressing steels cooled after heating to 500 °C can have a 30% loss of strength and heating to 600 °C can result in a 50% loss of strength (Gustaferro and Martin, 1988).
Loss of strength of the concrete itself is usually of less concern than loss of strength of the steel reinforcing. The heat affected region is often not very thick because of the low thermal conductivity of concrete. In simply supported flexural members, the compression zone on the top of the slab or beam is often not exposed to very high temperatures. Loss of strength of concrete near the surface can be estimated with an impact rebound hammer. Some types of concrete change colour after heating to elevated temperatures, depending on the aggregate.
Marchant (1972) describes a design procedure for reinstatement of fire damaged reinforced concrete buildings, and reports that typical concrete heated to less than 300 °C will have no colour change, concrete heated to 300–600 °C may be pink, concrete heated to 600–950 °C may be whitish‐grey and concrete heated over 950 °C may be a buff colour. Fire‐exposed concrete suffers no significant loss of residual strength when heated below 300 °C, whereas for higher temperatures the strength loss will depend on the concrete temperature as described in Chapter 7. When the concrete cools after heating, it regains strength slowly but never reaches the original strength (Lie, 1992).
Ceramic clay bricks lose very little strength after heating to temperatures as high as 1000 °C, but the mortar may suffer some damage. Reinforced concrete masonry will need to be assessed in the same way as normal reinforced concrete.
2.7.4 Timber
Because wood burns, fire damage to exposed timber surfaces is immediately visible. Heavy timber structural members such as beams, columns, or solid wood floors will be charred on the surface, with undamaged wood in the centre, as described in Chapter 9. The residual wood under the charred layer can be assumed to have full strength. The size of the residual cross section can be determined by scraping away the charred layer and any wood which is signifi- cantly discoloured. Fire‐exposed heavy timber members tend to deform much less than
unprotected steel members. Fire‐damaged timber members do not need to be replaced if the residual cross section has sufficient strength to carry the design loads. For future fire resis- tance it may be necessary to apply additional protection such as new layers of wood, or gypsum board, because of loss of the sacrificial wood. Severely damaged members will need to be replaced.
Light timber frame structures are protected from fire by linings of non‐combustible material such as gypsum board, as described in Chapter 10. After a severe fire, the linings on the under- side of ceilings and the fire side of walls will certainly be damaged. Some linings may have fallen off due to the effects of the fire or fire‐fighting activities. All damaged linings should be removed to inspect damage to the studs or joists. Any charred timber will have reduced load capacity. Calculations will be necessary to assess the strength of the residual members.
Inspection of gypsum board can give an indication of the duration of fully developed burning. When gypsum board is exposed to fire it dehydrates steadily from the hot surface.
The depth of dehydration can be observed by breaking open a small piece of board to locate the transition between the soft dehydrated plaster and the solid gypsum of the original board.
Typical gypsum board dehydrates at approximately 0.5 mm per minute.
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 discusses fires in rooms and heat transfer to structural members. It reviews the combustion of fuels in typical building fires, and the factors that affect fire growth. It also provides simple descriptions of pre‐ and post‐flashover fires. For more information on these topics, refer to Quintiere (1998), Karlsson and Quintiere (2000) or Drysdale (2011).