2.2 Behaviour of structural materials, buildings and building contents in a fire

Question 1

Timber

Answer 1

 Timber burns though the burning or charring rate is predictable and varies slightly with the type of timber and not with the severity of the fire.  The timber behind the charring plane is largely unaffected and able to perform structurally as intended.

Question 2

Stone

Answer 2

Building stone is typically granite, limestone or sandstone.  Granite contains free quartz which expands very rapidly at 575oC completely shattering the rock.  Limestone (calcium carbonate) decomposes into free lime and CO 2 at approx. 800oC. The interior is protected by the outer skin.  Sandstone will shrink and crack in a fire at temperatures between granite and limestone.

Question 3

Bricks

Answer 3

 Traditionally in the UK bricks were made of fired clay. Concrete bricks and sand-lime bricks (calcium silicate) are now also popular.  Brick performs better than stone in a fire.

Question 4

Concrete

Answer 4

Concrete consists of aggregates, cement and water mixed to form a mouldable material which sets hard with high compressive strength and durability. It is non-combustible with a slow rate of heat transfer and can achieve fire rating requirements of 1 to 4 hours. Concretes are termed normal weight concrete (NWC) or lightweight concrete (LWC), depending on the density of the aggregates used. High strength concrete (HSC) uses a low water/cement ratio through the addition of fly ash, slag or superplasticisers, or additives such as silica fume to enhance its strength. Temperatures up to 95oC (200oF) have little effect on concrete. Above this threshold cement paste shrinks due to dehydration and aggregates expand due to temperature rise. For normal weight concrete the aggregate expansion exceeds the paste shrinkage resulting in an overall expansion of concrete.

Question 5

Concrete blocks

Answer 5

Blocks made with limestone aggregates have higher fire resistance. The fire resistance is greatly improved if plastered both sides with a lightweight gypsum plaster.

Question 6

Reinforced or pre-stressed concrete

Answer 6

 Fire resistance depends on the mass of concrete around the steel reinforcement.  Critical temperature (loss of 50% of cold strength) for mild steel is 550oC and 400oC for high tensile steel.  Reinforced concrete will deflect considerably under load but is unlikely to collapse suddenly.

Question 7

Structural Steel

Answer 7

 Structural steel loses 2/3 of its strength at 593oC and will sag and twist in the direction of, and in proportion to, the load applied.  Steel joists expand on heating (a 10m joist will expand 600mm at 500oC), which may cause load bearing support walls to collapse.

Question 8

Other metals

Answer 8

 Lead flashings and plumbing melt at 327oC.  Aluminium alloy cladding: stability affected at 100oC to 225oC; high expansion rate; high thermal conductivity; melts at 660oC.

Question 9

Glass

Answer 9

Non-combustible. May be a weak point in a fire barrier as it will break at high temperatures.

Question 10

Plasterboard

Answer 10

 Consists of a core of set gypsum or anhydrite plaster bonded to external facings of heavy paper.  Gypsum is non-combustible. Will retard fire spread until paper burns and gypsum core breaks up.

Question 11

Asbestos sheets and boards

Answer 11

 Asbestos cement sheets contain up to 15% asbestos. Shatter in the early stages of fire.  Asbestos insulating / wall boards are 80% asbestos 20% lime-silica bonding agent. They are non-combustible but will contract and bow away from heat source.

Question 12

Plastics

Answer 12

The term covers a broad range of chemicals. Generally plastics are combustible and the combustion products are likely to be toxic.

Question 13

Fire resistance is determined in accordance with performance over three criteria:

Answer 13

 Resistance to collapse (Ioad-bearing capacity) - which applies to loadbearing elements only, denoted R in the European classification of the resistance to fire performance (BS EN 13501).  Resistance to fire penetration (integrity) - denoted E  Resistance to the transfer of excessive heat (insulation) - denoted I

Question 14

Classification of linings

Answer 14

Small rooms of area not more than: (a) 4 m2 in residential accommodation (b) 30m2 in non-residential accommodation 3 Other rooms (including garages) Circulation spaces within dwellings 1 Other circulation spaces, including the common areas of blocks of flats 0

Question 15

Fire load density refers to

Answer 15

the quantity of fuel per unit area. It is normally expressed in terms of MJ/m2 or kg/m2 of wood equivalent.