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itself on the floors below the neutral plane. In the case of
hot smoke, buoyancy forces can counteract normal
reverse stack effect causing the smoke to move up a
shaft. See Figure 3.
The neutral plane of a building or space is defined as the
elevation where the hydrostatic pressure inside the
building equals the outside pressure. Normally the
neutral plane is located near the midpoint of the building,
but can occur at any floor and depends upon building
design. The neutral plane of a building should be
determined prior to the design of a smoke control
system. ASHRAE's Design of Smoke Management
Systems contains methods for calculating the neutral
plane of a building or space.
Figure 3: Reverse stack effect
Outside
92°F (33.3°C)
Neutral
plane
Wind load
All buildings are to some extent leaky and wind
penetration through these leaks contributes to internal
air movement. Wind can have a dramatic effect on
smoke movement depending upon the wind speed and
direction, the characteristics of the surrounding terrain
(including the shielding effect of adjacent buildings), and
the building shape and height. See Figure 4below.
VM-1 Smoke Management Application Guide
Inside
75°F (23.9 C)
°
Chapter 1: Fire geometry and smoke movement in buildings
Figure 4: Wind effects on a building
Outside
75°F (23.9°C)
Neutral
plane
In fires if a window breaks or is left open in a fire
compartment, it has an effect on smoke movement. If
the opening is on the windward side of the building, the
wind causes a buildup of pressure in the fire
compartment and forces smoke throughout the floor and
possibly to other floors. Pressures caused by the wind in
this condition can be large and easily dominate smoke
movement through the building.
If the opening is on the leeward side of the building, the
reverse is true. The negative pressure created by the
wind vents the smoke from the fire compartment, greatly
reducing the smoke movement through the building.
HVAC systems
Mechanical air handling systems inside a building
condition and move air under normal conditions and can
affect the movement of smoke in a fire. Before we
reached our current understanding of smoke movement
in buildings, most HVAC Systems were shut down when
fires occurred for two primary reasons:
•
The HVAC system rapidly advanced smoke
movement from the room of fire origin to every area
the system served.
Inside
75°F (23.9 C)
°
5
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