Building Construction and Fire Resistance Flashcards
List the factors which have a bearing on the behaviour materials during a fire.
7 factors
(i) Severity of the fire.
(ii) Time the material has been involved.
(iii) How the material is fixed to adjoining materials.
(iv) Reaction between adjoining materials.
(v) Position of the material - wall, floor, roof, cladding etc.
(vi) Reaction of the material to extinguishing medium being used.
(vii) The standard of construction
Describe how steel behaves when exposed to fire/high heat conditions.
- Steel loose two thirds of its strength at ~550c.
- If structural or loading bearing it will twist in proportion to the direction of the load.
- Steel expands ~60mm for a 500c rise in temperature, therefore if positioned in a landing bearing wall it could force them outwards contributing to structural collapse.
- can be strengthened by flat plates reverted to flanges.
Describe the methods by which steel can be protected from fire/heat.
Intumescent Paints
- spray or brush, these paints expand (upto 50 times there original thickness) on exposure to heat in fire conditions and char in due course sealing off the oxygen supply around the steel from the fire and protects the steel from the rise in temperature produced by the fire.
Cladding
- with fire resistant boards, care must be taken to seal all joints and make sure the steel has been completely surrounded by the boards.
Encapsulation
- via concrete encasement, concrete and steel have similar expansion rates therefore they work well together, the concrete protects the steel from the rise in temperature.
Describe how aluminium behaves when exposed to fire/high heat conditions
Very rapid loss in strength in fire.
Stability affected anywhere from 100-225c
Has a high expansion rate, twice that of steel.
Very low melting point (pure Al 660c)
Describe how timber behaves when exposed to fire/high heat conditions
- Ingnites at 260-427c.
- Does not expand very quickly, may even shrink. Therefore if structural there is not normally a risk of immediate collapse.
- Very predictable burn and charring rates.
- Chars as it burns forming a protective layer limiting the amount of oxygen the wood fibres beneath are exposed to thereby insulating them from the fire.
- Not easily ignitable in sizes used for construction.
- Moisture content will affect fire performance with drier wood charring more readily.
Describe the methods by which the fire resistance of timber can be increased.
Intumescent paints
brush or sprayed on the expand and char insulation the timber from the effects of fire and heat.
Impregnation
fire resistant chemical are injected into the wood fibres via a pressure and vacuum process, timber should not be cut or sanded after this process is complete.
Cladding
with fire resistant boards, must be sure all timber has be sufficiently covered and joints (of the boards) are sealed appropriately.
Sacrificial timber
due to the known charring rate extra timber can be built into the fabric of the structure if involved in fire this timer sacrifices itself while the integrity of the timber require to maintain the structure is maintained, allowing ample time for evacuation before structural integrity is compromised.
What are the three certifications of fire rated glazing.
E - Integrity - provides integrity only.
EW - Integrity and low heat insulation
EI - Insulating and Integrity - provides integrity and insulation to radiant heat.
Describe the five categories of fire rate glazing
(i) Non-insulating Glass
able to resist to passage of smoke and flames but not satisfy insulation criteria.
two types:
Wired - can resist thermal shock from extinguishing media,
- suitable for fire rated doors and windows.
Special composite glass - does not break on exposure to fire due to low co-efficient of thermal expansion.
- suitable for applications involving rapid changes in temperature eg cookware, light fitting.
example Borosilicate glass.
(ii) Partially Insulating Glass
Usually laminate planes incorporating one intumescent interlayer that becomes opaque (via charring) on exposure to heat.
This glass is resistant to the passage of smoke, flames and hot gases and meets insulation criteria for heat up to 15 minutes.
after 15 minutes the unexposed side begins to rise in temperature above accept level albeit slower than non-insulating glass.
(iii) Insulating Glass Products
able resist the passage of smoke, flames and hot gases and are rated to up to 30 minutes for stopping the transfer of heat.
two types:
a. Laminated intumescent glass
b. gel inter-layered glass - fire resistance related to the thickness of the gel between 2 layers of glass, held by metal spacers and sealed at edges.
