NFPA Journal - September/October 2019 - 48

Challenge:
Atrium design and smoke control

T

O LEND ORDER to the
complex array of spaces
and functions inside the
museum, we created a
vertical hub-and-spoke
circulation system within a large, central
atrium that connected all the stories
and encouraged occupants to use the
monumental stair rather than wait for
elevators. The primary circulation core
is a calm, bright space that provides
visitors both a place of respite and an
opportunity to reorient themselves in
the building. The core ultimately leads
to the rooftop addition, which includes

48 | NFPA JOURNAL * S E P T E M B E R / O C T O B E R 2 0 1 9

a restaurant, special event spaces, and a
large performing arts theater.
With a vertical opening that is
only slightly larger than the stair, we
assumed that traditional hand calculations would be inadequate for designing
the smoke control system. NFPA 92,
Standard for Smoke Control Systems,
contains several algebraic formulas for
calculating the volume of smoke produced by a given design fire, but there
are limitations to the applicability of
those hand calculations.
Indeed, NFPA 92 (Section A.5.9 in
the 2018 edition) explicitly indicates
that empirical equations are not adequate
for atriums with "varying cross-sections and
complex geometry" and
recommends alternative
approaches such as computational fluid dynamics
(CFD) models in those
cases. Since the interaction between the smoke
plume and adjacent
balconies would not be
captured by hand calculations or simple models
such as zone models,
we believed that a CFD
model would be the most
appropriate method for
evaluating the smoke
exhaust system design.
Our theory was
confirmed when early
computer-based smoke
modeling results from
our consultant, Jensen

Hughes, using the fire dynamic simulator (FDS) developed by the US National
Institute of Standards and Technology
(NIST), indicated accumulation of
smoke on levels four and five, even
while the main smoke layer remained
above the top of the stair. Analysis of
those results suggested that the geometry of the space caused eddies of smoke
on those levels, which drove the visibility below the minimum acceptable
threshold of 30 feet.
In order to mitigate the eddies of
smoke on those levels, overhead-coiling
smoke curtains were designed to prevent smoke movement into the adjacent,
occupied spaces. Upon activation of
smoke detectors in the atrium, these
curtains drop at a controlled rate to
completely seal the openings into the
atrium. When retracted, they blend into
the ceiling and are nearly invisible.
The make-up air design also contributed to an effective, efficient
smoke-control system. Since exhausting
air from a space without replacing it
will create negative pressures, make-up
air must be provided to balance the
exhaust air.
We have found that make-up air
ductwork, grilles, and openings often
have a more significant architectural
impact than the exhaust fans and
openings. While exhaust openings are
generally located at the top of the space
and can exhaust a significant volume
of smoke through each opening, the
make-up air inlets are usually larger and
located on the lower levels. This often
requires shafts, ductwork, and wall area
for inlets that are in direct competition
NFPA Journal - September/October 2019

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