Joint (building)
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A building joint is a junction where building elements meet without applying a static load from one element to another. When one or more of these vertical or horizontal elements that meet are required by the local building code to have a fire-resistance rating, the resulting opening that makes up the joint must be firestopped in order to restore the required compartmentalisation.
Qualification requirements
Such joints are often subject to movement, as a function of the building's design basis. For a sample certification listing indicating fire tested motion, click here. Firestops must be able to demonstrate the ability to withstand operational movement prior to fire testing. Firestops for such building joints can be qualified to UL 2079 -- Tests for Fire Resistance of Building Joint Systems.
Whether or not the building elements forming the joint have a fire-resistance rating, the joint design must still consider the anticipated operational movement of each joint. Timing is also important, as freshly poured concrete shrinks [1] particularly during the first few months of a new building, potentially causing joint size changes.
Perimeter slab edge
Firestopping at the "perimeter slab edge", which is gaps between the floors and the backpan of the curtain wall or precast concrete panels, are essential to slow the passage of fire and combustion gases between floors. It is important to note that the firestop at the perimeter slab edge is considered a continuation of the fire-resistance rating of the floor slab. The curtain wall itself, however, is not ordinarily required to have a rating. This causes a quandary as Compartmentalization (fire protection) is typically based upon closed compartments to avoid fire and smoke migrations beyond each engaged compartment. A curtain wall by its very nature prevents the complete compartment (or envelope). The use of fire sprinklers has been shown to mitigate this matter. As such, unless the building is equipped with sprinklers, fire may still travel up the curtain wall, if the glass on the exposed floor is shattered due to fire influence, causing flames to lick up the outside of the building. Falling glass can endanger pedestrians, firefighters and firehoses below. An example of this is the First Interstate Bank Fire in Los Angeles, California. [2] The fire here leapfrogged up the tower by shattering the glass and then consuming the aluminium skeleton holding the glass. Aluminium's melting temperature is 660°C, whereas building fires can reach 1,100°C (see fire-resistance rating article). The melting point of aluminium is typically reached within minutes of the start of a fire. Sprinklering of each floor has a profoundly positive effect on the fire safety of buildings with curtain walls. In the case of the aforementioned fire, it was specifically the activation of the newly installed sprinkler system, which halted the advance of the fire and allowed effective suppression. Had the sprinkler system not become operational at that point in time, the tower would have collapsed onto fire crews and into an adjacent building, while on fire. Exceptionally sound cementitious spray fireproofing also helped to delay and ultimately to avoid the possible collapse of the building, due to having the structural steel skeleton of the building avoid reaching the critical temperature, at which steel normally softens and can no longer hold its intended load, as the post-mortem fire investigation report indicated. This fire proved the positive collective effect of both active fire protection (sprinklers) and passive fire protection (fireproofing).
Head-of-Wall (HOW)
Where fire-resistance rated wall assemblies, be they masonry or drywall, meet the underside of the floor slab above, a movement joint results, which can be subject to compression, as the freshly placed concrete cures and shrinks all over a new building. This joint must be firestopped in a flexible manner. For concrete block walls that are not load-bearing, the contractor is often instructed to stop the wall about 25mm short of the slab above. The drywaller has to fasten a track to the underside of the slab above, which holds the metal studs. The drywall is then either stopped short allowing for a flexible buffer to be inserted or caulked in place, or one may use purpose-made tracks that hold the studs but permit up and down movement [3] without damage to the system.
See also
- Concrete
- Penetration (firestop)
- Sealant
- Firestop
- Curtain wall
- Passive fire protection
- Active fire protection
- Mineral wool
- Packing (firestopping)
- Fire sprinkler
- Articulation
References
External links