(iv) Multi-laminate Glass
Consists of multiple layers of glass and clear intumescent interlayers.
On exposure to heat the intumescent layers char turning opaque preventing the transfer of radiant and conductive heat.
(v) Heat treated fire glass
wire free, fractures early if subjected to thermal shock.
key aspect is edges must be protected from the fire.
Imperative with all fire resistant glass that the frame and method of fixing must have the same or high fire resistance.
Describe the behaviour of concrete in fire and the two structural types most commonly used.
Weak and prone to cracking if not reinforced.
Fire resistance determined by the aggregate used.
Structural concrete fire resistance determined primarily by the amount of concrete covering the embedded steel.
Generally does not collapse suddenly but may deflect considerably under load and floors may suffer local breakdown.
Critical Temps:
Mild steel 550c
High tensile steel 400c
Types:
Reinforced
- mild or high tensile steel bars encased in concrete.
- not stressed until load is imposed on the structural members.
Pre-stressed
- of which two types Pre and Post tensioned.
Pre-tensioned - steel cables/tendons anchored at both ends and tensioned prior to being encased in concrete. Once concrete has cured to a determine level tension at the anchor points is released, allowing the rods to return to their original length.
Post-tensioned
- concrete cast with ducts in the middle through which the cables are then threaded through and stressed. The cables are then normally grouted so they bond to the precast concrete.
Describe the construction of Sandwich Panels and state the risks associated with them for firefighters in the event of a fire.
Consists of an inner core, (normally PIR or PUR) sandwiched between and bonded to a membrane (normally galvanised steel).
Can be used internally and externally for cladding, attached via lightweight fixings and hangers.
Internally - often used to provide enclosures for chilled or sub-zero environments or where a hygienic environment is required.
Issues for Ff’s:
(i) De-lamination the outer layer from the inner core due to the adhesive being affected by by fire heat.
(ii) loses structural integrity rapidly if involved/affect by fire.
(iii) increased chance of flashover due to insulating properties.
(iv) Increased chance of hidden fire spread due to insulating and acoustic properties masking fire spread.
(v) If used as an internal standalone (chilled/sub-zero) structure in warehouses there is a risk of structural collapse from refrigeration appliances housed on top of the sub-zero structure if integrity is compromised by fire.
Also leading to release of refrigeration gases.
(vi) Hard to detect fire external with TIC due to insulating properties.
(vii) Produce large amounts of black toxic smoke if involved in fire.
(viii) Most jointing/fixing elements have extremely limited structural performance in fire conditions leading to a real chance of structural collapse.
State the reason why external cladding is added to buildings and explain how cladding can affect external fire spread.
Added to:
(i) Improve thermal efficiency (insulation)
(ii) Improve Aestetics
(iii) Weather protection (eg rain screen)
How it affects external fire spread:
(i) Rain screens may be fitted with cavuty barriers to allow ventilation and drainage, providing the fire a route to spread.
(ii) Cladding can increase the fire load if it is not produce from fire resistant materials, especially if windows fail and the Coanda effect promotes the spread of heat and fire gases externally.
(iii) If poorly fitted there may be gaps in the cladding reducing integrity and providing a route for fire to spread.
Define the Categories of Fire protection/resistance for Construction/structural materials and give and example of each.
(i) BOX application - using a fire resistant board system to enclosing the material, reduces surface area but does not allow services to be passed through web openings
e.g. fire resistant cladding made from gypsm or vermiculite.
(ii) PROFILE application - using intumescent of cementious coatings follwoing the shape of the section. Provides higher section factor than box application.
e.g. solvent and expoy based intumescent coatings.
(iii) SOLID Application - uses webbing to surround the steel section and the application of a non-reactive cementious coating or concrete is used to fill in the webbed area.
e.g. vermiculite cement spray coating.
